Youn-Seok Choi

Reliable low-power control of ultrafast vortex-core switching with the selectivity in an array of vortex states by in-plane circular-rotational magnetic fields and spin-polarized currents

The authors investigated the technological utility of counterclockwise (CCW) and clockwise (CW) circular-rotating fields (HCCW and HCW) and spin-polarized currents with an angular frequency ωH close to the vortex eigenfrequency ωD, for the reliable, low-power, and selective switching of the bistate magnetization (M) orientations of a vortex core (VC) in an array of soft magnetic nanoelements. CCW and CW circular gyrotropic motions in response to HCCW and HCW, respectively, show remarkably contrasting resonant behaviors, (i.e., extremely large-amplitude resonance versus small-amplitude nonresonance), depending on the M orientation of a given VC. Owing to this asymmetric resonance characteristics, the HCCW(HCW) with ωH∼ωD can be used to effectively switch only the up (down) core to its downward (upward) M orientation, selectively, by sufficiently low field (∼10Oe) and current density (∼107A∕cm2). This work provides a reliable, low power, effective means of information storage, information recording, and inf...

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 is investigated by micromagnetic simulations along with vortex-core switching that occurs whenever the velocity of vortex-core motion reaches its critical velocity, upsilon cri = (1.66 +/- 0.18) gamma mean square root of Aex (e.g., upsilon cri = 330 +/- 37 m/s for Permalloy), with the exchange stiffness Aex and the gyromagnetic ratio gamma. On the basis of the universality of upsilon cri, phase diagrams for the vortex-core switching event and switching time with respect to both the amplitude and frequency of a circularly rotating magnetic field are calculated.

Electric-current-driven vortex-core reversal in soft magnetic nanodots

The authors report on electric-current-driven vortex-core (VC) reversal (switching) and the accompanying spin-wave emission, driven by spin-polarized ac currents of different amplitudes and frequencies, investigated by micromagnetic calculations of the dynamic evolution of a magnetic vortex in Permalloy nanodots. The magnetization orientation of the VC is effectively switchable between its upward and downward bistates and controllable by applying current above its threshold density, but with sufficiently small magnitude at frequencies close to the vortex eigenfrequency. This VC reversal phenomenon occurs through the creation of a vortex-antivortex pair and the subsequent annihilation of the initial vortex and the created antivortex, when the velocity of the initial VC reaches its critical value of approximately 340±20m∕s for the given material and geometry. In the course of these serial processes and immediately after VC switching, strong spin waves are emitted. These results provide physical insights int...

Logic Operations Based on Magnetic-Vortex-State Networks

Logic operations based on coupled magnetic vortices were experimentally demonstrated. We utilized a simple chain structure consisting of three physically separated but dipolar-coupled vortex-state Permalloy disks as well as two electrodes for application of the logical inputs. We directly monitored the vortex gyrations in the middle disk, as the logical output, by time-resolved full-field soft X-ray microscopy measurements. By manipulating the relative polarization configurations of both end disks, two different logic operations are programmable: the XOR operation for the parallel polarization and the OR operation for the antiparallel polarization. This work paves the way for new-type programmable logic gates based on the coupled vortex-gyration dynamics achievable in vortex-state networks. The advantages are as follows: a low-power input signal by means of resonant vortex excitation, low-energy dissipation during signal transportation by selection of low-damping materials, and a simple patterned-array structure.

Out-of-plane current controlled switching of the fourfold degenerate state of a magnetic vortex in soft magnetic nanodots

We report on an observation of transitions of the fourfold degenerate state of a magnetic vortex in soft magnetic nanodots by micromagnetic numerical calculations. The quaternary vortex states in patterned magnetic dots were found to be controllable by changing the density of out-of-plane dc or pulse currents applied to the dots. Each vortex state can be switched to any of the other states by applying different sequence combinations of individual single-step pulse currents. Each step pulse has a characteristic threshold current density and direction. This work offers a promising way for manipulating both the polarization and chirality of magnetic vortices.

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).

Underlying mechanism 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 remarkable reductions 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 underlying mechanism of this unexpected behavior. We associate the mechanism with the serial dynamic processes of the nucleation of vortex-antivortex pairs inside the stripe or at its edges, the nonlinear gyrotropic motions of vortices and antivortices, and their annihilation process. Moreover, this work evidences that a two-dimensional soliton model is required for adequate interpretation and understanding of DW motions in the linear- and oscillatory-DW-motion regimes as well as in the beyond-velocity-breakdown regime.

Negative refraction of dipole-exchange spin waves through a magnetic twin interface in restricted geometry

A phenomenon of negative refraction of dipole-exchange spin waves (DESWs) was demonstrated by micromagnetic modeling, based on the fact that the DESWs’ dispersion is anisotropic according to the relative orientation of the DESW propagation direction with respect to the orientation of local static magnetizations. Using this anisotropic dispersion behavior, the negative refraction of the DESWs was reproduced through a magnetic twin interface in a geometrically restricted medium of cubic in-plane anisotropy. This work verifies that the negative refraction of electromagnetic waves can also occur for DESWs propagating in restricted geometry through a specially designed magnetic interface.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.

Understanding eigenfrequency shifts observed in vortex gyrotropic motions in a magnetic nanodot driven by spin-polarized out-of-plane dc current

We observed sizable eigenfrequency shifts in spin-polarized dc-current-driven vortex gyrotropic motions in a soft magnetic nanodot, and clarified the underlying physics through micromagnetic numerical calculations. It was found that the vortex eigenfrequency is changed to higher (lower) values with increasing Oersted field (OH) strength associated with the out-of-plane dc current for the vortex chirality parallel (antiparallel) to the rotation sense of the OH circumferential in-plane orientation. The eigenfrequency shift was found to be linearly proportional to the current density j0 in the linear regime as in ΔνD≃±ηj0/|G|, where G is the gyrovector constant and η is a positive constant, e.g., 1.9×10−8 erg/A for a model Permalloy dot of 300 nm diameter and 20 nm thickness. This behavior originates from the sizable contribution of the OH to the effective potential energy of a displaced vortex core in the gyrotropic motion. The present results reveal that νD, an intrinsic dynamic characteristic of a given n...

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

Polarization-selective vortex-core switching by tailored orthogonal Gaussian-pulse currents Young-Sang Yu, 1 Ki-Suk Lee, 1 Hyunsung Jung, 1 Youn-Seok Choi, 1 Myoung-Woo Yoo, 1 Dong-Soo Han, 1 Mi-Young Im, 2 Peter Fischer, 2 and Sang-Koog Kim 1,2* National Creative Research Initiative Center for Spin Dynamics & Spin-Wave Devices, and Nanospinics Laboratory, Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA We experimentally demonstrate low-power-consumption vortex-core switching in magnetic nanodisks using tailored rotating magnetic fields produced with orthogonal and unipolar Gaussian-pulse currents. The optimal width of the orthogonal pulses and their time delay are found, from analytical and micromagnetic numerical calculations, to be determined only by the angular eigenfrequency  D for a given vortex-state disk of polarization p, such that   1  D and  t   p  D . The estimated optimal pulse parameters are in good agreement with the experimental results. This work lays a foundation for energy-efficient information recording in vortex-core cross-point architecture.