Dr. Jürgen Lindner

Tel.: +49 351 260 3221

Recent publications of the FWIN (magnetism) division

complete FWIN publication list

HZDR publication database


Static and dynamic properties of noncollinear magnetized ferromagnetic films

J. Jiménez-Bustamante, A. A. Lindner, H. N. Koyun Yücel, R. Salikhov, K. Lenz, J. Lindner, R. A. Gallardo


The dynamic matrix method was employed to perform theoretical calculations for investigating both static and
dynamic characteristics of thick ferromagnetic films. This approach considers situations where a noncollinear
equilibrium magnetization exists along the thickness due to a thickness-dependent uniaxial anisotropy and inter-
facial interactions in a synthetic antiferromagnet. In the former scenario, the study exposes a correlation between
noncollinear static magnetization and a nonmonotonic dependence of ferromagnetic resonance frequency, where
a frequency decrease is observed at low fields in the unsaturated regime. Regarding the synthetic antiferromagnet
structure, the research demonstrates noncoherent magnetization rotation in the spin-flop regime, with twisted
magnetization states influencing the critical and nucleation fields that define the spin-flop region. The results of
the investigation were compared to the macrospin approach, where the magnetization is assumed to be uniform
along the thickness. The study suggests that the contribution of noncollinear magnetic moments may mimic the
role of the biquadratic interaction in the macrospin model, implying that such a biquadratic term may be over-
estimated in coupled ferromagnetic films with thicknesses exceeding the material’s intrinsic exchange length.
Finally, the model was compared with experimental data obtained from a Py/Ir/Py synthetic antiferromagnet,
demonstrating that the theoretical consideration of a twisting equilibrium state of the magnetization precisely
reproduces the observed dynamic and static properties of the nanostructure.

Verknüpfte Publikationen


  • Zweitveröffentlichung erwartet ab 04.03.2025

Permalink: https://www.hzdr.de/publications/Publ-39061

Irradiation-induced enhancement of Fe and Al magnetic polarizations in Fe60Al40 films

A. Smekhova, T. Szyjka, E. La Torre, K. Ollefs, B. Eggert, B. Cöster, F. Wilhelm, R. Bali, J. Lindner, A. Rogalev, D. Többens, E. Weschke, C. Luo, K. Chen, F. Radu, C. Schmitz-Antoniak, H. Wende


The rise of Fe magnetic moment, changes in Al electronic structure and a variation of Al magnetic polarization in thin films of transition metal aluminide Fe60Al40 have been probed through the order-disorder phase transition by soft x-ray absorption spectroscopy and x-ray resonant magnetic reflectivity in the extreme ultraviolet regime. In a course of the transition induced by 20 keV Ne+ irradiation with low fluences (∼1014 ions cm−2), x-ray magnetic circular dichroism spectra taken at the Fe L2,3 absorption edges at room and low temperatures revealed a pronounced increase of Fe 3d states spin-polarization. X-ray resonant magnetic reflectivity applied to the Al L2,3 and Fe M2,3 edges allowed to detect the magnetic polarization of Al atoms in the films. The changes in Al electronic structure have been seen by alteration of Al K edge x-ray absorption near edge structure. A difference in anisotropy fields for films before and after irradiation has been observed by element-specific hysteresis loops recorded at low temperatures in absorption and reflection geometries at the Fe L2,3 and M2,3 edges, respectively. An attempt to reduce the top oxide layer by an inductively coupled hydrogen plasma has shown a possibility to recover the chemically ordered

Keywords: chemical disorder; ion-irradiation; XANES; XMCD; plasma treatment

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Verknüpfte Publikationen

Permalink: https://www.hzdr.de/publications/Publ-39038

Generation of localized, half-frequency spin waves in micron sized ferromagnetic stripes: Experiments and simulations

M. G. Copus, T. Hula, C. Heins, L. Flacke, M. Weiler, K. Schultheiß, H. Schultheiß, 
. E. Camley


We demonstrate the nonlinear generation of spin-wave edge modes with half the frequency of the applied oscillating field in a Co25Fe75 ferro- magnetic stripe through micromagnetic simulations and experiments. The generation of half-frequency modes depends on the simultaneous presence of resonances near both the driving frequency and the half-frequency in different regions of the material. The half-frequency genera- tion occurs in a system that is thin enough that typical three-magnon decay would not be allowed in a ferromagnetic resonance experiment in an extended film. We find that a limited range of driving frequencies will produce a half-frequency for a given set of system parameters. This range can be tuned by the strength of the oscillating field and the strength of the static external field. Our experimental results agree well with the findings from the simulations.

Keywords: spin waves; magnons; nonlinearity; Brillouin light scattering; micromagnetic simulations

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Verknüpfte Publikationen


  • Zweitveröffentlichung erwartet ab 10.04.2025

Permalink: https://www.hzdr.de/publications/Publ-39007

Small-scale functional fatigue of a Ni-Mn-Ga Heusler alloy

A. Fareed, J. M. Rosalie, S. Kar, S. Fähler, R. Maaß


Functional fatigue of shape-memory alloys is a considerable threat to reliable service of actuation devices. Here, we demonstrate the essentially degradation-free cyclic phase-transformation behavior of Ni-Mn-Ga microcrystals up to one million stress-driven superelastic cycles. Cyclic dissipation amounts to about 1/5 of the bulk counterpart and remains unaffected during cycling, even after the introduction of dislocation structures via plastic straining. Plastic yielding and the transformation stress largely exceed the known bulk values. However, the transformation-stress is found to strongly depend on plastic pre-straining, which suggests that the size-affected transformation stress is sensitive to the initial defect structure and that it can be tuned by a targeted introduction of dislocations. These findings demonstrate the high suitability of Ni-Mn-Ga as a robust shape-memory alloy in small-scale functional device engineering.

Keywords: Functional fatigue; Magnetic shape memory alloys; Deformation

Permalink: https://www.hzdr.de/publications/Publ-38745

Parametric magnon transduction to spin qubits

M. Bejarano, F. J. T. Goncalves, T. Hache, M. Hollenbach, C. Heins, T. Hula, L. Körber, J. Heinze, Y. Berencen, M. Helm, J. Faßbender, G. Astakhov, H. Schultheiß


The integration of heterogeneous modular units for building large-scale quantum networks requires engineering mechanisms that allow a suitable transduction of quantum information. Magnon-based transducers are especially attractive due to their wide range of interactions and rich nonlinear dynamics, but most of the work to date has focused on linear magnon transduction in the traditional system composed of yttrium iron garnet and diamond, two materials with difficult integrability into wafer-scale quantum circuits. In this work, we present a different approach by utilizing wafer-compatible materials to engineer a hybrid transducer that exploits magnon nonlinearities in a magnetic microdisc to address quantum spin defects in silicon carbide. The resulting interaction scheme points to the unique transduction behavior that can be obtained when complementing quantum systems with nonlinear magnonics.

Keywords: quantum; magnon; nanotechnology; qubit; transduction; defects; spins

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Verknüpfte Publikationen

Permalink: https://www.hzdr.de/publications/Publ-38554

Coherent Magnons with Giant Nonreciprocity at Nanoscale Wavelengths

R. A. Gallardo, M. Weigand, K. Schultheiß, A. Kakay, R. Mattheis, J. Raabe, G. Schütz, A. M. Deac, J. Lindner, S. Wintz


Non-reciprocal wave propagation arises in systems with broken time-reversal symmetry and is key to the functionality of devices, such as isolators or circulators, in microwave, photonic and acoustic applications. In magnetic systems, collective wave excitations known as magnon quasiparticles so far yielded moderate non-reciprocities, mainly observed by means of incoherent thermal magnon spectra, while their occurrence as coherent spin waves (magnon ensembles with identical phase) is yet to be demonstrated. Here, we report the direct observation of strongly non-reciprocal propagating coherent spin waves in a patterned element of a ferromagnetic bilayer stack with antiparallel magnetic orientations. We use time-resolved scanning transmission x-ray microscopy (TR-STXM) to directly image the layer-collective dynamics of spin waves with wavelengths ranging from 5 µm down to 100 nm emergent at frequencies between 500 MHz and 5 GHz. The experimentally observed non-reciprocity factor of these counter-propagating waves is greater than 10 with respect to both group velocities and specific wavelengths. Our experimental findings are supported by the results from an analytic theory and their peculiarities are further discussed in terms of caustic spin-wave focusing.

Keywords: spin wave; magnon; non-reciprocity; magnetic vortex; scanning transmission X-ray microscopy; caustics

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Verknüpfte Publikationen

Permalink: https://www.hzdr.de/publications/Publ-38354

Resonance-Based Sensing of Magnetic Nanoparticles Using Microfluidic Devices with Ferromagnetic Antidot Nanostructures

R. Dowling, R. Narkovic, K. Lenz, A. Oelschlägel, J. Lindner, M. Kostylev


We demonstrated resonance-based detection of magnetic nanoparticles employing novel designs based upon planar (on-chip) microresonators that may serve as alternatives to conventional magnetoresistive magnetic nanoparticle detectors. We detected 130 nm sized magnetic nanoparticle clusters immobilized on sensor surfaces after flowing through PDMS microfluidic channels molded using a 3D printed mold. Two detection schemes were investigated: (i) indirect detection incorporating ferromagnetic antidot nanostructures within microresonators, and (ii) direct detection of nanoparticles without an antidot lattice. Using scheme (i), magnetic nanoparticles noticeably downshifted the resonance fields of an antidot nanostructure by up to 207 G. In a similar antidot device in which nanoparticles were introduced via droplets rather than a microfluidic channel, the largest shift was only 44 G with a sensitivity of 7.57 G/ng. This indicated that introduction of the nanoparticles via microfluidics results in stronger responses from the ferromagnetic resonances. The results for both devices demonstrated that ferromagnetic antidot nanostructures incorporated within planar microresonators can detect nanoparticles captured from dispersions. Using detection scheme (ii), without the antidot array, we observed a strong resonance within the nanoparticles. The resonance’s strength suggests that direct detection is more sensitive to magnetic nanoparticles than indirect detection using a nanostructure, in addition to being much simpler.

Keywords: nanoparticles; sensors; fluids; ferromagnetic resonance; magnetism; microfluidics

Permalink: https://www.hzdr.de/publications/Publ-38241

Three-dimensional magnetic nanotextures with high-order vorticity in soft magnetic wireframes

O. Volkov, O. Pylypovskyi, F. Porrati, F. Kronast, J. A. Fernandez Roldan, A. Kakay, A. Kuprava, S. Barth, F. N. Rybakov, O. Eriksson, S. Lamb-Camarena, P. Makushko, M.-A. Mawass, S. Shakeel, O. V. Dobrovolskiy, M. Huth, D. Makarov


Additive nanotechnology enable curvilinear and three-dimensional (3D) magnetic architectures with tunable topology and functionalities surpassing their planar counterparts. Here, we experimentally reveal that 3D soft magnetic wireframe structures resemble compact manifolds and accommodate magnetic textures of high order vorticity determined by the Euler characteristic, $\chi$. We demonstrate that self-standing magnetic tetrapods (homeomorphic to a sphere; $\chi=+2$) support six surface topological solitons, namely four vortices and two antivortices, with a total vorticity of +2 equal to its Euler characteristic. Alternatively, wireframe structures with one loop (homeomorphic to a torus; $\chi = 0$) possess equal number of vortices and antivortices, which is relevant for spin-wave splitters and 3D magnonics. Subsequent introduction of $N$ holes into the wireframe geometry (homeomorphic to an $N$-torus; $\chi < 0$) enables the accommodation of a virtually unlimited number of antivortices, which suggests their usefulness for non-conventional (e.g. reservoir) computation. Furthermore, complex stray-field topologies around these objects are of interest for superconducting electronics, particle trapping and biomedical applications.

Keywords: Additive nanotechnology; Cuvilinear magnetism; 3D magnetic architectures; Topology; Magnetic wireframes

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  • Rechenzentrum

Permalink: https://www.hzdr.de/publications/Publ-37958

Laser-Induced Positional and Chemical Lattice Reordering Generating Ferromagnetism

T. Pflug, J. Pablo-Navarro, M. S. Anwar, M. Olbrich, C. Magén, M. R. Ibarra, K. Potzger, J. Faßbender, J. Lindner, A. Horn, R. Bali


Atomic scale reordering of lattices can induce local modulations of functional material properties, such as reflectance and ferromagnetism. Pulsed femtosecond laser irradiation enables lattice reordering in the picosecond range. However, the dependence of the phase transitions on the initial lattice order as well as the temporal dynamics of these transitions remain to be understood. This study investigates the laser-induced atomic reordering and the concomitant onset of ferromagnetism in thin Fe-based alloy films with vastly differing initial atomic orders. The optical response to single fs laser pulses on selected prototype systems, one that initially possesses positional disorder, Fe60V40, and a second system initially in a chemically ordered state, Fe60Al40, has been tracked with time. Despite the vastly different initial atomic orders the structure in both systems converges to a positionally ordered but chemically disordered state, accompanied by the onset of ferromagnetism. Time-resolved measurements of the transient reflectance combined with simulations of the electron and phonon temperature reveal that the reordering processes occur via the formation of a transient molten state with an approximate lifetime of 200 ps. These findings provide insights into fundamental processes involved in laser-induced atomic reordering, paving the way for controlling material properties in the picosecond range.

Keywords: positional order; chemical order; atomic reordering; ferrmagnetism; pump-probe reflectometry

Permalink: https://www.hzdr.de/publications/Publ-37775

Multilayer Metamaterials with Ferromagnetic Domains Separated by Antiferromagnetic Domain Walls

R. Salikhov, F. Samad, S. Schneider, D. Pohl, B. Rellinghaus, B. Böhm, R. Ehrler, J. Lindner, N. S. Kiselev, O. Hellwig


Magnetic nano-objects possess great potential for more efficient data processing, storage, and neuromorphic-type applications. Using high perpendicular magnetic anisotropy synthetic antiferromagnets in the form of multilayer-based metamaterials, the antiferromagnetic interlayer exchange energy is purposefully reduced below the out-of-plane demagnetization energy, which controls magnetic domain formation. In this unusual magnetic energy regime, as demonstrated via macroscopic magnetometry and microscopic Lorentz transmission electron microscopy, it becomes possible to stabilize nanometer-scale stripe and bubble textures consisting of ferromagnetic out-of-plane domain cores separated by antiferromagnetic in-plane Bloch-type domain walls. This unique coexistence of mixed ferromagnetic/antiferromagnetic order on the nanometer scale opens so far unexplored perspectives in the architecture of magnetic domain landscapes as well as the design and functionality of individual magnetic textures, such as bubble domains with depth-wise alternating chirality.

Keywords: Magnetizm; Magnetic Domains; Synthetic antiferromagnets; Transmission electron microscopy; micromagnetic simulations

Permalink: https://www.hzdr.de/publications/Publ-37543


Piezostrain as a Local Handle to Control Gyrotropic Dynamics of Magnetic Vortices

V. Iurchuk, S. Sorokin, J. Lindner, J. Faßbender, A. Kakay


We present a study of the piezostrain-tunable gyrotropic dynamics in Co40Fe40B20 vortex microstructures fabricated on a 0.7Pb[Mg1/3Nb2/3]O3-0.3PbTiO3 single-crystal substrate. Using field-modulated-spin-rectification measurements, we demonstrate large frequency tunability (up to 45%) in individual microdisks accessed locally with low surface voltages, and magnetoresistive readout. With increased voltage applied to the substrate, we observe a gradual decrease of the vortex-core gyrotropic frequency associated with the contribution of the strain-induced magnetoelastic energy. The frequency tunability strongly depends on the disk size, with increased frequency downshift for disks with larger diameter. Micromagnetic simulations suggest that the observed size effects originate from the joint action of the strain-induced magnetoelastic and demagnetizing energies in large magnetic disks. These results enable a selective energy-efficient tuning of the vortex gyrotropic frequency in individual vortex-based oscillators with all-electrical operation.

Keywords: Magnetic vortices; Magnetoelastic effect; Magnetoresistance; Spin dynamics

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Permalink: https://www.hzdr.de/publications/Publ-37975

Nontrivial Aharonov-Bohm effect and alternating dispersion of magnons in cone-state ferromagnetic rings

V. Uzunova, L. Körber, A. Kavvadia, G. Quasebarth, H. Schultheiß, A. Kakay, B. Ivanov


Soft magnetic dots in the form of thin rings have unique topological properties. They can be in a vortex state with no vortex core. Here, we study the magnon modes of such systems both analytically and numerically. In an external magnetic field, magnetic rings are characterized by easy-cone magnetization and shows a giant splitting of doublets for modes with the opposite value of the azimuthal mode quantum number. The effect of the splitting can be refereed as a magnon analog of the topology-induced Aharonov-Bohm effect. For this we develop an analytical theory to describe the non-monotonic dependence of the mode frequencies on the azimuthal mode number, influenced by the balance between the local exchange and non-local dipole interactions.

Keywords: Spin waves; Topology; Vortex; Magnetism; Aharonov-Bohm effect; Micromagnetism


Permalink: https://www.hzdr.de/publications/Publ-37849

Influence of oxidic and metallic interfaces on the magnetic damping of Permalloy thin films

V. Ney, R. Salikhov, K. Lenz, O. Hellwig, J. Lindner, A. Ney


Magnetic damping within Permalloy (Py) thin films is studied via temperature- and frequencydependent ferromagnetic resonance (FMR) experiments. While the Py thickness is kept constant at 20 nm, the environment at the film interfaces was systematically varied by fabricating a set of Py thin films grown on widely used substrates and capped with common layers, which are assumed to be suitable to prevent oxidation. The resulting frequency- and temperature-dependence of the FMR linewidth significantly deviates from the expected Gilbert-like behavior and especially for oxidic interfaces unwanted non-Gilbert-like contributions to the magnetic damping appear, in particular at low temperatures. It turns out that Py sandwiched in-between metallic capping and buffer layers
of Al exhibits the smallest magnetic damping of purely Gilbert-like nature.

Keywords: Thin films; ferromagnetism; ferromagnetic resonance; anisotropy; linewidth

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  • Zweitveröffentlichung erwartet ab 11.12.2024

Permalink: https://www.hzdr.de/publications/Publ-37626

Influence of Dielectric Capping on the Optical Properties of Two-Dimensional Transition Metal Dichalcogenides: Implications for nano optoelectronics

Y. Li, O. Steuer, K. Lin, F. Samad, D. Sokolova, A. Erbe, M. Helm, S. Zhou, S. Prucnal


The properties of transition metal dichalcogenides (TMDCs) are highly sensitive to doping and surface-state defects, making it crucial to fabricate high-performance nanoelectronic devices from defect-free materials and gate dielectrics that have a low interface-state density. In this work, the optical and structural properties of mechanically exfoliated mono-, bi- and trilayer thick TMDCs with Al2O3, Si3N4 or SiO2 as a potential gate dielectric layer are investigated. The photoluminescence (PL) and micro-Raman results indicate that all the dielectrics investigated increase the doping of the TMDCs monolayers, quench the emission of neutral excitons and enhance the trion emission. Plasma enhanced chemical vapour deposition was found to generate more defects in the monolayer TMDCs than atomic layer deposition. We establish the relationship between the dielectric deposition process and the optical properties of TMDCs, which could be of interest for future nanoelectronics based on 2D materials.

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Permalink: https://www.hzdr.de/publications/Publ-37605

A strain-controlled magnetostrictive pseudo spin valve

V. Iurchuk, J. Bran, M. Acosta, B. Kundys


Electric-field control of magnetism via an inverse magnetostrictive effect is an alternative path toward improving energy-efficient storage and sensing devices based on a giant magnetoresistance effect. In this Letter, we report on lateral electric-field driven strain-mediated modulation of magnetotransport properties in a Co/Cu/Py pseudo spin valve grown on a ferroelectric 0.7Pb[Mg1/3Nb2/3]O3–0.3PbTiO3 substrate. We show a decrease in the giant magnetoresistance ratio of the pseudo spin valve with the increase in the electric field, which is attributed to the deviation of the Co layer magnetization from the initial direction due to strain-induced magnetoelastic anisotropy contribution. Additionally, we demonstrate that strain-induced magnetic anisotropy effectively shifts the switching field of the magnetostrictive Co layer, while keeping the switching field of the nearly zero-magnetostrictive Py layer unaffected due to its negligible magnetostriction. We argue that magnetostrictively optimized magnetic films in properly engineered multilayered structures can offer a path to enhancing the selective magnetic switching in spintronic devices.

Keywords: Inverse magnetostrictive effect; Magnetoresistance; Ferroelectric materials; Magnetic anisotropy; Pseudo spin valve; Spintronics

Permalink: https://www.hzdr.de/publications/Publ-37582

Control of Four-Magnon Scattering by Pure Spin Current in a Magnonic Waveguide

T. Hache, L. Körber, T. Hula, K. Lenz, A. Kakay, O. Hellwig, J. Lindner, J. Faßbender, H. Schultheiß


We use a pure spin current originating from the spin Hall effect to generate a spin-orbit torque strongly reducing the effective damping in an adjacent ferromagnet. Because of additional microwave excitation, large spin-wave amplitudes are achieved exceeding the threshold for four-magnon scattering, thus resulting in additional spin-wave signals at discrete frequencies. Two or more modes are generated below and above the directly pumped mode with equal frequency spacing. It is shown how this nonlinear process can be controlled in magnonic waveguides by the applied dc current and the microwave pumping power. The sudden onset of the nonlinear effect after exceeding the thresholds can be interpreted as a spiking phenomenon, which makes the effect potentially interesting for neuromorphic computing applications. Moreover, we investigated this effect under microwave frequency and external field variation. The appearance of the additional modes was investigated in the time domain, revealing a time delay between the directly excited and the simultaneously generated nonlinear modes. Furthermore, spatially resolved measurements show different spatial decay lengths of the directly pumped mode and nonlinear modes.

