Juergen Fassbender

Despite extensive studies of epitaxial Fe(110) films on GaAs(110), the magnetic properties of films thicker than 20 nm remain largely unexplored, even though this thickness range exhibits rich and unconventional magnetic behavior. In this work, we investigate epitaxial, Ag-capped bcc Fe(110) films with thicknesses ranging from 5 to 100 nm grown by molecular beam epitaxy. Their magnetic and structural properties are characterized using ferromagnetic resonance, vibrating sample magnetometry, x-ray diffraction, and microfocused Brillouin light scattering. Our measurements reveal the coexistence of cubic and uniaxial magnetic anisotropies, that in combination generate an easy axis aligned along the in-plane 〈 001 〉 directions, which persists throughout the film volume even for thicknesses as large as 100 nm, having a magnitude comparable to that of the cubic anisotropy. To quantify the uniaxial volume anisotropy in thicker films, we fit the experimental data using a layer-resolved magnetic model that allows for a depth-dependent variation of magnetic properties across the film thickness. The precession-amplitude profiles of the perpendicular standing spin-wave modes across the film thickness lead to a depth-selective sensitivity of different modes, enabling the sensing of the local anisotropy landscape, from which a depth profile of anisotropy can be retrieved. X-ray diffraction measurements demonstrate that anisotropic strain—characterized by lateral anisotropic expansion accompanied by vertical compression—is the microscopic origin of the observed in-plane uniaxial anisotropy, persisting even in films with thicknesses up to 100 nm.

Ultrafast magnetization switching is essential for next-generation memory and logic devices operating at terahertz (THz) frequencies. Presently, efficient picosecond spin-orbit torque (SOT) switching relies on heavy metals (HMs), which are costly and environmentally burdensome. Self-generated SOT in nanostructures comprising a single conducting ferromagnetic layer offers a promising, cost-effective alternative, but ultrafast SOT has not been demonstrated in such systems and the underlying torque mechanisms remain unclear. Here we demonstrate SOT-driven picosecond magnetization dynamics in oxide-capped Ni ₈₁ Fe ₁₉ with efficiencies comparable to benchmark Ni ₈₁ Fe ₁₉ /HM heterostructures. By probing spin-charge interconversion on the picosecond timescale, we show that SOT is active only at off-stoichiometric oxide interfaces and negligible at stoichiometric ones, identifying interfacial atomic-scale oxidation of Ni ₈₁ Fe ₁₉ as the key ingredient for SOT efficiency. Moreover, the magnitude and sign of the torques are tunable via the oxidation level and oxide type. These results establish atomic-layer oxidation engineering as a viable, heavy-metal-free route to ultrafast SOT for THz spintronic technologies.

Layers of body‐centered cubic ( bcc ) Fe ₆₀ V ₄₀ are generated in short‐range ordered (SRO) Fe ₆₀ V ₄₀ precursors through atomic displacements caused by the irradiation of light noble gas ions. The structural change leads to the onset of ferromagnetism confined to the bcc layers. Here, the variation of ferromagnetic resonance response as a function of Ne ⁺ ‐ions of energies varying from 5 to 30 keV, while keeping the fluence fixed at 7 × 10 ¹⁵ ions/cm ² is investigated. The irradiation was performed on SRO Fe ₆₀ V ₄₀ films grown at 300 K as well as at 573 K. Shifts of the resonance line position as a function of ion energy are observed. As the ion energy is increased, the atomic displacements are distributed deeper within the film, thereby increasing the thickness of the induced bcc layers. The results can be understood in terms of a saturation magnetization ( M _s ) that is fixed for a given growth temperature, and an increasing effective magnetic thickness ( t _eff ) with increasing ion‐energy. Despite the varying t _eff , the Landé g ‐factor and Gilbert damping tend to remain relatively stable, respectively at 2.099 ± 0.006 and 0.003 ± 0.001, thus providing a material system well‐suited for microwave applications, that can be reliably modified using ion‐irradiation.

Forecasting complex, chaotic signals is a central challenge across science and technology, with implications ranging from secure communications to climate modeling. Here we demonstrate that magnons, the collective spin excitations in magnetically ordered materials, can serve as an efficient physical reservoir for predicting such dynamics. Using a magnetic vortex-state microdisk as a magnon-scattering reservoir, we show that intrinsic nonlinear interactions transform a simple microwave input into a high-dimensional spectral output suitable for time-series predictions. Trained on the Mackey-Glass benchmark, which generates a cyclic yet aperiodic time series, the system achieves accurate and reliable predictions that rival other state-of-the-art physical reservoirs. We further identify key design principles: spectral resolution governs the tradeoff between dimensionality and accuracy, while combining multiple device geometries systematically improves performance. These results establish magnonics as a promising platform for unconventional computing, offering a path toward scalable and CMOS-compatible hardware for real-time prediction tasks.

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

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

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

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

Physical reservoir computing has emerged as a powerful framework for exploiting the inherent nonlinear dynamics of physical systems to perform computational tasks. Recently, we presented the magnon-scattering reservoir, whose internal nodes are given by the fundamental wavelike excitations of ferromagnets called magnons. These excitations can be geometrically quantized and, in response to an external stimulus, show transient nonlinear scattering dynamics that can be harnessed to perform memory and nonlinear transformation tasks. Here, we test a magnon-scattering reservoir in a single magnetic disk in the vortex state against two key performance indicators for physical reservoir computing: the short-term memory and parity-check tasks. Using time-resolved Brillouin light scattering microscopy, we measure the evolution of the reservoir’s spectral response to an input sequence consisting of random binary inputs encoded in microwave pulses with two distinct frequencies. Two different output spaces of the reservoir are defined: one based on the time-averaged frequency spectra and another based on temporal multiplexing. Our results demonstrate that the memory and nonlinear transformation capability do not depend on the chosen readout scheme, provided that the dimension of the output space is large enough to capture all nonlinear features provided by the magnon-magnon interactions. This further shows that it is solely the nonlinear magnons in the physical system—not the readout—that determine the reservoir’s capacity. Published by the American Physical Society 2025

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 paper, 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 . 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.