The magnetic damping of Ni80Fe20 (Permalloy, Py) thin films is studied via temperature- and frequency-dependent ferromagnetic resonance (FMR) experiments as a function of the Py layer thickness. The Py films are protected from oxidation by an Al cap layer. Two different thickness series are investigated, Al/Py/sapphire, where Py is in direct contact with the oxidic substrate, and Al/Py/Al/sapphire, where an Al spacer separates the Py film from the substrate. Additional magnetic and structural characterization is done by superconducting quantum interference device (SQUID) magnetometry as well as x-ray reflectivity and transmission electron microscopy. The full FMR dataset allows to separate the Gilbert-like contributions to the FMR linewidth from non-Gilbert-like ones like two magnon scattering (TMS) processes via the frequency dependence of the FMR linewidth. In addition, the Py thickness series ranging from 3.4 nm to 37.1 nm allows to separate the derived magnetic parameters such as saturation magnetization Ms from SQUID measurements, and TMS contribution Gamma and, most importantly, Gilbert damping parameter alpha from FMR measurements into their respective bulk and interfacial contributions and their respective temperature dependencies. While the bulk contribution alpha_bulk monotonously decreases with temperature from 0.0061(1) down to 0.0054(1) for both samples series, the interfacial contribution interface shows a subsequent increase at low temperature, which is rather pronounced for the Al/Py/sapphire series, caused by a temperature dependent TMS contribution. For the Al/Py/Al/sapphire series this increase at low temperatures is much less pronounced and a temperature independent TMS contribution is only found for the thinnest sample due to a wavy Py film morphology due to the islandlike growth of the Al spacer. Correcting for the interfacial and/or TMS contributions allows to extract alpha_bulk(T ) which reflects the intrinsic magnetic damping properties of sputtered Py thin films.

Magnetic shape memory alloys, owing to their multifunctional properties, are a promising material system for integration into microsystems. Their multifunctionality arises from the coexistence of ferroelasticity and ferromagnetism. While size-effects in ferromagnetic microstructure are well understood, corresponding experiments on the ferroelastic martensite microstructure are sparse. In this study, we use epitaxially grown Ni-Mn-Ga-based films as a model system to investigate the influence of finite size on the martensite microstructure under constrained and freestanding conditions. The results show that the microfabricated patterns, in both conditions, retain the characteristics of their continuous film microstructures. Film thickness has a strong influence, as this is the smallest extension investigated in our study. Our analysis reveals similarities and differences between finite size effects in ferromagnetic and ferroelastic microstructure, which is crucial for using these multifunctional materials in microsystems

The experimental investigation of coherent magnon modes in a truly three-dimensional (3D) magnonic crystal has not yet been accomplished. This scientific gap exists despite the numerous theoretical predictions about miniband formation and edge magnon modes with topological protection. The latter aspects are key to advance nanoscale electronics for ultrafast on-chip information technologies. In this work, we make use of a scalable nanotechnology and integrate a micrometer-sized magnonic crystal to an on-chip microresonator. The 3D ferromagnetic structure consists of a nanostructured woodpile unit cell with a lattice constant of 1 μm. It was made by two-photon lithography and subsequent atomic layer deposition of 30-nm-thick polycrystalline nickel. The nickel-coated woodpile was transferred by micromanipulation into the planar microresonator to perform angle-dependent ferromagnetic resonance measurements under coherent excitation. We explored two frequencies in the GHz frequency regime. We detected rich spectra of resonances with angular dependencies, which reflect the face-centered cubic lattice geometry of the woodpile. Micromagnetic simulations for quantitative comparison and the high edge sensitivity of the microresonator allow us to identify modes in the curved nanocaps which are relatively robust against variations in the applied field orientation and exhibit a counter-intuitive phase evolution across the edges. Our findings advance the integration of 3D nanomagnonics into functional microwave circuits and the visionary prospects of edge-dominated magnon modes in 3D nanoengineered magnets.

Using Brillouin light scattering microscopy, we study the rich dynamics in magnetic disks and rings governed by non-linear interactions, focusing on the role of vortex core dynamics on the spin-wave eigenmode spectrum. By strongly exciting quantized magnon modes in magnetic vortices, self-induced magnon Floquet states are populated by the intrinsic nonlinear coupling of magnon modes to the vortex core gyration. In magnetic rings, however, this generation is suppressed even when exciting the system over a large power range. To retrieve the rich nonlinear dynamics in rings, we apply external in-plane magnetic fields by which the vortex core is restored. Our findings demonstrate how to take active control of the nonlinear processes in magnetic structures of different topology.

Driving condensed matter systems with periodic electromagnetic fields can result in exotic states not found in equilibrium. Termed Floquet engineering, such periodic driving applied to electronic systems can tailor quantum effects to induce topological band structures and control spin interactions. However, Floquet engineering of magnon band structures in magnetic systems has proven challenging so far. Here, we present a class of Floquet states in a magnetic vortex that arise from nonlinear interactions between the vortex core and microwave magnons. Floquet bands emerge through the periodic oscillation of the core, which can be initiated by either driving the core directly or pumping azimuthal magnon modes. For the latter, the azimuthal modes induce core gyration through nonlinear interactions, which in turn renormalizes the magnon band structure. This represents a self-induced mechanism for Floquet band engineering and offers new avenues to study and control nonlinear magnon dynamics.