Keywords: spin waves; magnetism; BLS; four-magnon scattering; spin current; spintronics; magnonics

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Permalink: https://www.hzdr.de/publications/Publ-37519

Efficient ultrafast field-driven spin current generation for spintronic terahertz frequency conversion

I. Ilyakov, A. Brataas, T. de Oliveira, O. Ponomaryov, J.-C. Deinert, O. Hellwig, J. Faßbender, J. Lindner, R. Salikhov, S. Kovalev


Efficient generation and control of spin currents launched by terahertz (THz) radiation with subsequent ultrafast spin-to-charge conversion is the current challenge for the next-generation of high-speed communication and data processing units. Here, we demonstrate that THz light can efficiently drive coherent angular momentum transfer in nanometer-thick ferromagnet/heavy-metal heterostructures. This process is non-resonant and does neither require external magnetic fields nor cryogenics. The efficiency of this process is more than one order of magnitude higher as compared to the recently observed THz induced spin-pumping in MnF2 antiferromagnet. The coherently driven spin currents originate from the ultrafast spin Seebeck effect, caused by a THz-induced temperature imbalance in electronic and magnonic temperatures and fast relaxation of the electron-phonon system. Owing to the fact that the electron-phonon relaxation time is comparable with the period of a THz wave, the induced spin current results in THz second harmonic generation and THz optical rectification, providing a spintronic basis for THz frequency mixing and rectifying components.

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  • T-ELBE

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Permalink: https://www.hzdr.de/publications/Publ-37375

All-electrical operation of a Curie-switch at room temperature

V. Iurchuk, O. Kozlov, S. Sorokin, S. Zhou, J. Lindner, S. Reshetniak, A. Kravets, D. Polishchuk, V. Korenivski


We present all-electrical operation of a Fe$_x$Cr$_{1-x}$-based Curie switch at room temperature. More specifically, we study the current-induced thermally-driven transition from ferromagnetic to antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect coupling in a Fe/Cr/Fe$_{17.5}$Cr$_{82.5}$/Cr/Fe multilayer. Magnetometry measurements at different temperatures show that the transition from the ferromagnetic to the antiferromagnetic coupling at zero field is observed at $\sim$325K. Analytical modelling confirms that the observed temperature-dependent transition from indirect ferromagnetic to indirect antiferromangetic interlayer exchange coupling originates from the modification of the effective interlayer exchange constant through the ferromagnetic-to-paramagnetic transition in the Fe$_{17.5}$Cr$_{82.5}$ spacer with minor contributions from the thermally-driven variations of the magnetization and magnetic anisotropy of the Fe layers. Room-temperature current-in-plane magnetotransport measurements on the patterned Fe/Cr/Fe$_{17.5}$Cr$_{82.5}$/Cr/Fe strips show the transition from the 'low-resistance' parallel to the 'high-resistance' antiparallel remanent magnetization configuration, upon increased probing current density. Quantitative comparison of the switching fields, obtained by magnetometry and magnetotransport, confirms that the Joule heating is the main mechanism responsible for the observed current-induced resistive switching.

Keywords: RKKY coupling; Interlayer exchange; Spin valve; Magnetoresistance; Joule heating

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Permalink: https://www.hzdr.de/publications/Publ-37306

Microfabrication Approaches on Magnetic Shape Memory Films

S. Kar, K. Nielsch, S. Fähler, H. Reith


Magnetic shape memory alloys are emerging multifunctional materials that enable applications
like high-stroke actuation, solid-state refrigeration, and energy harvesting of waste heat. Thin
films of these alloys promise integration in microsystems to exploit their multifunctional
properties at the microscale. However, the microfabrication process of these Heusler alloys is
difficult. Here, we investigate different etching techniques for the microfabrication of epitaxial
Ni-Mn-Ga films, explain the encountered challenges, and demonstrate ways to overcome them.
Our results show that wet chemical etching is suitable for large patterned structures, while
reactive ion etching of Ni-Mn-Ga films is unsuitable due to redeposition. For patterning
structures below 10 μm with clean and sharp edges, the best results are obtained by ion-beam
etching with adjusted sample-stage tilt. Finally, we demonstrate a microfabrication process
using Si microtechnology to fabricate partially free-standing structures. Our findings give
guidelines for the fabrication and integration of these smart materials in Si-based microsystems.

Keywords: magnetic shape memory alloys; epitaxial films; microfabrication; redeposition; ion-beam etching; freestanding structures

Permalink: https://www.hzdr.de/publications/Publ-37300

Guided acoustic waves in thin epitaxial films: experiment and inverse problem solution for NiTi

T. Grabec, Z. Soudná, K. Repček, K. Lünser, S. Fähler, P. Stoklasováa, P. Sedlák, H. Seiner


Despite the fundamental and technological importance of the elastic constants, a suitable method
for their full characterization in epitaxial films is missing. Here we show that transient grating
spectroscopy (TGS) with highly 𝑘−vector-selective generation and detection of acoustic waves
is capable of determination of all independent elastic coefficients of an epitaxial thin film
grown on a single-crystalline substrate. This experimental setup enables detection of various
types of guided acoustic waves and evaluation of the directional dependence of their speeds
of propagation. For the studied model system, which is a 3 μm thin epitaxial film of the NiTi
shape memory alloy on an MgO substrate, the TGS angular maps include Rayleigh-type surface
acoustic waves as well as Sezawa-type and Love-type modes, delivering rich information on
the elastic response of the film under different straining modes. The resulting inverse problem,
which means the calculation of the elastic constants from the TGS maps, is subsequently solved
using the Ritz-Rayleigh numerical method. Using this approach, tetragonal elastic constants of
the NiTi film and their changes with the austenite→martensite phase transition are analyzed.

Keywords: guided acoustic waves; elastic anisotropy; epitaxial thin film; shape memory alloys; transient grating spectroscop; Ritz-Rayleigh method

Permalink: https://www.hzdr.de/publications/Publ-37254

Solving the puzzle of hierarchical martensitic microstructures in NiTi by (111)-oriented epitaxial films

K. Lünser, A. Undisz, M. F.-X. Wagner, K. Nielsch, S. Fähler


The martensitic microstructure decides on the functional properties of shape memory alloys. However, for the most commonly used alloy, NiTi, it is still unclear how its microstructure is built up because the analysis is hampered by grain boundaries of polycrystalline samples. Here, we eliminate grain boundaries by using epitaxially grown films in (111)B2 orientation. By combining scale-bridging microscopy with integral inverse pole figures, we solve the puzzle of the hierarchical martensitic microstructure. We identify two martensite clusters as building blocks and three kinds of twin boundaries. Nesting them at different length scales explains why habit plane variants with 〈011〉B19' twin boundaries and {942} habit planes are dominant; but also some incompatible interfaces occur. Though the observed hierarchical microstructure agrees with the phenomenological theory of martensite, the transformation path decides which microstructure forms. The combination of local and global measurements with theory allows solving the scale bridging 3D puzzle of the martensitic microstructure in NiTi exemplarily for epitaxial films.

Keywords: NiTi; martensitic microstructure; epitaxial film

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Permalink: https://www.hzdr.de/publications/Publ-37198

Incipient stress-induced phase transformation of a Ni-Mn-Ga Heusler alloy: A small-scale design challenge

A. Fareed, J. Rosalie, S. Kumar, S. Kar, T. Hickel, S. Fähler, R. Maaß


NiMnGa shape-memory alloys are promising candidates for large strain actuation or magnetocaloric cooling devices. In view of potential small-scale applications, we probe here nanomechanically the stress-induced austenite-martensite transition in single crystalline austenitic thin films as a function of temperature. In 0.5 µm thin films, a marked incipient phase transformation to martensite is observed during nanoindentation, leaving behind pockets of residual martensite after unloading. These nanomechanical instabilities occur irrespective of deformation rate and temperature, are Weibull distributed, and reveal large spatial variations in transformation stress. In contrast, at a larger film thickness of 2 m fully reversible transformations occur, and mechanical loading remains entirely smooth. Ab-initio simulations demonstrate how an in-plane constraint can considerably increase the martensitic transformation stress, explaining the thickness-dependent nanomechanical behavior. These findings give insights into how reduced dimensions and constraints can lead to unexpectedly large transformation stresses in the studied shape-memory Heusler alloy.

Keywords: Ni2MnGa; Nanoindentation; Epitaxial Film

Permalink: https://www.hzdr.de/publications/Publ-37187

Integration of Multifunctional Epitaxial (Magnetic) Shape Memory Films in Silicon Microtechnology

L. Fink, S. Kar, K. Lünser, K. Nielsch, H. Reith, S. Fähler


Magnetic shape memory alloys exhibit various multifunctional properties, which range from high stroke actuation and magnetocaloric refrigeration to thermomagnetic energy harvesting. Most of these applications benefit from miniaturization and a single crystalline state. Epitaxial film growth is so far only possible on some oxidic substrates, but they are expensive and incompatible with standard microsystem technologies. Here, we demonstrate epitaxial growth of Ni-Mn-based Heusler alloys with single crystal-like properties on silicon substrates by using a SrTiO3 buffer. We show that this allows using standard microfabrication technologies to prepare partly freestanding patterns. Our approach is versatile, as we demonstrate its applicability for the NiTi shape memory alloy and discuss for spintronic and thermoelectric Heusler alloys. This paves the way for integrating additional multifunctional effects into state-of-the-art microelectronic and micromechanical technology, which is based on silicon.

Keywords: Magnetic shape memory alloys; Silicon microtechnology; Ni2MnGa; NiTi; Epitaxial film growth

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Permalink: https://www.hzdr.de/publications/Publ-37186

Determining the preferred directions of magnetisation in cubic crystals using symmetric polynomial inequalities

F. Samad, O. Hellwig


For a magnetic material, the easy and hard magnetic axes describe the directions of favourable respectively unfavourable alignment of the magnetisation. In this article, we describe how to determine these axes for cubic magnetic crystals. Usually it is assumed without further reasoning that they coincide with some principal symmetry directions of the crystal [Bozorth, Phys. Rev. 50, 1076–1081 (1936)], which is however invalid in general. In contrast, we present a full and elementary analysis using symmetric polynomial inequalities, which are well suited to the symmetry of the problem.

Keywords: Anisotropy; cubic magnetic anisotropy; symmetric polynomial inequalities; symmetry; cubic crystal


Permalink: https://www.hzdr.de/publications/Publ-37018

Combining x-ray real and reciprocal space mapping techniques to explore the epitaxial growth of semiconductors

S. Magalhaes, J. Salgado Cabaco, O. Concepcion, D. Buca, M. Stachowicz, F. Oliveira, M. F. Cerqueira, K. Lorenz, E. Alves


In the present work, the importance of determining the strain states of semiconductor compounds with high accuracy is
demonstrated. For the matter in question, new software titled LAPAs, the acronym for LAttice PArameters is presented. The
lattice parameters as well as the chemical composition of Al1−xIn x N and Ge1−xSn x compounds grown on top of GaN- and
Ge- buffered c-Al2O3 and (001) oriented Si substrates, respectively, are calculated via the real space Bond’s method. The
uncertainties in the lattice parameters and composition are derived, compared and discussed with the ones found via x-ray
diffraction reciprocal space mapping. Broad peaks lead to increased centroid uncertainty and are found to constitute up to
99% of the total uncertainty in the lattice parameters. Refraction correction is included in the calculations and found to have
an impact of 0.001 Å in the lattice parameters of both hexagonal and cubic crystallographic systems and below 0.01% in the
quantification of the InN and Sn contents. Although the relaxation degrees of the nitride and tin compounds agree perfectly
between the real and reciprocal-spaces methods, the uncertainty in the latter is found to be ten times higher. The impact of
the findings may be substantial for the development of applications and devices as the intervals found for the lattice match
the condition of Al1−xIn x N grown on GaN templates vary between ∼1.8% (0.1675-0.1859) and 0.04% (0.1708-0.1712) if
derived via the real- and reciprocal spaces methods. © 2023 The Author(s). Published by IOP Publishing Ltd.

Keywords: Bond's method; Chemical composition; Lattice parameters; Uncertainties; Gallium Nitride; III-V Semiconductors

Permalink: https://www.hzdr.de/publications/Publ-37002

Interaction of domain walls with grain boundaries in uniaxial insulating antiferromagnets

O. Pylypovskyi, N. Hedrich, A. Tomilo, T. Kosub, K. Wagner, R. Hübner, B. Shields, D. Sheka, J. Faßbender, P. Maletinsky, D. Makarov


A search for high-speed and low-energy memory devices puts antiferromagnetic thin films at the forefront of spintronic research. Here, we develop a material model of a granular antiferromagnetic thin film with uniaxial anisotropy and provide fundamental insight into the interaction of antiferromagnetic domain walls with grain boundaries. This model is validated on thin films of the antiferromagnetic insulator \ch{Cr2O3}, revealing complex maze-like domain patterns hosting localized nanoscale domains down to 50 nm. We show that the inter-grain magnetic parameters can be estimated based on an analysis of high-resolution images of antiferromagnetic domain patterns examining the domain patterns' self-similarity and the statistical distribution of domain sizes. Having a predictive material model and understanding of the pinning of domain walls on grain boundaries, we put forth design rules to realize granular antiferromagnetic recording media.

Keywords: antiferromagnetism; granular media; spin-lattice simulations; Nitrogen vacancy magnetometry

Permalink: https://www.hzdr.de/publications/Publ-36982

How to grow single-crystalline and epitaxial NiTi films in (100)- and (111)-orientation

K. Lünser, A. Undisz, K. Nielsch, S. Fähler


Understanding the martensitic microstructure in NiTi thin films helps to optimize their properties for applications in microsystems. Epitaxial and single-crystalline films can serve as model systems to understand the microstructure, as well as to exploit the anisotropic mechanical properties of NiTi. Here, we analyze the growth of NiTi on single-crystalline MgO(100) and Al2O3(0001) substrates and optimize film and buffer deposition conditions to achieve epitaxial films in (100)- and (111)-orientation. On MgO(100), we compare the transformation behavior and crystal quality of (100)-oriented NiTi films on different buffer layers. We demonstrate that a vanadium buffer layer helps to decrease the low-angle grain boundary density in the NiTi film, which inhibits undesired growth twins and leads to higher transformation temperatures. On Al2O3(0001), we analyze the orientation of a chromium buffer layer and find that it grows (111)-oriented only in a narrow temperature range around 500 °C. By depositing the Cr buffer below the NiTi film, we can prepare (111)-oriented, epitaxial films with transformation temperatures above room temperature. Transmission electron microscopy (TEM) confirms a martensitic microstructure with Guinier Preston (GP)-zone precipitates at room temperature. We identify the deposition conditions to approach the ideal single crystalline state, which is beneficial for the analysis of the martensitic microstructure and anisotropic mechanical properties in different film orientations.

Keywords: NiTi films; shape memory alloys; epitaxial film growth; Nitinol

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Permalink: https://www.hzdr.de/publications/Publ-36955

Structural and magnetic properties of thin cobalt films with mixed hcp and fcc phases

G. I. Patel, F. Ganss, R. Salikhov, S. Stienen, L. Fallarino, R. Ehrler, R. A. Gallardo, O. Hellwig, K. Lenz, J. Lindner


Cobalt is a magnetic material that finds extensive use in various applications, ranging from magnetic storage to ultrafast spintronics. Usually, it exists in two phases with different crystal lattices, namely in hexagonal close packed (hcp) or face-centered cubic (fcc) structure. The crystal structure of Co films significantly influences the magnetic and spintronic properties. We report on the thickness dependence of the structural and magnetic properties of sputter-deposited Co on a Pt seed layer. It grows in an hcp lattice at lower thickness. In thicker films it becomes a mixed hcp-fcc phase due to a stacking fault progression along the growth direction. The x-ray-based reciprocal space map technique has been employed to distinguish and confirm the presence of both phases. Moreover, the precise determination of Land ́e’s g-factor by ferromagnetic resonance provides valuable insights into the structural properties. In our detailed experiments, we observed that a structural variation results in a nonmonotonic variation of the magnetic anisotropy along the thickness. The work offers information of great significance in terms of practical application, for both fundamental physics and potential applications of thin films with perpendicular magnetic anisotropy.

Keywords: Magnetic Anisotropy; Gradient Anisotropy; Ferromagnetic Resonance


  • Zweitveröffentlichung erwartet ab 27.11.2024

Permalink: https://www.hzdr.de/publications/Publ-36824

Direct magnetic manipulation of a permalloy nanostructure by a focused cobalt ion beam

J. Pablo-Navarro, N. Klingner, G. Hlawacek, A. Kakay, L. Bischoff, R. Narkovic, P. Mazarov, R. Hübner, F. Meyer, W. Pilz, J. Lindner, K. Lenz


We present results of direct maskless magnetic patterning of ferromagnetic nanostructures using a cobalt focused ion beam (FIB) system. The liquid metal ion source of the FIB was made of a Co36Nd64 alloy. A Wien mass filter allows for selecting the ion species. Using the FIB, we implanted narrow tracks of Co ions into a nominal 5000×1000×50 nm3 permalloy strip. We observed the Co-induced changes of the magnetic properties by measuring the sample with microresonator ferromagnetic resonance before and after the implantation. Regions as small as 50 nm can be implanted up to concentrations of at.-10 % near the surface. This allows for easy magnetic modification of edge-localized spin waves with a lateral resolution otherwise hard to reach. The direct-write maskless FIB process is quick and convenient for optical measurement techniques, as it does not involve the virtually impossible removal of ion-hardened resist masks one would face when using lithography with broad-beam ion implantation

Keywords: Ferromagnetic resonance; nanostructures; ferromagnetism; focused ion beams; spin-wave dynamics

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  • Zweitveröffentlichung erwartet ab 26.10.2024

Permalink: https://www.hzdr.de/publications/Publ-36822

Quantifying the spin-wave asymmetry in single and double rectangular Ni80Fe20 microstrips by TR-STXM, FMR and micromagnetic simulations

S. Pile, A. Ney, K. Lenz, R. Narkovic, J. Lindner, S. Wintz, J. Förster, S. Mayr, M. Weigand


The asymmetry of spin-wave patterns in confined rectangular Ni80Fe20 microstrips, both in single and double-strip geometries, is quantified. The results of time-resolved scanning transmission x-ray microscopy (TR-STXM) and micromagnetic simulations are compared. The micromagnetic simulations were set up based on the parameters determined from ferromagnetic resonance measurements at 14.015 GHz. For the TR-STXM measurements and the corresponding simulations the excitation was a uniform microwave field with a fixed frequency of 9.43 GHz, while the external static magnetic field was swept. In the easy axis orientation of the analyzed microstrip, the results show a higher asymmetry for the double microstrip design, indicating an influence of the additional microstrip placed in close proximity to the analyzed one.

Keywords: ferromagnetic resonance; magnonics; micromagnetic simulations; mumax3; spin-wave dynamics; spin-wave imaging; scanning transmission x-ray microscopy

Permalink: https://www.hzdr.de/publications/Publ-36808

Spin pumping into a partially compensated antiferromagnetic/paramagnetic insulator

M. Buchner, K. Lenz, V. Ney, J. Lindner, A. Ney


Spin pumping from a metallic ferromagnet (FM) into an insulating antiferromagnet has been studied across the magnetic phase transition by means of temperature-dependent, broad-band ferromagnetic resonance (FMR) experiments. A set of spin pumping heterostructures consisting of Permalloy (Ni80Fe20) as FM and Zn1−xCoxO with x = 0.3, 0.5 and 0.6 (Co:ZnO) as antiferromagnetic insulator has been used where previous experiments have already pointed out the possibility of the existence of spin-pumping. The present experiment allow to reliably separate the various contributions of the temperature-dependent Gilbert damping parameter to the FMR line-width. A careful analysis of the obtained data demonstrates a significant increase of the temperature-dependence of the Gilbert damping parameter alpha(T ) around the magnetic phase transition of Co:ZnO which extends up to room temperature, confirming spin pumping into the fluctuating spin sink of an antiferromagnetic/paramagnetic insulator.

