arXiv subjects

Jakub Holobrádek

Publications and source records attributed to Jakub Holobrádek.

11 recordsLinked to original sources

arXiv2026

Ultrafast formation of a large dynamic magnetic soliton

Nonlinear magnetization dynamics offers a rich variety of phenomena ranging from bistability to chaos. Here, we report the ultrafast formation of a dynamic magnetic soliton in thin ferrimagnetic garnet films with perpendicular magnetic anisotropy, driven by the microwave magnetic field of a microstrip antenna. Using time-resolved Brillouin light scattering microscopy and scanning transmission X-ray microscopy, we directly track the build-up of the large-angle precession state. The observed soliton is distinct from other nonlinear magnetic excitations in two key aspects: (i) it forms inside the linear spin-wave frequency band, and (ii) it is exceptionally large, reaching tens of microns beyond the antenna. We explain the soliton formation by the self-limiting mechanism upon a positive nonlinear frequency shift and the spatial extent of the near-field of the antenna. At large distances from the drive, the soliton collapses and emits short-wavelength spin waves via almost instantaneous spatial wavenumber conversion. Time-resolved measurements further reveal a small finite delay during soliton formation, while coherent long-range oscillations appear essentially simultaneously over distances up to 40 micrometers. These results establish microwave-driven solitons as a robust nonlinear phenomenon in thin-film garnets and suggest opportunities for fast, nonlocal manipulation of magnetic states and for applications in novel computational schemes.

cond-mat.mes-hall↗

arXiv2026

Precise control of crystallography and magnetism in focused-ion-beam transformed iron-nickel thin films

Focused ion beam irradiation of metastable Fe$_{78}$Ni$_{22}$ thin films grown on Cu(100) substrates results in the localized transformation of the originally paramagnetic, face-centered-cubic continuous film into ferromagnetic patterns with body-centered-cubic structure. The direction of the magnetic easy axis can be controlled by the focused ion beam scanning strategy, resulting in eight differently oriented crystallographic domains with different magnetic properties. We study the local crystallographic orientations of the transformed areas by electron backscatter diffraction and correlate these results with local magnetometry measurements. The observed magnetic anisotropy can be explained as a result of residual lattice strain after the fcc$\to$bcc transformation. These results extend the understanding of this material system and its transformation and allow for the patterning of high-quality magnetic nanostructures with precisely controlled magnetization landscapes.

cond-mat.mtrl-sci↗

arXiv2026

SpinWaveToolkit: Python package for (semi-)analytical calculations in the field of spin-wave physics

We present an open-source Python package, SpinWaveToolkit (SWT), for (semi-)analytical modeling of spin-wave dynamics in thin ferromagnetic films and exchange-coupled magnetic bilayers. SWT combines analytical models based on the Kalinikos-Slavin theory with a semi-analytical dynamic-matrix approach, enabling the calculation of dispersion relations, group velocities, decay lengths, mode profiles, and static equilibrium magnetization states. In addition, SWT implements a quantitative model of micro-focused Brillouin light scattering (BLS) that incorporates vectorial optical focusing, spin-wave Bloch functions, magneto-optical coupling, and Green-function propagation to simulate experimentally measured BLS spectra. The package is validated against finite-element dynamic-matrix simulations performed with TetraX for Damon-Eshbach, backward-volume, forward-volume, and oblique-field geometries, showing excellent agreement while reducing computation times by nearly two orders of magnitude in comparison to the numerical simulations. Thanks to the easiness of the use and fast calculation times, SWT can be used not only for exploratory mapping of the parameter space, but also for the fitting of the measured dispersion relations and related parameters. Thus, it provides a versatile and efficient framework for experiment design, interpretation, and parameter optimization for magnonics research.

