arXiv subjects

Sang-Koog Kim

Publications and source records attributed to Sang-Koog Kim.

36 records · Page 2Linked to original sources

arXiv2009

Quantitative understanding of magnetic vortex oscillations driven by spin-polarized out-of-plane dc current: Analytical and micromagnetic numerical study

We studied magnetic vortex oscillations associated with vortex gyrotropic motion driven by spin-polarized out-of-plane dc current by analytical and micromagnetic numerical calculations. Reliable controls of the tunable eigenfrequency and orbital amplitude of persistent vortex oscillations were demonstrated. This work provides an advanced step towards the practical application of vortex oscillations to persistent vortex oscillators in a wide frequency (f) range of 10 to 2000 MHz and with high values of f/(delta f).

cond-mat.mtrl-sci↗

arXiv2009

Physical Origin and Generic Control of Magnonic Band Gaps of Dipole-Exchange Spin Waves in Width-Modulated-Nanostrip Waveguides

We report, for the first time, on a novel planar structure of magnonic-crystal waveguides, made of a single magnetic material, in which the allowed and forbidden bands of propagating dipole-exchange spin-waves can be manipulated by the periodic modulation of different widths in thin-film nanostrips. The origin of the presence of several magnonic wide band gaps and the crucial parameters for controlling those band gaps of the order of ~10 GHz are found by micromagnetic numerical and analytical calculations. This work can offer a route to the potential application to broad-band spin-wave filters in the GHz frequency range.

cond-mat.mtrl-sci↗

arXiv2009

A spin-wave frequency doubler by domain wall oscillation

We present a new mechanism for spin-wave excitation using a pinned domain wall which is forced to oscillate at its eigenfrequency and radiates spin waves. The domain wall acts as a frequency doubler, as the excited spin waves have twice the frequency of the domain wall oscillation. The investigations have been carried out using micromagnetic simulations and enable the determination of the main characteristics of the excited spin-waves such as frequency, wavelength, and velocity. This behavior is understood by the oscillation in the perpendicular magnetization which shows two anti-nodes oscillating out of phase with respect to each other.

cond-mat.mtrl-sci↗

arXiv2009

Refractive index and Snell's Law for Dipolar-Exchange Spin-Waves in a Confined Planar Structure

We derived the analytical forms of refractive index and Snell law for dipolar-exchange spin waves of reflection and refraction at a magnetically heterogeneous interface in a geometrically confined planer structure composed of different magnetic thin films from a microscopic scattering approach. A novel behavior, the optical total reflection, was demonstrated for spin waves with a specific interface between Yttrium iron garnet (Y3Fe5O12) and Permalloy (Ni80Fe20), by analytical and micromagnetic numerical calculations.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic domain-wall motion by propagating spin waves

We found by micromagnetic simulations that the motion of a transverse wall (TW) type domain wall in magnetic thin-film nanostripes can be manipulated via interaction with spin waves (SWs) propagating through the TW. The velocity of the TW motion can be controlled by changes of the frequency and amplitude of the propagating SWs. Moreover, the TW motion is efficiently driven by specific SW frequencies that coincide with the resonant frequencies of the local modes existing inside the TW structure. The use of propagating SWs, whose frequencies are tuned to those of the intrinsic TW modes, is an alternative approach for controlling TW motion in nanostripes.

cond-mat.mtrl-sci↗

arXiv2008

Vortex dynamics in confined stratified conditions

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions.

cond-mat.stat-mech↗

arXiv2008

Criterion for transformation of transverse domain wall to vortex or antivortex wall in soft magnetic thin-film nanostripes

We report on the criterion for the dynamic transformation of the internal structure of moving domain walls (DWs) in soft magnetic thin-film nanostripes above the Walker threshold field, Hw. In order for the process of transformation from transverse wall (TW) to vortex wall (VW) or antivortex wall (AVW) occurs, the edge-soliton core of the TW-type DW should grow sufficiently to the full width at half maximum of the out-of-plane magnetizations of the core area of the stabilized vortex (or antivortex) by moving inward along the transverse (width) direction. Upon completion of the nucleation of the vortex (antivortex) core, the VW (AVW) is stabilized, and then its core accompanies the gyrotropic motion in a potential well (hill) of a given nanostripe. Field strengths exceeding the Hw, which is the onset field of DW velocity breakdown, are not sufficient but necessary conditions for dynamic DW transformation.

cond-mat.mtrl-sci↗

arXiv2008

Layer-by-layer growth and growth-mode transition of SrRuO3 thin films on atomically flat single-terminated SrTiO3 (111) surfaces

