UCLA

Spin accumulation originating from relativistic spin-orbit interaction can exert spin-orbit torque (SOT) on adjacent magnetization and switch the direction of the magnetization. Such current-driven magnetization switching holds promise for miniaturized magnetic memory devices with high speed, low power consumption, and non-volatility. One of the promising material candidates is topological insulator (TI). TI is a novel quantum state of matter with gapped bulk bands and gapless surface states protected by topology. The spin and momentum of the carriers from the surface states are locked, and the spin-momentum locked carriers can drive giant SOT on neighboring magnetization. However, there are still several issues in the study of SOT from TIs. First, there have been large discrepancies in the reported values of SOT from TIs. Second, the magnetization switching by TIs requires the assistance of an external magnetic field, which results in limited applicability.

In this dissertation, I systematically investigate magnetically doped TI thin films and determine the SOT via both transport and optic approaches, collaborating with other group members. Large SOT generated by the topological surface states with consistent results is observed. The experimental results reveal a strong dependence of SOT on temperature and surface state carrier concentration. The SOT decreases drastically as temperature increases and can be manipulated by tuning the surface state carrier concentration. A competition between the top surface and bottom surface in contributing to SOT is also observed. The above phenomena could account for the large discrepancies in the reported SOT values. Utilizing the SOT from surface states, I am able to achieve current-driven magnetization switching in magnetically doped TIs. I also investigate TI/antiferromagnetic material heterostructures and demonstrate the realization of field-free magnetization switching in this material system. Accomplished by symmetry breaking with interfacial exchange-bias, the field-free switching can be driven by pulsed current with ultra-low current density. The study in this dissertation advances the understanding of SOT from TI as well as the implementation of practical and energy-efficient magnetic random-access memory.