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. The efficiency enhancement is attributed to a scattering-mediated extrinsic spin Hall contribution arising from the granular Pt morphology, an effect not previously proposed or observed. 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 unconventional SOT pathways in ultrathin heterostructures and provide new insights for optimizing spin–orbitronic device performance and enabling energy-efficient magnetic switching.