In this study, the spin-wave spectrum in multilayer synthetic antiferromagnets is calculated. The analysis focuses on the effects of varying both the thicknesses and the number of ferromagnetic layers within these structures. The results reveal that a non-reciprocal spin-wave dispersion occurs in structures with an even number of layers, while a reciprocal dispersion of two counterpropagating waves is observed for systems with an odd number of layers. As the number of layers and their thickness increase, the study identifies the distinctive presence of bulk and surface modes, with the latter being strongly affected by dynamic dipolar interactions. In multilayers with an even number of layers, such surface modes exhibit nonreciprocal behavior, maintaining their surface character only in one propagation direction. Conversely, in odd-layer systems, the symmetric counterpropagating surface modes have similar properties. Additionally, the bulk modes for both even and odd numbers of layers converge towards similar dynamic behavior as the thickness and number of layers increase. As the thickness of the ferromagnetic layers increases, the surface modes in multilayers with an odd number of layers remain localized at either the top or bottom, depending on the sign of the wave vector. In contrast, for the even case, the surface modes appear in both the top and bottom ferromagnetic layers when the layers are thin or ultrathin. However, as the ferromagnetic layer thickness increases, these modes gradually become predominantly localized at either the top or bottom of the multilayer. Finally, the study explores the application of an external magnetic field, demonstrating that surface chiral modes are absent in the saturated state, resulting in a reciprocal spin-wave dispersion. This establishes a magnetic field-mediated control over non-reciprocal localized surface modes.

The nonreciprocity created by dipolar coupling, electric currents, and Dzyaloshinskii-Moriya interactions is discussed in cases where the magnon propagation direction has a component parallel to the toroidal moment. A criterion for calculating the toroidal moments is established, addressing the issue of correct origin selection by considering compensated and uncompensated magnetization distributions. This criterion is then applied to various nonreciprocal magnetic systems, with the results of calculations consistent with those reported in the literature and predicting the existence of nonreciprocity in a more general manner. These results broaden the physical significance of the toroidal moment and facilitate the identification and estimation of nonreciprocity in magnonic systems. This work also clarifies the interrelations between different definitions of the toroidal moment for confined structures, where a surface term arising from surface-bound currents connects these definitions without the need for time averaging. Comparing these definitions of the toroidal moment applied to different magnetic textures demonstrates that they are always parallel but may differ in magnitude and sign. The discrepancy in the different definitions is deemed irrelevant since its direction, rather than its magnitude, primarily predicts the existence of magnon nonreciprocity.

In this study, we systematically compare the impact of a 1.5-nm-thin Ta adhesion layer on the structural and
magnetic behavior of Co/Pt multilayers (MLs) sputter-deposited at a low and a high Ar gas pressure. Our focus
is a comprehensive structural analysis, centered on real-space cross-sectional scanning transmission electron
microscopy (STEM) images, which are then correlated with complementary, reciprocal-space measurements
from x-ray reflectometry (XRR) and x-ray diffraction (XRD). Drawing on this comparison and supplemented
by atomic force microscopy (AFM), we develop a simplified model to fit the XRR data. Fits and data agree
excellently, while the extracted density profiles match well with the STEM data. The structural features are then
correlated with the magnetic ones, as determined by magnetic reversal loops and magnetic force microscopy
(MFM) images. We find considerably better texture with the adhesion layer, as well as vastly different structural
features at the higher deposition pressure between the different underlayers. At the lower pressure, the influence
of the underlayer is much less pronounced. Magnetically, the Ta layer results in a marked improvement in
the effective perpendicular magnetic anisotropy (PMA), increasing it up to an impressive 340% at the higher
pressure, while remaining comparable at the lower one. We attribute the changes in structure and magnetism
to two different factors: (1) The adhesion layer stops the implantation of Pt atoms into the substrate, improving
layer smoothness and texture. (2) The momentum transfer to the film is reduced at higher sputter pressures, which
reduces intermixing between Co and Pt, but increases overall roughness. Combining both, the roughness remains
small enough to enable well-defined grains with sharper Co/Pt interfaces and lower intragrain curvature, resulting
in the increased PMA. Our study provides a practical approach for controlling anisotropy in sputter-deposited
ML systems containing heavy atom species, utilizing different combinations of adhesion layer and sputter
pressure.

Waste heat harvesting of industrial processes is necessary to improve energy efficiency,
reduce environmental impact, and in the best of cases it also pays off economically.
Consequently, many different harvesting systems were developed, but for
temperatures below 100 °C, a mature technology that reaches sufficient efficiency and
output power is not yet available. Here, we focus on thermoelastic harvesting, which
uses shape-memory alloy (SMA) wires as active materials. We demonstrate that
the cycle frequency and thus output power of a thermoelastic harvesting system can
be increased by replacing the common fluid flow axial to the wire by a transverse
flow. This results in a simultaneous heat exchange along the length of the wire. This
improvement in timing allows one to increase the wire length and cycle frequency,
which both increases output power, effectively enabling the up-scaling of thermoelastic
devices. We validate our approach by in-operando experiments for a 1mm thick
wire and demonstrate that transverse flow allows us to increase the cycle frequency
from 1 to 11 Hz, which increases output power by a factor of 4.3. As slow frequency
is also a shortcoming of other ferroic harvesting and cooling devices, we discuss the
feasibility of our timing approach for these emerging technologies as well.

At equiatomic composition, FeV alloys form metastable phases such as the B2 phase or a short-range ordered phase in favor of the equilibrium phase. In this regime, structural, magnetic, and vibrational properties are strongly influenced by atomic disorder. For thermally co-evaporated FeV thin-films, the non-equilibrium phase diagram is explored and the short-range ordered phase that forms at equiatomic compositions is compared to its base-centered cubic counterpart. The results indicate that within the short-range ordered thin-films, the structural disorder is accompanied by an increase in chemical order, i.e. Fe and V atoms are their respective preferential nearest neighbors. These observations are important for a better understanding of recrystallization in nonmagnetic, short-range ordered templates, in which crystallization can be selectively triggered.

Voltage control of exchange bias (EB) is an important
technological goal for low-power spintronic sensor and memory devices. The
magneto-ionic (MI) approach for voltage-controlled EB is a promising
strategy to achieve this goal, utilizing electrochemical reactions at low
operational voltages. In typical MI devices, however, the sensitive EB layers
are directly targeted by the electrochemical reactions, which often impairs
reversibility. Here, we introduce an alternative device structure by isolating
the EB layers from the active MI layer. Making use of the interlayer (IL)
coupling through a spacer layer in an IrMn/Fe/Au/Fe spin-valve-like
heterostructure, we show that EB can be reversibly controlled by an
electrochemical modification of the top layer. Using the same device
structure, we also realize an MI switching between single-step and double-step
hysteresis loops. We interpret the observed MI effects via an increasing top Fe
layer thickness, caused by the electrochemical reduction of FeOx to
ferromagnetic Fe. Modeling of hysteresis loops as a function of top layer thickness in an extended Stoner−Wohlfarth approach
corroborates this interpretation. Our results highlight an advanced strategy for improving reversibility in MI devices and open a
novel pathway toward voltage-controlled spin valves.

