arXiv subjects

Jürgen Fassbender

Publications and source records attributed to Jürgen Fassbender.

At least 19 recordsLinked to original sources

arXiv2026

Electrical magnon spectroscopy with foundry-fabricated nanoscale magnetic tunnel junctions

Magnonics uses spin waves and their quanta, magnons, to carry and process information and offers new opportunities for computing and signal processing. Its integration with conventional electronics, however, requires scalable electrical interfaces for magnon readout. Here, we demonstrate electrical magnon spectroscopy using a 70-nm magnetic tunnel junction fabricated on a commercial 300-mm semiconductor production line. Dynamic dipolar fields generated by magnons in a magnetic vortex drive the free layer of the tunnel junction and are transduced into electrical signals through tunnelling magnetoresistance. By exploiting three-magnon splitting, we spectrally separate the magnon response from direct microwave excitation and resolve nonlinear magnon modes with linewidths down to 188 kHz at 3.59 GHz. The electrical measurements reveal spectral structure that remains unresolved in parallel Brillouin light scattering measurements. Our results establish foundry-fabricated magnetic tunnel junctions as nanoscale electrical interfaces for magnonic systems and provide a route towards their integration with semiconductor electronics.

cond-mat.mes-hall↗

arXiv2026

Spatially resolved quantum magnetometry and stray-field reconstruction of permalloy microdisks using boron-vacancy centers in hexagonal boron nitride

Transferable hexagonal boron nitride (hBN) hosting negatively charged boron-vacancy (VB$^{-}$) spin defects offers a versatile platform for integrated quantum magnetometry, yet quantitative imaging of magnetic microstructures remains challenging. Here, we integrate a transferred hBN flake with a 4 $μ$m-diameter permalloy (Py = Ni${81}$Fe${19}$) microdisk and perform spatially resolved optically detected magnetic resonance measurements at room temperature. An applied in-plane magnetic field distorts the vortex-state magnetization, generating edge-localized magnetic surface charges and pronounced stray-field signatures at opposite disk edges. By referencing each pixel to its local zero-field splitting and correcting for a residual out-of-plane bias field, we quantitatively reconstruct the out-of-plane stray-field distribution, revealing peak fields of approximately 11.2 mT. An edge-charge model reproduces the spatial distribution and amplitude of the reconstructed field, linking the ODMR response to the field-driven evolution of the vortex state. These results establish transferred hBN VB$^{-}$ sensors for quantitative magnetometry of magnetic microstructures.

cond-mat.mes-hall↗

arXiv2026

Local multiferroic ordering at room temperature in collinear magnetoelectric antiferromagnets induced by flexo-Zeeman coupling

Spin-driven multiferroicity attracts significant interest due to its tunability and inherently strong magnetoelectric coupling. While this mechanism induces sizeable electric polarization, it typically occurs at low temperatures and in complex materials. In the simple oxide, the magnetoelectric antiferromagnet Cr$_2$O$_3$, we experimentally demonstrate the existence of specific domain walls that act as room-temperature multiferroic regions. This behavior stems from an anisotropic crystal-symmetry-dependent mechanism of exchange origin, which is applicable to a broad class of bipartite antiferromagnets. The key signature is the magnetization occurring at antiferromagnetic textures, driven by the flexo-Zeeman interaction. These findings establish a foundation for exploring high-temperature spin-driven multiferroicity for magnetoelectric spin-orbit memory and logic applications.

cond-mat.mtrl-sci↗

arXiv2025

Coherent control of Floquet-engineered magnon frequency combs

Frequency combs represent a hallmark of coherence emerging from nonlinear dynamics, where periodic driving organizes energy into a precisely spaced spectral structure. Extending this concept to collective excitations in solids such as magnons, the quanta of spin waves in magnetically ordered materials, offers a powerful route to control energy flow, coherence, and information processing in condensed matter systems. Here, we demonstrate deterministic control of Floquet-engineered magnon frequency combs in magnetic vortices using nanosecond voltage pulses. By tuning the pulse duration and timing, we control the nonlinear energy transfer between magnons and the vortex core, enabling the Floquet-engineered initiation or suppression of magnon frequency combs far below their spontaneous instability threshold. This pulse-programmable interaction allows the vortex to sustain magnon-driven auto-oscillation with high phase stability, or to revert to its static ground state on demand. Our results establish vortex-based magnetic systems as a robust solid-state platform for Floquet engineering, bridging nonlinear spin dynamics with frequency conversion and coherent spin-based quantum devices.

