Enhanced Spin-Orbit Torque Efficiency by the Insertion of a Thin NiO Underlayer in Pt/Co/Ta Films

In this study, significant modulations of perpendicular magnetic anisotropy, spin-orbit torque (SOT) efficiency (ξ_SH), and magnetic damping have been achieved in Pt/Co/Ta films after inserting an insulating NiO underlayer. As the NiO thickness (t_NiO) varies from 0 to 2 nm, ξ_SH exhibits a nonmonotonic variation trend, initially increasing and then decreasing after reaching a maximum value at t_NiO = 1 nm. The maximum SOT efficiency is as high as 0.60, nearly twice as large as that in the sample without NiO. Based on the NiO thickness dependencies of effective perpendicular anisotropy field and magnetic damping factor, we can conclude that the complex variation in ξ_SH is attributable to the combined effects of modified interfacial orbital hybridization and spin absorption by the NiO layer. Significantly, the latter is identified as the primary enhancement mechanism at t_NiO < 1 nm, as it prevents spins with adverse polarization from reflecting back to the magnetic layer. Our findings demonstrate an alternative approach toward high SOT efficiency by engineering heterostructures with a magnetic insulator underlayer, which may play a powerful role in developing advanced energy-efficient spintronic devices.