arXiv subjects

Witold Skowroński

Publications and source records attributed to Witold Skowroński.

At least 19 recordsLinked to original sources

arXiv2026

Modeling of a magnetic field sensor based on spin Hall magnetoresistance

Next-generation spintronic sensors aim to overcome the limitations of traditional tunneling-magnetoresistance (TMR) devices, such as complex manufacturing, high $1/f$ noise, and significant offsets. This work presents a comprehensive modeling and experimental validation of a magnetic field sensor based on Spin Hall Magnetoresistance (SMR) in a Wheatstone bridge configuration. Utilizing a multiphysics approach, we simulate the interplay between SMR, Anisotropic Magnetoresistance (AMR), and Spin-Orbit Torque (SOT) using a Stoner-Wohlfarth model complemented by a Fuchs-Sondheimer analysis of current distribution. To account for the presence of magnetic domains, we incorporate a modified Stoner-Wohlfarth framework that considers non-uniform magnetization and domain wall motion through a "truncated astroid" approach, allowing for a statistical distribution of single-domain particles. The model is validated against experimental measurements of Pt/$\text{Fe}_{60}\text{Co}_{20}\text{B}_{20}$ and Ta/$\text{Fe}_{60}\text{Co}_{20}\text{B}_{20}$ bilayers patterned into Hall bars and Wheatstone bridges. The model provides critical design guidelines for optimizing material properties, layer thickness, and device layout to minimize power consumption and maximize sensitivity in SMR-based sensing applications.

cond-mat.mes-hall↗

arXiv2025

Reinforcement learning for spin torque oscillator tasks

We address the problem of automatic synchronisation of the spintronic oscillator (STO) by means of reinforcement learning (RL). A numerical solution of the macrospin Landau-Lifschitz-Gilbert-Slonczewski equation is used to simulate the STO and we train the two types of RL agents to synchronise with a target frequency within a fixed number of steps. We explore modifications to this base task and show an improvement in both convergence and energy efficiency of the synchronisation that can be easily achieved in the simulated environment.

physics.app-ph↗

arXiv2025

Towards mutual synchronization of serially connected Spin Torque Oscillators based on magnetic tunnel junctions

Multiple neuromorphic applications require the tuning of two or more devices to a common signal. Various types of neuromorphic computation can be realized using spintronic oscillators, where the DC current induces magnetization precession, which turns into an AC voltage generator. However, in spintronics, synchronization of two oscillators using a DC signal is still a challenging problem because it requires a certain degree of similarity between devices that are to be synchronized, which may be difficult to achieve due to device parameter distribution during the fabrication process. In this work, we present experimental results on the mechanisms of synchronization of spin-torque oscillators. Devices are based on magnetic tunnel junction with a perpendicularly magnetized free layer and take advantage of a uniform magnetization precision in the presence of the magnetic field and a DC bias. By using an external microwave source, we show the optimal condition for the synchronization of the magnetic tunnel junctions. Finally, we present results on the in-series connection of two junctions and discuss the possible path towards improving oscillation power and linewidth. In addition, using numerical simulations of the coupled oscillators model, we aim to reproduce the conditions of the experiments and determine the tolerance for achieving synchronization.

physics.app-ph↗

arXiv2024

Gate-tunable Exchange Bias and Voltage-controlled Magnetization Switching in a van der Waals Ferromagnet

The discovery of van der Waals magnets has established a new domain in the field of magnetism, opening novel pathways for the electrical control of magnetic properties. In this context, Fe3GeTe2 (FGT) emerges as an exemplary candidate owing to its intrinsic metallic properties, which facilitate the interplay of both charge and spin degrees of freedom. Here, the bidirectional voltage control of exchange bias (EB) effect in a perpendicularly magnetized all-van der Waals FGT/O-FGT/hBN heterostructure is demonstrated. The antiferromagnetic O-FGT layer is formed by naturally oxidizing the FGT surface. The observed EB magnitude reaches 1.4 kOe with a blocking temperature (150 K) reaching close to the Curie temperature of FGT. Both the exchange field and the blocking temperature values are among the highest in the context of layered materials. The EB modulation exhibits a linear dependence on the gate voltage and its polarity, observable in both positive and negative field cooling (FC) experiments. Additionally, gate voltage-controlled magnetization switching, highlighting the potential of FGT-based heterostructures is demonstrated in advanced spintronic devices. These findings display a methodology to modulate the magnetism of van der Waals magnets offering new avenues for the development of high-performance magnetic devices.

