Thickness dependent spin to charge interconversion efficiency in polycrystalline BiSb layers deposited on Si substrate

Abstract

The strong spin–orbit torque generated by topological insulators (TIs) interfaced with ferromagnetic layers paves the path toward the low-power, high-speed spintronic device applications. To date, large charge-spin or spin-charge conversion efficiency (ξ) of TIs is accomplished on high-quality epitaxially grown TI thin films on specifically oriented substrates. Here, we report the sputtering growth of polycrystalline BiSb on an industrially adaptable Si substrate, preserving the topological surface states (TSS) in BiSb. We have performed spin pumping and inverse spin Hall effect measurements on BiSb(x)/Ti(y)/Py(10 nm) stacks by varying the thicknesses of BiSb and Ti insertion layers (ILs). The ξ has improved from 8 to 12 nm of BiSb, and we found ξ of 3.27 in the BiSb(12 nm)/Ti(3 nm)/Py(10 nm) sample. This increasing trend in ξ with the thickness of BiSb is also consistently observed in spin-torque ferromagnetic resonance measurements. The improvement in ξ is attributed to the improved stability of TSS in BiSb when the thickness increased from 8 to 12 nm. The Ti IL thickness dependence of the ξ study has shown that a 3 nm Ti layer has successfully achieved a trade-off to hinder the interdiffusion between BiSb and NiFe and promote the efficient spin current transport, highlighting the critical role of IL thickness. Our results demonstrate the process involved in depositing a homogeneous BiSb layer directly on the Si substrate and the impact of the thickness of the IL and BiSb on the ξ in topological insulator/ferromagnet bilayer systems.