Asymmetric Spin-Orbit-Torque-Induced Magnetization Switching With a Noncollinear In-Plane Assisting Magnetic Field

The behaviors of spin-orbit-torque-(SOT) induced magnetization switching under an in-plane assisting magnetic field with varied directions are investigated in heavy metal (Pt, Ta)/Co40Fe40B20/MgO perpendicularly magnetized multilayers. When the assisting field direction is not collinear with that of the electric current, asymmetric SOT-induced switching between up-to-down and down-to-up magnetized states is observed and this is more prominent for the Ta sample. Such asymmetry is attributed to the transverse SOT effective field H-T, which is -566 (278) Oe per 10(8) A/cm(2) for the Ta (Pt) sample as characterized by the harmonic Hall voltage measurements. The atomistic simulations using the Landau-Lifshitz-Gilbert equation with inclusion of the SOT terms adequately explain the asymmetric SOT switching. By exploring the SOT-induced magnetization switching in more general scenarios with arbitrary direction of the in-plane assisting field, our results have important implications for optimizing the functionality and efficiency of SOT devices.