We report on growth-mode transitions in the growth of SrRuO3 thin films on atomically flat Ti4+ single-terminated SrTiO3 (111) substrates, investigated by reflection high-energy electron diffraction and atomic force microscopy. Over the first ~9 unit cells, the dominant growth mode changes from island to layer-by-layer for the growth rate of 0.074 unit cells/sec and the growth temperature of 700 C. Moreover, in the course of growing SrRuO3 films, the governing growth mode of interest can be manipulated by changing the growth temperature and the growth rate, which change allows for the selection of the desired layer-by-layer mode. The present study thus paves the way for integrations of SrRuO3 thin layers into (111)-orientated oxide heterostructures, and hence multi-functional devices, requiring control of the sharp atomic-level interfaces and the layer-by-layer growth mode.

cond-mat.mtrl-sci↗

arXiv2008

Universal criterion and phase diagram for switching a magnetic vortex core in soft magnetic nanodots

The universal criterion for ultrafast vortex-core switching between the up- and down-core bistates in soft magnetic nanodots was investigated by micromagnetic simulations along with analytical calculations. Vortex-core switching occurs whenever the velocity of vortex-core motion reaches the critical velocity that is expressed as (e.g. m/s for Permalloy), where Aex is the exchange stiffness, and is the gyromagnetic ratio. On the basis of the above results, phase diagrams for the vortex-core switching event and switching times with respect to both the amplitude and frequency of applied circularly rotating magnetic field were calculated, which offer practical guidance for implementing nanodots in vortex states into future solid-state information-storage devices.

cond-mat.mtrl-sci↗

arXiv2008

Dynamic origin of azimuthal modes splitting in vortex-state magnetic dots

A spin wave theory explaining experimentally observed frequency splitting of dynamical excitations with azimuthal symmetry of a magnetic dot in a vortex ground state is developed. It is shown that this splitting is a result of the dipolar hybridization of three spin wave modes of a dot having azimuthal indices |m|=1: two high-frequency azimuthal excitation modes of the in-plane part of the vortex with indices m = +/-1 and a low-frequency m= +1 gyrotropic mode describing the translational motion of the vortex core. The analytically calculated magnitude of the frequency splitting is proportional to the ratio of the dot thickness to its radius and quantitatively agrees with the results of time resolved Kerr experiments.

cond-mat.mtrl-sci↗

arXiv2008

Origin of the increased velocities of domain wall motions in soft magnetic thin-film nanostripes beyond the velocity-breakdown regime

It is known that oscillatory domain-wall (DW) motions in soft magnetic thin-film nanostripes above the Walker critical field lead to a remarkable reduction in the average DW velocities. In a much-higher-field region beyond the velocity-breakdown regime, however, the DW velocities have been found to increase in response to a further increase of the applied field. We report on the physical origin and detailed mechanism of this unexpected behavior. We associate the mechanism with the serial dynamic processes of the nucleation of vortex-antivortex (V-AV) pairs inside the stripe or at its edges, the non-linear gyrotropic motions of Vs and AVs, and their annihilation process. The present results imply that a two-dimensional soliton model is required for adequate interpretation of DW motions in the linear- and oscillatory-DW-motion regimes as well as in the beyond-velocity-breakdown regime.

cond-mat.mtrl-sci↗

arXiv2008

Two Circular-Rotational Eigenmodes in Vortex Gyrotropic Motions in Soft Magnetic Nanodots

We found, by micromagnetic numerical and analytical calculations, that the clockwise (CW) and counterclockwise (CCW) circular-rotational motions of a magnetic vortex core in a soft magnetic circular nanodot are the elementary eigenmodes existing in the gyrotropic motion with respect to the corresponding CW and CCW circular-rotational-field eigenbasis. Any steady-state vortex gyrotropic motions driven by a linearly polarized oscillating in-plane magnetic field in the linear regime can be perfectly understood according to the superposition of the two circular eigenmodes, which show asymmetric resonance characteristics reflecting the vortex polarization. The relative magnitudes in the amplitude and phase between the CCW and CW eigenmodes determine the elongation and orientation of the orbital trajectories of the vortex core motions, respectively, which trajectories vary with the polarization and chirality of the given vortex as well as the field frequency across the resonance frequency.

cond-mat.mtrl-sci↗

arXiv2007

Dynamics of magnetic topological solitons in soft magnetic nanostripes

The motions of domain walls driven by magnetic field in soft magnetic nanostripes were calculated. The domain walls reveal steady motions in the low fields and oscillations of their internal structure above a critical field. A developed soliton model of the walls explains its dynamics by the motions of a limited number of magnetic topological solitons such as vortex and antivortex. We predict the reduced wall velocity and critical field in the low-field regime, and increased wall-oscillation frequency in nanostripes, compared to 1D Walker solution for bulk magnets. The critical field and velocity are determined by the nanostripe sizes, whereas the oscillation frequency depends only on the field strength.

