Spin-torque efficiency enhanced in sputtered topological insulator by interface engineering

The way inter-mixing chemical state influences the damping-torque efficiency of a spin-orbit torque (SOT) heterostructure is a matter of debate. This is because it acts as a local symmetry-breaking that differs from the global symmetry-breaking at the interface. A combination of angle-resolved X-ray photoelectron spectroscopy (AR-XPS), X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) was utilized to investigate the composition-gradient effects on the damping-torque efficiency of the Bi2Se3/insertion (Ta, Pt, Cu)/CoFeB SOT devices, with the insertion to be 0.5 and 1.0 nm. These embedded metals have the advantages of (i) Ta: suppressing CoFeB magnetic dead-layer (ii) Pt: large spin-hall angle (iii) Cu: larger spin-diffusion length. From AR-XPS we observed a sharper composition gradient for Pt-0.5 nm than Pt-1.0 nm in terms of the Se-Pt state. By XAS branching ratio and XMCD sum-rule analyses, we obtained larger spin-orbit coupling (SOC) and spin-polarization (SP) effects on the Pt-0.5, Pt-1.0 nm samples than Cu & Ta insertion. The sharper composition gradient and promoted SOC and SP validate the larger damping-torque efficiency in the Pt-0.5 nm than the Pt-1.0 nm sample. This work manifests interfacial engineering as a powerful avenue for modulating SOT efficiency in spintronic-based applications.