Wedge-shaped HfO₂ buffer layer-induced field-free spin-orbit torque switching of HfO₂/Pt/Co structure

Field-free spin–orbit torque (SOT) switching of perpendicular magnetization is essential for future spintronic devices. This study demonstrates the field-free switching of perpendicular magnetization in an HfO₂/Pt/Co/TaO_x structure, which is facilitated by a wedge-shaped HfO₂ buffer layer. The field-free switching ratio varies with HfO₂ thickness, reaching optimal performance at 25 nm. This phenomenon is attributed to the lateral anisotropy gradient of the Co layer, which is induced by the wedge-shaped HfO₂ buffer layer. The thickness gradient of HfO₂ along the wedge creates a corresponding lateral anisotropy gradient in the Co layer, correlating with the switching ratio. These findings indicate that field-free SOT switching can be achieved through designing buffer layer, offering a novel approach to innovating spin–orbit device.