Observation of Out-of-Plane Antidamping Torque at the Platinum/Permalloy Interface

Achieving electrical control of ferromagnets without magnetic fields is crucial for the dense integration of nanodevices in modern memory and computing technologies. Current methods using spin orbit torques from the spin Hall effect and interfacial Rashba effect are limited to in-plane magnetized ferromagnets. Out-of-plane antidamping torque is essential for the electrical only control of ferromagnets with perpendicular magnetic anisotropy. In this work, we report the observation of out-of-plane polarized spin currents in platinum/permalloy bilayers, linked to interfacial perpendicular magnetic anisotropy at the interface between two metallic layers, as revealed by polarized neutron reflectometry. In-plane angle-resolved spin-torque ferromagnetic resonance measurements characterized the out-of-plane damping-like torque, constituting about 12% of the total torque in ultrathin Pt films, which vanishes when platinum thickness exceeds 4 nm, confirming its interfacial origin. This interfacial perpendicular magnetic anisotropy-induced torque is significant compared to the bulk spin Hall effect, which can be obtained in a typical heavy metal/ferromagnet bilayer. This advancement holds promise for enhancing the efficiency and reliability of spin orbit torque magnetic random-access memory (SOT-MRAM), spin Hall oscillators, and other spintronic devices.