Keywords: spin pumping; ferromagnetic resonance; magnetic oxides

Permalink: https://www.hzdr.de/publications/Publ-36803

Growth twins and premartensite microstructure in epitaxial Ni-Mn-Ga films

S. Kar, Y. Ikeda, K. Lünser, T. George Woodcock, K. Nielsch, H. Reith, R. Maaß, S. Fähler


Magnetic shape memory alloys have been examined intensively due to their multifunctionality and multitude of
physical phenomena. For both areas, epitaxial films are promising since the absence of grain boundaries is beneficial for applications in microsystems and they also allow to understand the influence of a reduced dimension on the physical effects. Despite many efforts on epitaxial films, two particular aspects remain open. First, it is not
clear how to keep epitaxial growth up to high film thickness, which is required for most microsystems. Second, it
is unknown how the microstructure of premartensite, a precursor state during the martensitic transformation,
manifests in films and differs from that in bulk.
Here, we focus on micrometer-thick austenitic Ni-Mn-Ga films and explain two distinct microstructural features
by combining high-resolution electron microscopy and X-ray diffraction methods. First, we identify pyramid-
shaped defects, which originate from {1 1 1} growth twinning and cause the breakdown of epitaxial growth.
We show that a sufficiently thick Cr buffer layer prevents this breakdown and allows epitaxial growth up to a
thickness of at least 4 μm. Second, premartensite exhibits a hierarchical microstructure in epitaxial films. The reduced dimension of films results in variant selection and regions with distinct premartensite variants, unlike its
microstructure in bulk.

Keywords: Epitaxial films; Magnetic shape memory alloy; Twinning; Premartensite; Hierarchical microstructure

Permalink: https://www.hzdr.de/publications/Publ-36786

Chirality coupling in topological magnetic textures with multiple magnetochiral parameters

O. Volkov, D. Wolf, O. Pylypovskyi, A. Kakay, D. D. Sheka, B. Büchner, J. Faßbender, A. Lubk, D. Makarov


Chiral effects originate from the lack of inversion symmetry within the lattice unit cell or sample’s shape. Being mapped onto magnetic ordering, chirality enables topologically non-trivial textures with a given handedness. Here, we demonstrate the existence of a static 3D texture characterized by two magnetochiral parameters being magnetic helicity of the vortex and geometrical chirality of the core string itself in geometrically curved asymmetric permalloy cap with a size of 80 nm and a vortex ground state. We experimentally validate the nonlocal chiral symmetry breaking effect in this object, which leads to the geometric deformation of the vortex string into a helix with curvature 3 μm−1 and torsion 11 μm−1. The geometric chirality of the vortex string is determined by the magnetic helicity of the vortex texture, constituting coupling of two chiral parameters within the same texture. Beyond the vortex state, we anticipate that complex curvilinear objects hosting 3D magnetic textures like curved skyrmion tubes and hopfions can be characterized by multiple coupled magnetochiral parameters, that influence their statics and field- or current-driven dynamics for spin-orbitronics and magnonics.

Keywords: Non-local chiral symmetry breaking; Magnetic vortex

Permalink: https://www.hzdr.de/publications/Publ-36672

Terahertz harmonic generation from graphite pencil drawings

A. Arshad, H. N. Koyun, R. Salikhov, M. Gensch, I. Ilyakov, O. Ponomaryov, G. L. Prajapati, K. Mavridou, J. Lindner, J.-C. Deinert, C. Ünlü, T. de Oliveira, S. Kovalev


We study the third harmonic generation (THG) of graphite layers on paper substrate upon excitation with intense (up to 100 kV/cm) narrowband terahertz (THz) pulses. Highest THG efficiencies are comparable with those of CVD-grown single-layer graphene. Samples were hand-drawn, using commercially available pencils. The THG response showed a high sensitivity regarding the hatching direction relative to the THz polarization orientation. Using Raman spectroscopy, we confirmed the occurrence of graphene-like structures in the samples. Our findings demonstrate the feasibility of virtually no cost and easy to fabricate materials for THz nonlinear optics.

Keywords: Graphite; terahertz harmonic generation; Dirac materials; nonlinear terahertz optics; Raman Spectroscopy

Permalink: https://www.hzdr.de/publications/Publ-36523

Tailoring of thermomagnetic properties in Ni-Mn-Ga films through Cu addition

L. Fink, K. Nielsch, S. Fähler


Thermomagnetic generators enable the conversion of low-grade waste heat into electric energy. The performance of a generator is intimately connected with the active thermomagnetic material used. Heusler alloys had been proposed as ideal systems for thermomagnetic microsystems, as they comprise a tuneable transition temperature just above room temperature, a steep change of magnetization within a narrow temperature change, a low heat capacity, and are easily processable by common deposition techniques.
In this work, we present a path to optimize Heusler films for thermomagnetic applications, which need different properties compared to magnetocaloric applications. We focus on the key thermomagnetic properties like 1) the thermomagnetic working temperature T* and 2) the change of magnetization with the change of temperature ∆M/∆T and correlate them with common properties like 3) crystal structure, 4) martensitic transition temperature, 5) Curie temperature and 6) spontaneous magnetization M0073. We systematically examine all these properties on polycrystalline Ni-Mn-Ga-Cu films prepared by combinatorial sputter deposition and subjected to a heat treatment. Our analysis allows disentangling the effects in changing the number of valence electrons trough the addition of Cu and the alteration of chemical order before and after heat treatment.

Keywords: Heusler alloys; Curie temperature; Magnetic films; Thermomagnetic material; Energy harvesting

Permalink: https://www.hzdr.de/publications/Publ-36512

Transport properties of Fe60Al40 during the B2 to A2 structural phase transition

S. Sorokin, M. S. Anwar, G. Hlawacek, R. Boucher, J. Salgado Cabaco, K. Potzger, J. Lindner, J. Faßbender, R. Bali


The variation of transport behaviour in a mesoscopic Fe60Al40 wire, initially possessing the ordered B2-phase structure, has been observed while inducing a phase transition to the disordered A2 structure. Gradual disordering was achieved using a highly focused beam of Ne+-ions. Both electrical resistance and anomalous Hall effect were measured in parallel with the local ion irradiation. Both the normal and Hall resistivity show a peak as a function of fluence. Moreover, the relationship between Hall resistivity and normal resistivity reconfirms the presence of two distinct regimes in the transition. Furthermore, field-dependence and temperature-dependence measurements were used to identify that it is necessary to consider the effect of scattering from magnetic clusters to understand these different regimes in transport properties.

Keywords: ion beams; magnetic materials; phase transitions; transport properties; ion microscope; magnetic clusters; ion irradiation

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Permalink: https://www.hzdr.de/publications/Publ-36384

Growth and Martensitic Transformation of Ferromagnetic Co-Cr-Ga-Si Epitaxial Films

Y. Ge, K. Lünser, F. Ganss, P. Gaal, L. Fink, S. Fähler


During cooling, conventional martensitic transformation can only be realized from austenite to martensite. Recently, a so-called reentrant martensitic transformation obtained much interest due to an additional transformation from martensite to austenite during further cooling. Obviously, materials with this reentrant transformation will increase the number of physical effects and possible applications. However, until now, only bulk samples are reported available, which are not suitable for applications in micro-devices. In this work, ferromagnetic Co-Cr-Ga-Si films were selected as a model system to explore the reentrant transformation behavior in thin films. We observed that the films grow epitaxially on MgO (100) substrates and exhibit a martensitic transformation if deposited at a sufficiently high temperature or with an additional heat treatment. Film within the austenite state are ferromagnetic while films within the martensitic state just exhibit a very low ferromagnetism order.

Keywords: Co-Cr-Ga-Si; martensitic transformation; reentrant martensite; epitaxial film; ferromagnetism

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Permalink: https://www.hzdr.de/publications/Publ-36257

Interpretation of spin-wave modes in Co/Ag nanodot arrays probed by broadband ferromagnetic resonance

D. Markó, R. Cheenikundil, J. Bauer, K. Lenz, W.-C. Chuang, K.-W. Lin, J.-C. Wu, M. D’Aquino, R. Hertel, D. S. Schmool


Ferromagnetic resonance (FMR) and the measurement of magnetization dynamics in general have become sophisticated tools for the study of magnetic systems at the nanoscale. Nanosystems, such as the nanodots of this study, are technologically important structures, which find applications in a number of devices, such as magnetic storage and spintronic systems. In this work, we describe the detailed investigation of cobalt nanodots with a \SI{200}{\nm} diameter arranged in a square pitch array with a periodicity of \SI{400}{\nm}. Due to their size, such structures can support standing spin-wave modes, which can have complex spectral responses. To interpret the experimentally measured broadband FMR, we are comparing the spectra of the nanoarray structure with the unpatterned film of identical thickness. This allows us to obtain the general magnetic properties of the system, such as the magnetization, $g$-factor and magnetic anisotropy. We then use state-of-the-art simulations of the dynamic response to identify the nature of the excitation modes. This allows us to assess the boundary conditions for the system. We then proceed to calculate the spectral response of our system, for which we obtained good agreement. Indeed, our procedure provides a high degree of confidence, since we have interpreted all the experimental data to a good degree of accuracy. In presenting this work, we provide a full description of the theoretical framework and its application to our system, and we also describe in detail the novel simulation method used.

Keywords: spin waves; ferromagnetic resonance; nanodot arrays


Permalink: https://www.hzdr.de/publications/Publ-35813

Modification of three-magnon splitting in a flexed magnetic vortex

L. Körber, C. Heins, I. Soldatov, R. Schäfer, A. Kakay, H. Schultheiß, K. Schultheiß


We present an experimental and numerical study of three-magnon splitting in a micrometer-sized magnetic disk with the vortex state strongly deformed by static in-plane magnetic fields. Excited with a large enough power at frequency fRF, the primary radial magnon modes of a cylindrical magnetic vortex can decay into secondary azimuthal modes via spontaneous three-magnon splitting. This nonlinear process exhibits selection rules leading to well-defined and distinct frequencies fRF/2±Δf of the secondary modes. Here, we demonstrate that three-magnon splitting in vortices can be significantly modified by deforming the magnetic vortex with in-plane magnetic fields, leading to a much richer three-magnon response. We find that, with increasing field, an additional class of secondary modes is excited which are localized to the highly-flexed regions adjacent to the displaced vortex core. While these modes satisfy the same selection rules of three-magnon splitting, they exhibit a much lower three-magnon threshold power compared to regular secondary modes of a centered vortex. The applied static magnetic fields are small (≃ 10 mT), providing an effective parameter to control the nonlinear spectral response of confined vortices. Our work expands the understanding of nonlinear magnon dynamics in vortices and advertises these for potential neuromorphic applications based on magnons.

Keywords: spin wave; magnon; vortex; BLS; micromagnetic modeling; Kerr microscopy; three-magnon splitting; reservoir computing

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Permalink: https://www.hzdr.de/publications/Publ-35559

Coupling of terahertz light with nanometre-wavelength magnon modes via spin–orbit torque

R. Salikhov, I. Ilyakov, L. Körber, A. Kakay, R. A. Gallardo, O. Ponomaryov, J.-C. Deinert, T. de Oliveira, K. Lenz, J. Faßbender, S. Bonetti, O. Hellwig, J. Lindner, S. Kovalev


Spin-based technologies can operate at terahertz frequencies but require manipulation techniques that work at ultrafast timescales to become practical. For instance, devices based on spin waves, also known as magnons, require efficient generation of high-energy exchange spin waves at nanometre wavelengths. To achieve this, a substantial coupling is needed between the magnon modes and an electro-magnetic stimulus such as a coherent terahertz field pulse. However, it has been difficult to excite non-uniform spin waves efficiently using terahertz light because of the large momentum mismatch between the submillimetre-wave radiation and the nanometre-sized spin waves. Here we improve the light–matter interaction by engineering thin films to exploit relativistic spin–orbit torques that are confined to the interfaces of heavy metal/ferromagnet heterostructures. We are able to excite spin-wave modes with frequencies of up to 0.6 THz and wavelengths as short as 6 nm using broadband terahertz radiation. Numerical simulations demonstrate that the coupling of terahertz light to exchange-dominated magnons originates solely from interfacial spin–orbit torques. Our results are of general applicability to other magnetic multilayered structures, and offer the prospect of nanoscale control of high-frequency signals.

Keywords: Magnonics; Spintronics; Terahertz; Magnetism

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Permalink: https://www.hzdr.de/publications/Publ-35382

Tailoring crosstalk between localized 1D spin-wave nanochannels using focused ion beams

V. Iurchuk, J. Pablo-Navarro, T. Hula, R. Narkovic, G. Hlawacek, L. Körber, A. Kakay, H. Schultheiß, J. Faßbender, K. Lenz, J. Lindner


1D spin-wave conduits are envisioned as nanoscale components of magnonics-based logic and computing schemes for future generation electronics. A-la-carte methods of versatile control of the local magnetization dynamics in such nanochannels are highly desired for efficient steering of the spin waves in magnonic devices. Here, we present a study of localized dynamical modes in 1-$\mu$m-wide Permalloy conduits probed by microresonator ferromagnetic resonance technique. We clearly observe the lowest-energy edge mode in the microstrip after its edges were finely trimmed by means of focused Ne+ ion irradiation. Furthermore, after milling the microstrip along its long axis by focused ion beams, creating consecutively ~50 and ~100 nm gaps, additional resonances emerge and are attributed to modes localized at the inner edges of the separated strips. To visualize the mode distribution, spatially resolved Brillouin light scattering microscopy was used showing an excellent agreement with the ferromagnetic resonance data and confirming the mode localization at the outer/inner edges of the strips depending on the magnitude of the applied magnetic field. Micromagnetic simulations confirm that the lowest-energy modes are localized within $\sim$15-nm-wide regions at the edges of the strips and their frequencies can be tuned in a wide range (up to 5 GHz) by changing the magnetostatic coupling (i.e. spatial separation) between the microstrips.

Keywords: Ferromagnetic resonance; Helium-ion microscope; Brillouin light scattering; Micromagnetic modelling; Magnetostatic coupling; Edge modes

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Permalink: https://www.hzdr.de/publications/Publ-35208

Spin wave non-reciprocity at the spin-flop transition region in synthetic antiferromagnets

O. Gladii, R. Salikhov, O. Hellwig, H. Schultheiß, J. Lindner, R. Gallardo


We investigate the frequency non-reciprocity in CoFeB/Ru/CoFeB synthetic antiferromagnets near the spin-flop transition region, where the magnetic moments in the two ferromagnetic layers are non-collinear. Using conventional Brillouin light scattering, we perform systematic measurements of the frequency non-reciprocity as a function of an external magnetic field. For the antiparallel alignment of the magnetic moments in the two layers, we observe a significant frequency non-reciprocity of up to a few GHz, which vanishes when the relative magnetization orientation switches into the parallel configuration at saturation. A non-monotonous dependence of the frequency non-reciprocity is found in the region where the system transitions from the antiparallel to the parallel orientation, with a maximum frequency shift around the spin-flop critical point. This non-trivial dependence of the non-reciprocity is attributed to the non-monotonous dependence of the dynamic dipolar interaction, which is the main factor that causes asymmetry in the dispersion relation. Furthermore, we found that the sign of the frequency shift changes even without switching the polarity of the bias field. These results show that one can precisely control the non-reciprocal propagation of spin waves via field-driven magnetization reorientation.

Keywords: Spin waves; Magnetization switching; Magnetization dynamics; Dipolar interaction; Magnetic multilayers; Synthetic antiferromagnets

Permalink: https://www.hzdr.de/publications/Publ-35185

Pattern recognition in reciprocal space with a magnon-scattering reservoir

L. Körber, C. Heins, T. Hula, J.-V. Kim, S. Thlang, H. Schultheiß, J. Faßbender, K. Schultheiß


Magnons are elementary excitations in magnetic materials and undergo nonlinear multimode scattering processes at large input powers. In experiments and simulations, we show that the interaction between magnon modes of a confined magnetic vortex can be harnessed for pattern recognition. We study the magnetic response to signals comprising sine wave pulses with frequencies corresponding to radial mode excitations. Three-magnon scattering results in the excitation of different azimuthal modes, whose amplitudes depend strongly on the input sequences. We show that recognition rates above 95\% can be attained for four-symbol sequences using the scattered modes, with strong performance maintained with the presence of amplitude noise in the inputs.

Keywords: spin wave; magnon; vortex; Brillouin-light scattering; BLS; reservoir computing; neuromorphic computing; nonlinear; three-magnon scattering; micromagnetic simulations

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Permalink: https://www.hzdr.de/publications/Publ-34945


Spin-wave study of magnetic perpendicular surface anisotropy in single crystalline MgO/Fe/MgO films

J. Solano, O. Gladii, P. Kuntz, Y. Henry, D. Halley, M. Bailleul


Broadband ferromagnetic resonance is measured in single crystalline Fe films of varying thickness sandwiched between
MgO layers. An exhaustive magnetic characterization of the films (exchange constant, cubic, uniaxial and surface
anisotropies) is enabled by the study of the uniform and the first perpendicular standing spin wave modes as a function of
applied magnetic field and film thickness. Additional measurements of nonreciprocal spin-wave propagation allow us to
separate each of the two interface contributions to the total surface anisotropy. The results are consistent with the model of a
quasi-bulk film interior and two magnetically different top and bottom interfaces, a difference ascribed to different oxidation

Keywords: Interface states; spin wave; Magnetic anisotropy; Broadband ferromagnetic resonance


Permalink: https://www.hzdr.de/publications/Publ-36276

Effects of the rf current and bias field direction on the transition from linear to non-linear gyrotropic dynamics in magnetic vortex structures

L. Ramasubramanian, V. Iurchuk, S. Sorokin, O. Hellwig, A. M. Deac


We present a frequency-domain study of the dynamic behavior of a magnetic vortex core within a single Permalloy disk by means of electrical detection and micromagnetic simulations. When exciting the vortex core dynamics in a nonlinear regime, the lineshape of the rectified dc signal reveals a resonance peak splitting which depends on the excitation amplitude. Using micromagnetic simulations, we show that at high excitation power the peak splitting originates from the nanosecond time scale quasiperiodic switching of the vortex core polarity. Using lock-in detection, the rectified voltage is integrated over a ms time scale, so that the net signal detected between the two resonant peaks for a given range of parameters cancels out. The results are in agreement with the reported effects of the in-plane static field magnitude on the gyration dynamics, and complement them by detailed analysis of the effects of the rf current amplitude and the azimuthal angle of the in-plane bias magnetic field. Systematic characterization shows that a transition from linear to nonlinear dynamical regime can be controlled by rf current as well as by varying the magnitude and the direction of the bias magnetic field.

Keywords: Spintronics; Magnetization dynamics; Magnetic vortex; Non-linear dynamics; Spin rectification effect; ED-FMR


Permalink: https://www.hzdr.de/publications/Publ-36131

Antiskyrmions and their electrical footprint in crystalline mesoscale structures of Mn1.4PtSn

M. Winter, F. J. Trindade Goncalves, I. Soldatov, Y. He, B. E. Zuniga Cespedes, P. Milde, K. Lenz, S. Hamann, M. Uhlarz, P. Vir, M. König, P. J. W. Moll, R. Schlitz, S. T. B. Goennenwein, L. M. Eng, R. Schäfer, J. Wosnitza, C. Felser, J. Gayles, T. Helm


Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effect. However, the electrical detection of antiskyrmions, in both thin films and bulk samples has been challenging to date. Here, we apply magneto-optical microscopy combined with electrical transport to explore the antiskyrmion phase as it emerges in crystalline mesoscale structures of the Heusler magnet Mn1.4PtSn. We reveal the Hall signature of antiskyrmions in line with our theoretical model, comprising anomalous and topological components. We examine its dependence on the vertical device thickness, field orientation, and temperature. Our atomistic simulations and experimental anisotropy studies demonstrate the link between antiskyrmions and a complex magnetism that consists of competing ferromagnetic, antiferromagnetic, and chiral exchange interactions, not captured by micromagnetic simulations.

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Permalink: https://www.hzdr.de/publications/Publ-35816

Finite-element dynamic-matrix approach for propagating spin waves: Extension to mono- and multilayers of arbitrary spacing and thickness

L. Körber, A. Hempel, A. Otto, R. A. Gallardo, Y. Henry, J. Lindner, A. Kakay


In our recent work [L. Körber, AIP Advances 11, 095006 (2021)], we presented an efficient numerical method to compute dispersions and mode profiles of spin waves in waveguides with translationally invariant equilibrium magnetization. A finite-element method (FEM) allowed to model two-dimensional waveguide cross sections of arbitrary shape but only finite size. Here, we extend our FEM propagating-wave dynamic-matrix approach from finite waveguides to the important cases of infinitely-extended mono- and multilayers of arbitrary spacing and thickness. To obtain the mode profiles and frequencies, the linearized equation of motion of magnetization is solved as an eigenvalue problem on a one-dimensional line-trace mesh, defined along the normal direction of the layers. Being an important contribution in multilayer systems, we introduce interlayer exchange into our FEM approach. With the calculation of dipolar fields being the main focus, we also extend the previously presented plane-wave Fredkin-Koehler method to calculate the dipolar potential of spin waves in infinite layers. The major benefit of this method is that it avoids the discretization of any non-magnetic material like non-magnetic spacers in multilayers. Therefore, the computational effort becomes independent on the spacer thicknesses. Furthermore, it keeps the resulting eigenvalue problem sparse, which therefore, inherits a comparably low arithmetic complexity. As a validation of our method (implemented into the open-source finite-element micromagnetic package \textsc{TetraX}), we present results for various systems and compare them with theoretical predictions and with established finite-difference methods. We believe this method offers an efficient and versatile tool to calculate spin-wave dispersions in layered magnetic systems.

