arXiv subjects

Myoung-Woo Yoo

Publications and source records attributed to Myoung-Woo Yoo.

14 recordsLinked to original sources

arXiv2026

Micromagnetic simulations for magnetic multipoles

Cluster magnetic multipoles are order parameters that characterize the symmetry of spin arrangements in magnetic materials. In particular, high-order multipoles play a pivotal role in altermagnets and non-collinear antiferromagnets where they govern electrical and optical phenomena. While spatially non-uniform multipole textures have been observed on the micrometer scale, their behavior at mesoscopic lengths remains largely unexplored. Here, we introduce a comprehensive micromagnetic framework for vector-like cluster magnetic multipoles, enabling quantitative, spatially resolved analysis of non-uniform multipole systems. As a demonstration, we apply the framework to magnetic-octupole domain-wall motion in the non-collinear antiferromagnet $\text{Mn}_3\text{Sn}$. Our simulations capture key features of domain-wall dynamics, including profile deformation and the emergence of an effective inertial mass. This work provides a unified approach for investigating the mesoscopic dynamics of high-order cluster multipoles, and opens new avenues for understanding and engineering the physical properties of functional magnetic materials, such as altermagnets and non-collinear antiferromagnets, for advanced spintronic technologies.

cond-mat.mes-hall↗

arXiv2024

Thermal contribution to current-driven antiferromagnetic-order switching

In information technology devices, current-driven state switching is crucial in various disciplines including spintronics, where the contribution of heating to the switching mechanism plays an inevitable role. Recently, current-driven antiferromagnetic order switching has attracted considerable attention due to its implications for next-generation spintronic devices. Although the switching mechanisms can be explained by spin dynamics induced by spin torques, some reports have claimed that demagnetization above the Neel temperature due to Joule heating is critical for switching. Here we present a systematic method and an analytical model to quantify the thermal contribution due to Joule heating in micro-electronic devices, focusing on current-driven octupole switching in the non-collinear antiferromagnet, Mn3Sn. The results consistently show that the critical temperature for switching remains relatively constant above the Neel temperature, while the threshold current density depends on the choice of substrate and the base temperature. In addition, we provide an analytical model to calculate the Joule-heating temperature which quantitatively explains our experimental results. From numerical calculations, we illustrate the reconfiguration of magnetic orders during cooling from a demagnetized state of polycrystalline Mn3Sn. This work not only provides deeper insights into magnetization switching in antiferromagnets, but also a general guideline for evaluating the Joule-heating temperature excursions in micro-electronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Epitaxial growth and magnetic properties of kagome metal FeSn/elemental ferromagnet heterostructures

Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flat bands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high quality heterostructures is required. Here we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution X-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices.

cond-mat.mtrl-sci↗

arXiv2022

Structural and Magnetic Properties of Pt/Co/Mn-Based Multilayers

Magnetic multilayers are a rich class of materials systems with numerous highly tunable physical parameters that determine both their magnetic and electronic properties. Here we present a comprehensive experimental study of a novel system, Pt/Co/Mn, which extends the group of Pt/Co/X ($\mathrm{X}=$ metal) multilayers that have been investigated thus far. We demonstrate that an increasing Co layer thickness changes the magnetic anisotropy from out-of-plane to in-plane, whereas the deposition of thicker Mn layers leads to a decrease in the saturation magnetization. Temperature-dependent magnetometry measurements reinforce the hypothesis of antiferromagnetic coupling at the Co/Mn interfaces being responsible for the observed Mn thickness dependence of the magnetization reversal. Moreover, magneto-optical imaging experiments indicate systematic changes in magnetic domain patterns as a function of the Co and Mn layer thickness, suggesting the existence of bubble-like domains -- potentially even magnetic skyrmions -- in the case of sufficiently thick Mn layers, which are expected to contribute to a sizeable Dzyaloshinskii-Moriya interaction in the multilayer stacks. We identify Pt/Co/Mn as a highly complex multilayer system with strong potential for further fundamental studies and possible applications.

