Junya Shibata

Microscopic approach to current-driven domain wall dynamics

Abstract This review describes in detail the essential techniques used in microscopic theories on spintronics. We have investigated the domain wall dynamics induced by an electric current, based on the s – d exchange model. The domain wall is treated as rigid and planar and is described by two collective coordinates: the position and angle of wall magnetization. The effect of conduction electrons on the domain wall dynamics is calculated in the case of slowly varying spin structure (close to the adiabatic limit) by use of a gauge transformation. The spin-transfer torque and force on the wall are expressed by Feynman diagrams and calculated systematically using non-equilibrium Green’s functions, treating electrons fully quantum mechanically. The wall dynamics is discussed, based on two coupled equations of motion derived for two collective coordinates. The force is related to electron transport properties, resistivity, and the Hall effect. The effect of conduction electron spin relaxation on the torque and wall dynamics is also studied.

Current-induced magnetic vortex motion by spin-transfer torque

We investigate the dynamics of a magnetic vortex driven by spin-transfer torque due to spin current in the adiabatic case. The vortex core represented by collective coordinate experiences a transverse force proportional to the product of spin current and gyrovector, which can be interpreted as the geometric force determined by topological charges. We show that this force is just a reaction force of Lorentz-type force from the spin current of conduction electrons. Based on our analyses, we propose analytically and numerically a possible experiment to check the vortex displacement by spin current in the case of single magnetic nanodot.

Threshold current of domain wall motion under extrinsic pinning, β-term and non-adiabaticity

Threshold current of domain wall motion under spin-polarized electric current in ferromagnets is theoretically studied based on the equation of motion of a wall in terms of collective coordinates. Effects of non-adiabaticity and a so-called β-term in Landau–Lifshitz equation, which are described by the same term in the equation of motion of a wall, are taken into account as well as extrinsic pinning. It is demonstrated that there are four different regimes characterized by different dependence of threshold on extrinsic pinning, hard-axis magnetic anisotropy, non-adiabaticity and β.Threshold current of domain wall motion under spin-polarized electric current in ferromagnets is theoretically studied based on the equation of motion of a wall in terms of collective coordinates. Effects of non-adiabaticity and a so-called

Effect of spin current on uniform ferromagnetism: domain nucleation.

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Magnetic Vortex Dynamics

-term in Landau-Lifshitz equation, which are described by the same term in the equation of motion of a wall, are taken into account as well as extrinsic pinning. It is demonstrated that there are four different regimes characterized by different dependence of threshold on extrinsic pinning, hard-axis magnetic anisotropy, non-adiabaticity and

Spin Torque and Force due to Current for General Spin Textures

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Vortex motion in chilarity-controlled pair of magnetic disks

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Detection of magnetic state in a nanoscale ferromagnetic ring by using ballistic semiconductor two-dimensional electron gas

A large spin current applied to a uniform ferromagnet leads to a spin-wave instability as pointed out recently. In this Letter, it is shown that such spin-wave instability is absent in a state containing a domain wall, which indicates that nucleation of magnetic domains occurs above a certain critical spin current. This scenario is supported also by an explicit energy comparison of the two states under spin current.

A brief review of field- and current-driven domain-wall motion

We review the recent theoretical and experimental achievements on dynamics of spin vortices in patterned ferromagnetic elements. We first demonstrate the theoretical background of the research topic and briefly list the analytical and experimental approaches dealing with magnetic vortices. Then we report on the most remarkable studies devoted to steady state vortex excitations, switching processes, and coupled-vortex dynamic phenomena including the design of artificial crystals where the micromagnetic energy transfer takes place via the magnetic dipolar interaction among excited vortices. Finally we summarize the present state of the research with respect to novel prospects from both the fundamental and the application viewpoints.

Theory of Domain Wall Dynamics under Current

The nonadiabatic correction to spin transfer torque arising from fast-varying spin texture is calculated treating the conductions electron fully quantum mechanically. The torque is nonlocal in space, and is shown to be equivalent to a force (due to momentum transfer) acting on the center of mass of the texture. Another kind of force exists in the adiabatic regime, and it is identified to be of topological origin. These forces are shown to be the counter-reactions of electric transport properties, resistivity and the Hall effect.The nonadiabatic correction to spin transfer torque arising from fast-varying spin texture is calculated treating the conductions electron fully quantum mechanically. The torque is nonlocal in space, and is shown to be equivalent to a force (due to momentum transfer) acting on the center of mass of the texture. Another kind of force exists in the adiabatic regime, and it is identified to be of topological origin. These forces are shown to be the counter-reactions of electric transport properties, resistivity and the Hall effect.