cond-mat.mtrl-sci↗

arXiv2024

Substrate-induced spin-torque-like signal in spin-torque ferromagnetic resonance measurement

Oxide thin films and interfaces with strong spin-orbit coupling have recently shown exceptionally high charge-to-spin conversion, making them potential spin-source materials for spintronics. Epitaxial strain engineering using oxide substrates with different lattice constants and symmetries has emerged as a mean to further enhance charge-to-spin conversion. However, high relative permittivity and dielectric loss of commonly used oxide substrates, such as SrTiO3, can cause significant current shunting in substrates at high frequency, which may strongly affect spin-torque measurement and potentially result in an inaccurate estimation of charge-to-spin conversion efficiency. In this study, we systematically evaluate the influence of various oxide substrates for the widely-used spin-torque ferromagnetic resonance (ST-FMR) measurement. Surprisingly, we observed substantial spin-torque signals in samples comprising only ferromagnetic metal on oxide substrates with high relative permittivity (e.g., SrTiO3 and KTaO3), where negligible signal should be initially expected. Notably, this unexpected signal shows a strong correlation with the capacitive reactance of oxide substrates and the leakage radio frequency (RF) current within the substrate. By revising the conventional ST-FMR analysis model, we attribute this phenomenon to a 90-degree phase difference between the RF current flowing in the metal layer and in the substrate. We suggest that extra attention should be paid during the ST-FMR measurements, as this artifact could dominate over the real spin-orbit torque signal from high-resistivity spin-source materials grown on substrate with high relative permittivity.

cond-mat.mtrl-sci↗

arXiv2024

Magnetization dynamics driven by displacement currents across a magnetic tunnel junction

Understanding the high-frequency transport characteristics of magnetic tunnel junctions (MTJs) is crucial for the development of fast-operating spintronics memories and radio frequency devices. Here, we present the study of frequency-dependent capacitive current effect in CoFeB/MgO-based MTJs and its influence on magnetization dynamics using time-resolved magneto-optical Kerr effect technique. In our device operating at gigahertz frequencies, we find a large displacement current of the order of mA's, which does not break the tunnel barrier of the MTJ. Importantly, this current generates an Oersted field and spin-orbit torque, inducing magnetization dynamics. Our discovery holds promise for building robust MTJ devices operating under high current conditions, also highlighting the significance of capacitive impedance in high frequency magnetotransport techniques.

cond-mat.mes-hall↗

arXiv2024

Odd non-linear conductivity under spatial inversion in chiral Tellurium

Electrical transport in non-centrosymmetric materials departs from the well-established phenomenological Ohm's law. Instead of a linear relation between current and electric field, a non-linear conductivity emerges along specific crystallographic directions. This non-linear transport is fundamentally related to the lack of spatial inversion symmetry. However, the experimental implications of an inversion symmetry operation on the non-linear conductivity remain to be explored. Here, we report on a large, non-linear conductivity in chiral Tellurium. By measuring samples with opposite handedness, we demonstrate that the non-linear transport is odd under spatial inversion. Furthermore, by applying an electrostatic gate, we modulate the non-linear output by a factor of 300, reaching the highest reported value excluding engineered heterostructures. Our results establish chiral Te as an ideal compound not just to study the fundamental interplay between crystal structure, symmetry operations and non-linear transport, but also to develop wireless rectifiers and energy-harvesting chiral devices.

cond-mat.mes-hall↗

arXiv2023

Influence of Ferromagnetic Interlayer Exchange Coupling on Current-induced Magnetization Switching and Dzyaloshinskii-Moriya Interaction in Co/Pt/Co Multilayer System

