arXiv subjects

Lukáš Flajšman

Publications and source records attributed to Lukáš Flajšman.

16 recordsLinked to original sources

arXiv2026

Bismuth-substituted Lutetium Iron Garnet Films with Giant Visible-Range Magneto-Optical Sensitivity

Magneto-optical materials are indispensable across modern physics, serving as the foundation for precision magnetic sensing, nonreciprocal photonics, and optical isolation technologies. The continual pursuit of materials with high Verdet constants has driven the development of garnet-based compounds exhibiting giant magneto-optical sensitivity. In this work, we report the growth and comprehensive magneto-optical characterization of bismuth-substituted lutetium iron garnet (LuBiIG), a material that combines the large spin-orbit coupling of bismuth with the lattice stability of lutetium iron garnet. LuBiIG films with thicknesses between 85-220nm were grown by pulsed laser deposition and characterized at room temperature over the 500-820nm wavelength range. The films exhibit an exceptionally high Verdet constant of up to -1.37e8 degree/meter/Tesla, peaking in the visible spectral range near 520nm. These results position LuBiIG as a highly sensitive magneto-optical material suitable for advanced cryogenic detection and hybrid quantum applications.

cond-mat.mtrl-sci↗

arXiv2026

Spin-wave bandgap engineering via mode hybridization in dipolar-coupled YIG film/CoFeB nanodisk magnonic crystals

We investigate spin-wave transport in hybrid two-dimensional magnonic crystals comprising a low-damping yttrium iron garnet (YIG) film coupled to a periodic array of CoFeB nanodisks. Using propagating spin-wave spectroscopy, super-Nyquist magneto-optical Kerr effect microscopy, and micromagnetic simulations, we demonstrate the formation of pronounced and tunable bandgaps that do not originate from conventional Bragg scattering. Instead, these gaps arise from hybridization between the fundamental magnonic-crystal mode and in-plane transverse standing modes induced by the periodic nanodisk array. The spectral position and width of these gaps are controlled by geometric parameters and by the magnetic state of the nanodisks, including their vortex configuration, which governs both static and dynamic dipolar coupling. For larger lattice periods, additional gaps emerge through hybridization with modes quantized both transverse and parallel to the spin-wave propagation direction, reflecting dispersion folding in two dimensions. Our results establish mode hybridization as a versatile mechanism for engineering spin-wave band structures beyond the constraints of Bragg scattering and provide a pathway toward reconfigurable magnonic devices based on dipolar-coupled hybrid architectures.

cond-mat.mes-hall↗

arXiv2026

Nonlinear dynamics in magnonic Fabry-Pérot resonators: Low-power neuron-like activation and transmission suppression

We report on nonlinear spin-wave dynamics in magnonic Fabry-Pérot resonators composed of yttrium iron garnet (YIG) films coupled to CoFeB nanostripes. Using super-Nyquist sampling magneto-optical Kerr effect microscopy and micromagnetic simulations, we observe a systematic downshift of the spin-wave transmission gaps as the excitation power increases. This nonlinear behavior occurs at low power levels, reduced by a strong spatial concentration of spin waves within the resonator. The resulting power-dependent transmission enables neuron-like activation behavior and frequency-selective nonlinear spin-wave absorption. Our results highlight magnonic Fabry-Pérot resonators as compact low-power nonlinear elements for neuromorphic magnonic computing architectures.