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Permalink: https://www.hzdr.de/publications/Publ-35295

Stimulated Resonant Inelastic X-Ray Scattering in a Solid

D. J. Higley, Z. Chen, M. Beye, M. Hantschmann, A. H. Reid, V. Mehta, O. Hellwig, G. L. Dakovski, A. Mitra, R. Y. Engel, T. Maxwell, Y. Ding, S. Bonetti, M. Bucher, S. Carron, T. Chase, E. Jal, R. Kukreja, T. Liu, A. Föhlisch, H. A. Dürr, W. F. Schlotter, J. Stöhr


When materials are exposed to X-ray pulses with sufficiently high intensity, various nonlinear
effects can occur. The most fundamental one consists of stimulated electronic decays after
resonant absorption of X-rays. Such stimulated decays enhance the number of emitted
photons and the emission direction is confined to that of the stimulating incident photons
which clone themselves in the process. Here we report the observation of stimulated reso-
nant elastic (REXS) and inelastic (RIXS) X-ray scattering near the cobalt L3 edge in solid Co/
Pd multilayer samples. We observe an enhancement of order 106 of the stimulated over the
conventional spontaneous RIXS signal into the small acceptance angle of the RIXS spectro-
meter. We also find that in solids both stimulated REXS and RIXS spectra contain con-
tributions from inelastic electron scattering processes, even for ultrashort 5 fs pulses.
Our results reveal the potential and caveats of the development of stimulated RIXS in
condensed matter.

Permalink: https://www.hzdr.de/publications/Publ-35287

Cooperative Effect of Electron Spin Polarization in Chiral Molecules Studied with Non-Spin-Polarized Scanning Tunneling Microscopy

T. N. H. Nguyen, L. Rasabathina, O. Hellwig, A. Sharma, G. Salvan, S. Yochelis, Y. Paltiel, L. T. Baczewski, C. Tegenkamp


Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al2O3/Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered self-assembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

Permalink: https://www.hzdr.de/publications/Publ-35286

Role of vibrational properties and electron-phonon coupling on thermal transport across metal-dielectric interfaces with ultrathin metallic interlayers

S. M. Oommen, L. Fallarino, J. Heinze, O. Hellwig, S. Pisana


We systematically analyze the influence of 5 nm thick metal interlayers inserted at the interface of several sets of different metal-dielectric systems to determine the parameters that most influence interface transport. Our results show that despite the similar Debye temperatures of Al2O3 and AlN substrates, the thermal boundary conductance measured for the Au/Al2O3 system with Ni and Cr interlayers is ∼2× and >3× higher than the corresponding Au/AlN system, respectively. We also show that for crystalline SiO2 (quartz) and Al2O3 substrates having highly dissimilar Debye temperature, the measured thermal boundary conductance between Al/Al2O3 and Al/SiO2 are similar in the presence of Ni and Cr interlayers. We suggest that comparing the maximum phonon frequency of the acoustic branches is a better parameter than the Debye temperature to predict the change in the thermal boundary conductance. We show that the electron–phonon coupling of the metallic interlayers also alters the heat transport pathways in a metal-dielectric system in a nontrivial way. Typically, interlayers with large electron–phonon coupling strength can increase the thermal boundary conductance by dragging electrons and phonons into equilibrium quickly. However, our results show that a Ta interlayer, having a high electron–phonon coupling, shows a low thermal boundary conductance due to the
poor phonon frequency overlap with the top Al layer. Our experimental work can be interpreted in the context of diffuse mismatch theory and can guide the selection of materials for thermal interface engineering.

Keywords: hermal boundary conductance; metal-dielectric interfaces; Debye temperature; phonon frequencies; time-domain thermoreflectance

Permalink: https://www.hzdr.de/publications/Publ-35285

Tailoring exchange-dominated synthetic layered antiferromagnets: From collective reversal to exchange bias

B. Böhm, O. Hellwig


Not only since the progressive reduction of structure sizes in modern micro- and nanotechnology, surface and interface effects have played an ever-increasing role and nowadays often dominate the behavior of entire systems. Therefore, understanding the nature of surface and interface effects and being able to fully control them is of fundamental importance, in particular in modern thin-film technology. In this study, it is revealed how Co/Pt multi-layer-based synthetic antiferromagnets (SAFs) with perpendicular magnetic anisotropy in the regime of dominating antiferromagnetic interlayer exchange can be employed to control the collective magnetic reversal via systemati-cally altering surface and interface effects. The specifically designed samples and experiments highlight the superior tunability of synthetic systems as compared to their intrinsic stoichiometric counterparts, where the antiferro-magnetism is directly tied to the indivisible discrete atomic nature and crystal structure of the materials. Thus, it is demonstrated that in SAFs, it becomes possible to energetically heal the broken magnetic symmetry at the surface, thereby enabling either on demand suppression or controlled enhancement of respective surface and interface effects, as demonstrated here in this study for the surface spin-flop and the exchange bias effect.


Permalink: https://www.hzdr.de/publications/Publ-35284

Understanding structure–properties relationships of porphyrin linked to graphene oxide through π–π-stacking or covalent amide bonds

A. Lewandowska-Andralojc, E. Gacka, T. Pedzinski, G. Burdzinski, A. A. Lindner, J. M. O’Brien, M. O. Senge, A. Siklitskaya, A. G. Kubas, B. Marciniak, J. Walkowiak-Kulikowska


Two graphene oxide nanoassemblies using 5-(4-(aminophenyl)-10,15,20-triphenylporphyrin
(TPPNH2) were fabricated by two synthetic methods: covalent (GO-CONHTPP) and noncovalent
bonding. GO-CONHTPP was achieved through amide formation at the periphery of GO sheets and the
hybrid material was fully characterized by FTIR, XPS, Raman spectroscopy, and SEM. Spectroscopic
measurements together with theoretical calculations demonstrated that assembling TPPNH2
on the GO surface in DMF-H2O (1:2, v/v) via non-covalent interactions causes changes in the absorption
spectra of porphyrin, as well as efficient quenching of its emission. Interestingly, covalent binding
to GO does not affect notably neither the porphyrin absorption nor its fluorescence. Theoretical
calculations indicates that close proximity and π–π-stacking of the porphyrin molecule with the GO
sheet is possible only for the non-covalent functionalization. Femtosecond pump–probe experiments
revealed that only the non-covalent assembly of TPPNH2 and GO enhances the efficiency of the
photoinduced electron transfer from porphyrin to GO. In contrast to the non-covalent hybrid,
the covalent GO-CONHTPP material can generate singlet oxygen with quantum yields efficiency
(ΦΔ = 0.20) comparable to that of free TPPNH2 (ΦΔ = 0.26), indicating the possible use of covalent
hybrid materials in photodynamic/photothermal therapy. The spectroscopic studies combined with
detailed quantum-chemical analysis provide invaluable information that can guide the fabrication of
hybrid materials with desired properties for specific applications.

Keywords: porphyrin; graphene oxide; photochemistry

Permalink: https://www.hzdr.de/publications/Publ-35030

Application of a Microfabricated Microwave Resonator in a Co-Pd–Based Magnetic Hydrogen-Gas Sensor

T. A. Schefer, R. Narkovic, K. Lenz, F. Ganss, M. P. Roberts, O. Hellwig, M. Martyniuk, J. Lindner, M. Kostylev


We investigate the ferromagnetic resonance (FMR) response of microfabricated microwave resonators loaded with small Co16Pd84 alloy rectangles. A major increase in the FMR signal-to-noise ratio is achieved by employing the microwave-resonator structure. A FMR peak shift similar to that of Co16Pd84 continuous films is measured in the presence of hydrogen gas in the sample environment. We show that the very high sensitivity of the FMR signal of the Co16Pd84 alloy rectangle to hydrogen exposure can be used to measure relatively small hydrogen-concentration steps near 100% H2. Additionally, we also demonstrate that this structure can measure hydrogen over a concentration range from 3% to 100% H2 in N2. In time-dependent FMR measurements, we discover a temperature dependence of the FMR signal, which we relate to intrinsic temperature-dependent changes in saturation magnetization and the magnetic anisotropy of the Co-Pd alloy.

Keywords: hydrogen sensors; ferromagnetic resonance; CoPd alloys; Nanostructures; gas sensors


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Permalink: https://www.hzdr.de/publications/Publ-35018

Control and tunability of magnetic bubble states in multilayers with strong perpendicular magnetic anisotropy at ambient conditions

R. Salikhov, F. Samad, S. S. P. K. Arekapudi, R. Ehrler, J. Lindner, N. S. Kiselev, O. Hellwig


The reversal of magnetic bubble helicity through topologically trivial transient states provides an additional
degree of freedom that promises the development of multidimensional magnetic memories. A key requirement
for this concept is the stabilization of bubble states at ambient conditions on application-compatible substrates.
In the present work, we demonstrate a stabilization routine for remanent bubble states in high perpendicular magnetic anisotropy [Co(0.44 nm)/Pt(0.7 nm)]X , X = 48, 100, and 150 multilayers on Si/SiO2 substrates by exploring the effect of external magnetic fields (Hm) of different strength and angles (θ) with respect to the film surface normal. By systematic variation of these two parameters, we demonstrate that remanent bubble density and mean bubble diameter can be carefully tuned and optimized for each sample. Our protocol based on magnetometry only reveals the densest remanent bubble states at Hm = 0.87Hs (Hs is the magnetic saturation field) and θ = 60◦–75◦ for all X with a maximum of 3700 domains/100 μm2 for the X = 48 sample. The experimental observations are supported by micromagnetic simulations, taking into account the nanoscale lateral grain structure of multilayers synthesized by magnetron sputter deposition, and thus helping to understand the different densities of the bubble states found in these systems.


Permalink: https://www.hzdr.de/publications/Publ-35017

Spin-wave channeling in magnetization-graded nanostrips

R. A. Gallardo, P. Alvarado-Seguel, F. Brevis, A. Roldán-Molina, K. Lenz, J. Lindner, P. Landeros


Magnetization-graded ferromagnetic nanostrips are proposed as potential prospects to channel spin waves. Here, a controlled reduction of the saturation magnetization enables the localization of the propagating magnetic excitations in the same way that light is controlled in an optical fiber with a varying refraction index. The approach is based on the dynamic matrix method, where the magnetic nanostrip is divided into small sub-strips. The dipolar and exchange interaction between sub-strips is accounted to reproduce the spin-wave dynamics of the magnonic fiber. The transition from one strip to an infinite thin film is presented for the Damon-Eshbach geometry, where the nature of the spin-wave modes is discussed. An in-depth analysis of the spin-wave transport as a function of the saturation magnetization profile is provided. It is predicted that it is feasible to induce a remarkable channeling of the spin waves along the zones with a reduced saturation magnetization, even when such a reduction is tiny. The results are compared with micromagnetic simulations, where a good agreement is observed between both methods. The findings have relevance for envisioned future spin-wave-based magnonic devices operating at the nanometer scale.

Keywords: spin waves; ferromagnetic resonance; magnetization dynamics; magnonics; magnetic gradients; theory

Permalink: https://www.hzdr.de/publications/Publ-34978

Curvilinear spin-wave dynamics beyond the thin-shell approximation: Magnetic nanotubes as a case study

L. Körber, R. Verba, J. A. Otálora, V. Kravchuk, J. Lindner, J. Faßbender, A. Kakay


Surface curvature of magnetic systems can lead to many static and dynamic effects which are not present in flat systems of the same material. These emergent magnetochiral effects can lead to frequency nonreciprocity of spin waves, which has been shown to be a bulk effect of dipolar origin and is related to a curvature-induced symmetry breaking in the magnetic volume charges. So far, such effects have been investigated theoretically mostly for thin shells, where the spatial profiles of the spin waves can be assumed to be homogeneous along the thickness. Here, using a finite-element dynamic-matrix approach, we investigate the transition of the spin-wave spectrum from thin to thick curvilinear shells, at the example of magnetic nanotubes in the vortex state. With increasing thickness, we observe the appearance of higher-order radial modes which are strongly hybridized and resemble the perpendicular-standing-waves (PSSWs) in flat films. Along with an increasing dispersion asymmetry, we uncover the curvature-induced non-reciprocity of the mode profiles. This is explained in a very simple picture general for thick curvilinear shells, considering the inhomogeneity of the emergent geometric volume charges along the thickness of the shell. Such curvature-induced mode-profile asymmetry also leads to non-reciprocal hybridization which can facilitate unidirectional spin-wave propagation. With that, we also show how curvature allows for nonlinear three-wave splitting of a higher-order radial mode into secondary modes which can also propagate unidirectionally. We believe that our study provides a significant contribution to the understanding of the spin-wave dynamics in curvilinear magnetic systems, but also advertises these for novel magnonic applications.

Keywords: spin waves; nanotubes; curvilinear magnetism; curvature effects; micromagnetic modeling; tetrax; nonreciprocity

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Permalink: https://www.hzdr.de/publications/Publ-34720

Mode splitting of spin waves in magnetic nanotubes with discrete symmetries

L. Körber, I. Kézsmárki, A. Kakay


We investigate how geometry influences spin dynamics in polygonal magnetic nanotubes. We find that lowering the rotational symmetry of nanotubes, by decreasing the number of planar facets, splits an increasing number spin-wave modes, which are doubly degenerate in cylindrical tubes. This symmetry-governed splitting is distinct form the topological split recently observed in cylindrical nanotubes. Doublet modes, where the azimuthal period is half-integer or integer multiple of the number of facets, split to singlet pairs with lateral standing-wave profiles of opposing mirror-plane symmetries. Moreover, the polygonal geometry facilitates the hybridization of modes with different azimuthal periods but the same symmetry, manifested in avoided level crossings. These phenomena, unimaginable in cylindrical geometry, provide new tools to control spin dynamics on the nanoscale. Our concepts can be generalized to nano-objects of versatile geometries and order parameters, offering new routes to understand and engineer dynamic responses in mesoscale physics.

Keywords: Spin wave; magnon; micromagnetic modeling; symmetry; nanotubes; group theory; micromagnetism; tetrax

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Permalink: https://www.hzdr.de/publications/Publ-34719

Flexomagnetism and vertically graded Néel temperature of antiferromagnetic Cr2O3 thin films

P. Makushko, T. Kosub, O. Pylypovskyi, N. Hedrich, J. Li, O. Pashkin, S. Avdoshenko, R. Hübner, F. Ganss, M. O. Liedke, M. Butterling, A. Wagner, K. Wagner, B. J. Shields, P. Lehmann, I. Veremchuk, J. Faßbender, P. Maletinsky, D. Makarov


Antiferromagnetic insulators are a prospective material science platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored both fundamentally and technologically. Here, we discover a new member in the family of flexoeffects in thin films of technologically relevant antiferromagnetic Cr2O3. We demonstrate that a gradient of mechanical strain can impact the magnetic phase transition resulting in the distribution of the N ́eel temperature along the thickness of a 50-nm-thick film and induces a sizable flexomagnetic coefficient of about 15 μb/nm2 originating from the inhomogeneous reduction of the antiferromagnetic order parameter. The antiferromagnetic ordering in inhomogeneously strained thin films of Cr2O3 can persist up to 100◦ C, rendering Cr2O3 relevant for industrial electronics applications. The presence of a strain gradient in thin films of Cr2O3 may therefore allow for the realization of reconfigurable antiferromagnetic racetracks, magnonic waveguides and magnon crystals. The presence of a strain gradient in ultrathin films of Cr2O3 enables new fundamental research directions on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Keywords: antiferromagnetism; flexomagnetism; Cr2O3; Neel temperature; NV magnetometry; magnetotransport

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Permalink: https://www.hzdr.de/publications/Publ-34673

Tuning of the Dzyaloshinskii-Moriya interaction by He+ ion irradiation

H. T. Nembach, E. Jué, K. Potzger, J. Faßbender, T. J. Silva, J. M. Shaw


We studied the impact of He+ irradiation on the Dzyaloshinskii-Moriya interaction (DMI) in Ta/Co20Fe60B20/Pt/MgO samples. We found that irradiation of 40 keV He+ ions increases the DMI by approximately 20% for fluences up to 2 × 1016 ions/cm2 before it decreases for higher fluence values. In contrast, the interfacial anisotropy shows a distinctly different fluence dependence. To better understand the impact of the ion irradiation on the Ta and Pt interfaces with the Co20Fe60B20 layer, we carried out Monte-Carlo simulations, which showed an expected increase in disorder at the interfaces. A moderate increase in disorder increases the total number of triplets for the three-site exchange mechanism and consequently increases the DMI. Our results demonstrate the significance of disorder for the total DMI.

Keywords: Dzyaloshinskii-Moriya interaction; Fluence dependence; Ions irradiation

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Permalink: https://www.hzdr.de/publications/Publ-34640

Coupling between ferromagnetic and ferroelastic transitions and ordering in Heusler alloys produces new multifunctionality

H. Oleg, S. Hanuš, S. Fähler


The ability of Heusler alloys to accommodate broad variations of composition, doping
and ordering provides multiple options for tailoring their ferromagnetic and ferroelastic
properties. Moreover, existing coupling between these ferroic properties ranging from
coupled ferroic transitions to a coupling of their ferromagnetic and ferroelastic
microstructure allows for manifold multifunctionalities. Here we focus on ferromagnetic,
metamagnetic and reentrant shape memory alloys explaining the principles and sketch
effects’ rich susceptibility to temperature, magnetic field and stress. We illustrate how
these can provide a path to a multitude of emerging applications for actuation, sensing,
and energy use. As an outlook, we discuss time dependency, fatigue, and finite size
effects, which are not yet fully explored.

Keywords: Heusler Alloys; Functional magnetic materials; Magnetic shape memory alloys

Permalink: https://www.hzdr.de/publications/Publ-34579

Depth-Adjustable Magnetostructural Phase Transition in Fe₆₀V₄₀ Thin Films

M. S. Anwar, H. Cansever, B. Boehm, R. Gallardo, R. Hübner, S. Zhou, U. Kentsch, S. Rauls, B. Eggert, H. Wende, K. Potzger, J. Faßbender, K. Lenz, J. Lindner, O. Hellwig, R. Bali


Phase transitions occurring within spatially confined regions can be useful for generating nanoscale material property modulations. Here we describe a magneto-structural phase transition in a binary alloy, where a structural transition from short range order (SRO) to body centered cubic (bcc) results in the formation of depth-adjustable ferromagnetic layers, which reveal application-relevant magnetic properties of high saturation magnetitzation (Ms) and low Gilbert damping (α). Here we use Fe₆₀V₄₀ binary alloy films which transform from initially Ms = 17 kA/m (SRO structure) to 747 kA/m (bcc structure) driven by atomic displacements caused by penetrating ions. Simulations show that estimated ~1 displacement per atom triggers a structural transition, forming homogeneous ferromagnetic layers. The thickness of ferromagnetic layer increases as a step-like function of the ion-fluence. Microwave excitations of the ferromagnetic/non-ferromagnetic layered system reveals an α = 0.0027 ± 0.0001. The combination of nanoscale spatial confinement, low α and high Ms provide a pathway for the rapid patterning of magnetic and microwave device elements.

Keywords: Magneto-structural correlations; Phase transitions; Magnetic thin films; Ion-irradiation; Short-range order

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Permalink: https://www.hzdr.de/publications/Publ-34510

Magnetic Configuration Driven Femtosecond Spin Dynamics in Synthetic Antiferromagnets

D. Anulekha, S. S. P. K. Arekapudi, L. Koch, F. Samad, N. P. Surya, B. Benny, O. Hellwig, A. Barman


Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt]n−1/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

Keywords: synthetic antiferromagnets; magnetic multilayers; ultrafast demagnetization; spin transport

Permalink: https://www.hzdr.de/publications/Publ-34449

Mechanism of femtosecond laser induced ultrafast demagnetization in ultrathin film magnetic multilayers

S. Pan, F. Ganss, S. Panda, G. Sellge, C. Banerjee, J. Sinha, O. Hellwig, A. Barman


Ever since its discovery ultrafast demagnetization has remained one of the most
intriguing research areas in magnetism. Here, we demonstrate that in [Co (tCo )/
Pd (0.9 nm)] 8 multilayers, the characteristic decay time in femtosecond time-
scale varies non-monotonically with tCo in the range 0.07 nm B tCo B 0.75 nm.
Further investigation reveals higher spin fluctuation at higher ratio of electron to
Curie temperature to be responsible for this. Microscopic three-temperature
modelling unravels a similar trend in the spin–lattice interaction strength, which
strongly supports our experimental observation. The knowledge of the fem-
tosecond magnetization decay mechanism in ultrathin ferromagnetic films is
unique and important for the advancement of fundamental magnetism besides
their potential applications in ultrahigh speed spintronic devices.