cond-mat.mtrl-sci↗

arXiv2021

Nanocontact vortex oscillators based on Co$_2$MnGe pseudo-spin valves

We present an experimental study of vortex dynamics in magnetic nanocontacts based on pseudo spin valves comprising the Co$_2$MnGe Heusler compound. The films were grown by molecular beam epitaxy, where precise stoichiometry control and tailored stacking order allowed us to define the bottom ferromagnetic layer as the reference layer, with minimal coupling between the free and reference layers. 20-nm diameter nanocontacts were fabricated using a nano-indentation technique, leading to self-sustained gyration of the vortex generated by spin-transfer torques above a certain current threshold. By combining frequency- and time-domain measurements, we show that different types of spin-transfer induced dynamics related to different modes associated to the magnetic vortex configuration can be observed, such as mode hopping, mode coexistence and mode extinction appear in addition to the usual gyration mode.

cond-mat.mes-hall↗

arXiv2020

Analytical model of the inertial dynamics of a magnetic vortex

We present an analytical model to account for the inertial dynamics of a magnetic vortex. The model is based on a deformation of the core profile based on the Döring kinetic field, whereby the deformation amplitudes are promoted to dynamical variables in a collective-coordinate approach that provides a natural extension to the Thiele model. This extended model accurately describes complex transients due to inertial effects and the variation of the effective mass with velocity. The model also provides a quantitative description of the inertial dynamics leading up to vortex core reversal, which is analogous to the Walker transition in domain wall dynamics.

cond-mat.mes-hall↗

arXiv2019

Modulation and phase-locking in nanocontact vortex oscillators

We have conducted experiments to probe how the dynamics of nanocontact vortex oscillators can be modulated by an external signal. We explore the phase-locking properties in both the commensurate and chaotic regimes, where chaos appears to impede phase-locking while a more standard behavior is seen in the commensurate phase. These different regimes correspond to how the periodicity of the vortex core reversal relates to the frequency of core gyration around the nanocontact; a commensurate phase appears when the reversal rate is an integer fraction of the gyration frequency, while a chaotic state appears when this ratio is irrational. External modulation where the power spectral density exhibits rich features, appears due to the modulation between the external source frequency, gyration frequency, and core reversal frequency. We explain these features with first- or second-order modulation between the three frequencies. Phase-locking is also visible between the external source frequency and internal vortex modes (gyration and core reversal modes).

cond-mat.mes-hall↗

arXiv2019

Chaos in Magnetic Nanocontact Vortex Oscillators

We present an experimental study of spin-torque driven vortex self-oscillations in magnetic nanocontacts. We find that above a certain threshold in applied currents, the vortex gyration around the nanocontact is modulated by relaxation oscillations, which involve periodic reversals of the vortex core. This modulation leads to the appearance of commensurate but also more interestingly here, incommensurate states, which are characterized by devil's staircases in the modulation frequency. We use frequency- and time-domain measurements together with advanced time-series analyses to provide experimental evidence of chaos in incommensurate states of vortex oscillations, in agreement with theoretical predictions.

cond-mat.mes-hall↗

arXiv2019

Pattern generation and symbolic dynamics in a nanocontact vortex oscillator

Harnessing chaos or intrinsic nonlinear behaviours from dynamical systems is a promising avenue for the development of unconventional information processing technologies. However, the exploitation of such features in spintronic devices has not been attempted despite the many theoretical and experimental evidence of nonlinear behaviour of the magnetization dynamics in nanomagnetic systems. Here, we propose a first step in that direction by unveiling and characterizing the patterns and symbolic dynamics originating from the nonlinear chaotic time-resolved electrical signals generated experimentally by a nanocontact vortex oscillator (NCVO). We use advanced filtering methods to dissociate nonlinear deterministic patterns from thermal fluctuations and show that the emergence of chaos results in the unpredictable alternation of simple oscillatory patterns controlled by the NCVO's core-polarity switching. With phase-space reconstruction techniques, we perform a symbolic analysis of the time series to assess the level of complexity and entropy generated in the chaotic regime. We find that at the centre of its incommensurate region, it can exhibit maximal entropy and complexity. This suggests that NCVOs are promising nonlinear nanoscale source of entropy that could be harnessed for information processing.