cond-mat.mtrl-sci↗

arXiv2025

Reservoir computing in a lithium-based magneto-ionic device

In-materio computing exploits the intrinsic physical dynamics of materials to perform complex computations, enabling low-power, real-time data processing by embedding computation directly within physical layers. Here, we demonstrate a voltage-controlled magneto-ionic device that functions as a reservoir computer capable of forecasting chaotic time series. The device consists of a crossbar structure with a Ta/CoFeB/Ta/MgO/Ta bottom electrode and a LiPON/Pt top electrode. A chaotic Mackey-Glass time series is encoded into a voltage signal applied to the device, while 2D Fourier transforms of voltage-dependent magnetic domain patterns form the output. Performance is influenced by the input rate, smoothing of the output, the number of elements in the reservoir state vector, and the training duration. We identify two distinct computational regimes: short-term prediction is optimized using smoothed, low-dimensional states with minimal training, whereas prediction around the Mackey-Glass delay time benefits from unsmoothed, high-dimensional states and extended training. Reservoir computing metrics reveal that slower input rates are more tolerant to output smoothing, while faster input rates degrade both memory capacity and nonlinear processing. These findings demonstrate the potential of magneto-ionic systems for neuromorphic computing and offer design principles for tuning performance in response to input signal characteristics.

physics.app-ph↗

arXiv2025

Spin-wave propagation at low temperatures in YIG thin films on YSGG substrates

The use of spin waves in magnetic thin films at cryogenic temperatures has long been hindered by the lack of a suitable material platform. Yttrium iron garnet (YIG) is the leading candidate, yet it is typically grown on gadolinium gallium garnet (GGG) substrates, which develop a large paramagnetic moment at low temperatures. This substrate effect limits spin-wave propagation. In this work, we demonstrate that thin YIG films grown on yttrium scandium gallium garnet (YSGG) substrates support robust spin-wave propagation in the Damon-Eshbach geometry, measurable down to 2 K under applied magnetic fields up to 150 mT. Compared with YIG/GGG, YIG/YSGG films exhibit narrower ferromagnetic resonance (FMR) linewidths at low temperatures and are free from the atomic interdiffusion effects that degrade the performance of YIG/GGG systems. These results establish YIG/YSGG thin films as a promising low-temperature platform, overcoming the intrinsic limitations of YIG/GGG and opening new opportunities for scalable magnonic and hybrid quantum devices operating under cryogenic conditions.

cond-mat.mes-hall↗

arXiv2024

A magneto-ionic synapse for reservoir computing

Neuromorphic computing aims to revolutionize large-scale data processing by developing efficient methods and devices inspired by neural networks. Among these, the control of magnetism through ion migration has emerged as a promising approach due to the inherent memory and nonlinearity of ionically conducting and magnetic materials. In this work, we present a lithium-ion-based magneto-ionic device that uses applied voltages to control the magnetic domain state of a perpendicularly magnetized ferromagnetic layer. This behavior emulates the analog and non-volatile properties of biological synapses and enables the creation of a simple reservoir computing system. To illustrate its capabilities, the device is used in a waveform classification task, where the voltage amplitude range and magnetic bias field are tuned to optimize the recognition accuracy.

physics.app-ph↗

arXiv2024

Magnonic Fabry-Pérot resonators as programmable phase shifters

We explore the use of magnonic Fabry-Pérot resonators as programmable phase shifters for spin-wave computing. The resonator, composed of a yttrium iron garnet (YIG) film coupled with a CoFeB nanostripe, operates through dynamic dipolar coupling, leading to wavelength downconversion and the formation of a magnonic cavity. Using super-Nyquist sampling magneto-optical Kerr effect (SNS-MOKE) microscopy and micromagnetic simulations, we demonstrate that these resonators can induce a $π$ phase shift in the transmitted spin wave. The phase shift is highly sensitive to the magnetization alignment within the resonator, allowing for on-demand control via magnetic switching. This feature, combined with low-loss transmission, positions the magnonic Fabry-Pérot resonator as a promising component for reconfigurable magnonic circuits and spin-wave computing devices.

cond-mat.mtrl-sci↗

arXiv2024

Tuning of perpendicular magnetic anisotropy in Bi-substituted yttrium iron garnet films by He$^+$ ion irradiation

