The layered structure of MAX phases is associated with a number of functional properties and is the subject of extensive research. While the unit-cell layers of these structures have been well studied, much less is known about the distribution and manipulation of point defects within them. Here, we selected the prototype Cr2AlC system and, using variable energy positron beams, observed Doppler broadening and positron annihilation lifetimes to track the evolution of defects caused by the penetration of energetic transition metal ions (Co+ and Mn+) and noble gas ions (Ar+ and Ne+). In all cases an overall reduction of the open-volume defect concentration is observed post-irradiation. Atomic displacements induced by the penetrating ions drastically modify the defect distribution: the concentration of agglomerates of 9–15 vacancies (corresponding to positron lifetimes of 335–450 ps) in the precursor [Cr2C/Al]n layers is suppressed, whereas Al mono- and Al-Cr di-vacancy (lifetimes 217–231 ps) concentrations are enhanced. This breakdown of large defects into point defects scales with atomic displacements and is largely independent of the penetrating ion species, providing insights into the manipulation of point defects in nano-layered systems.

Emitters based on photoconductive materials excited by ultrafast lasers are well-
established and popular devices for THz generation. However, so far, these emitters – both
photoconductive antennas and large area emitters - were mostly explored using driving lasers
with moderate average powers (either fiber lasers with up to hundreds of milliwatts or Ti:Sapphire
systems up to few watts). In this paper, we explore the use of high-power, MHz repetition
rate Ytterbium (Yb) based oscillator for THz emission using a microstructured large-area
photoconductive emitter, consist of semi-insulating GaAs with a 10 × 10 mm2 active area. As a
driving source, we use a frequency-doubled home-built high average power ultrafast Yb-oscillator,
delivering 22 W of average power, 115 fs pulses with 91 MHz repetition rate at a central
wavelength of 516 nm. When applying 9 W of average power (after an optical chopper with
a duty cycle of 50%) on the structure without optimized heatsinking, we obtain 65 μW THz
average power, 4 THz bandwidth; furthermore, we safely apply up to 18 W of power on the
structure without observing damage. We investigate the impact of excitation power, bias voltage,
optical fluence, and their interplay on the emitter performance and explore in detail the sources
of thermal load originating from electrical and optical power. Optical power is found to have
a more critical impact on large area photoconductive emitter saturation than electrical power,
thus optimized heatsinking will allow us to improve the conversion efficiency in the near future
towards much higher emitter power. This work paves the way towards achieving hundreds of
MHz or even GHz repetition rates, high-power THz sources based on photoconductive emitters,
that are of great interest for example for future THz imaging applications.

Magnonics is a research field that has gained an increasing interest in both the fundamental and applied sciences in recent years. This field aims to explore and functionalize collective spin excitations in magnetically ordered materials for modern information technologies, sensing applications and advanced computational schemes. Spin waves, also known as magnons, carry spin angular momenta that allow for the transmission, storage and processing of information without moving charges. In integrated circuits, magnons enable on-chip data processing at ultrahigh frequencies without the Joule heating, which currently limits clock frequencies in conventional data processors to a few GHz. Recent developments in the field indicate that functional magnonic building blocks for in-memory computation, neural networks and Ising machines are within reach. At the same time, the miniaturization of magnonic circuits advances continuously as the synergy of materials science, electrical engineering and nanotechnology allows for novel on-chip excitation and detection schemes. Such circuits can already enable magnon wavelengths of 50 nm at microwave frequencies in a 5G frequency band. Research into non-charge-based technologies is urgently needed in view of the rapid growth of machine learning and artificial intelligence applications, which consume substantial energy when implemented on conventional data processing units. In its first part, the 2024 Magnonics Roadmap provides an update on the recent developments and achievements in the field of nano-magnonics while defining its future avenues and challenges. In its second part, the Roadmap addresses the rapidly growing research endeavors on hybrid structures and magnonics-enabled quantum engineering. We anticipate that these directions will continue to attract researchers to the field and, in addition to showcasing intriguing science, will enable unprecedented functionalities that enhance the efficiency of alternative information technologies and computational schemes.

Antiferromagnetic materials have unique properties due to their alternating spin arrangements. Their compensated magnetic order, robust against external magnetic fields, prevents long-distance crosstalk from stray fields. Furthermore, antiferromagnets with combined parity and time-reversal symmetry enable electrical control and detection of ultrafast exchange-field enhanced spin manipulation up to THz frequencies. Here we report the experimental realization of a nonvolatile antiferromagnetic memory mimicking an artificial synapse, in which the reconfigurable synaptic weight is encoded in the ratio between reversed antiferromagnetic domains. The non-volatile memory is “written” by spin-orbit torque-driven antiferromagnetic domain wall motion and “read” by nonlinear magnetotransport. We show that the absence of long-range interacting stray magnetic fields leads to very reproducible electrical pulse-driven variations of the synaptic weights.

