High-Efficiency Continuous Spin-Conduction through NiO/Cu Bilayer Structure.

Materials that effectively separate charge and spin currents are key to advancing spin-orbit torque-based switching devices for nanomagnet memory. NiO, an insulating yet spin-conducting material, is essential in such systems. Interfacing NiO with a heavy metal like Pt, confines charge current to Pt while allowing spin current to pass through NiO into an adjacent NiFe layer. Introducing a spin-transparent Cu layer between NiO and Py prevents exchange interactions, transmits spin torque, and ensures a uniform magnetic environment at the Py interface, ensuring device reliability. To study spin-current conduction, we use dc bias-dependent spin-torque ferromagnetic resonance (ST-FMR) on nanobridges patterned from a Pt/NiO/Cu/NiFe stack with varying NiO thickness. Results show that a highly spin-transparent (93%) Cu spacer enables >40% spin-current transmission through defect-free NiO/Cu bilayers for NiO thicker than 1.5 nm. This stack demonstrates effective charge-spin separation and flexibility, with seamless spin-torque conversion from magnonic to electronic transport, enabling new spin-current-based device designs.