Sok-Bong Choi

Time-resolved x-ray imaging shows that the magnetization dynamics of a micron-sized pattern containing a ferromagnetic vortex is determined by its handedness, or chirality. The out-of-plane magnetization in the nanometer-scale vortex core induces a three-dimensional handedness in the planar magnetic structure, leading to a precessional motion of the core parallel to a subnanosecond field pulse. The core velocity was an order of magnitude higher than expected from the static susceptibility. These

We report the experimental results on magnetization reversal in Co/Pd multilayers. Interestingly enough, reversal phenomena in this system were found to be strongly dependent on the magnetic properties varied with the Co-sublayer thickness. Direct domain observations and magnetization viscosity measurements revealed that wall motion was dominant in the samples having 2-\AA{}-thick Co sublayer, whereas nucleation was dominant in those having 4-\AA{}-thick Co sublayer. Magnetization reversal was t

The Dzyaloshinskii–Moriya interaction (DMI) generates intriguing chiral magnetic objects, such as magnetic skyrmions and chiral domain walls, that can be used as building blocks in emerging magnetic nanodevices. Precise control of the DMI strength is one of the key issues for achieving better stability and functionality of these chiral objects. In this paper, we report that in magnetic trilayer films, the DMI strength exhibits a noticeable correlation with the work functions of the non-magnetic

We report the difference in activation volumes of wall-motion and nucleation processes in Co/Pd multilayers. Each activation volume was estimated from field dependence of wall-motion speed and nucleation rate, obtained by real-time domain imaging using a magneto-optical Kerr effect microscope. Delicate analysis shows that the two activation volumes are generally unequal and the ratio between the volumes varies noticeably from 0.9 to 1.1 with change of the multilayered structure. We found that th

The design of a magneto-optical microscope magnetometer (MOMM) for simultaneously probing local magnetic properties is described. The MOMM consists of an optical polarizing microscope capable of magneto-optical contrast that is used as a magnetometer by sweeping a magnetic field from an electromagnet. Due to full-field optical imaging, as opposed to single photodiode detection, the system is capable of simultaneous measurement of magnetic hysteresis loops and magnetization viscosity curves on 80

A method to simultaneously determine both the wall-motion speed and the nucleation probability of magnetic films is presented, where the domain reversal patterns are modelized by time-dependent circular domains formed by the nucleation process and then, expanded by the wall-motion process. The present method has been applied to understand the contrasting reversal behaviors in the Co/Pd multilayered system and the difference in the nucleation probabilities of the samples was found to be a major o

In chiral magnetic materials, numerous intriguing phenomena such as built-in chiral magnetic domain walls (DWs) and skyrmions are generated by the Dzyaloshinskii–Moriya interaction (DMI). The DMI also results in asymmetric DW speeds under an in-plane magnetic field, which provides a useful scheme to measure the strength of the DMI. However, recent findings of additional asymmetries such as chiral damping have inhibited the unambiguous determination of the DMI strength, and the underlying mechani

We report experimental evidence that submicron-scale local coercivity variation determines magnetization reversal dynamics in ferromagnetic thin films. Local coercivity distribution is generated from a two-dimensional array of hysteresis loops each of $0.32\ifmmode\times\else\texttimes\fi{}0.32 \ensuremath{\mu}{\mathrm{m}}^{2}$ spots simultaneously measured using a magneto-optical microscope magnetometer. We directly demonstrate that domain reversal pattern is truly coincident with local coerciv

We present an analytical theory of the magnetic domain configuration in magnetic multilayers. The theory predicts a sharp transition from a large-areal domain pattern to a striped domain configuration as a function of increasing magnetic layer thickness. This transition was verified experimentally by direct magnetic domain observations in Co/Pd multilayers. This transition results from a competition between the magnetostatic energy and the exchange and anisotropy energies.

We present an analytical description of a magnetization reversal phase diagram of ferromagnetic thin films that have uniaxial perpendicular anisotropy. The phase equilibrium lines were calculated from a micromagnetic consideration of equilibrium conditions of the wall motion, dendritic growth, and nucleation processes. The phase diagram characterizes well simulated domain evolution patterns: typical domain evolution patterns are predicted accurately in the corresponding phases accompanied by gra

We present an analytic theory of the magnetization phase in ferromagnetic nanowires with perpendicular magnetic anisotropy. In nanowire geometry, the shape anisotropy is reduced considerably in contrast to continuous films. Consequently, the spin reorientation transition occurs from in plane to out of plane with respect to the wire width. The stable domain size in the out-of-plane phase is sensitive to the wire width and approaches the single domain state on narrowing the width further. A phase

Spin reversal of Co/Pd multilayers under various strengths of an applied field has been quantitatively investigated. The reversal behavior was found to be essentially the same irrespective of the magnitude of the applied field, but the rate of domain evolution was considerably accelerated by increasing the applied field. Quantitative analysis revealed that both the wall-motion speed and the nucleation rate were exponentially proportional to the applied field. The proportional coefficients of the