arXiv subjects

Hiroshi Kohno

Publications and source records attributed to Hiroshi Kohno.

At least 37 records · Page 2Linked to original sources

arXiv2017

Theory of electromagnetic wave propagation in ferromagnetic Rashba conductor

We present a comprehensive study of various electromagnetic wave propagation phenomena in a ferromagnetic bulk Rashba conductor from the perspective of quantum mechanical transport. In this system, both the space inversion and time reversal symmetries are broken, as characterized by the Rashba field $α$ and magnetization $M$, respectively. First, we present a general phenomenological analysis of electromagnetic wave propagation in media based on the dielectric tensor. Then, we calculate the microscopic electromagnetic response of the current and spin of conduction electrons subjected to $ α$ and $M$, based on linear response theory and the Green's function method. Firstly, it is found that a large $α$ enhances the anisotropic properties of the system and enlarges the frequency range in which the electromagnetic waves have hyperbolic dispersion surfaces and exhibit unusual propagations known as negative refraction and backward waves. Secondly, we consider the electromagnetic cross-correlation effects on the wave propagation. These effects stem from the lack of space inversion symmetry and yield $q$-linear off-diagonal components in the dielectric tensor. This induces a Rashba-induced birefringence. In the presence of $M$, there arises an anomalous Hall effect and the dielectric tensor acquires off-diagonal components linear in $M$. These components yield the Faraday effect and the Cotton-Mouton effect. When $α$ and $M$ are noncollinear, $M$- and $q$-induced optical phenomena, nonreciprocal directional dichroism is possible. In these nonreciprocal optical phenomena, a "toroidal moment," $α\times M$, and a "quadrupole moment," $α_i M_j + α_j M_i$, play central roles. These phenomena are strongly enhanced at the spin-split transition edge in the electron band.

cond-mat.mes-hall↗

arXiv2017

Intrinsic and Extrinsic Spin Hall Effects of Dirac Electrons

We investigate the spin Hall effect (SHE) of electrons described by the Dirac equation, which is used as an effective model near the $L$-points in bismuth. By considering short-range nonmagnetic impurities, we calculate the extrinsic as well as intrinsic contributions on an equal footing. The vertex corrections are taken into account within the ladder type and the so-called skew-scattering type. The intrinsic SHE which we obtain is consistent with that of Fuseya et al. [J. Phys. Soc. Jpn. 81, 93704 (2012)]. It is found that the extrinsic contribution dominates the intrinsic one when the system is (semi)metallic. The extrinsic SHE due to the skew scattering is proportional to $Δ/ n_{\rm i} u$, where $2Δ$ is the band gap, $n_{\rm i}$ is the impurity concentration, and $u$ is the strength of the impurity potential.

cond-mat.mes-hall↗

arXiv2015

Anomalous Hall effect driven by dipolar spin waves in uniform ferromagnets

A new type of anomalous Hall effect is shown to arise from the interaction of conduction electrons with dipolar spin waves in ferromagnets. This effect exists even in homogeneous ferromagnets without relativistic spin-orbit coupling. The leading contribution to the Hall conductivity is proportional to the chiral spin correlation of dynamical spin textures and is physically understood in terms of the skew scattering by dipolar magnons.

cond-mat.mtrl-sci↗

arXiv2015

Theory of anomalous optical properties of bulk Rashba conductor

The Rashba interaction induced when inversion symmetry is broken in solids is a key interaction connecting spin and charge for realizing novel magnetoelectric cross-correlation effects. Here, we theoretically explore the optical properties of a bulk Rashba conductor by calculating the transport coefficients at finite frequencies. It is demonstrated that the combination of direct and inverse Edelstein effects leads to a softening of the plasma frequency for the electric field perpendicular to the Rashba field, resulting in a hyperbolic electromagnetic metamaterial. In the presence of magnetization, a significant enhancement of anisotropic propagation (directional dichroism) is predicted because of interband transition edge singularity.Based on an effective Hamiltonian analysis, the dichroism is demonstrated to be driven by toroidal and quadratic moments of the magnetic Rashba system. The effective theory of the cross-correlation effects has the same mathematical structure as that of insulating multiferroics.