Permalink: https://www.hzdr.de/publications/Publ-34448

Terahertz charge and spin transport in metallic ferromagnets: The role of crystalline and magnetic order

K. Neeraj, A. Sharma, M. Almeida, P. Matthes, F. Samad, G. Salvan, O. Hellwig, S. Bonetti


We study the charge and spin dependent scattering in a set of CoFeB thin films whose crystalline order is systematically enhanced and
controlled by annealing at increasingly higher temperatures. Terahertz conductivity measurements reveal that charge transport closely
follows the development of the crystalline phase, with the increasing structural order leading to higher conductivity. The terahertz-induced
ultrafast demagnetization, driven by spin-flip scattering mediated by the spin–orbit interaction, is measurable in the pristine amorphous sam-
ple and much reduced in the sample with the highest crystalline order. Surprisingly, the largest demagnetization is observed at intermediate
annealing temperatures, where the enhancement in spin-flip probability is not associated with an increased charge scattering. We are able to
correlate the demagnetization amplitude with the magnitude of the in-plane magnetic anisotropy, which we characterize independently, sug-
gesting a magnetoresistance-like description of the phenomenon.

Permalink: https://www.hzdr.de/publications/Publ-34444

What is the speed limit of martensitic transformations?

S. Schwabe, K. Lünser, D. Schmidt, K. Nielsch, P. Gaal, S. Fähler


Structural martensitic transformations enable various applications, which range from high stroke actuation and sensing to energy efficient magnetocaloric refrigeration and thermomagnetic energy harvesting. All these emerging applications benefit from a fast transformation, but up to now the speed limit of martensitic transformations has not been explored. Here, we demonstrate that a martensite to austenite transformation can be completed in under ten nanoseconds. We heat an epitaxial Ni-Mn-Ga film with a laser pulse and use synchrotron diffraction to probe the influence of initial sample temperature and overheating on transformation rate and ratio. We demonstrate that an increase of thermal energy drives this transformation faster. Though the observed speed limit of 2.5 x 10^27 (Js)^-1 per unit cell leaves plenty of room for a further acceleration of applications, our analysis reveals that the practical limit will be the energy required for switching. Our experiments unveil that martensitic transformations obey similar speed limits as in microelectronics, which are expressed by the Margolus–Levitin theorem.

Keywords: Martensitic Transformations; Time dependency; Syncrotron Diffraction

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Permalink: https://www.hzdr.de/publications/Publ-34330

Roadmap on Spin-Wave Computing

A. V. Chumak, P. Kabos, M. Wu, et al., H. Schultheiß, K. Schultheiß


Magnonics addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operation in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the academic domain, the scientific and technological challenges of the field are being extensively investigated, and many proof-of-concept prototypes have already been realized in laboratories. This roadmap is a product of the collective work of many authors that covers versatile spin-wave computing approaches, conceptual building blocks, and underlying physical phenomena. In particular, the roadmap discusses the computation operations with Boolean digital data, unconventional approaches like neuromorphic computing, and the progress towards magnon-based quantum computing. The article is organized as a collection of sub-sections grouped into seven large thematic sections. Each sub-section is prepared by one or a group of authors and concludes with a brief description of current challenges and the outlook of further development for each research direction.

Keywords: Spin wave; Magnon; Magnonics; Computing; Data processing

Permalink: https://www.hzdr.de/publications/Publ-34291

State-resolved ultrafast charge and spin dynamics in [Co/Pd] multilayers

L. Le Guyader, D. J. Higley, M. Pancaldi, T. Liu, Z. Chen, T. Chase, P. W. Granitzka, G. Coslovich, A. A. Lutman, G. L. Dakovski, W. F. Schlotter, P. Shafer, E. Arenholz, O. Hellwig, M. L. M. Lalieu, B. Koopmans, A. H. Reid, S. Bonetti, J. Stöhr, H. A. Dürr


We use transient absorption spectroscopy with circularly polarized x-rays to detect laser-
excited hole states below the Fermi level and compare their dynamics with that of unoc-
cupied states above the Fermi level in ferromagnetic [Co/Pd] multilayers. While below
the Fermi level an instantaneous and significantly stronger demagnetization is observed,
above the Fermi level the demagnetization is delayed by 35 ± 10 fs. This provides a direct
visualization of how ultrafast demagnetization proceeds via initial spin-flip scattering of
laser-excited holes to the subsequent formation of spin waves

Permalink: https://www.hzdr.de/publications/Publ-34287

Dysprosium Liquid Metal Alloy Ion Source For Magnetic Nanostructures

L. Bischoff, N. Klingner, P. Mazarov, K. Lenz, R. Narkovic, W. Pilz, F. Meyer


Focused Ion Beam (FIB) processing has been established as a well-suited and promising technique in R&D in nearly all fields of nanotechnology for patterning and prototyping on the μm-scale and below. Liquid Metal Alloy Ion Sources (LMAIS) represent an alternative to expand the FIB application fields beside all other source concepts. Especially ions from the rare earth (RE) element Dy is very interesting for local modification of magnetic properties like RE-induced damping in metallic alloys. So various alloys for source preparation were investigated. A promising solution was found in a Cu30Dy70 based LMAIS which should be introduced in more detail.

Keywords: Focused Ion Beam; Liquid Metal Alloy Ion Source; Dysprosium; magnetic properties

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Permalink: https://www.hzdr.de/publications/Publ-34127

Superparamagnetism and ferrimagnetism in the Sr2FeMoO6–δ nanoscale powder

N. Kalanda, M. Yarmolich, A. Burko, A. Temirov, A. Kislyuk, S. Demyanov, K. Lenz, J. Lindner, D.-H. Kim


As a result of using combined synthesis modes and optimized conditions for ultrasonic dispersion, a single-phase nanosized Sr2FeMoO6–δ powder with a high degree of superstructural ordering of Fe/Mo cations (88%) with an average grain size of 70.8 nm was obtained. Based on the results of Mössbauer spectroscopy and magnetic measurements, it was established that the obtained nanosized strontium ferromolybdate powder is in a magnetically inhomogeneous state, consisting of superparamagnetic and ferrimagnetic phases. The ferrimagnetic component of magnetization is characterized by higher values of magnetization in comparison with the superparamagnetic component, and a smooth increase is noted for this component, which reaches saturation with decreasing temperature. It is shown that there is no exchange magnetic interaction between superparamagnetic grains in the superparamagnetic phase, which made it possible, based on the Neel-Brown model, to estimate the critical sizes of nanoparticles, dSPM, in the single-domain state. The obtained dSPM values are smaller than the sizes of single-domain particles, which confirms the absence of a frozen state in some of the superparamagnetic particles. The results of this work are important for understanding the effect of nanosized grains on the magnetic properties and features of magnetization of the Sr2FeMoO6–δ polydisperse powder that is very promising for spintronic device application.

Keywords: Perovskites; sol-gel processes; electron microscopy; magnetic properties


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Permalink: https://www.hzdr.de/publications/Publ-33836

Control of site occupancy by variation of the Zn and Al content in NiZnAl ferrite epitaxial films with low magnetic damping

J. Lumetzberger, V. Ney, A. Zhakarova, D. Primetzhofer, K. Lenz, A. Ney


The structural and magnetic properties of Zn/Al doped nickel ferrite thin films can be adjusted by changing the Zn and Al content. The films are epitaxially grown by reactive magnetron sputtering using a triple cluster system to sputter simultaneously from three different targets. Upon the variation of the Zn content the films remain fully strained with similar structural properties, while the magnetic properties are strongly affected. The saturation magnetization and coercivity as well as resonance position and linewidth from ferromagnetic resonance (FMR) measurements are altered depending on the Zn content in the material. The reason for these changes can be elucidated by investigation of the x-ray magnetic circular dichroism spectra to gain site and valence specific information with elemental specificity. Additionally, from a detailed investigation by broadband FMR a minimum in g-factor and linewidth could be found as a function of film thickness. Furthermore, the results from a variation of the Al content using the same set of measurement techniques is given. Other than for Zn, the variation of Al affects the strain and even more pronounced changes to the magnetic properties are apparent.

Keywords: Spinel; Ferromagnetic resonance; damping; epitaxial films; XMCD; Ferrites


Permalink: https://www.hzdr.de/publications/Publ-33797

Origin and avoidance of double peaks in the induced voltage of a thermomagnetic generator for harvesting low-grade waste heat

D. Dzekan, T. D. Kischnik, A. Diestel, N. Kornelius, S. Fähler


Thermomagnetic harvesting is an emerging approach to convert low-grade
waste heat to electricity, which recently obtained a boost due to the development of both,
more efficient functional materials and innovative device concepts. Here we examine a
thermomagnetic generator which utilizes Gadolinium as thermomagnetic material and report
on double peaks of the induced voltage. By a combination of experiments and theory we
show that these double peaks originate from the interaction of an asymmetric magnetization
curve and a pretzel like magnetic field topology. Double peaks are detrimental for the output
power and can be avoided by matching the magnetization change by adjusting cold and hot
fluid flow.

Keywords: Thermomagnetic Energy Harvesting; Thermomagnetic Generator; Magnetocaloric Refrigeration; Magnetic Materials


Permalink: https://www.hzdr.de/publications/Publ-33433

Curvature-induced drift and deformation of magnetic skyrmions: Comparison of the ferromagnetic and antiferromagnetic cases

K. Yershov, A. Kakay, V. P. Kravchuk


The influence of the geometrical curvature of chiral magnetic films on the static and dynamic properties of hosted skyrmions are studied theoretically. We predict the effects of the curvature-induced drift of skyrmions under the action of the curvature gradients without any external stimuli. The strength of the curvature-induced driving force essentially depends on the skyrmion type, N\'eel or Bloch, while the trajectory of motion is determined by the type of magnetic ordering: ferro- or antiferromagnetic. During the motion along the surface, skyrmions experience deformations which depend on the its type. In the small-curvature limit, using the collective-variable approach we show, that the driving force acting on a N{\'e}el skyrmion is linear with respect to the gradient of the mean curvature. The driving acting on a Bloch skyrmion is much smaller: it is proportional to the product of the mean curvature and its gradient. In contrast to the fast N{\'e}el skyrmions, the dynamics of the slow Bloch skyrmions is essentially affected by the skyrmion profile deformation. For the sake of simplicity we restrict ourselves to the case of zero Gaussian curvature and consider cylindrical surfaces of general type. Equations of motion for ferromagnetic and antiferromagnetic skyrmions in curved magnetic films are obtained in terms of collective variables. All analytical predictions are confirmed by numerical simulations.

Keywords: skyrmions; curvature effects; ferromagnetic; antiferromagnetic; dynamics


Permalink: https://www.hzdr.de/publications/Publ-33408

Spin pumping at interfaces with ferro- and paramagnetic Fe60Al40 films acting as spin source and spin sink

T. Strusch, K. Lenz, R. Meckenstock, R. Bali, J. Ehrler, J. Lindner, J. Faßbender, M. Farle, K. Potzger, A. Semisalova


We present a study of spin pumping efficiency and determine the spin mixing conductance and spin diffusion length in thin bilayer films based on 3d transition metal alloy Fe60Al40. Due to its magnetostructural phase transition, Fe60Al40 can be utilized as a ferromagnetic (FM) or paramagnetic (PM) material at the same temperature depending on its structural order, thus thin Fe60Al40 film can act as a spin source or a spin sink when interfaced with a paramagnet or a ferromagnet, correspondingly. Ferromagnetic resonance (FMR) measurements were performed in a frequency range of 5 - 35 GHz on bilayer films composed of FM-Fe60Al40 / Pd and PM-Fe60Al40 / permalloy Ni80Fe20. The increase of damping parameter with the thickness of paramagnetic layer was interpreted as a result of spin pumping into a paramagnet. In the first case, the FM-Fe60Al40 acts as a spin source and in the second case PM-Fe60Al40 serves as a spin sink. We determine the spin mixing conductance g↑↓ Pd=(3.8±0.5)×1018 m-2 at the FM-Fe60Al40/Pd interface and the spin diffusion length λPd=9.1 ±2.0 nm in Pd. For the PM-Fe60Al40/permalloy interface we find a spin mixing conductance g↑↓ FeAl=(2.1±0.2)×1018 m-2 and a spin diffusion length λFeAl=11.9 ±0.2 nm for PM-Fe60Al40. Demonstrated bi-functionality of Fe60Al40 alloy in spin pumping structures may be promising for spintronic applications

Keywords: Ferromagnetic resonance; spin pumping; ferromagnetic films; ferromagnetism; FeAl alloys; spin diffusion; damping; linewidth

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Permalink: https://www.hzdr.de/publications/Publ-32940

Nonstationary spin waves in a single rectangular permalloy microstrip under uniform magnetic excitation

S. Pile, S. Stienen, K. Lenz, R. Narkovic, S. Wintz, J. Förster, S. Mayr, M. Buchner, M. Weigand, V. Ney, J. Lindner, A. Ney


The ferromagnetic resonance modes in a single rectangular Py microstrip were directly imaged using timeresolved STXM-FMR measurements and the findings were corroborated by micromagnetic simulations. The spin wave resonance modes showed a nonstanding character, when the wave-vector is parallel to the external static magnetic field due to the highly inhomogeneous effective field inside the strip. The propagating character is observed for all the observed spin waves. The influence of the edge quality was analyzed using micromagnetic simulations.

Keywords: ferromagnetic resonance; scanning x-ray transmission microscopy; spin waves; nanostructures; micromagnetism

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Permalink: https://www.hzdr.de/publications/Publ-32909

Resonance behavior of embedded and freestanding microscale ferromagnets

H. Cansever, M. S. Anwar, S. Stienen, K. Lenz, R. Narkovic, G. Hlawacek, K. Potzger, O. Hellwig, J. Faßbender, J. Lindner, R. Bali


The ferromagnetic resonance of a disordered A2 Fe60Al40 of ferromagnetic strip, of dimensions 5 µm × 1 µm x 32 nm, has been observed in two vastly differing surroundings: in the first case, the ferromagnetic region was circumferenced by ordered B2 Fe60Al40, and in the second case it was free standing, adhering only to the oxide substrate. The embedded ferromagnet possesses a periodic magnetic domain structure, which transforms to a single domain structure in the freestanding case. The two cases differ in their dynamic response, for instance, the resonance field for the uniform (k = 0) mode at ~ 14 GHz excitation displays a shift from 209 to 194 mT, respectively for the embedded and freestanding cases, with the external magnetic field applied along the long axis. The resonant behavior of a microscopic ferromagnet can thus be finely tailored via control of its near-interfacial surrounding.

Keywords: Embedded nanomagnets; microresonator; ferromagnetic resonance; ion irradiation

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Permalink: https://www.hzdr.de/publications/Publ-32834

Spin-wave frequency combs

T. Hula, K. Schultheiß, F. J. Trindade Goncalves, L. Körber, M. Bejarano, M. Copus, L. Flacke, L. Liensberger, A. Buzdakov, A. Kakay, M. Weiler, R. Camley, J. Faßbender, H. Schultheiß


We experimentally demonstrate the generation of spin-wave frequency combs based on the non-
linear interaction of propagating spin waves in a microstructured waveguide. By means of time- and space-resolved Brillouin light scattering spectroscopy, we show that the simultaneous excita- tion of spin waves with different frequencies leads to a cascade of four-magnon scattering events which ultimately results in well-defined frequency combs. Their spectral weight can be tuned by the choice of amplitude and frequency of the input signals. Furthermore, we introduce a model for stimulated four-magnon scattering which describes the formation of spin-wave frequency combs in the frequency and time domain.

Keywords: magnetism; magnetization dynamics; spin waves; magnons; spin dynamics; micromagnetic modeling; Brillouin light scattering; spectroscopy

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Permalink: https://www.hzdr.de/publications/Publ-32565


Laser induced crystallization of Co–Fe–B films

M. Almeida, A. Sharma, P. Matthes, N. Köhler, S. Busse, M. Müller, O. Hellwig, A. Horn, D. R. T. Zahn, G. Salvan, S. E. Schulz


Local crystallization of ferromagnetic layers is crucial in the successful realization of miniaturized
tunneling magnetoresistance (TMR) devices. In the case of Co–Fe–B TMR devices, used most
successfully so far in applications and devices, Co–Fe–B layers are initially deposited in an amorphous
state and annealed post-deposition to induce crystallization in Co–Fe, thereby increasing the device
performance. In this work, first direct proof of locally triggered crystallization of 10 nm thick Co–Fe–B
films by laser irradiation is provided by means of X-ray diffraction (XRD) using synchrotron radiation.
A comparison with furnace annealing is performed for benchmarking purposes, covering different
annealing parameters, including temperature and duration in the case of furnace annealing, as
well as laser intensity and scanning speed for the laser annealing. Films of Co–Fe–B with different
stoichiometry sandwiched between a Ru and a Ta or MgO layer were systematically assessed by XRD
and SQUID magnetometry in order to elucidate the crystallization mechanisms. The transformation
of Co–Fe–B films from amorphous to crystalline is revealed by the presence of pronounced CoFe(110)
and/or CoFe(200) reflexes in the XRD θ-2θ scans, depending on the capping layer. For a certain window
of parameters, comparable crystallization yields are obtained with furnace and laser annealing.
Samples with an MgO capping layer required a slightly lower laser intensity to achieve equivalent Co–
Fe crystallization yields, highlighting the potential of laser annealing to locally enhance the TMR ratio.

Permalink: https://www.hzdr.de/publications/Publ-33873

Noncollinear Remanent Textures Induced by Surface Spin Flop in Synthetic Antiferromagnets with Perpendicular Anisotropy

B. Böhm, L. Fallarino, D. Pohl, B. Rellinghaus, O. Hellwig


The surface spin flop, observed in synthetic antiferromagnets (SAFs) with uniaxial anisotropy and
strong antiferromagnetic (AF) interlayer exchange coupling, can be considered as a laterally homoge-
neous, vertical AF domain wall pushed into the SAF from either the top or the bottom in the presence
of a strong external vertical magnetic field. As a result, the AF domain wall can be described as a one-
dimensional entity. In this work, we present a concept to stabilize laterally homogeneous vertical AF
domain walls by local variation of the perpendicular magnetic anisotropy in Co/Pt-based SAFs. Our
approach not only allows the stabilization of the vertical AF domain wall in the absence of any exter-
nal magnetic field, but furthermore enables a deterministic selection among four different remanent states,
each one stable within a broad external magnetic field range of almost one tesla. We also demonstrate an
extension to our concept by stabilizing two coexisting vertical AF domain walls, thus yielding a system
with a total of six different selectable (and reprogrammable) remanent states. The controlled stabiliza-
tion of noncollinear AF textures in the form of vertical AF domain walls at remanence could be used as
an infrastructure for propagating spin waves within the AF domain wall itself, as well as for tuning the
dynamic behavior of perpendicular standing spin wave modes existing vertically across the SAF.

Permalink: https://www.hzdr.de/publications/Publ-33872

CoCrFeNi High-Entropy Alloy Thin Films Synthesised by Magnetron Sputter Deposition from Spark Plasma Sintered Targets

H. Schwarz, T. Uhlig, N. Rösch, T. Lindner, F. Ganss, O. Hellwig, T. Lampke, G. Wagner, T. Seyller


Two magnetron sputter targets of CoCrFeNi High-Entropy Alloy (HEA), both in equal atomic ratio, were prepared by spark plasma sintering. One of the targets was fabricated from a homogeneous HEA powder produced via gas atomisation; for the second target, a mixture of pure element powders was used. Economic benefits can be achieved by mixing pure powders in the intended ratio in comparison to the gas atomisation of the specific alloy composition. In this work, thin films deposited via magnetron sputtering from both targets are analysed. The surface elemental composition is investigated by X-ray photoelectron spectroscopy, whereas the bulk stoichiometry is measured by X-ray fluorescence spectroscopy. Phase information and surface microstructure are investigated using X-ray diffraction and scanning electron microscopy, respectively. It is demonstrated that the stoichiometry, phase composition and microscopic structure of the as-deposited HEA thin films are almost identical if the same deposition parameters are used.

Keywords: high-entropy alloy; magnetron sputtering; spark plasma sintering; X-ray photoelectron spectroscopy; X-ray diffraction; scanning electron microscopy

Permalink: https://www.hzdr.de/publications/Publ-33871

Anisotropic microwave propagation in a reconfigurable chiral spin soliton lattice

Y. Shimamoto, F. J. Trindade Goncalves, T. Sogo, Y. Kousaka, Y. Togawa


We investigated microwave propagation in the chiral spin soliton lattice (CSL) phase of micrometer-sized crystals of the monoaxial chiral helimagnet
CrNb₃S₆. An advantage of the CSL is that its periodicity can be reconfigured over a macroscopic length scale by means of an external magnetic field. Using a two-antenna microwave spectroscopy technique, we measured the anisotropic response of the transmitted microwaves via the spin dynamics of the CSL. When propagating along the direction parallel to the helical axis, the microwave amplitude increased up to a factor of twenty with decreasing the number of chiral soliton kinks. When the propagation direction was rotated by 90 degrees with regards to the helical axis, the microwave amplitude increased by one order of magnitude upon formation of the chiral helimagnetic order in the vicinity of zero magnetic field, exceeding that of the ferromagnetic phase above the critical field. Our findings open a novel route for controlling the characteristics of microwave propagation using noncollinear spin textures.