cond-mat.mes-hall↗

arXiv2025

Predicting the future with magnons

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 microdisk in the vortex state as a magnon-scattering reservoir, we show that intrinsic nonlinear interactions transform a simple microwave input into a high-dimensional spectral output suitable for reservoir computing, in particular, for time series predictions. Trained on the Mackey-Glass benchmark, which generates a cyclic yet aperiodic time series widely used to test machine-learning models, the system achieves accurate and reliable predictions that rival state-of-the-art physical reservoirs. We further identify key design principles: spectral resolution governs the trade-off 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.

cond-mat.mes-hall↗

arXiv2025

Benchmarking a magnon-scattering reservoir with modal and temporal multiplexing

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

cond-mat.mes-hall↗

arXiv2025

Control of magnon frequency combs in magnetic rings

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

cond-mat.mes-hall↗

arXiv2024

Self-induced Floquet magnons in magnetic vortices

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

cond-mat.mes-hall↗

arXiv2024

Ultrafast unidirectional spin Hall magnetoresistance driven by terahertz light field

The ultrafast control of magnetisation states in magnetically ordered systems is a key technological challenge for developing memory devices operable at picosecond timescales or terahertz (THz) frequencies. Despite significant efforts in ultrafast magnetic switching, convenient ultrafast readout of magnetic states remains under investigation. Currently, many experiments exploit magneto-optical effects for detecting magnetisation states, necessitating laser sources and optical components. However, energy-efficient and cost-effective electrical detection is preferred for practical applications. Unidirectional spin-Hall magnetoresistance (USMR) was proposed as a simple two-terminal geometry for the electrical detection of the magnetisation state in magnetic heterostructures. Here, we demonstrate that USMR is active at THz frequencies for picosecond time readouts, and can be initiated with light fields. We detect ultrafast USMR in various types of ferromagnet/heavy metal thin film heterostructures through THz second harmonic generation. Our studies, combined with temperature-dependent measurements of USMR, reveal a significant contribution from electron-magnon spin-flip scattering. This suggests possibilities for all-electrical detection of THz magnon modes.

cond-mat.mes-hall↗

arXiv2024

Strain-induced frequency pulling in CoFeB/Cu/Py double-vortex oscillators

We demonstrate piezostrain-induced frequency pulling in stacked double-vortex structures, magnetostatically coupled through the nonmagnetic spacer. We study the effect of the Cu spacer thickness on the strain-induced gyrotropic frequency shift in double-vortex structures comprising of magnetostrictive CoFeB and nonmagnetostricitve Py layers. For the two stacked vortices with different eigen-frequencies, the strain-induced magnetoelastic anisotropy leads to the downshift of the gyration frequency of the magnetostricitve vortex. We show that for increased dipolar coupling between the layers (i.e. decreased spacer thickness), a strain-induced frequency pulling regime is obtained, where the resonance frequency of the nonmagnetostrictive Py vortex is upshifted towards the gyration resonance of the magnetostrictive CoFeB vortex. This result offers an additional degree of freedom for the manipulation of the dynamical regimes and synchronization conditions in spintronic oscillators, controlled by voltage and tunable by strain

cond-mat.mes-hall↗

arXiv2023

Excitation of the Gyrotropic Mode in a Magnetic Vortex by Time-Varying Strain

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.

