Thermal spin pumping mediated by magnons in the semiclassical regime

Abstract

We microscopically analyze thermal spin pumping mediated by magnons, at the interfacebetween a ferromagnetic insulator and a non-magnetic metal, in the semiclassical regime.The generation of a spin current is discussed by calculating the thermal spintransfer torque, which breaks the spin conservation law for conduction electrons andoperates the coherent magnon state. Inhomogeneous thermal fluctuations betweenconduction electrons and magnons induce a net spin current, which is pumped into theadjacent non-magnetic metal. The pumped spin current is proportional to thetemperature difference. When the effective temperature of magnons is lower than that ofconduction electrons, localized spins lose spin angular momentum by emittingmagnons and conduction electrons flip from down to up by absorbing all theemitted momentum, and vice versa. Magnons at the zero mode cannot contribute tothermal spin pumping because they are eliminated by the spin-flip condition.Consequently thermal spin pumping does not cost any kind of applied magneticfields.