This paper investigates the relationship among interlayer exchange coupling (IEC), Dzyaloshinskii-Moriya interaction (DMI), and multilevel magnetization switching within a Co/Pt/Co heterostructure, where varying Pt thicknesses enable control over the coupling strength. Employing Brillouin Light Scattering to quantify the effective DMI, we explore its potential role in magnetization dynamics and multilevel magnetization switching. Experimental findings show four distinct resistance states under an external magnetic field and spin Hall effect related spin current. We explain this phenomenon based on the asymmetry between Pt/Co and Co/Pt interfaces and the interlayer coupling, which, in turn, influences DMI and subsequently impacts the magnetization dynamics. Numerical simulations, including macrospin, 1D domain wall, and simple spin wave models, further support the experimental observations of multilevel switching and help uncover the underlying mechanisms. Our proposed explanation, supported by magnetic domain observation using polar-magnetooptical Kerr microscopy, offers insights into both the spatial distribution of magnetization and its dynamics for different IECs, thereby shedding light on its interplay with DMI, which may lead to potential applications in storage devices.

cond-mat.mes-hall↗

arXiv2023

Multiferroic Magnon Spin-Torque Based Reconfigurable Logic-In-Memory

Magnons, bosonic quasiparticles carrying angular momentum, can flow through insulators for information transmission with minimal power dissipation. However, it remains challenging to develop a magnon-based logic due to the lack of efficient electrical manipulation of magnon transport. Here we present a magnon logic-in-memory device in a spin-source/multiferroic/ferromagnet structure, where multiferroic magnon modes can be electrically excited and controlled. In this device, magnon information is encoded to ferromagnetic bits by the magnon-mediated spin torque. We show that the ferroelectric polarization can electrically modulate the magnon spin-torque by controlling the non-collinear antiferromagnetic structure in multiferroic bismuth ferrite thin films with coupled antiferromagnetic and ferroelectric orders. By manipulating the two coupled non-volatile state variables (ferroelectric polarization and magnetization), we further demonstrate reconfigurable logic-in-memory operations in a single device. Our findings highlight the potential of multiferroics for controlling magnon information transport and offer a pathway towards room-temperature voltage-controlled, low-power, scalable magnonics for in-memory computing.

physics.app-ph↗

arXiv2023

Spin Hall Induced Magnetization Dynamics in Multiferroic Tunnel Junction

The combination of spin-orbit coupling driven effects and multiferroic tunneling properties was explored experimentally in thin Pt/Co/BTO/LSMO multilayers. The presence of a Pt heavy metal allows for the spin current-induced magnetization precession of Co upon radio-frequency charge current injection. The utilization of a BTO ferroelectric tunnel barrier separating the Co and LSMO ferromagnetic electrodes gives rise to both tunneling-magnetoresistance and electroresistance. Using the spin-orbit torque ferromagnetic resonance, the maganetization dynamics of the Co/Pt bilayers was studied at room temperature. Unexpectedly the magnetization dynamics study in the same geometry performed at low temperature reveals the existence of both Co and LSMO resonance peaks indicating efficient spin current generation both using the spin Hall effect in Pt and spin pumping in LSMO that tunnel via the BTO barrier.

cond-mat.mes-hall↗

arXiv2022

A comprehensive simulation package for analysis of multilayer spintronic devices

We present cmtj - a comprehensive simulation package that allows large-scale macrospin simulations for a variety of multilayer spintronics devices. Apart from conventional static simulations, such as magnetoresistance and magnetisation hysteresis loops, cmtj implements a mathematical model of dynamic experimental techniques commonly used for spintronics devices characterisation, for instance: spin diode ferromagnetic resonance, pulse-induced microwave magnetometry, or harmonic Hall voltage measurements. We demonstrate the accuracy of the macrospin simulations on a variety of examples, accompanied by some experimental results.

