arXiv subjects

Se Kwon Kim

Publications and source records attributed to Se Kwon Kim.

At least 73 records · Page 4Linked to original sources

arXiv2021

Orbital angular momentum and current-induced motion of a Skyrmion-textured domain wall in a ferromagnetic nanotube

We theoretically study the current-induced dynamics of a domain wall in a ferromagnetic nanotube by developing a theory for the orbital angular momentum of a domain wall and the current-induced torque on it. Specifically, a domain wall with nontrivial magnetization winding along the circumference is shown to possess finite orbital angular momentum, which is proportional to the product of its Skyrmion charge and position, and the current is shown to exert a torque changing the orbital angular momentum of the domain wall and thereby drives it. The current-induced torque is interpreted as the transfer of orbital angular momentum from electrons to the domain wall, which occurs due to the emergent magnetic field associated with the Skyrmion charge. Our results reveal a hitherto unrecognized utility of the orbital degree of freedom of magnetic solitons.

cond-mat.mes-hall↗

arXiv2020

Quantum hydrodynamics of spin winding

An easy-plane spin winding in a quantum spin chain can be treated as a transport quantity, which propagates along the chain but has a finite lifetime due to phase slips. In a hydrodynamic formulation for the winding dynamics, the quantum continuity equation acquires a source term due to the transverse vorticity flow. The latter reflects the phase slips and generally compromises the global conservation law. A linear-response formalism for the nonlocal winding transport then reduces to a Kubo response for the winding flow along the spin chain, in conjunction with the parasitic vorticity flow transverse to it. One-dimensional topological hydrodynamics can be recovered when the vorticity flow is asymptotically small. Starting with a microscopic spin-chain formulation, we focus on the asymptotic behavior of the winding transport based on the renormalized sine-Gordon equation, incorporating phase slips as well as Gilbert damping. A generic electrical device is proposed to manifest this physics. We thus suggest winding conductivity as a tangible concept that can characterize low-energy dynamics in a broad class of quantum magnets.

cond-mat.mes-hall↗

arXiv2020

Transport signature of the magnetic Berezinskii-Kosterlitz-Thouless transition

Motivated by recent experimental progress in 2D magnetism, we theoretically study spin transport in 2D easy-plane magnets at finite temperatures across the Berezinskii-Kosterlitz-Thouless (BKT) phase transition, by developing a duality mapping to the 2+1D electromagnetism with the full account of spin's finite lifetime. In particular, we find that the non-conservation of spin gives rise to a distinct signature across the BKT transition, with the spin current decaying with distance power-law (exponentially) below (above) the transition; this is detectable in the proposed experiment with NiPS$_3$ and CrCl$_3$.

cond-mat.mes-hall↗

arXiv2020

SU(3) Topology of Magnon-Phonon Hybridization in 2D Antiferromagnets

Magnon-phonon hybrid excitations are studied theoretically in a two-dimensional antiferromagnet with an easy axis normal to the plane. We show that two magnon bands and one phonon band are intertwined by the magnetoelastic coupling through a nontrivial SU(3) topology, which can be intuitively perceived by identifying a skyrmion structure in the momentum space. We develop a continuum theory as the long-wavelength approximation to the tight-binding model, showing our results are insensitive to lattice details and generally applicable to two-dimensional antiferromagnets. The theoretical results can be probed by measuring the thermal Hall conductance as a function of the temperature and the magnetic field. We envision that the magnetoelastic coupling in antiferromagnets can be a promising venue in search of various topological excitations, which cannot be found in magnetic or elastic models alone.

cond-mat.mes-hall↗

arXiv2020

Driving a magnetized domain wall in an antiferromagnet by magnons

We theoretically study the interaction of magnons, quanta of spin waves, and a domain wall in a one-dimensional easy-axis antiferromagnet in the presence of an external magnetic field applied along the easy axis. To this end, we begin by obtaining the exact solution for spin waves in the background of a domain wall magnetized by an external field. The finite magnetization inside the domain wall is shown to give rise to reflection of magnons scattering off the domain wall, deviating from the well-known result of reflection-free magnons in the absence of a magnetic field. For practical applications of the predicted reflection of magnons, we show that the magnon reflection contributes to the thermally-driven domain-wall motion. Our work leads us to envision that inducing a finite magnetization in antiferromagnetic solitons such as vortices and skyrmions can be used to engender phenomena that do not occur in the absence of magnetization.

cond-mat.mes-hall↗

arXiv2020

The dynamics of a domain wall in ferrimagnets driven by spin-transfer torque

The spin-transfer-torque-driven (STT-driven) dynamics of a domain wall in an easy-axis rare-earth transition-metal ferrimagnet is investigated theoretically and numerically in the vicinity of the angular momentum compensation point $T_A$, where the net spin density vanishes. The particular focus is given on the unusual interaction of the antiferromagnetic dynamics of a ferrimagnetic domain wall and the adiabatic component of STT, which is absent in antiferromagnets but exists in the ferrimagnets due to the dominant coupling of conduction electrons to transition-metal spins. Specifically, we first show that the STT-induced domain-wall velocity changes its sign across $T_A$ due to the sign change of the net spin density, giving rise to a phenomenon unique to ferrimagnets that can be used to characterize $T_A$ electrically. It is also shown that the frequency of the STT-induced domain-wall precession exhibits its maximum at $T_A$ and it can approach the spin-wave gap at sufficiently high currents. Lastly, we report a numerical observation that, as the current density increases, the domain-wall velocity starts to deviate from the linear-response result, calling for a more comprehensive theory for the domain-wall dynamics in ferrimagnets driven by a strong current.

