Oxidation-Tuned CuO _<i>x</i> for Spin–Orbit Torque Efficiency Enhancement

In this study, we demonstrate that oxidation-controlled CuO_<i>x</i> layers can serve as effective sources of orbital current for enhancing damping-like spin-orbit torque (SOT) in CoFeB/Pt heterostructures. By reactively sputtering CuO_<i>x</i> under varied oxygen concentrations, we systematically tuned its oxidation state and evaluated its impact on SOT efficiency through harmonic Hall voltage measurements. A peak damping-like SOT efficiency of |ξ_<i>DL</i>| ≈ 0.30 was achieved at Q = 4% with a CuO_<i>x</i> thickness of 3 nm and a Pt thickness of 4 nm, representing a ∼76% enhancement over the Pt-only control structure. SOT efficiency exhibited a nonmonotonic dependence on CuOx thickness, peaking at 3 m and then decreasing, suggesting limited propagation or interfacial conversion saturation. Similarly, tuning Pt thickness revealed that orbital-to-spin conversion is most effective around 4 nm, consistent with the expected spin diffusion behavior. In contrast, naturally oxidized Cu samples reached a maximum |ξ_<i>DL</i>| of ≈0.23 (∼35% enhancement), with limited tunability. These results confirm that while both natural and reactive oxidation can induce orbital torques, only controlled reactive sputtering yields consistently strong and stable effects. The findings establish CuO_<i>x</i> as a scalable orbital torque source and underscore the importance of oxidation control and interfacial engineering in next-generation spintronic devices.