arXiv subjects

Gen Tatara

Publications and source records attributed to Gen Tatara.

At least 73 records · Page 4Linked to original sources

arXiv2009

Perturbation theory of the dynamic inverse spin Hall effect with charge conservation

We present gauge-invariant theory of the dynamic inverse spin Hall effect driven by the spin--orbit interaction in metallic systems. Charge conservation is imposed diagrammatically by including vertex corrections. We show the charge current is induced by an effective electric field that is proportional to the spin current pumped by the magnetization dynamics. The result is consistent with recent experiments.

cond-mat.mes-hall↗

arXiv2008

Spin-wave contributions to current-induced domain wall dynamics

We examine theoretically the role of spin-waves on current-induced domain wall dynamics in a ferromagnetic wire. At room temperature, we find that an interaction between the domain wall and the spin waves appears when there is a finite difference between the domain wall velocity $\dot{x}_0$ and the spin current $u$. Three important consequences of this interaction are found. Firstly, spin-wave emission leads to a Landau-type damping of the current-induced domain wall motion towards restoring the solution $\dot{x}_0 = u$, where spin angular momentum is perfectly transfered from the conduction electrons to the domain wall. Secondly, the interaction leads to a modification of the domain wall width and mass, proportional to the kinetic energy of the domain wall. Thirdly, the coupling by the electrical current between the domain wall and the spin waves leads to temperature-dependent effective wall mass.

cond-mat.mtrl-sci↗

arXiv2008

The anomalous Hall conductivity due to the vector spin chirality

We study theoretically the anomalous Hall effect due to the vector spin chirality carried by the local spins in the $s$-$d$ model. We will show that the vector spin chirality indeed induces local Hall effect in the presence of the electron spin polarization, while the global Hall effect vanishes if electron transport is homogeneous. This anomalous Hall effect can be interpreted in terms of the rotational component of the spin current associated with the vector chirality.

cond-mat.mes-hall↗

arXiv2008

Calculation of current-induced torque from spin continuity equation

Current-induced torque is formulated based on the spin continuity equation. The formulation does not rely on the assumption of separation of local spin and charge degrees of freedom, in contrast to approaches based on the $s$-$d$ model or mean-field approximation of itinerant ferromagnetism. This new method would be thus useful for the estimation of torques in actual materials by first-principles calculations. As an example, the formalism is applied to the adiabatic limit of the $s$-$d$ model in order to obtain the analytical expression for torques and corresponding $β$ terms arising from spin relaxation due to spin-flip scattering and spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2008

Microscopic approach to current-driven domain wall dynamics

This review describes in detail the essential techniques used in microscopic theories on spintronics. We have investigated the domain wall dynamics induced by 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. Effect of conduction electron spin relaxation on the torque and wall dynamics is also studied.

cond-mat.mes-hall↗

arXiv2008

Charge and Spin Currents Generated by Dynamical Spins

We demonstrate theoretically that a charge current and a spin current are generated by spin dynamics in the presence of spin-orbit interaction in the perturbative regime. We consider a general spin-orbit interaction including the spatially inhomogeneous case. Spin current due to spin damping is identified as one origin of generated charge current, but other contributions exist, such as the one due to an induced conservative field and the one arising from the inhomogeneity of spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2008

Current-induced domain wall motion in Rashba spin-orbit system

Current-induced magnetic domain wall motion, induced by transfer of spin transfer effect due to exchange interaction, is expected to be useful for next generation high-density storages. We here show that efficient domain wall manipulation can be achieved by introduction of Rashba spin-orbit interaction, which induces spin precession of conduction electron and acts as an effective magnetic field. Its effect on domain wall motion depends on the wall configuration. We found that the effect is significant for Bloch wall with the hard axis along the current, since the effective field works as $β$ or field-like term and removes the threshold current if in extrinsic pinning is absent. For Néel wall and Bloch wall with easy axis perpendicular to Rashba plane, the effective field induces a step motion of wall corresponding to a rotation of wall plane by the angle of approximately $π$ at current lower than intrinsic threshold. Rashba interaction would therefore be useful to assist efficient motion of domain walls at low current.

cond-mat.mes-hall↗

arXiv2008

Charge current driven by spin dynamics in disordered Rashba spin-orbit system

Pumping of charge current by spin dynamics in the presence of the Rashba spin-orbit interaction is theoretically studied. Considering disordered electron, the exchange coupling and spin-orbit interactions are treated perturbatively. It is found that dominant current induced by the spin dynamics is interpreted as a consequence of the conversion from spin current via the inverse spin Hall effect. We also found that the current has an additional component from a fictitious conservative field. Results are applied to the case of moving domain wall.