cond-mat.mes-hall↗

arXiv2014

Transport Properties of Dirac Ferromagnet

We propose a model ferromagnet based on the Dirac Hamiltonian in three spatial dimensions, and study its transport properties which include anisotropic magnetoresistance (AMR) and anomalous Hall (AH) effect. This relativistic extension allows two kinds of ferromagnetic order parameters, denoted by $\bm{M}$ and $\bm{S}$, which are distinguished by the relative sign between the positive- and negative-energy states (at zero momentum) and become degenerate in the non-relativistic limit. Because of the relativistic coupling between the spin and the orbital motion, both $\bm{M}$ and $\bm{S}$ induce anisotropic deformations of the energy dispersion (and the Fermi surfaces) but in mutually opposite ways. The AMR is determined primarily by the anisotropy of the Fermi surface (group velocity), and secondarily by the anisotropy of the damping; the latter becomes important for ${\bm M}=\pm{\bm S}$, where the Fermi surfaces are isotropic. Even when the chemical potential lies in the gap, the AH conductivity is found to take a finite non-quantized value, $σ_{ij} = -(α/3π^2 \hbar) ε_{ijk} S_k $, where $α$ is the (effective) fine structure constant. This offers an example of Hall insulator in three spatial dimensions.

cond-mat.mtrl-sci↗

arXiv2014

Current-driven dynamics of coupled domain walls in a synthetic antiferromagnet

We develop the theory of magnetic domain wall motion in coupled double-layer systems where electrons can hop between the layers giving rise to an antiferromagnetic coupling. We demonstrate that the force from the interlayer coupling drives the walls and the effect of the extrinsic pinning is greatly reduced if the domain walls are initially separated. The threshold current density for metastable spin-aligned configurations is also much lower. We conclude that the interlayer coupling has a significant effect on domain wall mobility in double-layer systems.

cond-mat.mes-hall↗

arXiv2012

Spin torque due to diffusive spin current in magnetic texture

We present a microscopic theory of spin torque due to diffusive spin currents induced by spin accumulation. The obtained expression is a natural extension of the existing one due to 'local' spin currents associated with ordinary electric currents, and is the reciprocal of the spin motive force which induces charge accumulation as studied recently [J. Shibata and H. Kohno, Phys. Rev. B84, 184408 (2011)]. The result is applied to a domain wall motion in a nonlocal spin injection system, and the torque and force due to diffusive spin current are evaluated.

cond-mat.mes-hall↗

arXiv2011

Spin and charge transport induced by gauge fields in a ferromagnet

We present a microscopic theory of spin-dependent motive force ("spin motive force") induced by magnetization dynamics in a conducting ferromagnet, by taking account of spin relaxation of conduction electrons. The theory is developed by calculating spin and charge transport driven by two kinds of gauge fields; one is the ordinary electromagnetic field $A^{\rm em}_μ$, and the other is the effective gauge field $A^{z}_μ$ induced by dynamical magnetic texture. The latter acts in the spin channel and gives rise to a spin motive force. It is found that the current induced as a linear response to $A^{z}_μ$ is not gauge-invariant in the presence of spin-flip processes. This fact is intimately related to the non-conservation of spin via Onsager reciprocity, so is robust, but indicates a theoretical inconsistency. This problem is resolved by considering the time dependence of spin-relaxation source terms in the "rotated frame", as in the previous study on Gilbert damping [J. Phys. Soc. Jpn. {\bf 76}, 063710 (2007)]. This effect restores the gauge invariance while keeping spin non-conservation. It also gives a dissipative spin motive force expected as a reciprocal to the dissipative spin torque ("$β$-term").

