arXiv subjects

Lukas Körber

Publications and source records attributed to Lukas Körber.

At least 19 recordsLinked to original sources

arXiv2026

Effective Theory of Ultrafast Skyrmion Nucleation

Laser-induced ultrafast skyrmion nucleation has been experimentally demonstrated in several materials. So far, atomistic models have been used to corroborate experimental results. However, such simulations do not provide a simple intuitive understanding of the underlying physics. Here, we propose a coarse-grained effective theory where skyrmions can be nucleated or annihilated by thermal activation over energy barriers. Evaluating these two processes during a heat pulse shows good agreement with atomistic spin dynamics simulations and experiments while drastically reducing computational complexity. Furthermore, the effective theory provides a direct guide for experimentally optimizing the number of nucleated skyrmions. Interestingly, the model also predicts a novel pathway for ultrafast annihilation of skyrmions. Our results pave the way for a deeper understanding of ultrafast nanomagnetism and the role of non-equilibrium physics.

cond-mat.mes-hall↗

arXiv2026

Altermagnetic Magnons in Dipolar Nanomagnet Arrays

Altermagnetism is conventionally understood in a spin-conserving framework, where symmetry-enforced momentum-dependent spin splitting emerges in collinear magnets with vanishing net magnetization. Here, we show that its defining signatures persist in nanomagnet arrays coupled exclusively by dipolar interactions, despite the intrinsic breaking of spin conservation. Using a macrospin theory of dipolar-coupled ferromagnetic nanoislands, corroborated by micromagnetic simulations, we demonstrate that arrays engineered with altermagnetic symmetries exhibit spin-split magnon bands whose eigenstates can partially remain strongly spin polarized. The resulting spin expectation value displays the characteristic $d$-wave pattern throughout the Brillouin zone, establishing a mesoscopic realization of altermagnetic magnons beyond the conventional spin-conserving paradigm. As a consequence, spin-wave propagation becomes strongly direction dependent, providing a highly tunable platform for anisotropic magnon transport and synthetic altermagnetic functionality.

cond-mat.mes-hall↗

arXiv2026

Double-pulse control of all optical magnetization reversal in Tb/Co multilayers

Recent experiments have shown that femtosecond laser pulse with a Gaussian intensity profile can induce magnetization reversal in Tb/Co multilayers with a ring-shaped switching pattern within the laser-irradiated area. Here, we investigate the ultrafast magnetization dynamics leading to such a ring-shaped switching by using double-pulse laser excitation. The laser pulses cause heat-induced quenching and subsequent recovery of the magnetic anisotropy in the multilayers and drive the precessional magnetization switching in the magnetic multilayers. By adjusting the delay between the two pump pulses, we demonstrate that the recovery process can be manipulated and show, experimentally and numerically, that this allows control over the final magnetic domain pattern.

cond-mat.mtrl-sci↗

arXiv2025

A fluctuation-free pathway for a topological magnetic phase transition

Topological magnetic textures are particle-like spin configurations stabilized by competing interactions. Their formation is commonly attributed to fluctuation-driven, first-order nucleation processes requiring activation over a topological energy barrier. Here, we demonstrate an alternative barrier- and fluctuation-free pathway for nucleating topological magnetic textures, triggered in our experiments by an excitation-induced spin reorientation transition. By combining x-ray imaging, scattering and micromagnetic simulations, we show that the system follows a deterministic cascade of symmetry-breaking phase transitions after excitation. First, the system undergoes a second-order phase transition from a homogeneous state to weak stripe domains, then a first-order transition to topologically trivial bubbles, and finally a topological switching event into skyrmionic textures. Through simulations, we generalize our findings and demonstrate that this pathway is active in a vast range of low-anisotropy materials. This previously unrecognized, spontaneous transition pathway suggests strategies for rapid, low-energy generation of topological spin textures and points to a general role of intrinsic modulational instabilities in phase transitions beyond magnetism.

cond-mat.mtrl-sci↗

arXiv2025

Spin-correlation dynamics: A semiclassical framework for nonlinear quantum magnetism