nlin.PS↗

arXiv2007

Total Reflection and Negative Refraction of Dipole-Exchange Spin Waves at Magnetic Interfaces: Micromagnetic Modeling Study

We demonstrated that dipole-exchange spin waves traveling in geometrically restricted magnetic thin films satisfy the same laws of reflection and refraction as light waves. Moreover, we found for the first time novel wave behaviors of dipole-exchange spin waves such as total reflection and negative refraction. The total reflection in laterally inhomogeneous thin films composed of two different magnetic materials is associated with the forbidden modes of refracted dipole-exchange spin waves. The negative refraction occurs at a 90 degree domain-wall magnetic interface that is introduced by a cubic magnetic anisotropy in the media, through the anisotropic dispersion of dipole-exchange spin waves.

cond-mat.other↗

arXiv2007

Dynamic Origin of Vortex Core Switching in Soft Magnetic Nanodots

The magnetic vortex with the in-plane curling magnetization and the out-of-plane magnetization at the core is a unique ground state in nanoscale magnetic elements. This kind of magnetic vortex can be used as a memory unit for information storage, through its downward or upward core-orientation and, thus, controllable core switching deserves some special attention. Our analytical and micromagnetic calculations reveal that the origin of the vortex core reversal is a gyrotropic field. This field is induced by vortex dynamic motion and is proportional to the velocity of the moving vortex. Our calculations elucidate the physical origin of the vortex core dynamic reversal and offer a key to effective manipulation of the vortex-core orientation.

cond-mat.mtrl-sci↗

arXiv2007

Soft X-ray Circular Reflectivity from Ferromagnetic Transition-Metal Films Near the Brewster's Angle: Theoretical and Numerical X-ray Resonant Magnetic Scattering Study

We first report a novel phenomenon that manifests itself in a colossal difference in soft x-ray reflectivity from ferromagnetic transition-metal films between the left- and right-handed circular polarization (LCP and RCP) modes at a resonance near normal Brewster's angle. Theoretical and numerical studies of sft x-ray resonant magnetic scattering using the circular-polarization-mode basis reveal that this effect arises from a totally destructive interference of photons scattered individually from chargem orbital, and spin degrees of freedom in magnetized thin films that selectively occurs only for one helicity of the opposite circular modes when the required criteria are fulfilled. Across the normal Brewster's angle. the polarization state of scattered soft x-ray is continuously variable from the RCP to the LCP mode (or vice versa) through the linear s polarization mode by changing the incidence angle of linear p-polarized x rays at the resonance.

cond-mat.other↗

arXiv2007

Understanding of complex periodic transformations of moving domain walls in magnetic nanostripes

The magnetic field (or electric current) driven domain-wall motion in magnetic nanostripes is of considerable interest because it is essential to the performance of information storage and logic devices. One of the currently key problems is to unveil the complex behaviors of oscillatory domain-wall motions under applied magnetic fields stronger than the so-called Walker field, beyond which the velocity of domain walls markedly drops. Here, we provide not only considerably better understandings but also new details of the complex domain-wall motions. In a certain range just above the Walker field, the motions are not chaotic but rather periodic with different unique periodicities of dynamic transformations of a moving domain wall between the different types of its internal structure. Three unique periodicities found, which consist of different types of domain wall that are transformed from type one to another. The transformation periods vary with the field strength and the nanostripe width. This novel phenomenon can be described by the dynamic motion of a limited number of magnetic topological solitons such as vortex and antivortex in nanostripes.

cond-mat.other↗

arXiv2007

Ultrafast Vortex-Core Reversal Dynamics in Ferromagnetic Nanodots

To verify the exact underlying mechanism of ultrafast vortex-core reversal as well as the vortex state stability we conducted numerical calculations of the dynamic evolution of magnetic vortices in Permalloy cylindrical nanodots under an oscillating in-plane magnetic field over a wide range of the frequency and amplitude. The calculated results reveal different kinds of the non-trivial dynamic responses of vortices to the driving external field. In particular, the results offer insight into the 10 ps scale underlying physics of the ulrafast vortex-core reversals driven by small amplitude (~10 Oe) in-plane fields. This work also provides fundamentals of how to manipulate effectively the dynamical switching of the vortex-core orientation.

cond-mat.mtrl-sci↗