cond-mat.mes-hall↗

arXiv2020

Tuning entanglement by squeezing magnons in anisotropic magnets

We theoretically study the entanglement between two arbitrary spins in a magnetic material, where magnons naturally form a general squeezed coherent state, in the presence of an applied magnetic field and axial anisotropies. Employing concurrence as a measure of entanglement, we demonstrate that spins are generally entangled in thermodynamic equilibrium, with the amount of entanglement controlled by the external fields and anisotropies. As a result, the magnetic medium can serve as a resource to store and process quantum information. We, furthermore, show that the entanglement can jump discontinuously when decreasing the applied magnetic field. This tunable entanglement can be potentially used as an efficient switch in quantum-information processing tasks.

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↗

arXiv2019

Compensated magnetic insulators for extremely fast spin-orbitronics

The fast spin dynamics provide many opportunities for the future communication and memory technologies. One of the most promising examples is the domain wall (DW) racetrack memory. To achieve fast device performances, the high-speed motion of DWs is naturally demanded that leaves antiferromagnets (AFMs) and compensated ferrimagnets (FIMs) as the promising materials. While controlling and probing the dynamics of DWs in AFMs remains challenging, the fast motion of DWs with velocities around 1500 m/s has been demonstrated in metallic FIMs. The velocity of DWs in metallic FIMs is, however, suppressed by the magnetic damping of conduction electrons, which motivates us to explore how fast DWs can move in insulating FIMs where the conduction electron is absent. In this work, through synthesizing compensated FIM insulator Gd3Fe5O12 thin films with a perpendicular magnetic anisotropy, we demonstrate that the spin-orbit torque (SOT) induced motion of DWs along the Gd3Fe5O12/Pt racetrack can approach 6000 m/s. Our results show that the exceptionally fast motion of DWs can be achieved in compensated FIM insulators owing to small damping inherent to magnetic insulators, which could potentially facilitate the emerging ultrahigh-speed spintronic logics and racetrack memories by introducing insulating compensated FIMs as a new material platform.

cond-mat.mtrl-sci↗

arXiv2019

Electrical manipulation of spin pumping signal through nonlocal thermal magnon transport

We study the magnon transport in the nonlocal configuration composed of two Pt strips on top of yttrium iron garnet, with and without the presence of RF microwave generated by an on-chip antenna. We find that the spin-Hall induced thermal magnon heating/cooling, the Oersted field as well as the Joule heating generated by the a.c. current in the Pt injector can significantly influence the spin-pumping signal measured by the Pt detector in the presence of RF microwave, forcing the spin-pumping voltage to show up in the first and second harmonic signals in the nonlocal magnon transport measurement. These results indicate that nonlocal magnon transport configuration can serve as a structure to electrically detect and manipulate the spin-pumping signal. Furthermore, certain caution is needed when studying the interplay between incoherent magnon and coherent magnon spin transport in the nonlocal transport configuration, since the change in microwave-induced spin-pumping voltage can overwhelm the incoherent magnon transport signals.

physics.app-ph↗

arXiv2019

Magnetic field control of topological magnon-polaron bands in two-dimensional ferromagnets

We theoretically study magnon-phonon hybrid excitations in a square lattice ferromagnet subjected to a magnetic field by varying the field direction. The bulk bands of hybrid excitations, which are referred to as magnon-polarons, are investigated by considering all three phonon modes: vertical phonon, transverse phonon, and longitudinal phonon. We show that the topological proprieties of three hybridizations are different in terms of the Berry curvature and the Chern numbers. We also find that the topological properties of the bands can be controlled by changing the direction of the magnetic field, exhibiting one or more topological phase transitions. The dependence of thermal Hall conductivity as a function of magnetic field direction is proposed as an experiment probe of our theoretical results.

cond-mat.str-el↗

arXiv2019

Topological waveguide based on metamaterials of magnetic solitons

We theoretically investigate coupled gyration modes of magnetic solitons whose distances to the nearest neighbors are staggered. In a one-dimensional bipartite lattice, analogous to the Su-Schrieffer-Heeger model, there is a mid-gap gyration mode bounded at the domain wall connecting topologically distinct two phases. As a technological application, we show that a one-dimensional domain-wall string in a two-dimensional soliton lattice can serve as a topological waveguide of magnetic excitations, which offers functionalities of a signal localization and a selective propagation of the frequency modes. Our result offers an alternative way to control of the magnetic excitation modes by using a magnetic metamaterial for future spintronic devices.

