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 $A^{\rm em}_{\mu}$, and the other is the effective gauge field $A^{z}_{\mu}$ 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 $A^{z}_{\mu}$ 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 ("$\beta$-term").Comment: 13 pages, 3 figures, submitted to PR

Aharonov-Bohm effect in a ferromagnetic thin ring in diffusive regime is theoretically studied by calculating the Cooperon and Diffuson. In addition to the spin-orbit interaction, we include the spin-wave excitation and the spin splitting, which are expected to be dominant sources of dephasing in ferromagnets at low temperatures. The spin splitting turns out to kill the spin-flip channel of Cooperon but leaves the spin-conserving channel untouched. For the experimental confirmation of interference effect (described by Cooperons) such as weak localization and Aharonov-Bohm oscillation with period h/2e, we need to suppress the dominant dephasing by orbital motion. To do this we propose experiments on a thin film or thin ring with magnetization and external field perpendicular to the film, in which case the effective field inside the sample is equal to the external field (magnetization does not add up). The field is first applied strong enough to saturate the magnetization and then carrying out the measurement down to zero field keeping the magnetization nearly saturated, in order to avoid domain formations (negative fields may also be investigated if the coercive field is large enough).

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 with broken space inversion and time reversal symmetries based on the dielectric tensor. The dependence of the dielectric tensor on the wave vector q and M are retained to first order. 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; the results are used to study the system optical properties. 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 (direct and inverse Edelstein 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 which the polarization vector rotates around the vector (α × q). In the presence of M , which breaks time reversal symmetry, there arises an anomalous Hall effect and the dielectric tensor acquires off-diagonal components linear in M . For α M , these components yield the Faraday effect for the Faraday configuration q M , and the Cotton-Mouton effect for the Voigt configuration (q ⊥ M ). When α and M are noncollinear, M -and q-induced optical phenomena are possible, which include nonreciprocal directional dichroism in the Voigt configuration. In these nonreciprocal optical phenomena, a "toroidal moment," α × M , and a "quadrupole moment," αiMj + αjMi, play central roles. These phenomena are strongly enhanced at the spin-split transition edge in the electron band.

We report on the observation of the acoustic spin Hall effect that facilitates lattice motion–induced spin current via spin-orbit interaction (SOI). Under excitation of surface acoustic wave (SAW), we find that a spin current flows orthogonal to the SAW propagation in nonmagnetic metals (NMs). The acoustic spin Hall effect manifests itself in a field-dependent acoustic voltage in NM/ferromagnetic metal bilayers. The acoustic voltage takes a maximum when the NM layer thickness is close to its spin diffusion length, vanishes for NM layers with weak SOI, and increases linearly with the SAW frequency. To account for these results, we find that the spin current must scale with the SOI and the time derivative of the lattice displacement. These results, which imply the strong coupling of electron spins with rotating lattices via the SOI, show the potential of lattice dynamics to supply spin current in strong spin-orbit metals.

No abstract

No abstract

We study transport phenomena cross-correlated among the heat and electric currents of magnons and Dirac electrons on the surface of ferromagnetic topological insulators. For a perpendicular magnetization, we calculate magnon-(electron-) drag anomalous Nernst/Seebeck (anomalous Ettingshausen/Peltier) effects and magnon-/electron-drag thermal Hall effects. The magnon-drag thermoelectric effects are interpreted to be caused by magnon-induced electromotive force. When the magnetization has in-plane components, there arise thermal/thermoelectric analogs of anisotropic magnetoresistance (AMR). In the insulating state, the thermal AMR is realized as a magnonic analog of AMR.

No abstract