Microscopic derivation of magnon spin current in a topological insulator/ferromagnet heterostructure

TL;DR

This paper derives a microscopic model for magnon spin currents in topological insulator/ferromagnet heterostructures, revealing how electric fields induce spin torques via electron-magnon interactions at room temperature.

Contribution

It provides a theoretical framework for calculating magnon spin currents induced by electric fields in topological insulator/ferromagnet interfaces, including temperature-dependent expressions.

Findings

Derived expressions for spin torque at different temperatures

Estimated magnitude of spin current for realistic materials

Identified the role of electron-magnon coupling in spin transfer

Abstract

We investigate a spin-electricity conversion effect in a topological insulator/ferromagnet heterostructure. In the spin-momentum-locked surface state, an electric current generates nonequilibrium spin accumulation, which causes a spin-orbit torque that acts on the ferromagnet. When spins in the ferromagnet are completely parallel to the accumulated spin, this spin-orbit torque is zero. In the presence of spin excitations, however, a coupling between magnons and electrons enables us to obtain a nonvanishing torque. In this paper, we consider a model of the heterostructure in which a three-dimensional magnon gas is coupled with a two-dimensional massless Dirac electron system at the interface. We calculate the torque induced by an electric field, which can be interpreted as a magnon spin current, up to the lowest order of the electron-magnon interaction. We derive the expressions for high and low temperatures and estimate the order of magnitude of the induced spin current for realistic materials at room temperature.