Efficient spin-orbit torque switching in a magnetic insulator via ultrathin Pt and light metal overlayers

Spin-orbit torques (SOTs) are essential for electrically controlling magnetic order in spintronic devices. Platinum (Pt) is ubiquitous for SOT generation due to its strong bulk spin Hall and interfacial Rashba-Edelstein effects. Here, we revisit this established viewpoint by investigating ultrathin Pt films interfaced with a typical magnetic insulator, terbium iron garnet. We find that few-atom-thick, nanogranular Pt exhibits exceptionally efficient SOT-induced switching that cannot be explained by these conventional mechanisms. This enhancement is attributed to the granular morphology of sputtered Pt, which activates two complementary mechanisms: enhanced spin-orbit scattering at grain boundaries, leading to an increased effective spin Hall angle, and localized current density amplification due to non-uniform conduction paths. Furthermore, adding a titanium (Ti) or manganese (Mn) overlayer to thin Pt enhances the switching efficiency, indicating an active contribution from light metals via orbital current generation. These findings uncover key SOT pathways in ultrathin heterostructures and provide insights for optimizing spin-orbitronic device performance and enabling energy-efficient magnetic switching.