Effects of metal, oxide, and hybrid metal-oxide interlayers on spin–orbit torque in BiSb topological insulator and magnetic interfaces

The charge-to-spin conversion efficiency at the interface between a topological insulator and a ferromagnetic (FM) layer can be enhanced by inserting an interlayer (ITL). However, the mechanism of this enhancement is unclear. In this work, we systematically investigate the ITL effects by introducing various metal, oxide, and hybrid metal/oxide ITL between a BiSb topological insulator and a CoFe ferromagnetic layer. Our findings revealed that using a metallic NiFeGe or insulating MgO ITL resulted in similarly high efficiencies, with the highest efficiency achieved when employing hybrid NiFeGe/MgO ITL. However, efficiency decreased when NiFeGe was combined with MgTiO with increasing TiO composition. Such behaviors can be qualitatively understood by considering the ITL intrinsic effect of enhancing the intrinsic spin Hall angle of BiSb via preventing Sb diffusion from BiSb and migration of the FM and the extrinsic effect where the ITL spin transparency is determined by the spin tunneling/diffusion across the ITL and the spin-flip/spin-transfer at the ITL/FM interface. This study provides valuable insights and a framework for understanding and optimizing interlayer materials for ultralow power spin–orbit torque applications.