We report the continuous tuning of magnetic anisotropy in perpendicularly magnetized bismuth-substituted yttrium iron garnet (Bi-YIG) films via He$^+$ ion irradiation. Our findings indicate that the magnetization direction of epitaxial Bi-YIG films on sGGG substrates transitions from out-of-plane in the as-grown state to in-plane after He+ ion irradiation at a fluence exceeding $2\times 10^{14}$ ions/cm$^2$. The reorientation is attributed to the relaxation of tensile film strain, which reduces the perpendicular magnetic anisotropy without affecting the saturation magnetization. The Gilbert damping parameter and the inhomogeneous broadening of the ferromagnetic resonance linewidth show only minimal increases with ion irradiation. Additionally, at a fluence of $5\times 10^{13}$ ions/cm$^2$, we observe the formation of magnetic bubble domains in the Bi-YIG films. Micromagnetic simulations estimate a Dzyaloshinskii-Moriya interaction of 0.006 mJ/m$^2$, which is insufficient for stabilizing Néel-type skyrmions. Finally, we demonstrate that the effects of He$^+$ ion irradiation can be largely reversed through thermal annealing in an oxygen atmosphere.

cond-mat.mtrl-sci↗

arXiv2023

Solid-state lithium-ion supercapacitor for voltage control of skyrmions

Ionic control of magnetism gives rise to high magneto-electric coupling efficiencies at low voltages, which is essential for low-power magnetism-based non-conventional computing technologies. However, for on-chip applications, magneto-ionic devices typically suffer from slow kinetics, poor cyclability, impractical liquid architectures or strong ambient effects. As a route to overcoming these problems, we demonstrate an LiPON-based solid-state ionic supercapacitor with a magnetic Pt/Co$_{40}$Fe$_{40}$B$_{20}$/Pt thin-film electrode which enables voltage control of a magnetic skyrmion state. Skyrmion nucleation and annihilation are caused by Li ion accumulation and depletion at the magnetic interface under an applied voltage. The skyrmion density can be controlled through dc applied fields or through voltage pulses. The skyrmions are nucleated by single 60-$μ$s voltage pulses and devices are cycled 750,000 times without loss of electrical performance. Our results demonstrate a simple and robust approach to ionic control of magnetism in spin-based devices.

cond-mat.mtrl-sci↗

arXiv2023

Electric field control of RKKY coupling through solid-state ionics

Placing a suitable spacer layer between two magnetic layers can lead to an interaction between the magnetic layers known as Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling. Controlling RKKY coupling, particularly the ability to switch between ferromagnetic and antiferromagnetic coupling, would enable novel magnetic data storage devices. By combining solid-state Li ion battery technology with an out-of-plane magnetized Co/Pt-based stack coupled through a Ru interlayer we investigate the effects of the insertion of Li ions on the magnetic properties of the stack. The RKKY coupling and its voltage dependence is measured as a function of the Ru interlayer thickness, along with the effects of repeated voltage cycling. The Li ions both change the amplitude of the RKKY coupling and its phase, leading to the ability to switch the RKKY coupling between ferromagnetic and antiferromagnetic with applied voltages.

cond-mat.mtrl-sci↗

arXiv2022

Wideband Brillouin light scattering analysis of spin waves excited by a white-noise RF generator

Spin waves are studied intensively for their intriguing properties and potential use in future technology platforms for the transfer and processing of information and microwave signals. The development of devices and materials for spin-wave systems requires a lot of measurement time and effort, and thus increasing the measurement throughput by extending the instrumentation capabilities is of the essence. In this letter, we report on a new and straightforward approach to increase the measurement throughput by fully exploiting the wideband detection nature of the Brillouin light scattering technique in single-shot experiments using a white-noise RF generator.