In low dimensional magnetic structures, magnetostatic energy minimization yields a uniform spin-texture, resembling that of a macroscopic bar-magnet, thereby limiting their scope for applications. Here, the formation of non-collinear spin-textures such as Néel-type domain walls with perpendicular core magnetization, stable under ambient conditions, in magnetic nanostructures with dimensions reduced to ~25 nm, is experimentally demonstrated. A single-step magnetic writing process is deployed, wherein a nano-focussed beam of penetrating ions displaces atoms locally and converts a short-range ordered precursor alloy that is paramagnetic to a strain-relaxed lattice that is ferromagnetic, thus manifesting a controllable spin-texture evolution. Simulations show that the spin-textures rely on the presence of symmetry breaking due to crystallographic misalignments within the confined geometries. Theoretical considerations yield the critical dimension below which perpendicular Néel walls are stabilized. These results provide a direct route to implementing spatially confined non-collinear spin-textures generating local perpendicular magnetic fields, that are necessary in applications such as spin-transport and spin-based quantum sensing.

Spintronic terahertz (THz) frequency conversion in ferromagnet/heavy metal (FM/HM) heterostructures has the potential to enhance high-speed data communication and advance ultrafast magnetic memory applications. By leveraging ultrafast spin currents and spin-orbit interactions in FM/HM systems, broadband THz generation can be achieved, with recent studies demonstrating spintronic THz second harmonic generation (TSHG) and optical rectification. We introduce concepts for controlling the frequency conversion and temporal characteristics of TSHG through active manipulation of FM magnetization, providing flexibility in second harmonic emission and waveform shaping. The TSHG valve is realized by employing THz metamaterials, consisting of hybrid FM/HM structures combined with subwavelength gold periodic arrays. Additionally, using microstructured gold periodic arrays, we investigate the TSHG field enhancement capability as a function of grating filling factor and explore the potential for TSHG cavity enhancement.

Nano-lamellar alloys known as MAX phases integrate ceramic structural properties with metallic conduction. Here we induce point defects in the prototype Cr2AlC system thereby locally perturbing the lamellar structure and track the changes to the magnetic and electron transport behavior. Systematic defect generation is achieved by the irradiation of energetic Co+. Cr+ , as well Ar+ as ions at fluences ranging from 1012 – 1015 ions cm−2.

The magnetic behavior is shown to consist of contributions from J = 1/2 quantum spins as well as J = infinity classical cluster-like behavior containing 80 muB spins. Both these magnetic defect types contribute to the transport properties, where the J = spins give rise to the Kondo effect with a characteristic temperature, TK~5 K. Kondo defects are present in the as-prepared alloy as well as post-irradiation. A low-field magnetoresistive switching is observed post-irradiation, showing a temperature dependence that is consistent with polaron hopping within defect clusters. The enhanced magnetization as well as spin-scattering occur regardless of the irradiated species, proving that the source of these effects are the displaced Cr atoms of the precursor alloy. These results demonstrate the electronic tunability of MAX phases making them promising materials for spin-transport devices.

In this study, structural and transport properties of Cr2AlC films of thickness t = 500 nm are compared with films of t = 50 nm. Both films exhibit a weak fiber texture, however, columnar grain growth is only observed for the 500 nm film. The films were systematically disordered through ion irradiation with energetic Ar+ and Co+ ions at fluences ranging from 1012 ions·cm−2 to 1015 ions·cm−2. The ion energy was adjusted to match the penetration depth to the film thickness. For both film thicknesses, a resistivity upturn at low temperatures was observed, with the thin films showing the resistivity minima at temperatures of 20 - 40 K, compared to the thick films with the resistivity minima close to 10 K. Further decrease of temperature results in an increase of resistivity, which is 10 times larger in the thin film case. The ρ(T) upturns are consistent with the Kondo effect, arising due to Cr atoms being displaced from their lattice sites. Field switching of the magnetoresistance has been observed previously in disordered thick films, but here, thin films show only a parabolic field dependence, which aligns with metal-like ordinary magnetoresistance.

Bottom-up synthetic approaches to synthesize stacks of individual quasi-two-dimensional materials remain a major experimental challenge. Here, we evaluate the applicability of the little-known technique of modulated elemental reactants (MER) for the synthesis of one-unit-cell-thick Bi2⁢Se3/MoSe2 bilayer stacks via the self-assembly of amorphous elemental Bi|Se and Mo|Se precursors. Using a two-step annealing approach, both layers can be crystallized in a single heterostructure, as evidenced by a combination of x-ray photoelectron spectroscopy, Raman spectroscopy, and x-ray diffraction. However, we observe a limited stability of the Bi2⁢Se3 layer, reflected in its partial decomposition under conditions required to crystallize MoSe2. The choice of substrate proves to be a crucial piece to control the thin film growth: We demonstrate the evolution of larger-scale epitaxial alignment within a heterostructure grown via MER by low-energy electron diffraction.