physics.app-ph↗

arXiv2023

Spin-orbit interaction driven terahertz nonlinear dynamics in transition metals

The interplay of electric charge, spin, and orbital polarizations, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz spintronics and orbitronics. The essential rules for how terahertz light interacts 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 non-equilibrium spin and orbital polarization dynamics at terahertz frequencies, which is relevant for potential applications of terahertz spin- and orbital-based devices.

physics.app-ph↗

arXiv2023

Piezostrain -- a local handle to control gyrotropic dynamics in magnetic vortices

We present a study of the piezostrain-tunable gyrotropic dynamics in Co$_{40}$Fe$_{40}$B$_{20}$ vortex microstructures fabricated on a 0.7PMN-0.3PT single crystalline 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 PMN-PT, we observe a gradual decrease of the vortex core gyrotropic frequency associated with the strain-induced magnetoelastic energy contribution. The frequency tunability strongly depends on the disk size, with increased frequency downshift for the 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.

cond-mat.mes-hall↗

arXiv2022

Pattern recognition with a magnon-scattering reservoir

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.

cond-mat.mes-hall↗

arXiv2022

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

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-$μ$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 $\sim$50 and $\sim$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.

cond-mat.mes-hall↗

arXiv2022

Electrokinetic Janus micromotors moving on topographically flat chemical patterns

Ionic and molecular selectivity is considered unique for the nanoscale and not realizable in microfluidics. This is due to the scale-matching problem -- a difficulty to match the dimensions of ions and electrostatic potential screening lengths with the micron-sized confinements. Here, we demonstrate a microscale realization of the ionic transport processes closely resembling those specific to ionic channels or in nanofluidic junctions, including selectivity, guidance through complex geometries and flow focusing. As a model system, we explore electrokinetic spherical Janus micromotors moving over charged surfaces with a complex spatial charge distribution and without any topographical wall. We discuss peculiarities of the long-range electrostatic interaction on the behavior of the system including interface crossing and reflection of positively charged particles from negatively charged interfaces. These results are crucial for understanding the electrokinetic transport of biochemical species under confinement, have the potential to increase the precision of lab-on-chip-based assays, as well as broadening use cases and control strategies of nano-/micromachinery.

cond-mat.soft↗

arXiv2022

Nonlinear magnon control of atomic spin defects in scalable quantum devices

Ongoing efforts in quantum engineering have recently focused on integrating magnonics into hybrid quantum architectures for novel functionalities. While hybrid magnon-quantum spin systems have been demonstrated with nitrogen-vacancy (NV) centers in diamond, they have remained elusive on the technologically promising silicon carbide (SiC) platform mainly due to difficulties in finding a resonance overlap between the magnonic system and the spin centers. Here we circumvent this challenge by harnessing nonlinear magnon scattering processes in a magnetic vortex to access magnon modes that overlap in frequency with silicon-vacancy ($\textrm{V}_{\mathrm{Si}}$) spin transitions in SiC. Our results offer a route to develop hybrid systems that benefit from marrying the rich nonlinear dynamics of magnons with the advantageous properties of SiC for scalable quantum technologies.

cond-mat.mes-hall↗

arXiv2022

Field-induced spin reorientation transitions in antiferromagnetic ring-shaped spin chains

Easy axis antiferromagnets are robust against external magnetic fields of moderate strength. Spin reorientations in strong fields can provide an insight into more subtle properties of antiferromagnetic materials, which are often hidden by their high ground state symmetry. Here, we investigate theoretically effects of curvature in ring-shaped antiferromagnetic achiral anisotropic spin chains in strong magnetic fields. We identify the geometry-governed helimagnetic phase transition above the spin-flop field between vortex and onion states. The curvature-induced Dzyaloshinskii-Moriya interaction results in the spin-flop transition being of first- or second-order depending on the ring curvature. Spatial inhomogeneity of the Néel vector in the spin-flop phase generates weak ferromagnetic response in the plane perpendicular to the applied magnetic field. Our work contributes to the understanding of the physics of curvilinear antiferromagnets in magnetic fields and guides prospective experimental studies of geometrical effects relying on spin-chain-based nanomagnets.

cond-mat.mes-hall↗