Femtosecond laser-induced ultrafast magnetization dynamics are all-optically probed for different remanent magnetic domain states of a [Co/Pt]22 multilayer sample, thus revealing the tunability of the direct transport of spin angular momentum across domain walls. A variety of different magnetic domain configurations (domain wall origami) at remanence achieved by applying different magnetic field histories are investigated by time-resolved magneto-optical Kerr effect magnetometry to probe the ultrafast magnetization dynamics. Depending on the underlying domain landscape, the spin-transport-driven magnetization dynamics show a transition from typical ultrafast demagnetization to being fully dominated by an anomalous transient magnetization enhancement (TME) via a state in which both TME and demagnetization coexist in the system. Thereby, the study reveals an extrinsic channel for the modulation of spin transport, which introduces a route for the development of magnetic spin-texture-driven ultrafast spintronic devices.

Perpendicular magnetic anisotropy thin film systems are well known for their periodic magnetic stripe domain structures. In this study, we focus on investigating the behavior of [Co(3.0 nm)/Pt(0.6 nm)]X multilayers within the transitional regime from preferred in-plane to out-of-plane magnetization orientation. Particularly, we examine the sample with X = 11 repetitions, which exhibits a remanent state characterized by a significant presence of both out-of-plane (OOP) and in-plane (IP) magnetization components, here referred to as the “tilted” stripe domain state. Vector vibrating sample magnetometry and magnetic force microscopy are used to investigate this specific sample and its unusual out-of-plane reversal behavior. Through experimental data analysis and micromagnetic simulations of the tilted magnetization system, we identify a single point of irreversibility during an out-of-plane external magnetic field sweep. This behavior is qualitatively similar to the reversal of a Stoner-Wohlfarth particle or of an IP magnetized disk with remanent vortex structure, since both show distinct points of irreversibility as well. Such a collective response to an external field is typically not observed in conventional OOP or IP systems, where the reversal process often involves independent nucleation, propagation, and annihilation of individual domains. Finally, we show that our findings are not at all restricted to Co/Pt multilayers, but are a quite general feature of transitional in-plane to out-of-plane magnetization systems.

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.

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
phase.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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 M_{0073. 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.}

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.

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.

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.

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.

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.

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.

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.

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.

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
states

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.

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 Mn_1.4PtSn. 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.

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.

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.

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.

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.

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.

A chiral spin soliton lattice (CSL), one of the representative systems of a magnetic superstructure, exhibits reconfigurability in periodicity over a macroscopic length scale. Such coherent and tunable characteristics of the CSL lead to an emergence of elementary excitation of the CSL as phononlike modes due to translational symmetry breaking and bring a controllability of the dispersion relation of the CSL phonon. Using a broadband microwave spectroscopy technique, we directly found that higher-order magnetic resonance modes appear in the CSL phase of a chiral helimagnet CrNb3S6, which is ascribed to the CSL phonon response. The resonance frequency of the CSL phonon can be tuned between 16 and 40 GHz in the vicinity of the critical field, where the CSL period alters rapidly. The frequency range of the CSL phonon is expected to extend over 100 GHz as extrapolated on the basis of the theoretical model. The present results indicate that chiral helimagnets could work as materials useful for broadband signal processing in the millimeter-wave band.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

A broken interfacial inversion symmetry in ultrathin ferromagnet/heavy metal (FM/HM) bilayers is generally believed to be a prerequisite for accommodating Dzyaloshinskii-Moriya interaction (DMI) and for stabilizing chiral spin textures. By contrast, we present an approach for engineering both the sign and amplitude of DMI in relatively thick films without involving interfacial asymmetry, which is achieved through incorporating the composition gradient-induced bulk magnetic asymmetry (BMA) combined with strong spin-orbit coupling (SOC). The pivotal roles of BMA and SOC are theoretically examined based on the three-site Fert-Lévy model and the first principles calculations. Experimentally, both the sign and amplitude of DMI in films with controllable composition gradients along the growth direction, in the presence/absence of SOC are studied by using a Brillouin light scattering spectroscopy. Our results suggest that the appreciable value of DMI (±0.15 mJ/m2) could be established through combining BMA and SOC into relatively thick films. It is expected that our findings may help to further understand chiral magnetism and to design novel non-collinear spin textures.