The transition from planar to three-dimensional (3D) magnetic nanostructures represents a significant advancement in both fundamental research and practical applications, offering vast potential for next-generation technologies like ultrahigh-density storage, memory, logic, and neuromorphic computing. Despite being a relatively new field, the emergence of 3D nanomagnetism presents numerous opportunities for innovation, prompting the creation of a comprehensive roadmap by leading international researchers. This roadmap aims to facilitate collaboration and interdisciplinary dialogue to address challenges in materials science, physics, engineering, and computing. The roadmap comprises eighteen sections, roughly divided into three parts. The first section explores the fundamentals of 3D nanomagnetism, focusing on recent trends in fabrication techniques and imaging methods crucial for understanding complex spin textures, curved surfaces, and small-scale interactions. Techniques such as two-photon lithography (TPL) and focused electron beam-induced deposition enable the creation of intricate 3D architectures, while advanced imaging methods like electron holography and synchrotron x-ray tomography provide nanoscale spatial resolution for studying magnetization dynamics in three dimensions. Various 3D magnetic systems, including coupled multilayer systems, artificial spin-ice, magneto-plasmonic systems, topological spin textures, and molecular magnets are discussed. The second section introduces analytical and numerical methods for investigating 3D nanomagnetic structures and curvilinear systems, highlighting geometrically curved architectures, interconnected nanowire systems, and other complex geometries. Finite element methods are emphasized for capturing complex geometries, along with direct frequency domain solutions for addressing magnonic problems. The final section focuses on 3D magnonic crystals and networks, exploring their fundamental properties and potential applications in magnonic circuits, memory, and spintronics. Computational approaches using 3D nanomagnetic systems and complex topological textures in 3D spintronics are highlighted for their potential to enable faster and more energy-efficient computing.

Physical reservoir computing has emerged as a powerful framework for exploiting the inherent nonlinear dynamics of physical systems to perform computational tasks. Recently, we presented the magnon-scattering reservoir, whose internal nodes are given by the fundamental wave-like excitations of ferromagnets called magnons. These excitations can be geometrically-quantized and, in response to an external stimulus, show transient nonlinear scattering dynamics that can be harnessed to perform memory and nonlinear transformation tasks. Here, we test a magnon-scattering reservoir in a single magnetic disk in the vortex state towards two key performance indicators for physical reservoir computing, the short-term memory and parity-check tasks. Using time-resolved Brillouin-light-scattering microscopy, we measure the evolution of the reservoir’s spectral response to an input sequence consisting of random binary inputs encoded in microwave pulses with two distinct frequencies. Two different output spaces of the reservoir are defined, one based on the time-averaged frequency spectra and another based on temporal multiplexing. Our results demonstrate that the memory and nonlinear transformation capability do not depend on the chosen read-out scheme as long as the dimension of the output space is large enough to capture all nonlinear features provided by the magnon-magnon interactions. This further shows that solely the nonlinear magnons in the physical system, not the read-out, determine the reservoir’s capacity.

Cryogenic magnetic refrigeration becomes more and more important nowadays, especially for the liquefaction of gases such as hydrogen. In this study, we have synthesized La_1–zCe_z(Fe_0.88–yMn_ySi_0.12)₁₃ samples and investigated their magnetic and magnetocaloric properties in order to assess their potential for cryogenic applications. By adjusting the Mn and Ce content and adding excess rare-earth elements, the first-order ferromagnetic transition was lowered from 200 to 40 K and the adiabatic temperature change of the samples was measured directly using pulsed magnetic fields. The sample with the lowest transition temperature still showed a significant adiabatic temperature change in magnetic fields up to 10 T, with an increasingly stronger first-order transition observed in samples with higher Ce substitution. In addition, we synthesized spherical powder with diameters between 20 and 120 μm using ultrasonic atomization while maintaining the magnetic transition, which is a promising starting material for future additive manufacturing of magnetocaloric materials.

Co thin films grown by thermal evaporation have been oxidized in-situ, in ambient conditions, as well as using a plasma device. In all cases, the hysteresis loops reveal exchange-bias coupling between the Co and the CoO layers. We show that the CoO/Co systems fabricated under ambient conditions and in a pure oxygen atmosphere couple magnetically in a similar way. Contrary, the CoO layer produced by plasma treatment shows a lower bias field, coercive field and blocking temperature. The systems also exhibit asymmetric hysteresis loops with different magnetization reversal for the lower descending and upper ascending magnetization branches. In one direction of the external magnetic field sweep the CoO/Co system switches mainly by domain wall motion, while for the opposite field, the influence of the coherent magnetization rotation on the reversal process is stronger. The magnitude of the asymmetry is dependent on the measurement temperature.

Today, a substantial fraction of the energy used for industrial and domestic
processes is wasted as heat, which contributes to global warming. Although
recuperation of waste heat at high temperatures is already feasible and applied,
this remains a challenge for low-grade waste heat below 100 °C. At
low-grade, most approaches are either inefficient or use harmful materials.
Here, we address this gap by examining thermoelastic harvesting, which utilizes
shape memory alloys. By coupled finite element simulations of a reciprocating
heat engine, we analyses the intimate connection between the functional
properties of shape memory alloys and a thermodynamic cycle, which
converts thermal to mechanical energy. Our approach allows for a systematic
optimization of efficiency by varying design and operation parameters,
which include fluid pressure, tube diameter, temperature, prestrain, spring
constant and damping. We obtain an efficiency of 8.6 %, which is equivalent
to 51% with respect to Carnot – an outstanding value for low-grade waste
heat of just 55 °C. We derive design guidelines, which connect both, system
engineering of thermoelastic harvesters and material science of thermoelastic
shape memory alloys.

Nonreciprocal spin-wave propagation in bilayer ferromagnetic systems has attracted significant attention due to its potential to precisely quantify material parameters as well as for applications in magnonic logic and information processing. In this study we investigate the nonreciprocity of spin-wave dispersions in heterostructures consisting of two distinct ferromagnetic materials, focusing on the influence of saturation magnetization and thickness of the magnetic layers. We exploit Brillouin light scattering to confirm numerical calculations which are conducted with the finite element software TETRAX. An extensive numerical analysis reveals that the nonreciprocal behavior is strongly influenced by the changing material parameters, with asymmetry in the spin-wave propagation direction reaching several GHz under optimized conditions. Our findings demonstrate that tailoring the bilayer composition enables precise control over nonreciprocity, providing a pathway for engineering efficient unidirectional spin-wave devices. These results offer a deeper understanding of hybrid ferromagnetic systems and open avenues for designing advanced magnonic circuits.

