Yuichi Ohnuma

Spin Seebeck effect in antiferromagnets and compensated ferrimagnets

We theoretically investigate the spin Seebeck effect (SSE) in antiferromagnets and ferrimagnets, and show that the SSE vanishes in antiferromagnets but survives in ferrimagnets even at the magnetization compensation point despite the absence of its saturation magnetization. The non-vanishing SSE in ferrimagnets stems from two non-degenerate magnons. We demonstrate that the magnitude of the SSE in ferrimagnets is unchanged across the magnetization compensation point.

Origin of the spin Seebeck effect in compensated ferrimagnets

Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE.

Enhanced dc spin pumping into a fluctuating ferromagnet near TC

A linear-response formulation of the dc spin pumping, i.e., a spin injection from a precessing ferromagnet into an adjacent spin sink, is developed in view of describing many-body effects caused by spin fluctuations in the spin sink. It is shown that, when an itinerant ferromagnet near Tc is used as the spin sink, the spin pumping is largely increased owing to the fluctuation enhancement of the spin conductance across the precessing ferromagnet/spin sink interface. As an example, the enhanced spin pumping from permalloy into nickel palladium alloy (Tc ~ 20K) is analyzed by means of a self-consistent renormalization scheme, and it is predicted that the enhancement can be as large as tenfold.

Magnon instability driven by heat current in magnetic bilayers

We theoretically demonstrate that, in a ferromagnet/paramagnet bilayer, a magnon instability accompanied by a gigahertz microwave emission can be driven simply by means of a temperature bias. Employing many-body theory for investigating the effects of a phonon heat current on the magnon lifetime, we show that the magnon instability occurs upon the suppression of the Umklapp scattering at low temperatures, leading to microwave emission. The present finding provides crucial information about the interplay of spin current and heat current.

Theory of the spin Peltier effect

A microscopic theory of the spin Peltier effect in a bilayer structure comprising a paramagnetic metal (PM) and a ferromagnetic insulator (FI) based on the nonequilibrium Greens function method is presented. Spin current and heat current driven by temperature gradient and spin accumulation are formulated as functions of spin susceptibilities in the PM and the FI, and are summarized by Onsagers reciprocal relations. By using the current formulae, we estimate heat generation and absorption at the interface driven by the heat-current injection mediated by spins from PM into FI.

Dirac surface state–modulated spin dynamics in a ferrimagnetic insulator at room temperature

Spin dynamics of magnetic insulator is markedly modulated by the Dirac surface states in the adjacent topological insulator. This work demonstrates markedly modified spin dynamics of magnetic insulator (MI) by the spin momentum–locked Dirac surface states of the adjacent topological insulator (TI), which can be harnessed for spintronic applications. As the Bi concentration x is systematically tuned in 5-nm-thick (BixSb1−x)2Te3 TI films, the weight of the surface relative to bulk states peaks at x = 0.32 when the chemical potential approaches the Dirac point. At this concentration, the Gilbert damping constant of the precessing magnetization in 10-nm-thick Y3Fe5O12 MI films in the MI/TI heterostructures is enhanced by an order of magnitude, the largest among all concentrations. In addition, the MI acquires additional strong magnetic anisotropy that favors the in-plane orientation with similar Bi concentration dependence. These extraordinary effects of the Dirac surface states distinguish TI from other materials such as heavy metals in modulating spin dynamics of the neighboring magnetic layer.

Theory of spin hydrodynamic generation

Spin-current generation by fluid motion is theoretically investigated. Based on quantum kinetic theory, the spin-diffusion equation coupled with fluid vorticity is derived. We show that spin currents are generated by the vorticity gradient in both laminar and turbulent flows and that the generated spin currents can be detected by the inverse spin Hall voltage measurements, which are predicted to be proportional to the flow velocity in a laminar flow. In contrast, the voltage in a turbulent flow is proportional to the square of the flow velocity. This study will pave the way to fluid spintronics.

Spin transport in half-metallic ferromagnets

We theoretically investigate spin transport in half-metallic ferromagnets at finite temperatures. The side-jump and skew-scattering contributions to spin Hall conductivity are derived using the Kubo formula. The electron-magnon interaction causes a finite density of states in the energy gap of the minority-spin band and induces spin Hall conductivity. We show that spin Hall conductivity is proportional to

Spin Current Noise of the Spin Seebeck Effect and Spin Pumping

T^{3/2}

Magnonic noise and Wiedemann-Franz law

, with