Stripe domains in thin films form through a complex competition of perpendicular anisotropy and demagnetizing energy and are still lacking a complete micromagnetic description, despite being investigated since 50 years. This work elucidates the formation of stripe domains with a special focus on the dependence of stripe domain width on film thickness with varying ratio of the two major energy contributions. An overview and review of the most established analytical models for the calculation of this dependency is given with respect to existing experimental data as well as to new experimental data on epitaxial Fe-Co-C films with perpendicular anisotropy. Since the analytical models are limited in their predictive power and compatibility in terms of the suitable material parameter range, an effcient but rigorous micromagnetic simulation method was developed, which proved to be comparable or better than
previous models in describing experimental finding, especially for films with strongly dominating demagnetizing. Comprehensive simulations where performed to determine thickness dependent stripe width for various material parameters which can serve as a benchmark for analytical theories or can be used directly for comparison with experimental results. At a given combination of exchange constant and saturation polarization there exists a specifc thickness at which the stripe width is independent of the uniaxial anisotropy.

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.

This work bridges two gaps in the magnetic separation of rare-earth ions. 1) A material property database is provided for the solutal expansion coefficient and the magnetic susceptibility of eleven out of seventeen trivalent rare-earths. 2) A novel protocol is developed to enhance and resolve the magnetic term of the Kelvin force. For that purpose, an assembly of partition magnets is created where the individual magnets function in the first quadrant of their magnetic hysteresis loop. The mutual reinforcement is quantified in a particle magnetic levitation system. Thus, compared to exisiting magnetic assemblies, an enhancement in $\frac{\partial B^2}{2 \mu_0 \partial z}$ as high as 2 orders of magnitudes is realized that covers 90\% of the normalized spatial scale and requires 1 order of magnitude less magnet mass. Modeling the energy density field makes it possible to quantify the equilibrium position of the particle cloud at rest, which is attained via magnetophoresis of the particles regardless of their initial position. This enables the magnetic trapping and manipulation of particles with small hydrodynamic diameters. Optically tracking the transient magnetophoresis enables a high-fidelity, sub-mm resolution of $\frac{\partial \bm{B}^2}{2 \mu_0 \partial z}$ which is further used to quantify the magnetic susceptibility of Ho(III), Tb(III), Er(III) and Gd(III).

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.

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.

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.

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.

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 λ_Pd=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 λ_FeAl=11.9 ±0.2 nm for PM-Fe60Al40. Demonstrated bi-functionality of Fe60Al40 alloy in spin pumping structures may be promising for spintronic applications

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.

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.

Traditionally, the primary field, where curvature has been at the heart of research, was the theory of general relativity. In recent studies, however, the
impact of curvilinear geometry enters various disciplines, ranging from solid-state physics over soft-matter physics, chemistry and biology to mathematics,
giving rise to a plethora of emerging domains such as curvilinear nematics, curvilinear studies of cell biology, curvilinear semiconductors,
superfluidity, optics, two-dimensional van der Waals materials, plasmonics, magnetism and superconductivity. Here, we summarize the state of the art
and outline prospects for future research in curvilinear solid-state systems exhibiting such fundamental cooperative phenomena as ferromagnetism,
antiferromagnetism and superconductivity. Highlighting the recent developments and current challenges in theory, fabrication and characterization of
curvilinear micro- and nanostructures, special attention is paid to perspective research directions entailing new physics and to their strong application
potential. Overall, the perspective is aimed at crossing the boundaries between the magnetism and superconductivity communities and drawing attention
to the conceptual aspects of how extension of structures into the third dimension and curvilinear geometry can modify existing and aid launching
novel functionalities. In addition, the perspective should stimulate the development and dissemination of R&D-oriented techniques to facilitate rapid
transitions from laboratory demonstrations to industry-ready prototypes and eventual products.

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.
Frequency

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The effects of hydrogen absorption on the effective magnetization (4πM_eff), gyromagnetic ratio (γ), Gilbert damping constant (α_G), and the inhomogeneous linewidth broadening in Py(x)/Y(16 nm)/Pd(15 nm) trilayer films (x = 2, 3, 5, 8, 10, 20, 40 nm) were investigated with ferromagnetic resonance (FMR), transmission electron microscopy, and vibrating sample magnetometry. In the presence of a hydrogen atmosphere, the samples show a reduction of their FMR linewidth which is found to stem purely from a reduction of the inhomogeneous linewidth broadening. This is attributed to a rearrangement of atoms at the Py/Y interface in the presence of hydrogen, making the Py/Y interface more homogeneous. In addition, a reduction of 4πM_eff was seen for all samples in the hydrogen atmosphere which is typical for an increase of the interfacial perpendicular magnetic anisotropy at the Py/Y interface.

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.

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.