Current-Induced Magnetization Switching by the High Spin Hall Conductivity α-W

The spin Hall effect originating from 5d heavy transition-metal thin films such as Pt, Ta, and W is able to generate efficient spin-orbit torques that can switch adjacent magnetic layers. This mechanism can serve as an alternative to conventional spin-transfer torque for controlling next-generation magnetic memories. Among all 5d transition metals, W in its resistive amorphous phase typically shows the largest spin-orbit torque efficiency approximate to 0.20-0.50. In contrast, its conductive and crystalline alpha phase possesses a significantly smaller efficiency of approximate to 0.03 and no spin-orbit torque switching is realized using alpha-W thin films as the spin Hall source. Herein, through a comprehensive study of high-quality W/CoFeB/MgO and the reversed MgO/CoFeB/W magnetic heterostructures, it is shown that although amorphous-W has a greater spin-orbit torque efficiency, the spin Hall conductivity of alpha-W (|sigma SH alpha-W|=3.71x105 omega-1 m-1) is approximate to 3.5 times larger than that of amorphous W (|sigma SHamorphous-W|=1.05x105 omega-1 m-1). Moreover, spin-orbit torque-driven magnetization switching using a MgO/CoFeB/alpha-W heterostructure is demonstrated. The findings suggest that the conductive and high spin Hall conductivity alpha-W is a potential candidate for future low-power consumption spin-orbit torque memory applications.