Enhancement of Spin-Orbit Torque in Pt/Co/HfO_x Heterostructures with Voltage-Controlled Oxygen Ion Migration

Spin-orbit torque (SOT) induced magnetization switching and SOT modulation have been intensely pursued and developed in recent years for efficient spintronic devices. Research has shown that the magnetic anisotropy of ferromagnet (FM) layers can be modified via inducing surface charges or orbital occupancy by interfacing with an oxide layer. Moreover, voltage-controlled oxygen migration can reduce the energy barrier for magnetization switching. The electric-field-driven interfacial oxygen mobility can affect both the SOT strength and sign. However, more research on the oxidation effects at the interfaces of the voltage-controlled magnetism in FM/oxide systems needs to be investigated. In this work, we report the enhancement of damping-like field and SOT efficiency of up to 60% and 23%, respectively, in perpendicularly magnetized Pt/Co/HfO_x heterostructures over Pt/Co system at an optimal thickness of 2 nm HfO_x. We further verify this enhanced SOT by applying voltages across the Pt/Co/HfO_x structure, which allows the O²⁻migration from HfO_x to the Co layer. By using voltage-controlled oxygen ion modulation of interfacial oxidation, we show that the gate voltage affects not only coercivity and anisotropy fields but also the SOT efficiency at room temperature. Our work promotes the SOT enhancement and modulation by oxidation effects for energy-efficient spintronic devices.