Thermally-driven spin torques in layered magnetic insulators

TL;DR

This paper explores two distinct mechanisms for thermally-driven spin-transfer torques in layered magnetic insulators, involving local magnon effects and nonlocal spin currents in spin valves, advancing understanding of thermal control in spintronics.

Contribution

It introduces two novel mechanisms for thermally-driven spin torques in insulating magnetic heterostructures, expanding the theoretical framework for thermal spin control.

Findings

Identification of a local magnon-mediated torque mechanism.

Proposal of a nonlocal spin torque via spin valve structures.

Insight into symmetry-breaking requirements for thermal torques.

Abstract

Thermally-driven spin-transfer torques have recently been reported in electrically insulating ferromagnet$|$normal-metal heterostructures. In this paper, we propose two physically distinct mechanisms for such torques. The first is a local effect: out-of-equilibrium, thermally-activated magnons in the ferromagnet, driven by a spin Seebeck effect, exert a torque on the magnetization via magnon-magnon scattering with coherent dynamics. The second is a nonlocal effect which requires an additional magnetic layer to provide the symmetry breaking necessary to realize a thermal torque. The simplest structure in which to induce a nonlocal thermal torque is a spin valve composed of two insulating magnets separated by a normal metal spacer; there, a thermal flux generates a pure spin current through the spin valve, which results in a torque when the magnetizations of the layers are misaligned.