Jisu Ryu

Magnetization dynamics induced by in-plane currents in ultrathin magnetic nanostructures with Rashba spin-orbit coupling

Recent experiments on ultrathin magnetic layers with broken inversion symmetry reported anomalous current-driven magnetization dynamics. We show that the spin-transfer torque can be significantly modified by Rashba spin-orbit coupling and the modified spin-transfer torque can explain the anomalous magnetization dynamics. This work will be valuable for the development of next generation spintronic devices based on ultrathin magnetic systems.

Spin-wave propagation in the presence of interfacial Dzyaloshinskii-Moriya interaction

In ferromagnetic thin films, broken inversion symmetry and spin-orbit coupling give rise to interfacial Dzyaloshinskii-Moriya interactions. Analytic expressions for spin-wave properties show that the interfacial Dzyaloshinskii-Moriya interaction leads to non-reciprocal spin-wave propagation, i.e. different properties for spin waves propagating in opposite directions. In favorable situations, it can increase the spin-wave attenuation length. Comparing measured spin wave properties in ferromagnet

Current-induced motion of a transverse magnetic domain wall in the presence of spin Hall effect

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Magnetic domain-wall motion in a nanowire: Depinning and creep

normal metal bilayers and other artificial layered structures with these calculations can provide a useful characterization of the interfacial Dzyaloshinskii-Moriya interactions.

Current-driven domain wall motion with spin Hall effect: Reduction of threshold current density

We theoretically study current-induced dynamics of a transverse magnetic domain wall in bi-layer nanowires consisting of a ferromagnetic layer on top of a nonmagnetic layer with strong spin-orbit coupling. Domain wall dynamics is characterized by two threshold current densities, JthWB and JthREV, where JthWB is a threshold for the chirality switching of the domain wall and JthREV is another threshold for the reversed domain wall motion caused by spin Hall effect. Domain walls with a certain chirality may move opposite to the electron-flow direction with high speed in the current range JthREV JthREV and α>β, where α is the Gilbert damping constant and β is the nonadiabaticity of spin torque. Micromagnetic simulations confirm the validity of analytical results.

Current-induced domain wall motion: Domain wall velocity fluctuations

The domain wall motion in a magnetic nanowire is examined theoretically in the regime where the domain wall driving force is weak and its competition against disorders is assisted by thermal agitations. Two types of driving forces are considered; magnetic field and current. While the field induces the domain wall motion through the Zeeman energy, the current induces the domain wall motion by generating the spin transfer torque, of which effects in this regime remain controversial. The spin transfer torque has two mutually orthogonal vector components, the adiabatic spin transfer torque and the nonadiabatic spin transfer torque. We investigate separate effects of the two components on the domain wall depinning rate in one-dimensional systems and on the domain wall creep velocity in two-dimensional systems, both below the Walker breakdown threshold. In addition to the leading order contribution coming from the field and/or the nonadiabatic spin transfer torque, we find that the adiabatic spin transfer torque generates corrections, which can be of relevance for an unambiguous analysis of experimental results. For instance, it is demonstrated that the neglect of the corrections in experimental analysis may lead to incorrect evaluation of the nonadiabaticity parameter. Effects of the Rashba spin-orbit coupling on the domain wall motion are also analyzed.

Universality classes of magnetic domain wall motion.

We theoretically study the current-driven domain wall motion in the presence of both the spin Hall effect and an extrinsic pinning potential. The spin Hall effect mainly affects the damping ratio of the domain wall precession in the pinning potential. When the pinning potential is not too strong, this results in a significant reduction of a threshold current density for the depinning of a domain wall with certain polarity. We also propose one way to distinguish the spin Hall effect induced spin-transfer torque from the one induced by the Rashba spin-orbit coupling experimentally.

Electric current effect on the energy barrier of magnetic domain wall depinning: origin of the quadratic contribution.

The spin transfer torque generated by a spin polarized current can generate the domain wall (DW) motion in a magnetic nanowire. We examine fluctuations of a DW velocity induced by uncontrolled edge roughness and defects. In the weak pinning regime (regime I), the fluctuation is small. In the intermediate pinning regime (regime II), the DW velocity fluctuates significantly as the density of pinning centers changes. Moreover the threshold current density for the finite density of pinning centers is found to be noticeably larger than the threshold current density for the escape from a single pinning center. Finally, in the strong pinning regime (regime III), the DW velocity fluctuates as the density of pinning centers changes. But the enhancement of the threshold current density does not occur. Thus the fluctuations of the DW velocity are different in the three pinning regimes.