Keywords: Spin waves; Chiral magnets; Ferromagnetic resonance

Permalink: https://www.hzdr.de/publications/Publ-33785

Nondiffusive Transport and Anisotropic Thermal Conductivity in High-Density Pt/Co Superlattices

M. Shahzadeh, O. Andriyevska, R. Salikhov, L. Fallarino, O. Hellwig, S. Pisana


Despite the numerous reports over the last two decades dedicated to the study of interfacial thermal transport,
physics of thermal transport across nanoscale metallic multilayers is less explored. This is in part due to the relatively
high conductance characteristic of these interfaces, which renders them difficult to characterize.
Interfacial transport in these systems has so far appeared to be diffusive, a surprising behavior when the interface density
increases and the layer thicknesses become comparable with the mean free path of electrons.
To address the limit of diffusive theories describing heat transport across high-density metallic interfaces,
we systematically investigate heat transport in and across Pt/Co multilayers via frequency domain thermoreflectance.
Sensitivity gained from offsetting the laser beam and reducing the laser spot size allows for the
measurement of anisotropic thermal conductivity of the multilayers. By changing the number of interfaces while keeping the overall
thickness of Pt and Co in the multilayer structure constant, the effect of interface density on the multilayers’ effective thermal
conductivity is studied. The extracted Pt/Co interface thermal boundary conductance is then compared to the calculations from the
electronic diffuse mismatch model and experimental data available in the literature. We show that as the multilayer period thickness
becomes much smaller than the electron mean free path, measurements markedly deviate from the diffusive transport theory. We
attribute this deviation to the nondiffusive nature of heat transport in subnanometric scales at interface densities above 1/nm.

Keywords: heat transport; metallic multilayers; anisotropic thermal conductivity; nondiffusive transport; frequency domain thermoreflectance

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Permalink: https://www.hzdr.de/publications/Publ-33503

Spin-wave focusing induced by dipole-dipole interaction in synthetic antiferromagnets

R. A. Gallardo, P. Alvarado-Seguel, A. Kákay, J. Lindner, P. Landeros


Under certain conditions, spin waves can be channeled into a broad angular spectrum of wave vectors, where the direction
of the group velocity becomes independent of those wave vectors. Such highly focused waves are called caustic waves,
whose properties can be manipulated by anisotropies or chiral interactions, like the Dzyaloshinskii-Moriya interaction. In this
paper, we theoretically study the focusing features of the spin waves induced by the dipole-dipole interaction in synthetic
antiferromagnets. For stacked systems, the dipolar interaction causes a noticeable frequency nonreciprocity when the
magnetizations in both films are antiparallelly aligned, and then the focusing properties of the spin waves are enhanced. The
role of thicknesses and magnetic graduation along the film's normal are systematically analyzed. We found that the degree
of focalization of the spin waves can be manipulated by increasing the layers' thickness. Also, we show that the low- and
high-frequency modes exhibit different focalization properties; the low-frequency mode manifests a similar behavior to the
heavy-metal/ferromagnet systems with interfacial Dzyaloshinskii-Moriya interaction, while the high-frequency one tends the
generate almost reciprocal interference patterns along one axis. In the case of magnetization-graded synthetic
antiferromagnets, we demonstrate that the graduation slightly influences the low-frequency mode, while the focusing and
nonreciprocal dynamic properties of the high-frequency ones are notoriously altered. The theoretical calculations are
compared with micromagnetic simulations, where a good agreement is found between both methods. Our results
demonstrate that a synthetic antiferromagnetic system allows for controlling the propagation of spin waves, assisting in the
transfer of angular momentum and energy.

Keywords: Spin waves; Wave focusing; synthetic antiferromagnets; Dzyaloshinskii-Moriya interaction; Spectrum analysis; dipole-dipole interaction; multilayers


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Permalink: https://www.hzdr.de/publications/Publ-33489

Robust formation of nanoscale magnetic skyrmions in easy-plane anisotropy thin film multilayers with low damping

L. Flacke, V. Ahrens, S. Mendisch, L. Körber, T. Böttcher, E. Meidinger, M. Yaqoob, M. Müller, L. Liensberger, A. Kakay, M. Becherer, P. Pirro, M. Althammer, S. Geprägs, H. Huebl, R. Gross, M. Weiler


We experimentally demonstrate the formation of room-temperature skyrmions with radii of about 25 nm in easy-plane anisotropy multilayers with an interfacial Dzyaloshinskii-Moriya interaction (DMI). We detect the formation of individual magnetic skyrmions by magnetic force microscopy and find that the skyrmions are stable in out-of-plane fields up to about 200 mT. We determine the interlayer exchange coupling as well as the strength of the interfacial DMI. Additionally, we investigate the dynamic microwave spin excitations by broadband
magnetic resonance spectroscopy. From the uniform Kittel mode we determine the magnetic anisotropy and lowdamping α < 0.04. We also find clear magnetic resonance signatures in the nonuniform (skyrmion) state. Our findings demonstrate that skyrmions in easy-plane multilayers are promising for spin-dynamical applications.

Keywords: skyrmion; ferromagnetic resonance; DMI; low damping


Permalink: https://www.hzdr.de/publications/Publ-33420

Control of Stripe-Domain-Wall Magnetization in Multilayers Featuring Perpendicular Magnetic Anisotropy

R. Salikhov, F. Samad, B. Böhm, S. Schneider, D. Pohl, B. Rellinghaus, A. Ullrich, M. Albrecht, J. Lindner, N. S. Kiselev, O. Hellwig


We report on the controlled switching of domain-wall (DW) magnetization in aligned stripe-domain
structures, stabilized in [Co(0.44 nm)/Pt(0.7 nm)]X (X = 48, 100, 150) multilayers with perpendicular
magnetic anisotropy. The switching process, induced by an external magnetic field, is monitored by measuring the evolution of the in-plane magnetization. We show that the remanent in-plane magnetization originates from the polarization of the Bloch-type DWs. With micromagnetic simulations, we reveal that
the reversal of the DW polarization is the result of the emergence and collapse of horizontal Bloch lines
within the DWs at particular strengths of the external magnetic field, applied opposite to the DW polarization. Our findings are relevant for DW-based magnonics and bubble-skyrmion applications in magnetic multilayers.

Keywords: Magnetic domains; Domain walls; Magnetization switching; Skyrmions; Spintronics; Micromagnetism


Permalink: https://www.hzdr.de/publications/Publ-33411

Symmetry and curvature effects on spin waves in vortex-state hexagonal nanotubes

L. Körber, M. Zimmermann, S. Wintz, S. Finizio, M. Kronseder, D. Bougeard, F. Dirnberger, M. Weigand, J. Raabe, J. A. Otálora, H. Schultheiß, E. Josten, J. Lindner, I. Kézsmárki, C. H. Back, A. Kakay


Analytic and numerical studies on curved magnetic nano-objects predict numerous exciting effects that can be referred to as magneto-chiral effects, which do not originate from intrinsic Dzyaloshinskii–Moriya interaction or interface-induced anisotropies. In constrast, these chiral effects stem from isotropic exchange or dipole-dipole interaction, present in all magnetic materials, which acquire asymmetric contributions in case of curved geometry of the specimen. As a result, for example, the spin-wave dispersion in round magnetic nanotubes becomes asymmetric, namely spin waves of the same frequency propagating in opposite directions along the nanotube exhibit different wavelenghts. Here, using time-resolved scanning transmission X-ray microscopy experiments, standard micromagntic simulations and a dynamic-matrix approach, we show that the spin-wave spectrum undergoes additional drastic changes when transitioning from a continuous to a discrete rotational symmetry, i.e. from round to hexagonal nanotubes, which are much easier to fabricate. The polygonal shape introduces localization of the modes both to the sharp, highly curved corners and flat edges. Moreover, due to the discrete rotational symmetry, the degenerate nature of the modes with azimuthal wave vectors known from round tubes is partly lifted, resulting in singlet and duplet modes. For comparison with our experiments, we calculate the microwave absorption from the numerically obtained mode profiles which shows that a dedicated antenna design is paramount for magnonic applications in 3D nano-structures. To our knowledge these are the first experiments directly showing real space spin-wave propagation in 3D nano objects.

Keywords: spin wave; dispersion; curvature; micromagnetic modeling; hexagonal; symmetry; STXM

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Permalink: https://www.hzdr.de/publications/Publ-33367

Numerical reverse engineering of general spin-wave dispersions: Bridge between numerics and analytics using a dynamic-matrix approach

L. Körber, A. Kakay


Modern problems in magnetization dynamics require more and more the numerical determination of the spin-wave spectra and -dispersion in magnetic systems where analytic theories are not yet available. Micromagnetic simulations can be used to compute the spatial mode profiles and oscillation frequencies of spin-waves in magnetic system with almost arbitrary geometry and different magnetic interactions. Although numerical approaches are very versatile, they often do not give the same insight and physical understanding as analytical theories. For example, it is not always possible to decide whether a certain feature (such as dispersion asymmetry, for example) is governed by one magnetic interaction or the other. Moreover, since numerical approaches typically yield the normal modes of the system, it is not always feasible to disentangle hybridized modes. In this manuscript, we build a bridge between numerics and analytics by presenting a methodology to calculate the individual contributions to general spin-wave dispersions in a fully numerical manner. We discuss the general form of any spin-wave dispersion in terms of the effective (stiffness) fields produced by the modes. Based on a special type of micromagnetic simulation, the numerical dynamic-matrix approach, we show how to calculate each stiffness field in the respective dispersion law, separately for each magnetic interaction. In particular, it becomes possible to disentangle contributions of different magnetic interactions to the dispersion asymmetry in systems where non-reciprocity is present. At the same time, dipolar-hybridized modes can be easily disentangled. Since this methodology is independent of the geometry or the involved magnetic interactions at hand, we believe it is attractive for experimental and theoretical studies of magnetic systems where there are no analytics available yet, but also to aid the development of new analytical theories.

Keywords: spin wave; Micromagnetic simulations; theory; dispersion; dynamic-matrix approach; normal modes; hybridization; numerics


Permalink: https://www.hzdr.de/publications/Publ-33366

Stress-induced modification of gyration dynamics in stacked double-vortex structures studied by micromagnetic simulations

V. Iurchuk, L. Körber, A. M. Deac, J. Faßbender, J. Lindner, A. Kakay


In this paper, using micromagnetic simulations, we investigate the stress-induced frequency tunability of double-vortex nano-oscillators comprising magnetostrictive and non-magnetostrictive ferromagnetic layers separated vertically by a non-magnetic spacer. We show that the relative orientations of the vortex core polarities p1 and p2 have a strong impact on the eigen-frequencies of the dynamic modes. When the two vortices with antiparallel polarities have different eigen-frequencies and the magnetostatic coupling between them is sufficiently strong, the stress-induced magnetoelastic anisotropy can lead to the single-frequency resonant gyration mode of the two vortex cores. Additionally, for the case of parallel polarities, we demonstrate that for sufficiently strong magnetostatic coupling, the magnetoelastic anisotropy leads to the coupled vortex gyration in the chaotic regime and to the lateral separation of the vortex core trajectories. These findings offer a path for achieving a fine control over gyration frequencies and trajectories in vortex-based oscillators via adjustable elastic stress, which can be easily generated and tuned electrically, mechanically or optically.

Keywords: Magnetic vortex; Magnetization dynamics; Magnetoelastic anisotropy; Micromagnetic modelling

Permalink: https://www.hzdr.de/publications/Publ-33127

Controlled and deterministic creation of synthetic antiferromagnetic domains by focused ion beam irradiation

F. Samad, G. Hlawacek, S. S. P. K. Arekapudi, X. Xu, L. Koch, M. Lenz, O. Hellwig


We study layered synthetic antiferromagnets (SAFs) with out-of-plane interface anisotropy, where the layer-wise antiferromagnetic (AF)
alignment is induced by interlayer exchange coupling (IEC). By applying low energy He+ focused ion beam irradiation to the SAF, a depth-dependent
reduction of the IEC and anisotropy can be achieved due to layer intermixing. As a consequence, after irradiation, a specific field
reversal sequence of the SAF is energetically preferred. When tuning the pristine SAF to exhibit an inverted field reversal, we are thus able to
create AF domains in the irradiated regions. When irradiated with a fluence gradient, these AF domains can be further deterministically
manipulated by an external magnetic field. Among other applications, this could be utilized for engineering a controllable and local magnetic
stray field landscape, for example, at AF domain walls, within the otherwise stray field free environment provided by the SAF.

Keywords: Bubble domains; Focused ion beam; Sputter deposition; Interlayer exchange coupling; Magnetic hysteresis

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Permalink: https://www.hzdr.de/publications/Publ-33090

Reconfigurable Spin-Wave Interferometer at the Nanoscale

J. Chen, H. Wang, T. Hula, C. Liu, S. Liu, T. Liu, H. Jia, Q. Song, C. Guo, Y. Zhang, J. Zhang, X. Han, D. Yu, M. Wu, H. Schultheiß, H. Yu


Spin waves with nanoscale wavelengths can transfer information free of electron transport and hence are promising for wave-based computing technologies with low-power consumption as a solution to the severe energy losses in modern electronics. Logic circuits based on the interference of spin waves have been proposed for more than a decade. However, spin-wave interference at the nanoscale has yet been realized. Here, we demonstrate experimentally the interference of spin waves with wavelengths down to 50 nm in a low-damping magnetic insulator. The constructive and destructive interference of spin waves is detected in the frequency domain using propagating spin-wave spectroscopy, which is further confirmed by the Brillouin light scattering. The interference pattern is found to be highly sensitive to the distance between two magnetic nanowires acting as spin-wave emitters. By controlling the magnetic configuration of the double-wire system, one can switch the spin-wave interferometer on and off. The observed phenomena are theoretically accounted for by the interlayer magnon-magnon coupling. Our demonstrations are thus key to the realization of spin-wave computing system based on non-volatile nanomagnets at the GHz frequencies.

Keywords: spin waves; magnetism; Brillouin spectroscopy; interferometer; interference; nanotechnologie

Permalink: https://www.hzdr.de/publications/Publ-32911

Nonreciprocity of spin waves in magnetic nanotubes with helical equilibrium magnetization

M. M. Salazar-Cardona, L. Körber, H. Schultheiß, K. Lenz, A. Thomas, K. Nielsch, A. Kakay, J. A. Otálora


Spin waves (SWs) in magnetic nanotubes have shown interesting nonreciprocal properties in their dispersion relation, group velocity, frequency linewidth, and attenuation lengths. The reported chiral effects are similar to those induced by the Dzyaloshinskii–Moriya interaction but originating from the dipole–dipole interaction. Here, we show that the isotropic-exchange interaction can also induce chiral effects in the SW transport; the so-called Berry phase of SWs. We demonstrate that with the application of magnetic fields, the nonreciprocity of the different SW modes can be tuned between the fully dipolar governed and the fully exchange governed cases, as they are directly related to the underlying equilibrium state. In the helical state, due to the combined action of the two effects, every single sign combination of the azimuthal and axial wave vectors leads to different dispersions, allowing for a very sophisticated tuning of the SW transport. A disentangle- ment of the dipole–dipole and exchange contributions so far was not reported for the SW transport in nanotubes. Furthermore, we propose a device based on coplanar waveguides that would allow to selectively measure the exchange or dipole induced SW nonreciprocities. In the context of magnonic applications, our results might encourage further developments in the emerging field of 3D magnonic devices using curved magnetic membranes.

Keywords: spin wave; nanotube; nonreciprocity; transducer


Permalink: https://www.hzdr.de/publications/Publ-32767

Multistate current-induced magnetization switching in Au/Fe/MgO(001) epitaxial heterostructures

P. Gospodarič, E. Młyńczak, I. Soldatov, A. Kakay, D. E. Bürgler, L. Plucinski, R. Schäfer, J. Faßbender, C. M. Schneider


Magnetization switching using in-plane charge current recently has been widely investigated in heavy metal/ferromagnet bilayers with the switching mechanism usually attributed to the action of the spin-orbit coupling. Here we study in-plane current induced magnetization switching in model epitaxial bilayers that consist of Au(001) and Fe(001) grown on MgO(001). We use the planar Hall effect combined with magnetooptical Kerr effect (MOKE) microscopy to investigate magnetic properties of the bilayers and current-induced switching. We show that a current density beyond 1.4×10^7 A/cm can be employed for reproducible electrical switching of the magnetization between multiple stable states that correspond to different arrangements of magnetic domains with magnetization direction along one of the in-plane easy magnetization axes of the Fe(001) film. Lower current densities result in stable intermediate transversal resistances which are interpreted based on MOKE-microscopy investigations as resulting from the current-induced magnetic domain structure that is formed in the area of the Hall cross. We find that the physical mechanism of the current-induced magnetization switching of the Au/Fe/MgO(001) system at room temperature can be fully explained by the Oersted field, which is generated by the charge current flowing mostly through the Au layer.

Permalink: https://www.hzdr.de/publications/Publ-32616

Spin polarization and magnetotransport properties of systematically disordered Fe60Al40 thin films

K. Borisov, J. Ehrler, C. Fowley, B. Eggert, H. Wende, S. Cornelius, K. Potzger, J. Lindner, J. Faßbender, R. Bali, P. Stamenov


We investigate the evolution of spin polarization, spontaneous Hall angle (SHA), saturation magnetization and Curie temperature of B2-ordered Fe60Al40 thin films under varying antisite disorder, induced by Ne+-ion irradiation. The spin polarization increases monotonically as a function of ion fluence. A relatively high polarization of 46 % and the SHA of 3.1 % are achieved on 40 nm thick films irradiated with 2 ⋅ 1016 ions/cm2 at 30 keV. An interesting divergence in the trends of the magnetization and SHA is observed for low disorder concentrations. The high spin polarization and its broad tunability range make ion-irradiated Fe60Al40 a promising material for application in spin electronic devices.

Keywords: Spin polarization; Iron-Aluminium; Spintronics; Anomalous Hall Effect; Topological Hall effect; Irradiation effects; Thin films; Magnetometry; Andreev reflection

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Permalink: https://www.hzdr.de/publications/Publ-32604

Finite-element dynamic-matrix approach for spin-wave dispersions in magnonic waveguides with arbitrary cross section

L. Körber, G. Quasebarth, A. Otto, A. Kakay


We present a numerical approach to efficiently calculate spin-wave dispersions and spatial mode profiles in magnetic waveguides of arbitrarily shaped cross section with any non-collinear equilibrium magnetization which is translationally invariant along the waveguide. Our method is based on the propagating-wave dynamic-matrix approach by Henry et al. and extends it to arbitrary cross sections using a finite-element method. We solve the linearized equation of motion of the magnetization only in a single waveguide cross section which drastically reduces computational effort compared to common three-dimensional micromagnetic simulations. In order to numerically obtain the dipolar potential of individual spin-wave modes, we present a plane-wave version of the hybrid finite-element/boundary-element method by Frekdin and Koehler which, for the first time, we extend to a modified version of the Poisson equation. Our method is applied to several important examples of magnonic waveguides including systems with surface curvature, such as magnetic nanotubes, where the curvature leads to an asymmetric spin-wave dispersion. In all cases, the validity of our approach is confirmed by other methods. Our method is of particular interest for the study of curvature-induced or magnetochiral effects on spin-wave transport but also serves as an efficient tool to investigate standard magnonic problems.

Keywords: spin wave; eigensolver; micromagnetic simulation; dispersion; finite-element method; FEM

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Permalink: https://www.hzdr.de/publications/Publ-32561

Agility of spin Hall nano-oscillators

F. J. Trindade Goncalves, T. Hache, M. Bejarano, T. Hula, O. Hellwig, J. Faßbender, H. Schultheiß


We investigate the temporal response of constriction-based spin Hall nano-oscillators driven by pulsed stimuli using time-resolved Brillouin light scattering microscopy. The growth rate of the magnetization auto-oscillations, enabled by spin Hall effect and spin orbit torque, is found to vary with the amplitude of the input voltage pulses, as well as the synchronization frequency set by an external microwave input. The combination of voltage and microwave pulses allows to generate auto-oscillation signals with multi-level amplitude and frequency in the time-domain. Our findings suggest that the lead time of processes such as synchronization and logic using spin Hall nano-oscillators can be reduced to the nanosecond time-scale.

Keywords: Spintronics; Magnons; Spin waves; Brillouin scattering & spectroscopy; Microwave techniques

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Permalink: https://www.hzdr.de/publications/Publ-32448

Robustness of the remanent magnetic domain pattern formation and associated stripe-bubble transitions in Co/Pt multilayers against field sequencing

A. Gentillon, C. Richards, L. A. Ortiz-Flores, J. Metzner, D. Montealegre, M. Healey, K. Cardon, A. Westover, O. Hellwig, K. Chesnel


Thin ferromagnetic [Co/Pt] multilayers with perpendicular magnetic anisotropy exhibit a variety of nanoscopic magnetic domain patternsat remanence, from long interlaced stripes to lattices of bubbles, depending on the multilayer structure but also on the magnetic historyof the sample. For optimized structural parameters, stripe-bubble transitions accompanied by drastic increases in domain density havebeen observed when the magnitude of the previously applied perpendicular fieldHmis finely tuned throughout the hysteresis loop. Here, we investigate the robustness of these morphological transitions against field sequencing and field cycling. We conducted this study on[Co(x)/Pt(7Å)]N=50where x varies from 4 to 60 Å. We mapped the morphological transition withHmvarying from 0 to 9 T, following bothan ascending sequence (0→9 T) and a descending sequence (9 T→0). We found that the optimal fieldHm=H∗at which the domain densityis maximized and its associated maximal density n∗ are not significantly affected by the field sequencing direction. We have also investigatedpossible pumping effects when cycling the applied field at the value H∗. We found that n∗ remains relatively stable through field cycling, andmuch more stable in the bubble state, compared to longer stripe states. The observed robustness of these morphological transitions againstfield sequencing and field cycling is of crucial importance for potential magnetic recording applications.

