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

摘要

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