The interplay of electronic charge, spin, and orbital currents, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz lightwave spintronics and orbitronics. The essential rules for how terahertz fields interact with these systems in a nonlinear way are still not understood. In this work, we demonstrate a universally applicable electronic nonlinearity originating from spin-orbit interactions in conducting materials, wherein the interplay of light-induced spin and orbital textures manifests. We utilized terahertz harmonic generation spectroscopy to investigate the nonlinear dynamics over picosecond timescales in various transition metal films. We found that the terahertz harmonic generation efficiency scales with the spin Hall conductivity in the studied films, while the phase takes two possible values (shifted by π), depending on the d-shell filling. These findings elucidate the fundamental mechanisms governing nonequilibrium spin and orbital polarization dynamics at terahertz frequencies, which is relevant for potential applications of terahertz spin- and orbital-based devices.

With the goal of creating an in-plane (IP) uniaxial anisotropy system, we deposited a thickness series of epitaxial films grown on Si(110) substrates with Ag(110) and Cr(211) buffer layers by magnetron sputtering. However, quantifying the IP magnetic anisotropy using ferromagnetic resonance measurements revealed a much more complex behavior than expected for a simple uniaxial system like hexagonally close-packed (hcp) Co. To understand the experimental results, an in-depth x-ray diffraction analysis of the film structure was performed. Even at a thickness of 100 nm, it revealed an anisotropic strain in the Co films, mainly within the Co basal plane, while the axis remained mostly unaffected. Calculations show that such unrelaxed strain induces a significant magnetoelastic anisotropy, which counteracts the magnetocrystalline one and, as a result, reduces the overall effective anisotropy. A detailed analysis revealed that mainly the compressive strain along the out-of-plane direction is responsible for the observed magnetoelastic anisotropy, while the tensile strain along the IP direction only plays a minor role.

Nanoscale magnetic patterning can lead to the formation of a variety of spin textures, depending on the intrinsic properties of the material and the microstructure. Here we report on the spin textures formed in laterally patterned antiferromagnetic (AF)/ferromagnetic (FM) thin film stripes with a period of 200 nm (100 nm FM/100 nm AF). We make use of the AF to FM phase transition in FeRh thin films at ~100 °C, thereby creating a nanoscale pattern that is thermally switchable between AF/FM stripes and uniformly FM. A combination of spin-resolved photoemission electron microscopy, magnetic force microscopy, and magnetometry measurements allow direct nanoscale observations of the stray magnetic fields emergent from the nanopattern as well as the underlying magnetization. Our measurements reveal pinning centres resistant to temperature cycling that govern the modulated spin-texture as well as a sub-texture consisting of grain-driven nanoscale magnetization structure directed out of the film plane. The nanoscale magnetic structure is thus strongly influenced by the film microstructure. Signatures of exchange bias are not observed, most likely due to the small contact area between the AF and FM regions. These results show that temperature controllable spin textures can be created in FeRh thin films which could find application in domain wall, microwave, or magnonic devices.

The ultrafast control of magnetisation states in magnetically ordered systems poses significant technological challenges yet is vital for the development of memory devices that operate at picosecond timescales or terahertz (THz) frequencies. Despite considerable efforts achieving convenient ultrafast readout of magnetic states remains an area of active investigation. For practical applications, energy-efficient and cost-effective electrical detection is highly desirable. In this context, unidirectional spin-Hall magnetoresistance (USMR) has been proposed as a straightforward two-terminal geometry for the electrical detection of magnetisation states in magnetic heterostructures. In this work, we demonstrate that USMR is effective at THz frequencies, enabling picosecond time readouts initiated by light fields. We observe ultrafast USMR in various ferromagnet/heavy metal thin film heterostructures via THz second-harmonic generation. Our findings, along with temperature-dependent measurements of USMR, reveal a substantial contribution from electron-magnon spin-flip scattering, highlighting the potential for all-electrical detection of THz magnon modes.

Shape memory alloys have a wide range of applications, including high stroke actuation, energy-efficient ferroic cooling, and energy harvesting. The use of these alloys is based on a reversible martensitic transformation, which leads to a complex microstructure in the martensitic state. Understanding the formation of this microstructure after fast heating and cooling is crucial, since all above mentioned applications benefit from a high cycle frequency, as it allows a high power density. Here, to study the formation of the martensitic microstructure after fast heating and cooling, we use epitaxial Ni-Mn-Ga film as a model system, since the high surface-to-volume ratio of thin films enables rapid heating and cooling. Furthermore, the formation of a multi-level hierarchical microstructure after slow cooling of this material system is well understood. We apply a millisecond flash lamp pulse on Ni-Mn-Ga films and analyse the hierarchical martensitic microstructure after the flash lamp pulse at different energy densities. We observe substantial changes compared to slow cooling, which we attribute to the limited time available for the microstructure to form and to the thermal stress between film and substrate during rapid temperature changes.

Ferroic materials enable a multitude of emerging applications, and optimum functional properties are achieved when ferromagnetic and ferroelectric properties are coupled to a first-order ferroelastic transition. In bulk materials, this first-order transition involves an invariant habit plane, connecting coexisting phases: austenite and martensite. Theory predicts that this plane should converge to a line in thin films, but experimental evidence is missing. Here, we analyze the martensitic and magnetic microstructure of a freestanding epitaxial magnetic shape memory film. We show that the martensite microstructure is determined by an invariant line constraint using lattice parameters of both phases as the only input. This line constraint explains most of the observable features, which differ fundamentally from bulk and constrained films. Furthermore, this finite-size effect creates a remarkable checkerboard magnetic domain pattern through multiferroic coupling. Our findings highlight the decisive role of finite-size effects in multiferroics.

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.

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.