cond-mat.mtrl-sci↗

arXiv2022

Numerical Model Of Harmonic Hall Voltage Detection For Spintronic Devices

We present a numerical macrospin model for harmonic voltage detection in multilayer spintronic devices. The core of the computational backend is based on the Landau-Lifshitz-Gilbert-Slonczewski equation, which combines high performance with satisfactory, for large-scale applications, agreement with the experimental results. We compare the simulations with the experimental findings in Ta/CoFeB bilayer system for angular- and magnetic field-dependent resistance measurements, electrically detected magnetisation dynamics, and harmonic Hall voltage detection. Using simulated scans of the selected system parameters such as the polar angle $θ$, magnetisation saturation ($μ_\textrm{0}M_\textrm{s}$) or uniaxial magnetic anisotropy ($K_\textrm{u}$) we show the resultant changes in the harmonic Hall voltage, demonstrating the dominating influence of the $μ_\textrm{0}M_\textrm{s}$ on the first and second harmonics. In the spin-diode ferromagnetic resonance (SD-FMR) technique resonance method the ($μ_\textrm{0}M_\textrm{s}$, $K_\textrm{u}$) parameter space may be optimised numerically to obtain a set of viable curves that fit the experimental data.

physics.app-ph↗

arXiv2021

Angular harmonic Hall voltage and magnetoresistance measurements of Pt/FeCoB and Pt-Ti/FeCoB bilayers for spin Hall conductivity determination

Materials with significant spin-orbit coupling enable efficient spin-to-charge interconversion, which can be utilized in novel spin electronic devices. A number of elements, mainly heavy-metals (HM) have been identified to produce a sizable spin current ($j_\mathrm{s}$), while supplied with a charge current ($j$), detected mainly in the neighbouring ferromagnetic (FM) layer. Apart from the spin Hall angle $θ_\mathrm{SH}$ = $j_\mathrm{s}$/$j$, spin Hall conductivity ($σ_\mathrm{SH}$) is an important parameter, which takes also the resistivity of the material into account. In this work, we present a measurement protocol of the HM/FM bilayers, which enables for a precise $σ_\mathrm{SH}$ determination. Static transport measurements, including resistivity and magnetization measurements are accompanied by the angular harmonic Hall voltage analysis in a dedicated low-noise rotating probe station. Dynamic characterization includes effective magnetization and magnetization damping measurement, which enable HM/FM interface absorption calculation. We validate the measurement protocol in Pt and Pt-Ti underlayers in contact with FeCoB and present the $σ_\mathrm{SH}$ of up to 3.3$\times$10$^5$ S/m, which exceeds the values typically measured in other HM, such as W or Ta.

cond-mat.mes-hall↗

arXiv2021

Study of Spin-Orbit Interactions and Interlayer Ferromagnetic Coupling in Co/Pt/Co Trilayers in Wide Range of Heavy Metal Thickness

The spin-orbit torque, a torque induced by a charge current flowing through the heavy-metal conducting layer with strong spin-orbit interactions, provides an efficient way to control the magnetization direction in heavy-metal/ferromagnet nanostructures, required for applications in the emergent magnetic technologies like random access memories, high-frequency nano oscillators, or bio-inspired neuromorphic computations. We study the interface properties, magnetization dynamics, magnetostatic features and spin-orbit interactions within the multilayer system Ti(2)/Co(1)/Pt(0-4)/Co(1)/MgO(2)/Ti(2) (thicknesses in nanometers) patterned by optical lithography on micrometer-sized bars. In the investigated devices, Pt is used as a source of the spin current and as a non-magnetic spacer with variable thickness, which enables the magnitude of the interlayer ferromagnetic exchange coupling to be effectively tuned. We also find the Pt thickness-dependent changes in magnetic anisotropies, magnetoresistance, effective Hall angle and, eventually, spin-orbit torque fields at interfaces. The experimental findings are supported by the relevant interface structure-related simulations, micromagnetic, macrospin, as well as the spin drift-diffusion models. Finally, the contribution of the spin-orbital Edelstein-Rashba interfacial fields is also briefly discussed in the analysis.

