Thermal spin pumping mediated by magnon in the semiclassical regime

TL;DR

This paper provides a microscopic analysis of thermal spin pumping via magnons at the interface of ferromagnetic insulators and non-magnetic metals, highlighting the role of thermal fluctuations and the absence of magnetic field requirements.

Contribution

It introduces a semiclassical framework for understanding thermal spin pumping mediated by magnons, emphasizing the role of temperature differences and inhomogeneous thermal fluctuations.

Findings

Spin current is proportional to temperature difference.

Magnons at zero mode do not contribute to spin pumping.

Thermal spin pumping occurs without applied magnetic fields.

Abstract

We microscopically analyze thermal spin pumping mediated by magnons, at the interface between a ferromagnetic insulator and a non-magnetic metal, in the semiclassical regime. The generation of a spin current is discussed by calculating the thermal spin transfer torque, which breaks the spin conservation law for conduction electrons and operates the coherent magnon state. Inhomogeneous thermal fluctuations between conduction electrons and magnons induce a net spin current, which is pumped into the adjacent non-magnetic metal. The pumped spin current is proportional to the temperature difference. When the effective temperature of magnons is lower than that of conduction electrons, localized spins lose spin angular momentum by emitting magnons and conduction electrons flip from down to up by absorbing all the emitted momentum, and vice versa. Magnons at the zero mode cannot contribute to thermal spin pumping because they are eliminated by the spin-flip condition. Consequently thermal spin pumping does not cost any kinds of applied magnetic fields. We have discussed the distinction from the theory proposed by Xiao et al. [Phys. Rev. B, 81 (2010) 214418], Adachi et al. [Phys. Rev. B, 83 (2011) 094410], and Bender et al. [arXiv:1111.2382].