Permalink: https://www.hzdr.de/publications/Publ-32342

Microresonators and Microantennas—tools to explore magnetization dynamics in single nanostructures

H. Cansever, J. Lindner


The phenomenon of magnetic resonance and its detection via microwave spectroscopy provide direct insight into the magnetization dynamics of bulk or thin film materials. This allows for direct access to fundamental properties, such as the effective magnetization, g-factor, magnetic anisotropy and the various damping (relaxation) channels that govern the decay of magnetic excitations. Cavity-based and broadband ferromagnetic resonance techniques that detect the microwave absorption of spin systems require a minimum magnetic volume to obtain a sufficient signal-to-noise ratio (S/N). Therefore, conventional techniques typically do not offer the sensitivity to detect individual micro- or nanostructures. A solution to this sensitivity problem is the so-called planar microresonator, which is able to detect even the tiniest absorption signals of magnetic nanostructures, including spin-wave or edge resonance modes. As an example, we describe the microresonator-based detection of spin-wave modes within microscopic strips of ferromagnetic A2 Fe60Al40 that are imprinted into a paramagnetic B2 Fe60Al40-matrix via focused ion-beam irradiation. While microresonators operate at a fixed microwave frequency, a reliable quantification of the key magnetic parameters like the g-factor or spin relaxation times requires investigations within a broad range of frequencies. In this study, we introduce and describe the step from microresonators towards a broadband microantenna approach. It allows for performing broadband magnetic resonance experiments on single nanostructured magnetic objects in a frequency range of 2-18 GHz. We employ this detection scheme to explore the influence of lateral structuring on the magnetization dynamics of a Permalloy strip.

Keywords: ferromagnetic resonance; microantenna; microresonator; magnetic relaxation; thin films; nanosctructures

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Permalink: https://www.hzdr.de/publications/Publ-32170

Theory of three-magnon interaction in a vortex-state magnetic nanodot

R. Verba, L. Körber, K. Schultheiß, H. Schultheiß, V. Tiberkevich, A. Slavin


We use vector Hamiltonian formalism (VHF) to study theoretically three-magnon parametric interaction (or three-wave splitting) in a magnetic disk existing in a magnetic vortex ground state. The three-wave splitting in a disk is found to obey two selection rules: (i) conservation of the total azimuthal number of the resultant spin-wave modes, and (ii) inequality for the radial numbers of interacting modes, if the mode directly excited by the driving field is radially symmetric (i.e. if the azimuthal number of the directly excited mode is m=0). The selection rule (ii), however, is relaxed in the "small" magnetic disks, due to the influence of the vortex core. We also found, that the efficiency of the three-wave interaction of the directly excited mode strongly depends on the azimuthal and radial mode numbers of the resultant modes, that becomes determinative in the case when several splitting channels (several pairs of resultant modes) simultaneously approximately satisfy the resonance condition for the splitting. The good agreement of the VHF analytic calculations with the experiment and micromagnetic simulations proves the capability of the VHF formalism to predict the actual splitting channels and the magnitudes of the driving field thresholds for the three-wave splitting.

Keywords: spin wave; nonlinear; three-magnon interaction; theory; micromagnetic simulation; vortex

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Permalink: https://www.hzdr.de/publications/Publ-32042

The 2021 Magnonics Roadmap

A. Barman, G. Gubbiotti, S. Ladak, A. O. Adeyeye, M. Krawczyk, J. Gräfe, A. V. Chumak, A. Khitun, D. Nikonev, I. A. Young, V. I. Vasyuchka, B. Hillebrands, S. A. Nikitov, H. Yu, D. Grundler, A. V. Sadovnikov, A. A. Grachev, S. E. Sheshukova, J.-Y. Duquesne, M. Marangolo, G. Csaba, W. Porod, V. E. Demidov, S. Urazhdin, S. O. Demokritov, E. Albisetti, D. Petti, R. Bertacco, H. Schultheiß, V. V. Kruglyak, V. D. Poimanov, S. Sahoo, J. Sinha, T. Moriyama, S. Mizukami, H. Yang, M. Münzenburg, P. Landeros, R. A. Gallardo, G. Carlotti, J.-V. Kim, R. L. Stamps, R. E. Camley, B. Rana, Y. Otani, W. Yu, T. Yu, G. E. W. Bauer, C. Back, G. S. Uhrig, O. V. Dobrovolskiy, S. van Dijken, B. Budinska, H. Qin, C. Adelmann, S. Cotofana, A. Naeemi, B. W. Zingsem, M. Winklhofer


Magnonics is a rather young physics research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. After several papers and review articles published in the last decade, with a steadily increase in the number of citations, we are presenting the first Roadmap on Magnonics. This a collection of 22 sections written by leading experts in this field who review and discuss the current status but also present their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and the interconnections to standard electronics. In this respect, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This Roadmap represents a milestone for future emerging research directions in magnonics and hopefully it will be followed by a series of articles on the same topic.

Keywords: magnonics; spin waves; roadmap; spin textures; skyrmions; computing

Permalink: https://www.hzdr.de/publications/Publ-31827

Highly Tunable Magnetic and Magnetotransport Properties of Exchange Coupled Ferromagnet/Antiferromagnet-based Heterostructures

S. S. P. K. Arekapudi, D. Bülz, F. Ganss, F. Samad, F. Radu, D. R. T. Zahn, K. Lenz, G. Salvan, M. Albrecht, O. Hellwig


Antiferromagnets (AFMs) with zero net magnetization are proposed as active elements in future spintronic devices. Depending on the critical thickness of the AFM thin films and the measurement temperature, bimetallic Mn-based alloys and transition metal oxide-based AFMs can host various coexisting ordered, disordered, and frustrated AFM phases. Such coexisting phases in the exchange coupled ferromagnetic (FM)/AFM-based heterostructures can result in unusual magnetic and magnetotransport phenomena. Here, we integrate chemically disordered AFM γ-IrMn3 thin films with coexisting AFM phases into complex exchange coupled MgO(001)/γ-Ni3Fe/γ-IrMn3/γ-Ni3Fe/CoO heterostructures and study the structural, magnetic, and magnetotransport properties in various magnetic field cooling states. In particular, we unveil the impact of rotating the relative orientation of the disordered and reversible AFM moments with respect to the irreversible AFM moments on the magnetic and magnetoresistance properties of the exchange coupled heterostructures. We further found that the persistence of AFM grains with thermally disordered and reversible AFM order is crucial for achieving highly tunable magnetic properties and multi-level magnetoresistance states. We anticipate that the introduced approach and the heterostructure architecture can be utilized in future spintronic devices to manipulate the thermally disordered and reversible AFM order at the nanoscale.


Permalink: https://www.hzdr.de/publications/Publ-31742

Frequency- and magnetic-field-dependent properties of ordered magnetic nanoparticle arrangements

N. Neugebauer, T. Hache, M. T. Elm, D. M. Hofmann, C. Heiliger, H. Schultheiß, P. J. Klar


We present a frequency and magnetic field dependent investigation of ordered arrangements of 20 nm mag-netic nanoparticles (MNPs) consisting of magnetite (Fe3O4) by employing micro Brillouin light scatteringmicroscopy. We utilized electron beam lithography to prepare hexagonally arranged, circularly shaped MNP-assemblies consisting of a single layer of MNPs using a variant of the Langmuir-Blodgett technique. Bycomparing the results with non-structured, layered superlattices of MNPs, further insight into the influenceof size and geometry of the arrangement on the collective properties is obtained. We show that at low staticexternal field strengths, two signals occur in frequency dependent measurements for both non-structured andstructured assemblies. Enlarging the static external field strength leads to a sharpening of the main signal,while the satellite signal decreases in its intensity and increases in its linewidth. The occurrence of multiplesignals at low external field strengths is also confirmed by sweeping the static external field and keeping theexcitation frequency constant. Micromagnetic simulations unravel the origin of the different signals and theirdependence on the static external field strength, enabling an interpretation of the observed characteristics interms of different local environments of an MNPs forming the MNP assembly.

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Permalink: https://www.hzdr.de/publications/Publ-31689

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A. Smith, K. Sobotkiewich, A. Khan, E. A. Montoya, L. Yang, Z. Duan, T. Schneider, K. Lenz, J. Lindner, K. An, X. Li, I. N. Krivorotov
Dimensional crossover in spin Hall oscillators
Phys. Rev. B 102, 054422 (2020)
DOI: 10.1103/PhysRevB.102.054422

T. Huang, V. R. Misko, S. Gobeil, X. Wang, F. Nori, J. Schütt, J. Fassbender, G. Cuniberti, D. Makarov, L. Baraban
Inverse Solidification Induced by Active Janus Particles
Adv. Funct. Mater., 2003851 (2020)
DOI: 10.1002/adfm.202003851

E. Baek, N. Ranjan Das, C. Vittorio Cannistraci, T. Rim, G. Santiago Cañón Bermúdez, K. Nych, H. Cho, K. Kim, C.-K. Baek, D. Makarov, R. Tetzlaff, L. Chua, L. Baraban, G. Cuniberti
Intrinsic plasticity of silicon nanowire neurotransistors for dynamic memory and learning functions
Nature Electronics 3, 398 (2020)
DOI: 10.1038/s41928-020-0412-1

T. Hula, K. Schultheiß, A. Buzdakov, L. Körber, M. Bejarano, L. Flacke, L. Liensberger, M. Weiler, J. M. Shaw, H. T. Nembach, J. Faßbender, H. Schultheiß
Nonlinear losses in magnon transport due to four-magnon scattering
Appl. Phys. Lett. 117, 042404 (2020)
DOI: 10.1063/5.0015269

J. Llandro, D. M. Love, A. Kovács, J. Caron, K. N. Vyas, A. Kakay, R. Salikhov, K. Lenz, J. Faßbender, M. R. J. Scherer, C. Cimorra, U. Steiner, C. H. W. Barnes, R. E. Dunin-Borkowski, S. Fukami, H. Ohno
Visualizing Magnetic Structure in 3D Nanoscale Ni–Fe Gyroid Networks
Nano Lett. 20, 3642 (2020)
DOI: 10.1021/acs.nanolett.0c00578

S. Pile, M. Buchner, V. Ney, T. Schaffers, K. Lenz, R. Narkovic, J. Lindner, H. Ohldag, A. Ney
Direct imaging of the ac component of the pumped spin polarization with element specificity
Phys. Rev. Appl. 14, 034005 (2020)
DOI: 10.1103/PhysRevApplied.14.034005

T. Huang, S. Gobeil, X. Wang, V. Misko, F. Nori, W. de Malsche, J. Faßbender, D. Makarov, G. Cuniberti, L. Baraban
Anisotropic exclusion effect between photocatalytic Ag/AgCl Janus particles and passive beads in a dense colloidal matrix
Langmuir 36, 7091 (2020)
DOI: 10.1021/acs.langmuir.0c00012

J. Lumetzberger, M. Buchner, S. Pile, V. Ney, W. Gaderbauer, N. Daffé, M. V. Moro, D. Primetzhofer, K. Lenz, A. Ney
Influence of structure and cation distribution on magnetic anisotropy and damping in Zn/Al doped nickel ferrites
Phys. Rev. B 102, 054402 (2020)
DOI: 10.1103/PhysRevB.102.054402

C. Wang, C.-H. Chang, A. Herklotz, C. Chen, F. Ganss, U. Kentsch, D. Chen, X. Gao, Y.-J. Zeng, O. Hellwig, M. Helm, S. Gemming, Y.-H. Chu, S. Zhou
Topological Hall effect in single thick SrRuO3 layers induced by defect engineering
Adv. Electron. Mater. (2020)
DOI: 10.1002/aelm.202000184

T. Hache, Y. Li, T. Weinhold, B. Scheumann, F. J. Trindade Goncalves, O. Hellwig, J. Faßbender, H. Schultheiß
Bipolar spin Hall nano-oscillators
Appl. Phys. Lett. 116, 192405 (2020)
DOI: 10.1063/5.0008988

B. Eggert, A. Schmeink, J. Lill, M. O. Liedke, U. Kentsch, M. Butterling, A. Wagner, S. Pascarelli, K. Potzger, J. Lindner, T. Thomson, J. Fassbender, K. Ollefs, W. Keune, R. Bali, H. Wende
Magnetic response of FeRh to static and dynamic disorder
RSC Adv. 10, 14386 (2020)
DOI: 10.1039/D0RA01410A

O. Pylypovskyi, V. P. Kravchuk, O. Volkov, J. Faßbender, D. Sheka, D. Makarov
Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii–Moriya interaction
Appl. Phys. Lett. 53, 395003 (2020)
DOI: 10.1088/1361-6463/ab95bd

L. Ramasubramanian, A. Kákay, C. Fowley, O. Yildirim, P. Matthes, S. Sorokin, A. Titova, D. Hilliard, R. Böttger, R. Hübner, S. Gemming, S. E. Schulz, F. Kronast, D. Makarov, J. Faßbender, A. M. Deac
Tunable magnetic vortex dynamics in ion-implanted permalloy disks
ACS Appl. Mater. Interfaces 12, 27812 (2020)
DOI: 10.1021/acsami.0c08024

O. Yildirim, D. Hilliard, S. S. P. K. Arekapudi, C. Fowley, H. Cansever, L. Koch, L. Ramasubramanian, S. Zhou, R. Böttger, J. Lindner, J. Faßbender, O. Hellwig, A. M. Deac
Ion-irradiation-induced cobalt/cobalt oxide heterostructures: printing 3D interfaces
ACS Appl. Mater. Interfaces 12, 9858 (2020)
DOI: 10.1021/acsami.9b13503

D. D. Sheka, O. Pylypovskyi, P. Landeros, Y. Gaididei, A. Kakay, D. Makarov
Nonlocal chiral symmetry breaking in curvilinear magnetic shells
Communications Physics 3, 128 (2020)
DOI: 10.1038/s42005-020-0387-2

H. Zhong, M. Ghorbani-Asl, K. H. Ly, J. Ge, J. Zhang, M. Wang, Z. Liao, D. Makarov, E. Zschech, E. Brunner, I. M. Weidinger, J. Zhang, A. Krasheninnikov, S. Kaskel, R. Dong, X. Feng
Synergistic Electroreduction of Carbon Dioxide to Carbon Monoxide on Bimetallic Layered Conjugated Metal-Organic Frameworks
Nat. Commun. 11, 1409 (2020)
DOI: 10.1038/s41467-020-15141-y

M. Melzer, D. Makarov, O. G. Schmidt
A review on stretchable magnetic field sensorics
J. Phys. D Appl. Phys. 53, 083002 (2020)
DOI: 10.1088/1361-6463/ab52cf

S. Sorokin, R. Gallardo, C. Fowley, K. Lenz, A. Titova, G. Dennehy, G. Atcheson, K. Rode, J. Faßbender, J. Lindner, A. M. Deac
Magnetization dynamics in synthetic antiferromagnets: the role of dynamical energy and mutual spin-pumping
Phys. Rev. B 101, 14410 (2020)
DOI: 10.1103/PhysRevB.101.144410

T. Hache, M. Vaňatka, L. Flajšman, T. Weinhold, T. Hula, O. Ciubotariu, M. Albrecht, B. Arkook, I. Barsukov, L. Fallarino, O. Hellwig, J. Faßbender, M. Urbánek, H. Schultheiß
Freestanding and positionable microwave-antenna device for magneto-optical spectroscopy experiments
Phys. Rev. Appl. 13, 054009 (2020)
DOI: 10.1103/PhysRevApplied.13.054009

C. Dubs, O. Surzhenko, R. Thomas, J. Osten, T. Schneider, K. Lenz, J. Grenzer, R. Hübner, W. Elke
Low damping and microstructural perfection of sub-40nm-thin yttrium iron garnet films grown by liquid phase epitaxy
Phys. Rev. Mater. 4, 024416 (2020)
DOI: 10.1103/PhysRevMaterials.4.024416

B. Dieny, I. L. Prejbeanu, K. Garello, P. Gambardella, P. Freitas, R. Lehndorff, W. Raberg, U. Ebels, S. O. Demokritov, J. Akerman, A. Deac, P. Pirro, C. Adelmann, A. Anane, A. V. Chumak, A. Hiroata, S. Mangin, M. Cengiz Onbaşlı, M. D’Aquino, G. Prenat, G. Finocchio, L. Lopez Diaz, R. Chantrell, O. Chubykalo-Fesenko, P. Bortolotti
Opportunities and challenges for spintronics in the microelectronic industry
Nature Electronics 3, 446 (2020)
DOI: 10.1038/s41928-020-0461-5

J. Ehrler, B. Sanyal, J. Grenzer, S. Zhou, R. Böttger, H. Wende, J. Lindner, J. Faßbender, C. Leyens, K. Potzger, R. Bali
Magneto-structural correlations in a systematically disordered B2 lattice
New J. Phys. 22, 073004 (2020)
DOI: 10.1088/1367-2630/ab944a


Concept of artificial magnetoelectric materials via geometrically controlling curvilinear helimagnets
O. Volkov, U. K. Rößler, J. Faßbender, D. Makarov
J. Phys. D Appl. Phys. 52, 345001 (2019)
DOI: 10.1088/1361-6463/ab2368

Experimental Observation of Exchange-Driven Chiral Effects in Curvilinear Magnetism
O. Volkov, A. Kakay, K. Florian, J. I. Mönch, M. Mohamad-Assaad, J. Faßbender, D. Makarov
Phys. Rev. Lett. 123, 077201 (2019)
DOI: 10.1103/PhysRevLett.123.077201

Thermodynamics and determination of the exchange stiffness of asymmetrically sandwiched ultrathin ferromagnetic films with perpendicular anisotropy
I. Iastremskyi, O. Volkov, M. Kopte, T. Kosub, S. Stienen, K. Lenz, J. Lindner, J. Faßbender, B. A. Ivanov, D. Makarov
Phys. Rev. Appl. 12, 064038 (2019)
DOI: 10.1103/PhysRevApplied.12.064038

Gilbert damping in NiFeGd compounds: Ferromagnetic resonance versus time-resolved spectroscopy
R. Salikhov, A. Alekhin, T. Parpiiev, T. Pezeril, D. Makarov, R. Abrudan, R. Meckenstock, F. Radu, M. Farle, H. Zabel, V. V. Temnov
Phys. Rev. B 99, 104412 (2019)
DOI: 10.1103/PhysRevB.99.104412

Finite-size effects in ultrafast remagnetization dynamics of FePt
L. Willig, A. von Reppert, M. Deb, F. Ganss, O. Hellwig, M. Bargheer
Phys. Rev. B 100, 224408 (2019)
DOI: 10.1103/PhysRevB.100.224408

Imaging and writing magnetic domains in the non-collinear antiferromagnet Mn₃Sn
H. Reichlova, T. Janda, J. Godinho, A. Markou, D. Kriegner, R. Schlitz, J. Zelezny, Z. Soban, M. Bejarano, H. Schultheiß, P. Nemec, T. Jungwirth, C. Felser, J. Wunderlich, S. Goennenwein
Nat. Commun. 10, 5459 (2019)
DOI: 10.1038/s41467-019-13391-z

Spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy study of thermally treated Co60Fe20B20 thin films
M. Hoffmann, A. Sharma, P. Matthes, S. Okano, O. Hellwig, R. Ecke, D. Zahn, G. Salvan, S. Schulz
J. Phys.: Condens. Matter 32, 055702 (2019)
DOI: 10.1088/1361-648X/ab4d2f

Femtosecond X-ray induced changes of the electronic and magnetic response of solids from electron redistribution
D. Higley, A. Reid, Z. Chen, L. Le Guyader, O. Hellwig, A. Lutman, T. Liu, P. Shafer, T. Chase, G. Dakovski, A. Mitra, E. Yuan, J. Schlappa, H. Dürr, W. Schlotter, J. Stöhr
Nat. Commun. 10, 5289 (2019)
DOI: 10.1038/s41467-019-13272-5

Antiferromagnetic domain wall control via surface spin flop in fully tunable synthetic antiferromagnets with perpendicular magnetic anisotropy
B. Böhm, L. Fallarino, D. Pohl, B. Rellinghaus, K. Nielsch, N. S. Kiselev, O. Hellwig
Phys. Rev. B 100, 140411 (2019)
DOI: 10.1103/PhysRevB.100.140411

Independent Geometrical Control of Spin and Charge Resistances in Curved Spintronics
K. S. Das, D. Makarov, P. Gentile, M. Cuoco, B. J. van Wees, C. Ortix, I. J. Vera-Marun
Nano Lett. 19, 6839 (2019)
DOI: 10.1021/acs.nanolett.9b01994