cond-mat.mes-hall↗

arXiv2021

Multi-state MRAM cells for hardware neuromorphic computing

Magnetic tunnel junctions (MTJ) have been successfully applied in various sensing application and digital information storage technologies. Currently, a number of new potential applications of MTJs are being actively studied, including high-frequency electronics, energy harvesting or random number generators. Recently, MTJs have been also proposed in designs of a new platforms for unconventional or bio-inspired computing. In the present work, it is shown that serially connected MTJs forming a multi-state memory cell can be used in a hardware implementation of a neural computing device. A behavioral model of the multi-cell is proposed based on the experimentally determined MTJ parameters. The main purpose of the mutli-cell is the formation of the quantized weights of the network, which can be programmed using the proposed electronic circuit. Mutli-cells are connected to CMOS-based summing amplifier and sigmoid function generator, forming an artificial neuron. The operation of the designed network is tested using a recognition of the hand-written digits in 20x20 pixel matrix and shows detection ratio comparable to the software algorithm, using the weight stored in a multi-cell consisting of four MTJs or more.

cs.ET↗

arXiv2020

Current-induced magnetization switching of exchange-biased NiO heterostructures characterized by spin-orbit torque

In this work, we study magnetization switching induced by spin-orbit torque in W(Pt)/Co/NiO heterostructures with variable thickness of heavy-metal layers W and Pt, perpendicularly magnetized Co layer and an antiferromagnetic NiO layer. Using current-driven switching, magnetoresistance and anomalous Hall effect measurements, perpendicular and in-plane exchange bias field were determined. Several Hall-bar devices possessing in-plane exchange bias from both systems were selected and analyzed in relation to our analytical switching model of critical current density as a function of Pt and W thickness, resulting in estimation of effective spin Hall angle and perpendicular effective magnetic anisotropy. We demonstrate in both the Pt/Co/NiO and the W/Co/NiO systems the deterministic Co magnetization switching without external magnetic field which was replaced by in-plane exchange bias field. Moreover, we show that due to a higher effective spin Hall angle in W than in Pt-systems the relative difference between the resistance states in the magnetization current switching to difference between the resistance states in magnetic field switching determined by anomalous Hall effect ($ΔR/ΔR_{\text{AHE}}$) is about twice higher in W than Pt, while critical switching current density in W is one order lower than in Pt-devices. The current switching stability and training process is discussed in detail.

physics.app-ph↗

arXiv2020

Spin-orbit torque induced magnetisation dynamics and switching in CoFeB/Ta/CoFeB system with mixed magnetic anisotropy

Spin-orbit torque (SOT) induced magnetisation switching in CoFeB/Ta/CoFeB trilayer with two CoFeB layers exhibiting in-plane magnetic anisotropy (IPMA) and perpendicular magnetic anisotropy (PMA) is investigated. Interlayer exchange coupling (IEC), measured using ferromagnetic resonance technique is modified by varying thickness of Ta spacer. The evolution of the IEC leads to different orientation of the magnetic anisotropy axes of two CoFeB layers: for thicker Ta layer where magnetisation prefers antiferromagnetic ordering and for thinner Ta layer where ferromagnetic coupling exists. Magnetisation state of the CoFeB layer exhibiting PMA is controlled by the spin-polarized current originating from SOT in $μm$ sized Hall bars. The evolution of the critical SOT current density with Ta thickness is presented, showing an increase with decreasing $t_\mathrm{Ta}$, which coincides with the coercive field dependence. In a narrow range of $t_\mathrm{Ta}$ corresponding to the ferromagnetic IEC, the field-free SOT-induced switching is achieved.

physics.app-ph↗

arXiv2019

Microwave magnetic field modulation of spin torque oscillator based on perpendicular magnetic tunnel junctions

Modulation of a spin-torque oscillator (STO) signal based on a magnetic tunnel junction (MTJ) with perpendicularly magnetized free layer is investigated. Magnetic field inductive loop was created during MTJ fabrication process, which enables microwave field application during STO operation. The frequency modulation by the microwave magnetic field of up to 3 GHz is explored, showing a potential for application in high-data-rate communication technologies. Moreover, an inductive loop is used for self-synchronization of the STO signal, which after field-locking exhibits significant improvement of the linewidth and oscillation power.

physics.app-ph↗