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.

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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