Since its discovery, graphene has gained considerable interest from scientists all over the world. For more than one decade, the scientific community has been spending notable amounts of intellectual and financial resources to study its properties, which paves a way towards the commercialization of graphene, other 2D materials and a manifold of their derivatives. In this review, the spectroscopic properties of porphyrin-functionalized 2D materials are comprehensively discussed, followed by an extensive presentation of state-of-the-art achievements in photocatalysis based on such composite/hybrid materials. The primary focus is on the fundamental understanding of the structure–property–performance relationship as well as its importance in the future target-oriented design and fabrication of photocatalysts with tailored properties. After a short introduction, different design strategies for the fabrication of porphyrin (Por)-functionalized graphene-based materials (GBMs) (covalent vs. non-covalent assemblies) are systematically summarized. Then, the photocatalysis-relevant properties of the composites are thoroughly discussed based on the experimental results provided by steady-state absorption spectroscopy (ground-state properties) and time-resolved absorption and emission spectroscopies (excited-state properties). The importance of appropriate data analysis, with particular respect to the photoemission processes, is brought to attention. Subsequently, the photocatalytic behavior towards hydrogen generation, CO2 reduction and pollutant degradation of Por and GBM hybrids are comprehensively reported and fundamental mechanisms of light-driven catalytic processes in such systems, together with efficiency-limiting steps, are highlighted. The role of spectroscopy as a very powerful tool that enables the determination of key photophysical properties essential for light-driven catalysis is emphasized. Finally, recent advances with respect to 2D materials beyond graphene and their assembly with Por as promising photocatalysts are presented. We believe that this review, which comprehensively presents the knowledge gained to date regarding composites based on 2D materials with porphyrins as promising photocatalysts, will stimulate further efforts of researchers to tackle the remaining challenges and contribute to taking a decisive step towards the commercialization of these photocatalysts in the future.

Research in cancer therapies is rapidly advancing and demands the
exploration of innovative approaches to further improve the efficacy of
treatment. Here a multimodal approach for cancer therapy is reported which
combines bioactive targeting, magnetic hyperthermia, and controlled drug
release. For this, a nanoformulation MNP-Chi-Dox-Ab, is bioengineered by
conjugating CA 15-3 antibodies to doxorubicin-loaded functionalized magnetic
nanoparticles (MNPs). Solvothermally synthesized MNPs of uniform
spherical shape and size are functionalized with thermo-pH-responsive
chitosan. The nanoformulation showed higher drug release of ≈65% at pH 5
and 42 °C temperature compared to the release at physiological pH and
temperature. Furthermore, in an alternating magnetic field drug release is
enhanced to 74%. Cytotoxicity studies in MCF-7 breast cancer cells confirm
the active targeting potential of the nanoformulation. For the nanoformulation
without bioactive molecule (anti-CA 15-3) only 18% cancer cell death is noted
whereas with the conjugation of anti-CA 15-3, 43% cell death is recorded.
Flow cytometry studies revealed an increased apoptotic population at
hyperthermic temperature (42 °C) compared to the physiological temperature.
These results suggest that MNP-Chi-Dox-Ab nanoformulation represents a
promising multimodal platform for synergistic breast cancer therapy by
combining active targeting, controlled drug release, and hyperthermia.

Yttrium iron garnet (YIG) is a prototypical material in spintronics due to its exceptional magnetic properties. To exploit these properties, high quality thin films need to be manufactured. Deposition techniques like sputter deposition or pulsed laser deposition at ambient temperature produce amorphous films, which need a postannealing step to induce crystallization. However, not much is known about the exact dynamics of the formation of crystalline YIG out of the amorphous phase. Here, we conduct extensive time and temperature series to study the crystallization behavior of YIG on various substrates and extract the crystallization velocities as well as the activation energies needed to promote crystallization. We find that the type of crystallization as well as the crystallization velocity depend on the lattice mismatch to the substrate. We compare the crystallization parameters found in literature with our results and find excellent agreement with our model. Our results allow us to determine the time needed for the formation of a fully crystalline film of arbitrary thickness for any temperature.

Graphite based conductors are promising low cost and light-weight alternatives to copper but current challenges
are among others the improvement of the conductivity of graphite films (GF). Therefore, in this work the in-
fluence of copper(II) chloride and nickel(II) chloride on the graphitization of graphene oxide and the electrical
conductivity of the resulting GF was studied. The electrical conductivity was measured at different scales by
contactless eddy current method and four point probe scanning tunneling microscopy transport measurements.
The macroscopic and nanoscopic transport measurements were complemented by network-based simulations,
which allowed us to estimate the microscopic material properties of the GF with and without additives.
Annealing temperatures between 1600 ◦C to 2850 ◦C and varying metal chloride concentrations revealed an
optimum at 2850 ◦C and a concentration of 8.8 mmol/l NiCl2 in the aqueous dispersion. These GFs displayed an
electrical conductivity of 609 kS/m, around 30 % higher than the GFs without any metal chloride addition. The
graphitization morphology was analyzed by x-ray diffraction and scanning electron microscopy respectively. The
precipitation effect of carbon from Ni supports the growth of graphitic structures, whereas the catalytic activity
of Cu known from chemical vapor deposition does not promote graphitic structures. Rather, the annealing
temperature is the crucial parameter for achieving highly conductive films. Furthermore, the influence of applied
pressure during compression was studied. High pressures of at least 250 MPa are needed to obtain compact GFs
with an electrical conductivity 3 times larger than before compression

Magneto-ionics, as the electrochemical reconfiguration
of magnetic materials at low gating voltages, is a highly energy-efficient
alternative to control magnetism. For the fastest magneto-ionic concept
based on hydrogen, fundamental mechanisms are currently under debate,
mainly because quantitative compositional information inside the magnetic
materials upon gating is lacking. Using the electrochemical hydrogen loading
of Co/Pd multilayers with perpendicular anisotropy, this study demonstrates
that the hydrogen concentration inside the magnetic material determines
its magnetic properties. Hydrogen concentrations up to a maximum of
(0.24 ± 0.01) hydrogen atoms per metal atom can be set deterministically by
voltage and are quantified via flow-cell coulometry. With increasing hydrogen
concentration, a continuous increase in coercivity of up to 15% and a decrease
in magnetic domain-wall velocity by an order of magnitude are observed using
in situ MOKE microscopy. This enables the voltage-controlled stop-and-go of
domain walls. These magneto-ionic effects can be explained by an increasing
perpendicular anisotropy with increasing hydrogen content in Co/Pd
multilayers, which is supported by theory. Importantly, the approach should
be transferable to other ionic systems, such as lithium- or oxygen-based
ones, where it can uncover the yet hidden effects of ionic concentration on the
magnetic properties and guide the design of functional magneto-ionic devices.

