Spin-orbit torque measurements in heavy metal/ferromagnet heterostructures using the magneto-optic Kerr effect

Abstract

Spin-orbit coupling in heavy-metal/ferromagnet (HM/FM) bilayer heterostructures has attracted considerable attention because it provides an efficient way to manipulate the magnetization with strong current-driven spin-orbit torques (SOTs), which may lead to new technologies for nonvolatile magnetic memory and logic devices. An electric current flowing through a heavy metal generates a field-like spin-orbit torque (FT) and a damping-like spin-orbit torque (DT) on the magnetization of a neighboring ferromagnet. Two mechanisms have been proposed to explain the generation of SOTs: the Rashba-Edelstein effect due to interfacial spin-orbit coupling and the spin Hall effect in the bulk of materials with strong spin-orbit coupling (SOC). Much effort has been dedicated to identifying the dominant mechanism of the SOTs; however, the underlying mechanism for the SOC-driven phenomena remained unsettled. In this thesis, we develop a sensitive SOT magnetometer based on the magneto-optic Kerr effect (MOKE) that measures the SOTs for HM/FM bilayers over a wide thickness range. We observe that the DT inversely scales with the ferromagnet thickness, and the FT has a threshold effect that appears only when the ferromagnetic layer is thinner than 1 nm. Through a thickness-dependence study with an additional copper insertion layer at the interface, we conclude that both SHE and Rashba effect exist in HM/FM heterostructures. The relative strengths of their contributions depend on the material system. ☐ We have also demonstrated that MOKE with normal incidence light can be used to obtain the DT and FT in HM/FM bilayers by analyzing the polar Kerr effect as well as the quadratic Kerr effect. The two effects can be distinguished by properly selecting the polarization of the incident light. We study a series of Pt/Py bilayers to verify the accuracy of this method. The angular dependence of SOTs in Ta (2 nm)/CoFeB (1 nm)/MgO (3 nm) trilayers with perpendicular magnetization is quantified based on polar MOKE with field calibration. A strong angular dependence is observed that is different from the previous experimental observations. Based on this strong angle dependence, we conclude there is a strong Rashba effect in this system. Simultaneous detection of current-driven DT and FT in HM/FM bilayers by measuring all three magnetization components , and using a vector-resolved MOKE technique based on quadrant detection has also been accomplished. The technique can be easily extended to measure SOTs in systems with perpendicular magnetization, as well as in systems with arbitrary magnetization direction.