Spin-Transfer Torques for Domain Wall Motion in Antiferromagnetically-Coupled Ferrimagnets

Abstract

Spin-transfer torque (STT) effects on DW motion in rare earth-transition metal (RE-TM) ferrimagnets are investigated both theoretically and experimentally across the angular momentum compensation temperature, \({T}_{\mathrm{A}}\). First, theoretical equation for the field-driven STT-assisted DW velocity in ferrimagnets is derived based on Landau–Lifshitz–Gilbert equation. Second, this theory is tested experimentally using the DW motion experiment on a ferrimagnetic GdFeCo alloy, where Gd and FeCo moments are coupled antiferromagnetically. The experimental results are well fitted with the theory, confirming that the adiabatic STT component in DW velocity reverses its sign across \({T}_{\mathrm{A}}\), whereas the non-adiabatic STT component is the maximum at \({T}_{\mathrm{A}}\) without a sign change. Given that STT effects at \({T}_{\mathrm{A}}\) in ferrimagnets represent those in antiferromagnets, this finding indicates that the non-adiabatic STT in antiferromagnets acts as a staggered magnetic field, which can induce antiferromagnetic DW motion. It is also found that non-adiabaticity parameter, \(\beta\), which characterizes the magnitude of non-adiabatic STT, is remarkably larger than Gilbert damping parameter, \(\alpha\). This finding challenges the conventional understanding of the non-adiabatic STT based on experiments on ferromagnets.KeywordsSpin-transfer torqueDomain wall motionAngular momentum compensation temperature of ferrimagnets