Classical nonlinear theories are highly successful in describing far-from-equilibrium dynamics of magnets, encompassing phenomena such as parametric resonance, ultrafast switching, and even chaos. However, at ultrashort length and time scales, where quantum correlations become significant, these models inevitably break down. While numerous methods exist to simulate quantum many-body spin systems, they are often limited to near-equilibrium conditions, capture only short-time dynamics, or obscure the intuitive connection between nonlinear behavior and its geometric origin in the su(2) spin algebra. To advance nonlinear magnetism into the quantum regime, we develop a theory in which semiclassical spin correlations, rather than individual spins, serve as the fundamental dynamical variables. Defined on the bonds of a bipartite lattice, these correlations are inherently nonlocal, with dynamics following through a semiclassical mapping that preserves the original spin algebra. The resulting semiclassical theory captures nonlinear dynamics that are entirely nonclassical and naturally accommodates phenomenological damping at the level of correlations, which is typically challenging to include in quantum methods. As an application, we focus on Heisenberg antiferromagnets, which feature significant quantum effects. We predict nonlinear scaling of the mean frequency of quantum oscillations in the Néel state with the spin quantum number S. These have no classical analog and exhibit features reminiscent of nonlinear parametric resonance, fully confirmed by exact diagonalization. The predicted dynamical features are embedded in the geometric structure of the semiclassical phase space of spin correlations, making their physical origin much more transparent than in full quantum methods. With this, semiclassical spin-correlation dynamics provide a foundation for exploring nonlinear quantum magnetism.

cond-mat.mes-hall↗

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

Toroidal Moments in Confined Nanomagnets and their Impact on Magnonics

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

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↗

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

Curvature-induced parity loss and hybridization of magnons: Exploring the connection of flat and tubular magnetic shells

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 hetero-symmetry 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 new theoretical insights into curvilinear magnetization dynamics and are relevant for interpreting and designing experiments in the field.

cond-mat.mes-hall↗

arXiv2023

Nontrivial Aharonov-Bohm effect and alternating dispersion of magnons in cone-state ferromagnetic rings

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.

cond-mat.mes-hall↗

arXiv2023

Modification of three-magnon splitting in a flexed magnetic vortex

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 $f_\mathrm{RF}$, 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 $f_\mathrm{RF}/2\pm Δf$ 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 ($\simeq$ 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.

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

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

Finite-element dynamic-matrix approach for propagating spin waves: Extension to mono- and multilayers of arbitrary spacing and thickness

In our recent work [AIP Adv. 11, 095006], we presented an efficient numerical method to compute dispersions and spatial mode profiles of spin waves propagating in waveguides with translationally invariant equilibrium magnetization. Using 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 practical 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 only 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 interaction into our FEM approach. With the calculation of dynamic dipolar fields being the main focus of this paper, 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, such as non-magnetic spacers in multilayers. Therefore, the computational effort becomes completely independent on the spacer thicknesses. Furthermore, it keeps the resulting discretized 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 TetraX), we present results for various systems and compare them with theoretical predictions as well as with established finite-difference numerical methods. We believe this method offers an efficient and versatile tool to calculate spin-wave dispersions in layered magnetic systems.

cond-mat.mes-hall↗

arXiv2022

Curvilinear spin-wave dynamics beyond the thin-shell approximation: Magnetic nanotubes as a case study

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.

cond-mat.mes-hall↗

arXiv2022

Spin-wave frequency combs

We experimentally demonstrate the generation of spin-wave frequency combs based on the nonlinear interaction of propagating spin waves in a microstructured waveguide. By means of time and space-resolved Brillouin light scattering spectroscopy, we show that the simultaneous excitation of spin waves with different frequencies leads to a cascade of four-magnon scattering events which ultimately results in well-defined frequency combs. Their spectral weight can be tuned by the choice of amplitude and frequency of the input signals. Furthermore, we introduce a model for stimulated four-magnon scattering which describes the formation of spin-wave frequency combs in the frequency and time domain.

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