arXiv subjects

Attila Kákay

Publications and source records attributed to Attila Kákay.

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

TetMaG-Guided Design and Operando Electron Holography Validation of Current-Induced Domain-Wall Motion in 3D Curved and Cornered Fe Nanobridges

Three-dimensional (3D) magnetic nanostructures offer new opportunities for controlling domain-wall (DW) configurations beyond the limitations of planar systems, providing promising architectures. However, the realization of reliable 3D magnetic devices requires precise control of geometry-dependent DW behaviour and quantitative experimental validation of the resulting magnetic states. Here, we combine TetMaG micromagnetic simulations, focused electron beam induced deposition (FEBID), and off-axis electron holography to investigate the influence of curvature and corner geometries on DW behaviour in 3D magnetic nanobridges. TetMaG simulations predict fundamentally different magnetic properties for curved and cornered geometries. Cornered nanobridges act as preferential DW pinning sites, stabilizing localized magnetic configurations and enabling controlled switching between neighbouring pinning positions. While curved nanobridges promote gradual magnetization rotation, reduced pinning, and smoother DW motion. These optimized geometries were fabricated with high structural fidelity using FEBID and subsequently characterized by quantitative electron holography. Electron holography measurements revealed magnetic induction maps that matched the simulated magnetization configurations, providing direct experimental validation of the TetMaG predictions. Curved and cornered geometries exhibited distinct DW characteristics governed by their local structural features, demonstrating the critical role of geometry in tailoring magnetic behaviour in 3D systems. Operando current-biasing experiments further revealed current-induced DW motion, including the displacement of a tail-to-tail DW into a head-to-tail configuration within corner structures.

cond-mat.mes-hall↗

arXiv2026

Nonreciprocal spin waves of helical magnetization states in CoFeB/NiFe bilayers

We investigated the nonreciprocal spin-wave properties, including the frequency shift, of a helical equilibrium state in a versatile CoFeB/NiFe bilayer. Through an extension of the dynamic matrix formalism (developed in this work) to an arbitrary non-collinear configuration along a heterostructured multilayered system thickness, we explained the frequency shift via differences in the dynamic dipolar and interlayer exchange interactions arising from the distinct spin-wave mode profiles across the bilayer thickness for counterpropagating modes at the same wave vector. In contrast to recent literature wherein the frequency shift is attributed solely to the dipolar interaction, our results and explanations hereby presented involve a starring role of the interlayer exchange interaction not accounted in current literature. Furthermore, we also found a combination of large frequency shift values and sub-100 nm spin wave wavelengths that can be tuned or even enhanced with the twisting degree of the helical magnetization state by the application of the external field, and with the thickness of the NiFe sublayer, which might be highly relevant for magnonic applications. We validated our model and the physical mechanism that explains the frequency shift using recent simulations and experimental results.

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arXiv2026

Equilibrium Magnetic Properties in Magnetic Nanoscrews

We investigate the equilibrium magnetization in ferromagnetic nanoscrews (NSw) using micromagnetic simulations. These systems consist of elongated three-dimensional magnetic membranes with helicoidal geometry, combining curvature, torsion ($\mathrm{w}$), and eccentricity ($ε$) along their length. We focus on the influence of these geometric parameters, together with membrane thickness and inner diameter, on remanent states and coercive fields. Our results, obtained over a broad range of eccentricities and torsions, reveal bistable magnetic behavior, with vortex-domain-wall propagation during magnetization reversal. We identify four degenerate configurations of a remarkably stable mixed remanent state. The coercive field is found to increase with eccentricity for structures with a major axis (larger inner diameter) approximately 30\% larger than the minor axis (smaller inner diameter), while remaining largely insensitive to variations in torsion. These findings are interpreted in terms of geometry-induced modifications of surface magnetostatic charges on the membrane mantle. Overall, our results demonstrate that nanoscrews exhibit robust bistability under systematic geometric deformation, together with enhanced coercivity, highlighting their potential for applications in three-dimensional nanomagnetism.