cond-mat.mes-hall↗

arXiv2009

Spin Hall Current and Spin-transfer Torque in Ferromagnetic Metal

We theoretically examine the spin-transfer torque in the presence of spin-orbit interaction (SOI) at impurities in a ferromagnetic metal on the basis of linear response theory. We obtained, in addition to the usual spin-transfer torque, a new contributioin $\sim {\bm j}_{\rm SH}^{\phantom{\dagger}} \cdot \nabla {\bm n}$ in the first order in SOI, where ${\bm j}_{\rm SH}^{\phantom{\dagger}}$ is the spin Hall current driven by an external electric field. This is a reaction to inverse spin Hall effect driven by spin motive force in a ferromagnet.

cond-mat.mes-hall↗

arXiv2009

Inverse Spin Hall Effect Driven by Spin Motive Force

The spin Hall effect is a phenomenon that an electric field induces a spin Hall current. In this Letter, we examine the inverse effect that, in a ferromagnetic conductor, a charge Hall current is induced by a spin motive force, or a spin-dependent effective ` electric' field ${\bm E}_{\rm s}$, arising from the time variation of magnetization texture. By considering skew-scattering and side-jump processes due to spin-orbit interaction at impurities, we obtain the Hall current density as $σ_{\rm SH} {\bm n}\times{\bm E}_{\rm s}$, where ${\bm n}$ is the local spin direction and $σ_{\rm SH}$ is the spin Hall conductivity. The Hall angle due to the spin motive force is enhanced by a factor of $P^{2}$ compared to the conventional anomalous Hall effect due to the ordinary electric field, where $P$ is the spin polarization of the current. The Hall voltage is estimated for a field-driven domain wall oscillation in a ferromagnetic nanowire.

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

Probing the spin polarization of current by soft X-ray imaging of current-induced magnetic vortex dynamics

Time-resolved soft X-ray transmission microscopy is applied to image the current-induced resonant dynamics of the magnetic vortex core realized in a micronsized Permalloy disk. The high spatial resolution better than 25 nm enables us to observe the resonant motion of the vortex core. The result also provides the spin polarization of the current to be 0.67 +/-0.16 for Permalloy by fitting the experimental results with an analytical model in the framework of the spin-transfer torque.

cond-mat.mtrl-sci↗

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↗

arXiv2007

Gauge Field Formulation of Adiabatic Spin Torques

Previous calculation of spin torques for small-amplitude magnetization dynamics around a uniformly magnetized state [J. Phys. Soc. Jpn. {\bf 75} (2006) 113706] is extended here to the case of finite-amplitude dynamics. This is achieved by introducing an `` adiabatic'' spin frame for conduction electrons, and the associated SU(2) gauge field. In particular, the Gilbert damping is shown to arise from the time variation of the spin-relaxation source terms in this new frame, giving a new physical picture of the damping. The present method will allow a `` first-principle'' derivation of spin torques without any assumptions such as rotational symmetry in spin space.

cond-mat.mes-hall↗

arXiv2007

Electrical switching of vortex core in a magnetic disk

A magnetic vortex is a curling magnetic structure realized in a ferromagnetic disk, which is a promising candidate of a memory cell for future nonvolatile data storage devices. Thus, understanding of the stability and dynamical behaviour of the magnetic vortex is a major requirement for developing magnetic data storage technology. Since the experimental proof of the existence of a nanometre-scale core with out-of-plane magnetisation in the magnetic vortex, the dynamics of a vortex has been investigated intensively. However, the way to electrically control the core magnetisation, which is a key for constructing a vortex core memory, has been lacking. Here, we demonstrate the electrical switching of the core magnetisation by utilizing the current-driven resonant dynamics of the vortex; the core switching is triggered by a strong dynamic field which is produced locally by a rotational core motion at a high speed of several hundred m/s. Efficient switching of the vortex core without magnetic field application is achieved thanks to resonance. This opens up the potentiality of a simple magnetic disk as a building block for spintronic devices like a memory cell where the bit data is stored as the direction of the nanometre-scale core magnetisation.

cond-mat.mtrl-sci↗

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↗