Field-Free Spin-Orbit Torque Switching of Perpendicular Magnetization by the Rashba Interface

Current-induced spin-orbit torques (SOTs) enable efficient electrical manipulation of the magnetization in heterostructures with a perpendicular magnetic anisotropy through the Rashba effect or spin-Hall effect. However, in conventional SOT-based heterostructures, an in-plane bias magnetic field along the current direction is required for the deterministic switching. Here, we report that the field-free SOT switching can be achieved by introducing a wedged oxide interface between a heavy metal and a ferromagnet. The results demonstrate that the field-free SOT switching is determined by a current-induced perpendicular effective field (H^eff) originating from the interfacial Rashba effect due to the lateral structural symmetry-breaking introduced by the wedged oxide layer. Furthermore, we show that the sign and magnitude of H^eff exhibit a significant dependence on the interfacial oxygen content, which can be controlled by the inserted oxide thickness. Our findings provide a deeper insight into the field-free SOT switching by the interfacial Rashba effect.