cond-mat.mes-hall↗

arXiv2008

Theory of Domain Wall Dynamics under Current

Microscopic theory of domain wall dynamics under electric current is reviewed. Domain wall is treated as rigid and planar. The spin-transfer torque and forces on the wall are derived based on the $s$-$d$ exchange interaction between localized spins and conduction electrons, treating non-adiabaticity expressed by the gauge field perturbatively. Effect of spin relaxation is also studied.

cond-mat.mes-hall↗

arXiv2007

Spin torque and force due to current for general spin textures

Non-adiabatic correction to spin transfer torque arising from fast-varying spin texture is calculated treating conduction electron fully quantum mechanically. The torque is non-local in space, and is shown to 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 is identified to be of topological origin. These forces are shown to be the counter reaction of electric transport properties, resistivity and Hall effect, respectively.

cond-mat.mes-hall↗

arXiv2006

Microscopic Calculation of Spin Torques in Disordered Ferromagnets

Effects of conduction electrons on magnetization dynamics, represented by spin torques, are calculated microscopically in the first order in spatial gradient and time derivative of magnetization. Special attention is paid to the so-called $β$-term and the Gilbert damping, $α$, in the presence of electrons' spin-relaxation processes, which are modeled by quenched magnetic (and spin-orbit) impurities. The obtained results such as $α\ne β$ hold for localized as well as itinerant ferromagnetism.

cond-mat.mes-hall↗

arXiv2006

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 $β$.

cond-mat.mes-hall↗

arXiv2005

Current-induced 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.

cond-mat.mes-hall↗

arXiv2005

Effect of Spin Current on Uniform Ferromagnetism: Domain Nucleation

Large spin current applied to a uniform ferromagnet leads to a spin-wave instability as pointed out recently. In this paper, 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.

cond-mat.mes-hall↗

arXiv2004

Domain wall displacement triggered by an AC current below threshold

It is theoretically demonstrated that a displacement of a pinned domain wall, typically of order of $μ$m, can be driven by use of an ac current which is below threshold value. The point here is that finite motion around the pinning center by a low current is enhanced significantly by the resonance if the frequency is tuned close to the pinning frequency as demonstrated by recent experiment.

cond-mat.mes-hall↗

arXiv2004

Universality of thermally assisted magnetic domain wall motion under spin torque

Thermally assisted motion of magnetic domain wall under spin torque is studied theoretically. It is shown that the wall velocity $v$ depends exponentially on the spin current, $\Is$, below the threshold value, in the same way as in a thermally activated motion driven by a force. A novel property of the spin torque driven case at low temperature is that the linear term in spin current is universal, i.e., $\ln v \sim \frac{π\hbar}{2e}(\Is/\kB T)$. This behavior, which is independent of pinning and material constants, could be used to confirm experimentally the spin torque as the driving mechanism.

cond-mat.mes-hall↗

arXiv2004

Anomalous Hall Effect and Skyrmion Number in Real- and Momentum-space

We study the anomalous Hall effect (AHE) for the double exchange model with the exchange coupling $|J_H|$ being smaller than the bandwidth $|t|$ for the purpose of clarifying the following unresolved and confusing issues: (i) the effect of the underlying lattice structure, (ii) the relation between AHE and the skyrmion number, (iii) the duality between real and momentum spaces, and (iv) the role of the disorder scatterings; which is more essential, $σ_H$ (Hall conductivity) or $ρ_H$ (Hall resistivity)? Starting from a generic expression for $σ_H$, we resolve all these issues and classify the regimes in the parameter space of $J_H τ$ ($τ$: elastic-scattering time), and $λ_{s}$ (length scale of spin texture). There are two distinct mechanisms of AHE; one is characterized by the real-space skyrmion-number, and the other by momentum-space skyrmion-density at the Fermi level, which work in different regimes of the parameter space.

cond-mat.mes-hall↗

arXiv2003

Theory of Current-Driven Domain Wall Motion: A Poorman's Approach

A self-contained theory of the domain wall dynamics in ferromagnets under finite electric current is presented. The current is shown to have two effects; one is momentum transfer, which is proportional to the charge current and wall resistivity ($\rhow$), and the other is spin transfer, proportional to spin current. For thick walls, as in metallic wires, the latter dominates and the threshold current for wall motion is determined by the hard-axis magnetic anisotropy, except for the case of very strong pinning. For thin walls, as in nanocontacts and magnetic semiconductors, the momentum-transfer effect dominates, and the threshold current is proportional to $\Vz/\rhow$, $\Vz$ being the pinning potential.

cond-mat.mes-hall↗