Strong variation of spin-orbit torques with relative spin relaxation rates in ferrimagnets

Spin-orbit torques (SOTs) have been widely understood as an interfacial transfer of spin that is independent of the bulk properties of the magnetic layer. Here, we report that SOTs acting on ferrimagnetic FexTb1-x layers decrease and vanish upon approaching the magnetic compensation point because the rate of spin transfer to the magnetization becomes much slower than the rate of spin relaxation into the crystal lattice due to spin-orbit scattering. These results indicate that the relative rates of competing spin relaxation processes within magnetic layers play a critical role in determining the strength of SOTs, which provides a unified understanding for the diverse and even seemingly puzzling SOT phenomena in ferromagnetic and compensated systems. Our work indicates that spin-orbit scattering within the magnet should be minimized for efficient SOT devices. We also find that the interfacial spin-mixing conductance of interfaces of ferrimagnetic alloys (such as FexTb1-x) is as large as that of 3d ferromagnets and insensitive to the degree of magnetic compensation. There has been a lot of interest in using antiferromagnets for magnetic memories, due to their fast dynamics, and resilience to stray fields. Such a memory was supposed to be switched by a spin-orbit torque. Here, Zhu and Ralph find that as a ferrimagnet approaches the magnetic compensation point, the spin-orbit torque acting on the ferrimagnet vanishes due to competing spin relaxation processes.

Automated summary

Spin-orbit torques acting on ferrimagnetic FexTb1-x layers decrease and vanish upon approaching the magnetic compensation point due to the competing spin relaxation processes. The relative rates of spin relaxation within magnetic layers play a critical role in determining the strength of SOTs, providing a unified understanding for diverse SOT phenomena. It is important to minimize spin-orbit scattering within the magnet for efficient SOT devices, and the interfacial spin-mixing conductance of ferrimagnetic alloys is large and insensitive to magnetic compensation.