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arXiv2026

Nanoscale magnetometry of a synthetic three-dimensional spin texture

Multilayered synthetic antiferromagnets (SAFs) are artificial three-dimensional (3D) architectures engineered to create novel, complex, and stable spin textures. Non-invasive and quantitative nanoscale magnetic imaging of the two-dimensional stray field profile at the sample surface is essential for understanding the fundamental properties of the spin-structure and being able to tailor them to achieve new functionalities. However, the deterministic detection of spin textures and their quantitative characterization at the nanoscale remain challenging. Here, we use nitrogen-vacancy scanning probe microscopy (NV-SPM) under ambient conditions to perform the first quantitative vector-field magnetometry measurements in the multilayered SAF [(Co/Pt)$_5$/Co/Ru]$_3$/(Co/Pt)$_6$. We investigate nanoscale static and dynamic properties of antiferromagnetic domains with boundaries hosting ``one-dimensional'' ferromagnetic stripes with ~ 100 nm of width and periodic modulation of the magnetization. By employing NV-SPM measurements in different imaging modes and involving NV-probes with various crystallographic orientations, we demonstrated distinct fingerprints emerging from GHz-range spin noise and constant stray fields on the order of several mT. This provides quantitative insights into the structure of domains and domain walls, as well as, into magnetic noise associated with thermal spin-waves. Our work opens up new opportunities for quantitative vector-field magnetometry of modern magnetic materials with tailored 3D spin textures and stray field profiles, and potentially novel spin-wave dispersions--in a quantitative and non-invasive manner, with exceptional magnetic sensitivity and nanometer scale spatial resolution.

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arXiv2026

Exchange-dominated frequency shift of spin-wave nonreciprocal dispersion relation in planar magnetic multilayers

Spin-wave nonreciprocity, manifested as a frequency difference between counterpropagating modes, underpins many proposed magnonic devices. While this effect is commonly attributed to dipolar interactions or interfacial chirality, the microscopic origin of the frequency shift in nonreciprocal dispersion in magnetic multilayers remains under debate. Here, we analyze the frequency shift of nonreciprocal spin-wave dispersion in planar multilayer heterostructures without Dzyaloshinskii-Moriya interaction. Using a frequency-shift dynamic matrix formalism, we show that the frequency asymmetry cannot generally be ascribed to dipolar effects alone. Instead, once counterpropagating modes differ in their geometric structure along the thickness, interlayer exchange dominates the frequency shift. Applied to representative multilayer systems, we find that the interlayer exchange contribution exceeds dipolar and intralayer exchange effects by up to two to three orders of magnitude over a broad wave-vector range. Our results establish interlayer exchange as the primary mechanism contributing to the frequency shift of nonreciprocal dispersion in multilayer magnonic systems and provide a quantitative framework for engineering large frequency shifts in nonreciprocal magnonic devices.

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arXiv2026

Controlling the Spin-Wave Nonreciprocity of a Crescent-Shaped Nanowire via Curvature and Magnetic Field

Recent studies on spin-wave propagation in ferromagnetic waveguides has highlighted the role of nonreciprocity resulting from the chiral nature of dipolar interactions in curved elements. However, the impact of spin-wave mode type on nonreciprocity remains unexplored. Using micromagnetic simulations supported by analytical modeling, we systematically analyzed the propagation of edge, fundamental, and width-quantized spin-wave modes in a ferromagnetic nanowire with a crescent-shaped cross-section. Our results show that the strength and sign of nonreciprocity depend on the mode type, as well as on the curvature magnitude of the nanowire's top and bottom surfaces and the strength of the external magnetic field. Interestingly, changing the mode type, for instance induced by altering the curvature or magnetic field, result in a significant change in the dispersion relation asymmetry. This effect underscores the important role of spin-wave profiles in nonreciprocity, deepens our fundamental understanding of spin-wave dynamics in curved geometries, and paves the way for designing magnonic waveguides with tailored properties.

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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.

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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.

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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.

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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.

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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

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arXiv2024

Anatomy of localized edge modes in laterally coupled waveguides

We present a systematic micromagnetic study of standing spin-wave modes in infinitely long Permalloy strips with rectangular cross-section. Using a finite-element dynamic-matrix method, we first calculate the eigenfrequencies and the corresponding eigenvectors (mode profiles), as a function of the in-plane magnetic field applied across the strip. The ferromagnetic resonance spectra is computed from the mode profiles, assuming a homogeneous radio-frequency excitation, equivalently to an experimental ferromagnetic resonance measurement. The investigation of the field-dependent mode profiles enables for the classification of the observed resonances, here focusing mostly on the true edge mode localized at the vicinity of strip edges. Furthermore, we study the mode localization in pairs of 50-nm-thick Permalloy strips as a function of the strip width and their lateral separation. For closely spaced strips, the spatial profile of the quasi-uniform mode is substantially modified due to a significant hybridization with the edge-localized standing spin-wave modes of the neighbouring strip. We show that a wide-range-tunability of the localized edge-mode resonances can be achieved with a precise control of the magnetostatic coupling between the strips. Extreme sensitivity of the edge mode frequency on the bias field demonstrates a potential of the edge resonances for field sensing. Furthermore, for narrow strips (~100 nm in width), due to the reduced number of the allowed confined modes, a field-controllable switching between the resonances localized either in the strip center or at the edges of the strips can be achieved.

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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.

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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

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.

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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.

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