The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons

TL;DR

This study reveals that Bi-Sb alloys exhibit high spin Hall conductivity driven by thermally excited Dirac-like electrons, with bulk contributions dominating and temperature enhancing the effect, useful for spintronic applications.

Contribution

It demonstrates that the spin Hall effect in Bi-Sb alloys is primarily due to bulk thermally excited electrons, with minimal influence from topological surface states, and shows temperature-dependent enhancement.

Findings

High spin Hall conductivity at Bi-rich compositions

SHC increases with alloy thickness until saturation

SHC increases with temperature due to thermally excited electrons

Abstract

We have studied the charge to spin conversion in Bi$_{1-x}$Sb$_x$/CoFeB heterostructures. The spin Hall conductivity (SHC) of the sputter deposited heterostructures exhibits a high plateau at Bi-rich compositions, corresponding to the topological insulator phase, followed by a decrease of SHC for Sb-richer alloys, in agreement with the calculated intrinsic spin Hall effect of Bi$_{1-x}$Sb$_x$ alloy. The SHC increases with increasing thickness of the Bi$_{1-x}$Sb$_x$ alloy before it saturates, indicating that it is the bulk of the alloy that predominantly contributes to the generation of spin current; the topological surface states, if present in the films, play little role. Surprisingly, the SHC is found to increase with increasing temperature, following the trend of carrier density. These results suggest that the large SHC at room temperature, with a spin Hall efficiency exceeding 1 and an extremely large spin current mobility, is due to increased number of Dirac-like, thermally-excited electrons in the $L$ valley of the narrow gap Bi$_{1-x}$Sb$_x$ alloy.