Collective induced antidiffusion effect and general magnon Boltzmann transport theory

Abstract

Magnon transport and its efficient manipulation in FIs (ferromagnetic insulators) are important issues in magnon spintronics research, while the relaxation and decay of magnons are still in debates. In this paper, starting from the Hamiltonian of a magnetically ordered system, we derive and solve the general magnon Boltzmann equation. It is found that there exists an ``antidiffusion driving effect'' in magnon transport, which is caused by the collective interaction of a high density (or strong magnetic field) magnon gas in the limited magnetic background. This effect acts as a ``spin pressure'' on magnons, resulting in a decrease in magnon propagation length and an extra accumulation near the magnon generation boundary. By applying the derived magnon Boltzmann equation to several cases and comparing with the experiments, it is shown that our theory cannot only describe the influence of the external magnetic field on magnon transport but also confirm the fast drop regime in magnon transport is attributed to the boundary effect. The magnon Boltzmann method also predicts a new pathway for manipulations of magnon transport in FIs.