Colossal spin–orbit torques in gradient oxidation of platinum/hafnium heterostructures

Efficient manipulation of orbital angular momentum offers a promising route toward low-power spin–orbitronic devices. However, the role of the interfacial orbital phenomena in Hf-based heterostructures remains largely unexplored. Here, we report colossal spin–orbit torques (SOTs) in gradient-oxidized Pt/HfOx heterostructures, enabling low-current magnetization switching. Spin-torque ferromagnetic resonance measurements reveal a maximum SOT efficiency (ξFMR) of 0.60 ± 0.082, approximately 750% higher than that of the unoxidized counterpart. Control experiments using unoxidized and Cu-inserted heterostructures, together with Pt thickness-dependent measurements, identify the Pt/HfOx interface as the origin of the enhanced SOT. We attribute this enhancement to the interfacial orbital Rashba–Edelstein effect activated by gradient oxidation, where orbital polarization can be converted into spin polarization through spin–orbit coupling in Pt. Consequently, the critical switching current density is reduced to 1.48–2.58 × 106 A/cm2, providing an effective strategy for interfacial orbital engineering and energy-efficient spin–orbitronic devices.