cond-mat.mes-hall↗

arXiv2025

Ultrafast propagation of magnon-polaritons

The manipulation of magnetization lies at the heart of spintronic and magnonic technologies, with the ultimate performance of such systems limited by the velocity at which magnetic excitations can propagate. Here, we demonstrate ultrafast propagation of magnon-polaritons-hybrid quasiparticles arising from the coupling between spin waves and electromagnetic fields in thin pure, bismuth-, and gallium substituted yttrium iron garnet (YIG, Bi:YIG and Ga:YIG) films. Using time- and phase-resolved Brillouin light scattering microscopy and time-resolved scanning transmission microscopy, we show that magnon-polaritons can propagate faster than 100 km/s, nearly three orders of magnitude more than conventional spin waves, and can be observed at distances exceeding 40 micrometers in 20 nm thick films. Analytical modeling based on retarded Maxwell equations and Polder tensor formalism confirms the hybridized nature of the excitations and captures the nontrivial dispersion and attenuation profiles. Notably, the magnon-polaritons maintain high initial magnetization amplitudes and long decay lengths, enabling ultrafast manipulation of the magnetization far away from the excitation source. We show, that they can move domain walls or stabilize nonlinear magnetization processes. The unprecedentedly high propagation velocities make magnon-polaritons promising candidates for high-speed information transfer in future spin-based computing architectures, potentially overcoming long-standing group delay bottlenecks in magnonic logic circuits.

physics.app-ph↗

arXiv2025

Mie-enhanced micro-focused Brillouin light scattering with wavevector resolution

Magnons, the quanta of spin waves, are magnetic excitations of matter spanning through the entire crystal's Brillouin zone and covering a wide range of frequencies ranging from sub-gigahertz to hundreds of terahertz. Magnons play a crucial role in many condensed matter phenomena, such as the reduction of saturation magnetization with increasing temperature or Bose-Einstein condensation. However, current experimental techniques cannot resolve magnons with wavevectors between 30 and 300$\,$rad$\,μ$m$^{-1}$. In this letter, we address this gap by tailoring the light in Brillouin light scattering process with dielectric periodic nanoresonators and thus gaining access to the previously unmeasurable spin waves with full wavevector resolution using table-top optical setup. Filling this gap can stimulate further experimental investigations of the fundamental phenomena associated with magnons but also stimulate the application of magnonics in computational and microwave devices. In addition, the same methodology can be applied to other excitations of matter, such as phonons, opening up new possibilities in e.g. mechanobiological studies.

cond-mat.mes-hall↗

arXiv2024

Unidirectional propagation of zero-momentum magnons

We report on experimental observation of unidirectional propagation of zero-momentum magnons in synthetic antiferromagnet consisting of strained CoFeB/Ru/CoFeB trilayer. Inherent non-reciprocity of spin waves in synthetic antiferromagnets with uniaxial anisotropy results in smooth and monotonous dispersion relation around Gamma point, where the direction of the phase velocity is reversed, while the group velocity direction is conserved. The experimental observation of this phenomenon by intensity-, phase-, and time-resolved Brillouin light scattering microscopy is corroborated by analytical models and micromagnetic simulations.

cond-mat.mes-hall↗

arXiv2023

Active Loss Engineering in Vanadium Dioxide Based BIC Metasurfaces

Metasurfaces have unlocked significant advancements across photonics, yet their efficient active control remains challenging. The active materials required often lack continuous tunability, exhibit inadequate refractive index (RI) changes, or suffer from high losses. These aspects pose an inherent limitation for resonance-shifting based switching: when RI changes are small, the resulting shift is also minor. Conversely, high RI changes typically come with high intrinsic losses necessitating broad modes because narrow ones cannot tolerate such losses. Therefore, larger spectral shifts are required to effectively detune the modes. This paper introduces a novel active metasurface approach that converts the constraint of high intrinsic losses into a beneficial feature. This is achieved by controlling the losses in a hybrid vanadium dioxide (VO$_{2}$) - silicon metasurface, supporting symmetry-protected bound states in the continuum (BICs) within the infrared spectrum. By leveraging the temperature-controlled losses in VO$_{2}$ and combining them with the inherent far-field-coupling tunability of BICs, we gain unprecedented precision in independently controlling both the radiative and nonradiative losses of the resonant system. Our dual-control mechanism allows us to optimize our metasurfaces and we experimentally demonstrate quality factors above 200, a maximum reflectance amplitude of 90%, a relative switching contrast of 78%, and continuous tuning from under- to over-coupling within the infrared spectral range. This study provides a foundation for experimentally and technologically simple, fine-tunable, active metasurfaces for applications ranging from molecular sensors to filters and optical modulators.