cond-mat.mes-hall↗

arXiv2019

Magnetic Domain Wall Motion due to AC Bias-Driven Resonances

Most of the existing researches on the dynamics of a domain wall (DW) have focused on the effect of DC biases, where the induced velocity is determined by the bias strength. Here we show that AC biases such as a field or a current are also able to move a DW via synchronization between the DW angle and the phase of the AC bias. The resulting DW velocity is proportional to the driving frequency of the AC bias, but independent of the bias strength, offering potentially low-power operations of DW devices. The AC-bias-driven DW motion is shown to exhibit a phase locking-unlocking transition, a critical phenomenon akin to the Walker breakdown of a DC-bias-driven DW motion. Our work shows that a DW can be driven resonantly by synchronizing its angle to AC biases, shedding a light on hitherto overlooked utility of internal degree of freedom for driving magnetic textures.

cond-mat.mes-hall↗

arXiv2019

Laser control of magnonic topological phases in antiferromagnets

We study the laser control of magnon topological phases induced by the Aharonov-Casher effect in insulating antiferromagnets (AFs). Since the laser electric field can be considered as a time-periodic perturbation, we apply the Floquet theory and perform the inverse frequency expansion by focusing on the high frequency region. Using the obtained effective Floquet Hamiltonian, we study nonequilibrium magnon dynamics away from the adiabatic limit and its effect on topological phenomena. We show that a linearly polarized laser can generate helical edge magnon states and induce the magnonic spin Nernst effect, whereas a circularly polarized laser can generate chiral edge magnon states and induce the magnonic thermal Hall effect. In particular, in the latter, we find that the direction of the magnon chiral edge modes and the resulting thermal Hall effect can be controlled by the chirality of the circularly polarized laser through the change from the left-circular to the right-circular polarization. Our results thus provide a handle to control and design magnon topological properties in the insulating AF.

cond-mat.mes-hall↗

arXiv2019

Temperature dependence of magnetic resonance in ferrimagnetic GdFeCo alloys

We provide a macroscopic theory and experimental results for magnetic resonances of antiferromagnetically-coupled ferrimagnets. Our theory, which interpolates the dynamics of antiferromagnets and ferromagnets smoothly, can describe ferrimagnetic resonances across the angular momentum compensation point. We also present experimental results for spin-torque induced ferrimagnetic resonance at several temperatures. The spectral analysis based on our theory reveals that the Gilbert damping parameter, which has been considered to be strongly temperature dependent, is insensitive to temperature. We envision that our work will facilitate further investigation of ferrimagnetic dynamics by providing a theoretical framework suitable for a broad range of temperatures.

cond-mat.mtrl-sci↗

arXiv2019

Topological Magnon-Phonon Hybrid Excitations in Two-Dimensional Ferromagnets with Tunable Chern Numbers

We theoretically investigate magnon-phonon hybrid excitations in two-dimensional ferromagnets. The bulk bands of hybrid excitations, which are referred to as magnon-polarons, are analytically shown to be topologically nontrivial, possessing finite Chern numbers. We also show that the Chern numbers of magnon-polaron bands and the number of band-crossing lines can be manipulated by an external magnetic field. For experiments, we propose to use the thermal Hall conductivity as a probe of the finite Berry curvatures of magnon-polarons. Our results show that a simple ferromagnet on a square lattice supports topologically nontrivial magnon-polarons, generalizing topological excitations in conventional magnetic systems.

cond-mat.str-el↗

arXiv2019

The dynamics of bimeron skyrmions in easy-plane magnets induced by a spin supercurrent

We theoretically study the interaction of an isolated bimeron skyrmion in quasi-two-dimensional easy-plane magnets with a surrounding spin superfluid associated with spontaneously broken U(1) spin-rotational symmetry, revealing that skyrmion energy depends on the local spin current flowing in its background. The finding leads us to propose to manipulate a skyrmion energy landscape via a spin supercurrent, which can be controlled non-locally by varying the magnitudes of spin-current injection and ejection through the boundaries. Two exemplary cases are discussed: a steady-state motion of a skyrmion induced by a uniform force and a skyrmion motion localized along a one-dimensional racetrack. We envision that a skyrmion interacting with a spin superfluid can serve as a robust point-like information carrier that can be operated with minimal dissipation.

cond-mat.mes-hall↗

arXiv2019

Bidirectional spin-wave-driven domain wall motion in antiferromagnetically coupled ferrimagnets

We investigate ferrimagnetic domain wall dynamics induced by circularly polarized spin waves theoretically and numerically. We find that the direction of domain wall motion depends on both the circular polarization of spin waves and the sign of net spin density of ferrimagnet. Below the angular momentum compensation point, left- (right-) circularly polarized spin waves push a domain wall towards (away from) the spin-wave source. Above the angular momentum compensation point, on the other hand, the direction of domain wall motion is reversed. This bidirectional motion originates from the fact that the sign of spin-wave-induced magnonic torque depends on the circular polarization and the subsequent response of the domain wall to the magnonic torque is governed by the net spin density. Our finding provides a way to utilize a spin wave as a versatile driving force for bidirectional domain wall motion.

cond-mat.mtrl-sci↗