Giant Spin-Orbit Torque in Sputter-Deposited Bi Films

Abstract

© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH. Bismuth (Bi) has the strongest spin-orbit coupling among non-radioactive elements and is thus a promising material for efficient charge-to-spin conversion. However, previous electrical detections have reported controversial results for the conversion efficiency. In this study, an optical detection of a spin-orbit torque is reported in a Bi/CoFeB bilayer with a polycrystalline texture of (012) and (003). Taking advantage of the optical detection, spin-orbit torque is accurately separated from the Oersted field and achieves a giant damping-like torque efficiency of +0.5, verifying efficient charge-to-spin conversion. This study also demonstrates a field-like torque efficiency of -0.1. For the mechanism of the charge-to-spin conversion, the bulk spin Hall effect and the interface Rashba-Edelstein effect are considered. Spin-orbit torque from polycrystalline bismuth (Bi) film is investigated using an optical method of magneto-optical Kerr effect (MOKE). Employing the spatial and vector resolution of the MOKE microscopy, damping-like torque (hDL) and field-like torque (hFL) are clearly separated from the Oersted field (hOe). Large torque efficiencies of +0.5 and -0.1, respectively for hDL and hFL, are achieved.image