On the origin of bulk spin-orbit torque in thickness-graded Co/Pt multilayers

<h2>Summary</h2> Bulk spin-orbit torque (BSOT) exhibits promise for low-power spintronics, but its fundamental properties, including its sign and magnitude, remain elusive. In this work, a spin-current-source capping layer with opposite sign (Pt and Ta) is introduced to thickness-graded Co/Pt multilayers with BSOT, and the effects of capping layers on magnetization switching and SOT efficiency are systematically explored. It is demonstrated that Ta enhances while Pt reduces SOT efficiency, with 1.8 nm graded Pt generating SOT comparable to 3 nm Pt. More importantly, BSOT primarily originates from graded Pt layers, while Co/Pt thickness grading synergistically enhances efficiency. Additionally, similar results were achieved using miscut Al₂O₃ substrates that allow full field-free switching of perpendicular magnetization, further validating the sign and magnitude of BSOT. Our results reveal the origin of BSOT in thickness-graded Co/Pt multilayers, enabling field-free switching via miscut substrate and advancing the potential applications of BSOT for low-power spintronic devices.