Strain Anisotropy and Magnetic Domains in Embedded Nanomagnets
M. Nord, A. Semisalova, A. Kákay, G. Hlawacek, I. Maclaren, V. Liersch, O. Volkov, D. Makarov, G. W. Paterson, K. Potzger, J. Lindner, J. Faßbender, D. Mcgrouther, R. Bali
Small 15, 1904738 (2019)
DOI: 10.1002/smll.201904738

Implantable highly compliant devices for heating of internal organs: towards cancer treatment
G. S. Cañón Bermudez, A. Kruv, T. Voitsekhivska, I. Hochnadel, A. Lebanov, A. Potthoff, J. Fassbender, T. Yevsa, D. Makarov
Adv. Eng. Mater. 21, 1900407 (2019)
DOI: 10.1002/adem.201900407

A bimodal soft electronic skin for tactile and touchless interaction in real time
J. Ge, X. Wang, M. Drack, O. Volkov, M. Liang, G. S. Cañón Bermúdez, R. Illing, C. Wang, S. Zhou, J. Fassbender, M. Kaltenbrunner, D. Makarov
Nat. Commun. 10, 4405 (2019)
DOI: 10.1038/s41467-019-12303-5

Spin Hall magnetoresistance in heterostructures consisting of noncrystalline paramagnetic YIG and Pt
M. Lammel, R. Schlitz, K. Geishendorf, D. Makarov, T. Kosub, S. Fabretti, H. Reichlova, R. Huebner, K. Nielsch, A. Thomas, S. T. B. Goennenwein
Appl. Phys. Lett. 114, 252402 (2019)
DOI: 10.1063/1.5090098

Magnetization Dynamics of an Individual Single-Crystalline Fe-Filled Carbon Nanotube
K. Lenz, R. Narkowicz, K. Wagner, C. F. Reiche, J. Körner, T. Schneider, A. Kákay, H. Schultheiss, D. Suter, B. Büchner, J. Fassbender, T. Mühl, J. Lindner
Small 15, 1904315 (2019)
DOI: 10.1002/smll.201904315

Strain-induced perpendicular magnetic anisotropy and Gilbert damping of Tm3Fe5O12 thin films
O. Ciubotariu, A. Semisalova, K. Lenz, M. Albrecht
Sci Rep 9, 17474 (2019)
DOI: 10.1038/s41598-019-53255-6

Synthesis of Mg and Zn diolates and their use in metal oxide deposition
P. Frenzel, A. Preuß, J. Bankwitz, C. Georgi, F. Ganss, L. Mertens, S. Schulz, O. Hellwig, M. Mehring, H. Lang
RSC Adv. 9, 10657 (2019)
DOI: 10.1039/C9RA00585D

High spin-wave propagation length consistent with low damping in a metallic ferromagnet
L. Flacke, L. Liensberger, M. Althammer, H. Huebl, S. Geprägs, K. Schultheiß, A. Buzdakov, T. Hula, H. Schultheiß, E. R. J. Edwards, H. T. Nembach, J. M. Shaw, R. Gross, M. Weiler
Appl. Phys. Lett. 115, 122402 (2019)
DOI: 10.1063/1.5102132

Nonlinear ferromagnetic resonance in the presence of 3-magnon scattering in magnetic nanostructures
D. V. Slobodianiuk, G. A. Melkov, K. Schultheiß, H. Schultheiß, R. V. Verba
IEEE Magn. Lett. 10, 6103405 (2019)
DOI: 10.1109/LMAG.2019.2913132

Nanomagnetism of Magnetoelectric Granular Thin-Film Antiferromagnets
P. Appel, B. J. Shields, T. Kosub, N. Hedrich, R. Hübner, J. Fassbender, D. Makarov, P. Maletinsky
Nano Lett. 19, 1682 (2019)
DOI: 10.1021/acs.nanolett.8b04681

Zero-field dynamics stabilized by in-plane shape anisotropy in MgO-based spin-torque oscillators
E. Kowalska, A. Kákay, C. Fowley, V. Sluka, J. Lindner, J. Fassbender, A. M. Deac
J. Appl. Phys. 125, 083902 (2019)
DOI: 10.1063/1.5081036

Effect of insertion layer on electrode properties in magnetic tunnel junctions with a zero-moment half-metal
A. Titova, C. Fowley, E. Clifford, Y.-C. Lau, K. Borisov, D. Betto, G. Atcheson, R. Hübner, C. Xu, P. Stamenov, M. Coey, K. Rode, J. Lindner, J. Fassbender, A. M. Deac
Sci Rep 9, 4020 (2019)
DOI: 10.1038/s41598-019-40609-3

Characterization of Continuous Wave Laser-Induced Thermal Gradients in Magnetic Tunnel Junctions Integrated Into Microresonators via COMSOL Simulations
H. Cansever, J. Lindner, T. Huebner, A. Niesen, G. Reiss, J. Faßbender, A. M. Deac
IEEE T. Magn. 55 (2019)
DOI: 10.1109/TMAG.2019.2891903

Tuning Ferromagnetic Resonance via Disorder/Order Interfaces
T. Schneider, K. Lenz, A. Semisalova, J. Gollwitzer, J. Heitler-Klevans, K. Potzger, J. Fassbender, J. Lindner, R. Bali
J. Appl. Phys. 125, 195302 (2019)
DOI: 10.1063/1.5088797

Reconfigurable spin-wave non-reciprocity induced by dipolar interaction in a coupled ferromagnetic bilayer
R. A. Gallardo, T. Schneider, A. K. Chaurasiya, A. Oelschlägel, S. S. P. K. Arekapudi, A. Roldáan-Molina, R. Hübner, K. Lenz, A. Barman, J. Fassbender, J. Lindner, O. Hellwig, P. Landeros
Phys. Rev. Appl. 12, 034012 (2019)
DOI: 10.1103/PhysRevApplied.12.034012

Ion induced ferromagnetism combined with self-assembly for large area magnetic modulation of thin films
M. Krupinski, R. Bali, D. Mitin, P. Sobieszczyk, J. Gregor-Pawlowski, A. Zarzycki, R. Böttger, M. Albrecht, K. Potzger, M. Marszałek
Nanoscale 11, 8930 (2019)
DOI: 10.1039/c8nr10011j

Combined frequency and time domain measurements on injection-locked, constriction-based spin Hall nano-oscillators
T. Hache, T. Weinhold, K. Schultheiss, J. Stigloher, F. Vilsmeier, C. Back, S. S. P. K. Arekapudi, O. Hellwig, J. Fassbender, H. Schultheiss
Appl. Phys. Lett. 114, 102403 (2019)
DOI: 10.1063/1.5082692

Domain wall-based spin-Hall nano-oscillators
N. Sato, K. Schultheiß, L. Körber, N. Puwenberg, T. Mühl, A. A. Awad, S. S. P. K. Arekapudi, O. Hellwig, J. Faßbender, H. Schultheiß
Phys. Rev. Lett. 123, 057204 (2019)
DOI: 10.1103/PhysRevLett.123.057204

Excitation of whispering gallery magnons in a magnetic vortex
K. Schultheiss, R. Verba, F. Wehrmann, K. Wagner, L. Körber, T. Hula, T. Hache, A. Kákay, A. A. Awad, V. Tiberkevich, A. N. Slavin, J. Fassbender, H. Schultheiss
Phys. Rev. Lett. 122, 097202 (2019)
DOI: 10.1103/PhysRevLett.122.097202

The role of open-volume defects in the annihilation of antisites in a B2-ordered alloy
J. Ehrler, M. O. Liedke, J. Cizek, R. Boucher, M. Butterling, S. Zhou, R. Böttger, E. Hirschmann, T. T. Trinh, A. Wagner, J. Lindner, J. Fassbender, C. Leyens, K. Potzger, R. Bali
Acta Mater. 176, 167 (2019)
DOI: 10.1016/j.actamat.2019.06.037

Highly compliant planar Hall effect sensor with sub 200 nT sensitivity
P. Granell, G. Wang, G. S. Canon Bermudez, T. Kosub, F. Golmar, L. Steren, J. Fassbender, D. Makarov
npj Flexible electronics 3, 3 (2019)
DOI: 10.1038/s41528-018-0046-9

Experimental and Theoretical Study of Curvature Effects in Parabolic Nanostripes
O. M. Volkov, F. Kronast, I. Mönch, M.-A. Mawass, A. Kákay, J. Fassbender, D. Makarov
Phys. Status Solidi 13, 1800309 (2019)
DOI: 10.1002/pssr.201800309

Spin-wave nonreciprocity on magnetization-graded ferromagnetic films
R. A. Gallardo, P. Alvarado-Seguel, T. Schneider, C. Gonzalez-Fuentes, A. Roldán-Molina, K. Lenz, J. Lindner, P. Landeros
New J. Phys. 21, 033026 (2019)
DOI: 10.1088/1367-2630/ab0449

Structure-property relationship of Co2MnSi thin films in response to He+-irradiation
F. Hammerath, R. Bali, R. Hübner, M. R. D. Brandt, S. Rodan, K. Potzger, R. Böttger, Y. Sakuraba, B. Büchner, S. Wurmehl
Sci Rep 9, 2766 (2019)
DOI: 10.1038/s41598-019-39435-4

Tunnel magnetoresistance angular and bias dependence enabling tuneable wireless communication
E. Kowalska, A. Fukushima, V. Sluka, C. Fowley, A. Kákay, Y. Aleksandrov, J. Lindner, J. Fassbender, S. Yuasa, A. M. Deac
Sci Rep 9, 9541 (2019)
DOI: 10.1038/s41598-019-45984-5

Emission and Propagation of Multi-Dimensional Spin Waves in Anisotropic Spin Textures
V. Sluka, T. Schneider, R. A. Gallardo, A. Kakay, M. Weigand, T. Warnatz, R. Mattheis, A. Roldan-Molina, P. Landeros, V. Tiberkevich, A. Slavin, G. Schütz, A. Erbe, A. Deac, J. Lindner, J. Fassbender, J. Raabe, S. Wintz
Nat. Nanotechnol. 14, 328 (2019)
DOI: 10.1038/s41565-019-0383-4

Flat Bands, Indirect Gaps, and Unconventional Spin-Wave Behavior Induced by a Periodic Dzyaloshinskii-Moriya Interaction
R. A. Gallardo, D. Cortés-Ortuno, T. Schneider, A. Roldán-Molina, F. Ma, K. Lenz, H. Fangohr, J. Lindner, P. Landeros
Phys. Rev. Lett. 122, 067204 (2019)
DOI: 10.1103/PhysRevLett.122.067204

Spin-Wave Modes in Transition from a Thin Film to a Full Magnonic Crystal
M. Langer, R. A. Gallardo, T. Schneider, S. Stienen, A. Roldán-Molina, Y. Yuan, K. Lenz, J. Lindner, P. Landeros, J. Fassbender
Phys. Rev. B 99, 024426 (2019)

Control of domain structure and magnetization reversal in thick Co/Pt multilayers
L. Fallarino, A. Oelschlägel, J. A. Arregi, A. Bashkatov, F. Samad, B. Böhm, K. Chesnel, O. Hellwig
Phys. Rev. B 99, 024431 (2019)

Dynamic Imaging of the Delay-and Tilt-Free Motion of Neel Domain Walls in Perpendicularly Magnetized Superlattices
S. Finizio, S. Wintz, K. Zeissler, A. V. Sadovnikov, S. Mayr, S. A. Nikitov, C. H. Marrows, J. Raabe
Nano Lett. 19, 375 (2019)


270. Hydrogen storage in Mg2FeSi alloy thin films depending on the Fe-to-Si ratio measured by conversion electron Mössbauer spectroscopy
T. T. Trinh, K. Asano, R. Heller, H. Reuther, J. Grenzer, H. Schreuders, B. Dam, K. Potzger
Nucl. Instrum. Meth. B 434, 109 (2018)

269.Chiral Skyrmion and Skyrmionium States Engineered by the Gradient of Curvature
O. V. Pylypovskyi, D. Makarov, V. P. Kravchuk, Y. Gaididei, A. Saxena, D. D. Sheka
Phys. Rev. Appl. 10, 064057 (2018)

268. Controlled coexcitation of direct and indirect ultrafast demagnetization in Co/Pd multilayers with large perpendicular magnetic anisotropy
S. Pan, O. Hellwig, A. Barman
Phys. Rev. B 98, 214436 (2018)

267. Anomalous Hall-like transverse magnetoresistance in Au thin films on Y3Fe5O12
T. Kosub, S. Velez, J. M. Gomez-Perez, L. E. Hueso, J. Fassbender, F. Casanova, D. Makarov
Appl. Phys. Lett. 113, 222409 (2018)

266. Electronic-skin compasses for geomagnetic field driven artificial magnetoception and interactive electronics
G. S. Cañón Bermúdez, H. Fuchs, L. Bischoff, J. Fassbender, D. Makarov
Nature Electronics 1, 589 (2018)

265. Beyond a phenomenological description of magnetostriction
A. H. Reid, X. Shen, P. Maldonado, T. Chase, E. Jal, P. W. Granitzka, K. Carva, R. K. Li, J. Li, L. Wu, T. Vecchione, T. Liu, Z. Chen, D. J. Higley, N. Hartmann, R. Coffee, J. Wu, G. L. Dakovski, W. F. Schlotter, H. Ohldag, Y. K. Takahashi, V. Mehta, O. Hellwig, A. Fry, Y. Zhu, J. Cao, E. E. Fullerton, J. Stöhr, P. M. Oppeneer, X. J. Wang, H. A. Dürr
Nat. Commun. 9, 388 (2018)

264. Measuring the thermal properties of anisotropic materials using beam-offset frequency domain thermoreflectance
M. Rahman, M. Shahzadeh, P. Braeuninger-Weimer, S. Hofmann, O. Hellwig, S. Pisana
J. Appl. Phys. 123, 245110 (2018)

263. High-frequency measurements of thermophysical properties of thin films using a modified broad-band frequency domain thermoreflectance approach
M. Shahzadeh, M. Rahman, O. Hellwig, S. Pisana
Rev. Sci. Instrum. 89, 084905 (2018)

262. Ultrafast laser generated strain in granular and continuous FePt thin films
A. von Reppert, L. Willig, J.-E. Pudell, M. Rössle, W. Leitenberger, M. Herzog, F. Ganss, O. Hellwig, M. Bargheer
Appl. Phys. Lett. 113, 123101 (2018)

261. Ultrafast Self-Induced X-Ray Transparency and Loss of Magnetic Diffraction
Z. Chen, D. J. Higley, M. Beye, M. Hantschmann, V. Mehta, O. Hellwig, A. Mitra, S. Bonetti, M. Bucher, S. Carron, T. Chase, E. Jal, R. Kukreja, T. Liu, A. H. Reid, G. L. Dakovski, A. Föhlisch, W. F. Schlotter, H. A. Dürr, J. Stöhr
Phys. Rev. Lett. 121, 137403 (2018)

260. Visible Light Actuated Efficient Exclusion Between Plasmonic Ag/AgCl Micromotors and Passive Beads
X. Wang, L. Baraban, V. R. Misko, F. Nori, T. Huang, G. Cuniberti, J. Fassbender, D. Makarov
Small 14, 1802537 (2018)

259. High-Motility Visible Light-Driven Ag/AgCl Janus Micromotors
X. Wang, L. Baraban, A. Nguyen, J. Ge, V. R. Misko, J. Tempere, F. Nori, P. Formanek, T. Huang, G. Cuniberti, J. Fassbender, D. Makarov
Small, 1803613 (2018)

258. Injection locking of multiple auto-oscillation modes in a tapered nanowire spin Hall oscillator
K. Wagner, A. Smith, T. Hache, J.-R. Chen, L. Yang, E. Montoya, K. Schultheiss, J. Lindner, J. Fassbender, I. Krivorotov, H. Schultheiss
Sci Rep 8, 16040 (2018)

257. Geometry-induced motion of magnetic domain walls in curved nanostripes
K. V. Yershov, V. P. Kravchuk, D. D. Sheka, O. V. Pylypovskyi, D. Makarov, Y. Gaididei
Physical Review B 98, 060409 (2018)

256. Localization of magnon modes in a curved magnetic nanowire
Y. Gaididei, V. P. Kravchuk, F. G. Mertens, O. V. Pylypovskyi, A. Saxena, D. D. Sheka, O. M. Volkov
Low Temperature Physics 44, 814 (2018)

255. Frequency linewidth and decay length of spin waves in curved magnetic membranes
J. A. Otalora, A. Kákay, J. Lindner, H. Schultheiss, A. Thomas, J. Fassbender, K. Nielsch
Physical Review B 98, 014403 (2018)

254. Local probe of irradiation induced structural changes and orbital magnetism in Fe60Al40 thin films via order-disorder phase transition
E. La Torre, A. Smekhova, C. Schmitz-Antoniak, K. Ollefs, B. Eggert, B. Cöster, D. Walecki, F. Wilhelm, A. Rogalev, J. Lindner, R. Bali, R. Banerjee, B. Sanyal, H. Wende
Physical Review B 98, 024101 (2018)

253. Investigating spin-transfer torques induced by thermal gradients in magnetic tunnel junctions by using micro-cavity ferromagnetic resonance
H. Cansever, R. Narkowicz, K. Lenz, C. Fowley, L. Ramasubramanian, O. Yildirim, A. Niesen, T. Huebner, G. Reiss, J. Lindner, J. Fassbender, A. M. Deac
Journal of Physics D: Applied Physics 51, 224009 (2018)

252. Unexpected field-induced dynamics in magnetostrictive microstructured elements under isotropic strain
S. Finizio, S. Wintz, S. Gliga, E. Kirk, A. K. Suszka, P. Wohlhüter, K. Zeissler, J. Raabe
Journal of Physics: Condensed Matter 30, 314001 (2018)

251. Origin and Manipulation of Stable Vortex Ground States in Permalloy Nanotubes
M. Zimmermann, T. N. Gerhard-Meier, F. Dirnberger, A. Kákay, M. Decker, S. Wintz, S. Finizio, E. Josten, J. Raabe, M. Kronseder, D. Bougeard, J. Lindner, C. H. Back
Nano Letters 18, 2828 (2018)

250. Laser-Rewriteable Ferromagnetism at Thin Film Surfaces
J. Ehrler, M. He, M. V. Shugaev, N. I. Polushkin, S. Wintz, V. Liersch, S. Cornelius, R. Hübner, K. Potzger, J. Lindner, J. Fassbender, A. A. Ünal, S. Valencia, F. Kronast, L. V. Zhigilei, R. Bali
ACS Applied Materials and Interfaces 10, 15232 (2018)

249. Thick Permalloy films for the imaging of spin texture dynamics in perpendicularly magnetized systems
S. Finizio, S. Wintz, D. Bracher, E. Kirk, A. S. Semisalova, J. Förster, K. Zeissler, T. Weßels, M. Weigand, K. Lenz, A. Kleibert, J. Raabe
Physical Review B 98, 104415 (2018)

248. Multiplet of skyrmion states on a curvilinear defect: Reconfigurable skyrmion lattices
V. P. Kravchuk, D. D. Sheka, A. Kákay, O. M. Volkov, U. K. Rößler, J. van den Brink, D. Makarov, Y. Gaididei
Physical Review Letters 120, 067201 (2018)

247. Symmetries and localization properties of defect modes in metamaterial magnonic superlattices
R. A. Gallardo, T. Schneider, A. Roldán-Molina, M. Langer, A. S. Núñez, K. Lenz, J. Lindner, P. Landeros
Physical Review B 97, 174404 (2018)

246. Magnetosensitive e-skins with directional perception for augmented reality
G. S. Cañón Bermúdez, D. D. Karnaushenko, D. Karnaushenko, A. Lebanov, L. Bischoff, M. Kaltenbrunner, J. Fassbender, O. G. Schmidt, D. Makarov
Science Advances 4, eaao2623 (2018)

245. Mesoscale Dzyaloshinskii-Moriya interaction: geometrical tailoring of the magnetochirality
O. M. Volkov, D. D. Sheka, V. P. Kravchuk, Y. Gaididei, U. K. Rößler, J. Faßbender, D. Makarov
Scientific Reports 8, 866 (2018)

244. Dipolar interaction induced band gaps and flat modes in surface-modulated magnonic crystals
R. A. Gallardo, T. Schneider, A. Roldan-Molina, M. Langer, J. Fassbender, K. Lenz, J. Lindner, P. Landeros
Physical Review B 97, 144405 (2018)

243. Positron Annihilation Studies using a Superconducting Electron LINAC
A. Wagner, M. Butterling, E. Hirschmann, R. Krause-Rehberg, M. O. Liedke, K. Potzger
AIP Conference Proceedings 1970, 040003 (2018)

242. Ultra-dense planar metallic nanowire arrays with extremely large anisotropic optical and magnetic properties
Q. Jia, X. Ou, M. Langer, B. Schreiber, J. Grenzer, P. F. Siles, R. D. Rodriguez, K. Huang, Y. Yuan, A. Heidarian, R. Hübner, T. You, W. Yu, K. Lenz, J. Lindner, X. Wang, S. Facsko
Nano Research 11, 3519 (2018)

241. Interplay between magnetic domain patterning and anisotropic magnetoresistance probed by magnetooptics
J. Osten, K. Lenz, H. Schultheiss, J. Lindner, J. McCord, J. Fassbender
Physical Review B 97, 014415 (2018)

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