cond-mat.mes-hall↗

arXiv2020

Spin wave propagation in corrugated waveguides

Curvature-induced effects allow us to tailor the static and dynamic response of a magnetic system with a high degree of freedom. We study corrugated magnonic waveguides deposited on a sinusoidally modulated substrate prepared by focused electron beam deposition. The surface curvature in films with thicknesses comparable to the amplitude of modulation locally modifies the contributions of dipolar and exchange energies and results in an effective anisotropy term which can be tuned on-demand based on the exact geometry. We show, by Brillouin light scattering microscopy, that without the presence of an external magnetic field, spin waves propagate over a distance 10$\times$larger in the corrugated waveguide than in the planar waveguide. Further, we analyze the influence of the modulation amplitude on spin-wave propagation and conclude that for moderate modulation amplitudes, the spin-wave decay length is not affected. For larger amplitudes, the decay length decreases linearly with increasing modulation.

physics.app-ph↗

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↗

arXiv2019

Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing

Metastable face-centered-cubic Fe78Ni22 thin films grown on Cu(001) substrates are excellent candidates for focused ion beam direct writing of magnonic structures due to their favorable magnetic properties after ion-beam-induced transformation. The focused ion beam transforms the originally nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control over the direction of uniaxial magnetic in-plane anisotropy. The magnetocrystalline anisotropy in transformed areas is strong enough to stabilize the magnetization in transverse direction to the long axis of narrow waveguides. Therefore, it is possible to propagate spin waves in these waveguides without the presence of an external magnetic field in the favorable Demon-Eshbach geometry. Phase-resolved micro-focused Brillouin light scattering yields the dispersion relation of these waveguides in zero as well as in nonzero external magnetic fields.

physics.app-ph↗

arXiv2018

The growth of metastable fcc Fe78Ni22 thin films on H-Si(100) substrates suitable for focused ion beam direct magnetic patterning

We have studied the growth of metastable face-centered-cubic, non-magnetic Fe78Ni22 thin films on silicon substrates. These films undergo a magnetic (paramagnetic to ferromagnetic) and structural (fcc to bcc) phase transformation upon ion beam irradiation and thus can serve as a template for direct writing of magnetic nanostructures by the focused ion beam. So far, these films were prepared only on single crystal Cu(100) substrates. We show that transformable Fe78Ni22 thin films can also be prepared on a hydrogen-terminated Si(100) with a 130-nm-thick Cu(100) buffer layer. The H-Si(100) substrates can be prepared by hydrofluoric acid etching or by annealing at 1200°C followed by adsorption of atomic hydrogen. The Cu(100) buffer layer and Fe78Ni22 fcc metastable thin film were deposited by thermal evaporation in an ultra-high vacuum. The films were consequently transformed in-situ by 4keV Ar+ ion irradiation and ex-situ by a 30 keV Ga+ focused ion beam, and their magnetic properties were studied by magneto-optical Kerr effect magnetometry. The substitution of expensive copper single crystal substrate by standard silicon wafers dramatically expands application possibilities of metastable paramagnetic thin films for focused ion beam direct magnetic patterning.

cond-mat.mtrl-sci↗

arXiv2018

Focused ion beam direct writing of magnetic patterns with controlled structural and magnetic properties

Focused ion beam irradiation of metastable Fe$_{78}$Ni$_{22}$ thin films grown on Cu(100) substrates is used to create ferromagnetic, body-centered-cubic patterns embedded into paramagnetic, face-centered-cubic surrounding. The structural and magnetic phase transformation can be controlled by varying parameters of the transforming gallium ion beam. The focused ion beam parameters as ion dose, number of scans, and scanning direction can be used not only to control a degree of transformation, but also to change the otherwise four-fold in-plane magnetic anisotropy into the uniaxial anisotropy along specific crystallographic direction. This change is associated with a preferred growth of specific crystallographic domains. The possibility to create magnetic patterns with continuous magnetization transitions and at the same time to create patterns with periodical changes in magnetic anisotropy makes this system an ideal candidate for rapid prototyping of a large variety of nanostructured samples. Namely spin-wave waveguides and magnonic crystals can be easily combined into complex devices in a single fabrication step.

physics.app-ph↗