cond-mat.mes-hall↗

arXiv2017

Current-driven skyrmion expulsion from magnetic nanostrips

We study the current-driven skyrmion expulsion from magnetic nanostrips using micromagnetic simulations and analytic calculations. We explore the threshold current density for the skyrmion expulsion, and show that this threshold is determined by the critical boundary force as well as the spin-torque parameters. We also find the dependence of the critical boundary force on the magnetic parameters; the critical boundary force decreases with increasing the exchange stiffness and perpendicular anisotropy constants, while it increases with increasing Dzyaloshinskii-Moriya interaction and saturation magnetization constants. Using a simple model describing the skyrmion and locally-tilted edge magnetization, we reveal the underlying physics of the dependence of the critical boundary force on the magnetic parameters based on the relation between the scaled Dzyaloshinskii-Moriya-interaction parameter and the critical boundary force. This work provides a fundamental understanding of the skyrmion expulsion and the interaction between the skymion and boundaries of devices and shows that the stability of the skyrmion in devices can be related to the scaled Dzyaloshinskii-Moriya-interaction parameter of magnetic materials.

cond-mat.mes-hall↗

arXiv2017

Current-driven skyrmion dynamics in disordered films

A theoretical study of the current-driven dynamics of magnetic skyrmions in disordered perpendicularly-magnetized ultrathin films is presented. The disorder is simulated as a granular structure in which the local anisotropy varies randomly from grain to grain. The skyrmion velocity is computed for different disorder parameters and ensembles. Similar behavior is seen for spin-torques due to in-plane currents and the spin Hall effect, where a pinning regime can be identified at low currents with a transition towards the disorder-free case at higher currents, similar to domain wall motion in disordered films. Moreover, a current-dependent skyrmion Hall effect and fluctuations in the core radius are found, which result from the interaction with the pinning potential.

cond-mat.mtrl-sci↗

arXiv2015

Resonantly exited precession motion of three-dimensional vortex core in magnetic nanospheres

We found resonantly excited precession motions of a three-dimensional vortex core in soft magnetic nanospheres and controllable precession frequency with the sphere diameter 2R, as studied by micromagnetic numerical and analytical calculations. The precession angular frequency for an applied static field $H_{DC}$ is given as $ω_{MV}= γ_{eff} H_{DC}$, where $γ_{eff} = γ $ is the effective gyromagnetic ratio in collective vortex dynamics, with the gyromagnetic ratio $γ$ and the average magnetization component $ $ of the ground-state vortex in the core direction. Fitting to the micromagnetic simulation data for $ $ yields a simple explicit form of $ = (73.6 \pm 3.4)(l_{ex}/2R)^{2.20 \pm 0.14}$, where $l_{ex}$ is the exchange length of a given material. This dynamic behavior might serve as a foundation for potential bio-applications of size-specific resonant excitation of magnetic vortex-state nanoparticles, for example, magnetic particle resonance imaging.

cond-mat.mes-hall↗

arXiv2011

Polarization-selective vortex-core switching by orthogonal Gaussian-pulse currents

We experimentally demonstrate low-power-consumption vortex-core switching in magnetic nanodisks using tailored rotating magnetic fields that are produced with orthogonal and unipolar Gaussian-pulse currents. Optimal width of the orthogonal pulses and their time delay are found to be determined only by the angular eigenfrequency ω_D for a given vortex-state disk of its polarization p, such that σ = 1/ω_D and Δt = πp/2ω_D, as studied from analytical and micromagnetic numerical calculations. The estimated optimal pulse parameters are in good agreements with the experimentally found results. This work provides a foundation for energy-efficient information recording in vortex-core cross-point architecture.

cond-mat.mtrl-sci↗

arXiv2010

Edge-Soliton-Mediated Vortex-Core Reversal Dynamics

We report a new reversal mechanism of magnetic vortex cores in nanodot elements driven by out-of-plane currents, occurring through two coupled edge-solitons via dynamic transformations between magnetic solitons of different topological charges. This mechanism differs completely from the well known switching process mediated by the creation and annihilation of vortex-antivortex pairs in terms of the associated topological solitons, energies, and spin-wave emissions. Strongly localized out-of-plane gyrotropic fields induced by the fast motion of the two coupled edge-solitons enable a magnetization dip that plays a crucial role in the formation of the reversed core magnetization. This work provides a new physical insight into the dynamic transformations of magnetic solitons in nanoelements.

cond-mat.mtrl-sci↗