Using all-optical time-resolved magneto-optical Kerr effect measurements we demonstrate an
efficient modulation of the spin–wave (SW) dynamics via the bias magnetic field orientation
around nanoscale diamond shaped antidots that are arranged on a square lattice within a
[Co(0.75 nm)/Pd(0.9 nm)]8 multilayer with perpendicular magnetic anisotropy (PMA).
Micromagnetic modeling of the experimental results reveals that the SW modes in the lower
frequency regime are related to narrow shell regions around the antidots, where in-plane (IP)
domain structures are formed due to the reduced PMA, caused by Ga+ ion irradiation during the
focused ion beam milling process of antidot fabrication. The IP direction of the shell
magnetization undergoes a striking change with magnetic field orientation, leading to the sharp
variation of the edge localized (shell) SW modes. Nevertheless, the coupling between such edge
localized and bulk SWs for different orientations of bias field in PMA systems gives rise to
interesting Physics and attests to new prospects for developing energy efficient and
hybrid-system-based next-generation nanoscale magnonic devices.

The chirality induced spin selectivity (CISS) effect has been up to now measured in a wide variety of systems but its exact mechanism is still under debate. Whether the spin polarization occurs at an interface layer or builds up in the helical molecule is yet not clear. Here we have investigated the current transmission through helical polyalanine molecules as a part of a tunnel junction realized with a scanning tunneling microscope. Depending on whether the molecules were chemisorbed directly on the magnetic Au/Co/Au substrate or at the STM Au-tip, the magnetizations of the Co layer had been oriented in the opposite direction in order to preserve the symmetry of the IV-curves. This is the first time that the CISS effect is demonstrated for a tunneling junction without a direct interface between the helical molecules and the magnetic substrate. Our results can be explained by a spin-polarized or spin-selective interface effect, induced and defined by the helicity and electric dipole orientation of the molecule at the interface. In this sense, the helical molecule does not act as a simple spin-filter or spin-polarizer and the CISS effect is not limited to spinterfaces.

Nonreciprocity, unidirectionality, and channeling are essential concepts for potential magnonic applications. Nonreciprocity and unidirectionality ensure the efficient propagation of spin waves along predetermined paths with preferential directions, disrupting the symmetry of counterpropagating waves. Channeling fosters the development of intricate spin-wave networks, enabling more sophisticated functionalities. Integrating these concepts into practical applications will shape the future of spin-wave-based information processing devices. This article theoretically studies the dynamics of spin waves in a ferromagnetic strip coupled to a heavy-metal strip, where the nonreciprocity, unidirectionality, and channeling effects are analyzed. Both backward volume
(BV) and Damon–Eshbach (DE) configurations are considered, where the lateral dimensions of the heavy-metal and ferromagnetic strips can differ. Calculations show notable nonreciprocal channeling of spin waves in both DE and BV modes. In the BV configuration, the dispersion is reciprocal with nontrivial localization of lateral confined modes. It is shown that the waves can be channeled into the zones in contact with the HM, where the Dzyaloshinskii–Moriya interaction is active. In the DE configuration, the waves exhibit nonreciprocal spin-wave dispersion, allowing unidirectional and channeled spin-wave propagation. The main results are compared to micromagnetic simulations, where an excellent agreement between both methods is obtained. These findings are relevant for envisioning advanced magnonic devices, enabling precise control over spin-wave propagation for innovative, low-power, high-speed information processing.

This paper delves into the connection between flat and curvilinear magnetization dynamics. For this, we numerically study the evolution of the magnon spectrum of rectangular waveguides upon rolling its cross section up to a full tube. Magnon spectra are calculated over a wide range of magnetization states using a finite-element dynamic-matrix method, which allows us to trace the evolution of the magnon frequencies and several critical magnetic fields with increasing curvature. By analyzing the parity of the higher-order magnon modes, we find a curvature-induced mode heterosymmetry that originates from a chiral contribution to the exchange interaction and is related to the Berry phase of magnons in closed loops. Importantly, this curvature-induced parity loss has profound consequences for the linear coupling between different propagating magnons, allowing for hybridization between initially orthogonal modes. In this context, we demonstrate the integral role of edge modes in forming the magnon spectrum in full tubes. Our findings provide theoretical insights into curvilinear magnetization dynamics and are relevant for interpreting and designing experiments in the field.

Magnetism of oxide antiferromagnets (AFMs) has been studied in single crystals and extended thin films. The properties of AFM nanostructures still remain underexplored. Here, we report on the fabrication and magnetic imaging of granular 100-nm-thick magnetoelectric \ch{Cr2O3} films patterned in circular bits with diameters ranging from 500 down to 100\,nm. With the change of the lateral size, the domain structure evolves from a multidomain state for larger bits to a single domain state for the smallest bits. Based on spin-lattice simulations, we show that the physics of the domain pattern formation in granular AFM bits is primarily determined by the energy dissipation upon cooling, which results in motion and expelling of AFM domain walls of the bit. Our results provide a way towards the fabrication of single domain AFM-bit-patterned memory devices and the exploration of the interplay between AFM nanostructures and their geometric shape.

In this Letter, we demonstrate terahertz (THz) magnetic field detection in fused silica with sensitivity that can be easily controlled by sample tilting (for both amplitude and polarization). The proposed technique remains in the linear regime at magnetic fields exceeding 0.3 T (0.9 MV/cm of equivalent electric field) and allows the use of low-cost amorphous materials. Furthermore, the demonstrated effects should be present in a wide variety of materials used as substrates in different THz-pump laser–probe experiments and need to be considered in order to disentangle different contributions to the measured signals.

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.

