arXiv subjects

Hee-Sung Han

Publications and source records attributed to Hee-Sung Han.

5 recordsLinked to original sources

arXiv2024

Skyrmion Emergence via Domain Wall Anchoring through Vertical Bloch Line

Skyrmions, topologically stable magnetic solitons characterized by whirling magnetization in nanoscale magnetic elements, show promise information carriers in spintronics and spin-based quantum computing due to their unique properties: small size, stability, and controllability. In this study, we introduce a novel method of skyrmion generation through domain wall deformation dynamics. Our analytical and micromagnetic simulations demonstrate that domain wall motion exceeding the Walker threshold induces topological deformation of magnetic domain walls exhibiting Dzyaloshinskii-Moriya interaction. This deformation process catalyzes the emergence of skyrmions from magnetic domain wall structure distortion, specifically through the Anchoring of domain walls due to the vertical Bloch line. We elucidate the underlying mechanism of skyrmion generation, correlating it with topological transitions accompanied by burst energy dissipation through spin-wave radiation. Notably, we present robust skyrmion generation conditions through a comprehensive classification of domain wall distortion, including vertical Bloch line generation and annihilation in magnetic domain wall dynamics within a DMI system. These findings provide noble insights into topological behaviors of spin structures and offer a potential pathway for efficient, controlled skyrmion creation in the next-generation spintronic devices.

cond-mat.mes-hall↗

arXiv2023

Beyond Walker Breakdown through the Resonant Dissipation: Dramatic Enhancement of Magnetic Domain Wall Velocity via Resonant Excitation of Standing Wave Modes of Domain Wall Structure

The dynamic behaviors of magnetic domain walls have significant implications for developing advanced spintronic devices. In this study, we investigate the intriguing resonance phenomenon within the magnetic domain wall structure and its profound influence on dynamic motion, focusing on the dissipation mechanism. By applying a static external magnetic field, we observe a remarkable amplification of domain wall velocity, surpassing the limitations of the conventional one-dimensional model. To quantify this enhancement, we introduce a novel parameter, the distortion variation rate, which captures the rapid and pronounced changes occurring within the domain wall structure. Through comprehensive micromagnetic simulations, we establish a robust relationship between speed and distortion variation rate, thereby validating our theoretical framework. Our findings provide crucial insights into the underlying mechanisms governing domain wall dynamics while paving the way for developing and optimizing next-generation spintronic devices boasting unparalleled speed and efficiency.

cond-mat.mtrl-sci↗

arXiv2020

Thermal generation, manipulation and detection of skyrmions

Recent years have witnessed significant progresses in realizing skyrmions in chiral magnets1-4 and asymmetric magnetic multilayers5-13, as well as their electrical manipulation2,7,8,10. Equally important, thermal generation, manipulation and detection of skyrmions can be exploited for prototypical new architecture with integrated computation14 and energy harvesting15. It has yet to verify if skyrmions can be purely generated by heating16,17, and if their resultant direction of motion driven by temperature gradients follows the diffusion or, oppositely, the magnonic spin torque17-21. Here, we address these important issues in microstructured devices made of multilayers: (Ta_CoFeB_MgO)15, (Pt_CoFeB_MgO_Ta)15 and (Pt_Co_Ta)15 integrated with on-chip heaters, by using a full-field soft X-ray microscopy. The thermal generation of densely packed skyrmions is attributed to the low energy barrier at the device edge, together with the thermally induced morphological transition from stripe domains to skyrmions. The unidirectional diffusion of skyrmions from the hot region towards the cold region is experimentally observed. It can be theoretically explained by the combined contribution from repulsive forces between skyrmions, and thermal spin-orbit torques in competing with magnonic spin torques17,18,20,21 and entropic forces22. These thermally generated skyrmions can be further electrically detected by measuring the accompanied anomalous Nernst voltages23. The on-chip thermoelectric generation, manipulation and detection of skyrmions could open another exciting avenue for enabling skyrmionics, and promote interdisciplinary studies among spin caloritronics15, magnonics24 and skyrmionics3,4,12.

cond-mat.mes-hall↗

arXiv2019

Direct demonstration of topological stability of magnetic skyrmions via topology manipulation

Topological protection precludes a continuous deformation between topologically inequivalent configurations in a continuum. Motivated by this concept, magnetic skyrmions, topologically nontrivial spin textures, are expected to exhibit the topological stability, thereby offering a prospect as a nanometer-scale non-volatile information carrier. In real materials, however, atomic spins are configured as not continuous but discrete distribution, which raises a fundamental question if the topological stability is indeed preserved for real magnetic skyrmions. Answering this question necessitates a direct comparison between topologically nontrivial and trivial spin textures, but the direct comparison in one sample under the same magnetic fields has been challenging. Here we report how to selectively achieve either a skyrmion state or a topologically trivial bubble state in a single specimen and thereby show how robust the skyrmion structure is in comparison with the bubbles for the first time. We demonstrate that topologically nontrivial magnetic skyrmions show longer lifetimes than trivial bubble structures, evidencing the topological stability in a real discrete system. Our work corroborates the physical importance of the topology in the magnetic materials, which has hitherto been suggested by mathematical arguments, providing an important step towards ever-dense and more-stable magnetic devices.

cond-mat.mtrl-sci↗

arXiv2017

Spin-orbit torque-driven skyrmion dynamics revealed by time-resolved X-ray microscopy

Magnetic skyrmions are topologically-protected spin textures with attractive properties suitable for high-density and low-power spintronic device applications. Much effort has been dedicated to understanding the dynamical behaviours of the magnetic skyrmions. However, experimental observation of the ultrafast dynamics of this chiral magnetic texture in real space, which is the hallmark of its quasiparticle nature, has so far remained elusive. Here, we report nanosecond-dynamics of a 100 nm-size magnetic skyrmion during a current pulse application, using a time-resolved pump-probe soft X-ray imaging technique. We demonstrate that distinct dynamic excitation states of magnetic skyrmions, triggered by current-induced spin-orbit torques, can be reliably tuned by changing the magnitude of spin-orbit torques. Our findings show that the dynamics of magnetic skyrmions can be controlled by the spin-orbit torque on the nanosecond time scale, which points to exciting opportunities for ultrafast and novel skyrmionic applications in the future.

cond-mat.mtrl-sci↗