physics.optics↗

arXiv2023

Zero-field spin wave turns

Spin-wave computing, a potential successor to CMOS-based technologies, relies on the efficient manipulation of spin waves for information processing. While basic logic devices like magnon transistors, gates, and adders have been experimentally demonstrated, the challenge for complex magnonic circuits lies in steering spin waves through sharp turns. In this study we demonstrate with micromagnetic simulations and Brillouin light scattering microscopy experiments, that dipolar spin waves can propagate through 90-degree turns without distortion. The key lies in carefully designed in-plane magnetization landscapes, addressing challenges posed by anisotropic dispersion. The experimental realization of the required magnetization landscape is enabled by spatial manipulation of the uniaxial anisotropy using corrugated magnonic waveguides. The findings presented in this work should be considered in any magnonic circuit design dealing with anisotropic dispersion and spin wave turns.

cond-mat.mes-hall↗

arXiv2023

Phase-resolved optical characterization of nanoscale spin waves

We study theoretically and experimentally the process of Brillouin light scattering on an array of silicon disks on a thin Permalloy layer. We show that phase-resolved Brillouin light scattering microscopy performed on an array of weakly interacting dielectric nanoresonators can detect nanoscale waves and measure their dispersion. In our experiment, we were able to map the evolution of the phase of the spin wave with a wavelength of 209 nm with a precision of 6 nm. These results demonstrate the feasibility of all-optical phase-resolved characterization of nanoscale spin waves.

physics.optics↗

arXiv2022

Observing high-k magnons with Mie-resonance-enhanced Brillouin light scattering

Magnonics is a prospective beyond CMOS technology which uses magnons, the quanta of spin waves, for low-power information processing. Many magnonic concepts and devices were recently demonstrated at macro- and microscale, and now these concepts need to be realized at nanoscale. Brillouin light scattering spectroscopy and microscopy (BLS) has become a standard technique for spin wave visualization and characterization, and enabled many pioneering magnonic experiments. However, due to its fundamental limit in maximum detectable magnon momentum, the conventional BLS cannot be used to detect nanoscale spin waves. Here we show that optically induced Mie resonances in dielectric nanoparticles can be used to extend the range of accessible spin wave wavevectors beyond the BLS fundamental limit. The method is universal and can be used in many magnonic experiments dealing with thermally excited as well as coherently excited high-momentum, short-wavelength spin waves. This discovery significantly extends the usability and relevance of the BLS technique for nanoscale magnonic research.

cond-mat.mes-hall↗

arXiv2020

Propagation of spin waves through a Néel domain wall

Spin waves have the potential to be used as a new platform for data transfer and processing as they can reach wavelengths in the nanometer range and frequencies in the terahertz range. To realize a spin-wave device, it is essential to be able to manipulate the amplitude as well as the phase of spin waves. Several theoretical and recently also experimental works have shown that the spin-wave phase can be manipulated by the transmission through a domain wall (DW). Here, we study propagation of spin waves through a DW by means of micro-focused Brillouin light scattering microscopy ($μ$BLS). The acquired 2D spin-wave intensity maps reveal that spin-wave transmission through a Néel DW is influenced by a topologically enforced circular Bloch line in the DW center and that the propagation regime depends on the spin-wave frequency. In the first regime, two spin-wave beams propagating around the circular Bloch line are formed, whereas in the second regime, spin waves propagate in a single central beam through the circular Bloch line. Phase-resolved $μ$BLS measurements reveal a phase shift upon transmission through the domain wall for both regimes. Micromagnetic modelling of the transmitted spin waves unveils a distortion of their phase fronts which needs to be taken into account when interpreting the measurements and designing potential devices. Moreover, we show, by means of micromagnetic simulations, that an external magnetic field can be used to move the circular Bloch line within the DW and to manipulate spin-wave propagation.

physics.app-ph↗