Partial disordering of Fe60Al40 thin films was achieved during neon ion irradiation through nanosphere shadow masks or by adjusting the ion energy for near-surface penetration only. Both approaches lead to adjacent chemically disordered and ordered areas. The magnetic behaviour of the films reveals a low-magnetization and high-coercive chemically ordered phase (non-irradiated ferromagnetic area, NIFM), as well as a high-magnetization and low-coercive chemically disordered phase (irradiated ferromagnetic area, IMF). It was shown that the modulated films of coexisting magnetic phases do not lead to an exchange coupling in most cases. Evidence for exchange-spring behaviour, however, was found. Moreover, both magneto-structural phases show at low temperatures spin-glass like properties.

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.

The magnetic damping of Ni80Fe20 (Permalloy, Py) thin films is studied via frequency-dependent ferromagnetic resonance (FMR) experiments. The thickness of the Py films is kept constant and they are all protected from oxidation by an identical 5-nm-thick Al cap layer. To separate the Py film from the oxidic sapphire substrates a systematic variation of the thickness of an additional Al spacer layer was carried out. Py sandwiched in Al exhibits a low, purely Gilbert-like magnetic damping when the Al spacer layer thickness is kept below 3 nm. Above this thickness the magnetic damping is strongly increased due to a pronounced two-magnon contribution. A detailed investigation of the temperature dependence as well as full angular dependence of the FMR allows for correlating the magnetic properties of the Py with the microscopic structural properties of the films as studied by x-ray reflectivity and transmission electron microscopy. It turns out that the detrimental twomagnon processes are activated by an island-like growth of the Al spacer layers, which leads to rough, wavy interfaces with characteristic length scales of the order of ten nanometers. Nevertheless, for Al spacer layers with a thickness below 3 nm a low, purely Gilbert-like magnetic damping can be observed.

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.

Martensitic transformations enable various emerging applications like the shape memory effect and elastocaloric applications in NiTi. Increasing the speed of this transformation can shorten the response time for actuation and increase the power density of caloric cooling systems. Up to now, research on the speed and possible time limits of the martensitic transformation in NiTi has been limited to milli- and microsecond experiments. The dynamics of the transformation for shorter time scales are therefore unknown. Here, we report the fastest transformations in NiTi so far by heating an epitaxial NiTi film with a ns laser pulse and tracking the martensitic transition with in-situ synchrotron X-ray diffraction. We find that the martensite to austenite transition upon heating can proceed within the 7 ns pulse duration of the laser, but it requires substantial overheating as the rate of the transformation increases with the driving energy. The austenite to martensite transition is slower because cooling proceeds by conductive heat transfer, but with appropriate undercooling, the complete transformation from martensite to austenite and back only takes 200 ns. We compare our results to previous experiments on the Heusler alloy Ni-Mn-Ga and (K,Na)NbO3 and find very similar trends, which reveal that fast martensitic transformations in general follow a universal scaling law.

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.

Thin films of short-range ordered Fe60V40 alloy undergo a transition to the bcc structure, caused by the penetration of energetic ions. The magnetic behavior correlates to the lattice order, where the short-range ordered alloy is paramagnetic, and the bcc structure ferromagnetic. We show that during the reordering there occurs a process of point defect agglomeration along with the formation of vacancy clusters. The reordering occurs at different rates, thus the distribution of bond-distances in the final bcc structure correlates with the initial monovacancy concentration and the initial degree of short-range ordering. The degree of ordering after Ne+ ion irradiation depends on the elements involved in the bonds, i.e., Fe-Fe bonds show higher ordering as compared to V-V bonds. The fraction of the crystalline, ferromagnetic bcc phase, however, increases on expense of the short-range ordered paramagnetic phase with increasing ion fluence applied.

Time-resolved x-ray microscopy is used in a low-alpha synchrotron operation mode to image spin dynamics at an unprecedented combination of temporal and spatial resolution. Thereby, nanoscale spin waves with wavelengths down to 70 nm and frequencies up to 30 GHz are directly observed in ferromagnetic thin film microelements with spin vortex ground states. In an antiparallel ferromagnetic bilayer system, we detect the propagation
of both optic and acoustic modes; the latter exhibiting even a strong non-reciprocity. In single layer systems, quasi-uniform spin waves are observed together with modes of higher order (up to the 4th order), bearing precessional nodes over the thickness of the film. Furthermore, the effects from magnetic material properties, film thickness and magnetic fields on the spin-wave spectrum are experimentally determined. Our experimental results were found to be consistent with those from numerically solving an analytic micromagnetic theory even on these so-far unexplored time- and length scales.

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.

We demonstrate excitation of the gyrotropic mode in a magnetostrictive vortex by time-varying strain. The vortex dynamics is driven by a time-varying voltage applied to the piezoelectric substrate and detected electrically by spin rectification at subthreshold values of rf current. When the frequency of the time-varying strain matches the gyrotropic frequency at given in-plane magnetic field, the strain-induced in-plane magnetic anisotropy leads to a resonant excitation of the gyration dynamics in a magnetic vortex. We show that nonlinear gyrotropic dynamics can be excited already for moderate amplitudes of the time-varying strain.

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.

The efficient excitation of spin waves is a key challenge in the realization of magnonic devices. We demonstrate the current-driven generation of spin waves in antiferromagnetically coupled magnetic vortices. We employ time-resolved scanning transmission X-ray microscopy (TR-STXM) to directly image the emission of spin waves upon the application of an alternating current flowing directly through the magnetic stack. Micromagnetic simulations allow us to identify the origin of the excitation to be the current-driven Oersted field, which in the present system proves to be orders of magnitude more efficient than the commonly used excitation via stripline antennas. Our numerical studies also reveal that the spin-transfer torque can lead to the emission of spin waves as well, yet only at much higher current amplitudes. By using magnetostrictive materials, we futhermore demonstrate that the direction of the magnon propagation can be steered by increasing the excitation amplitude, which modifies the underlying magnetization profile through an additional anisotropy in the magnetic layers. The demonstrated methods allow for the efficient and tunable excitation of spin waves, marking a significant advance in the generation and control of spin waves in magnonic devices.

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.

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.