Spin-orbit torques and spin Hall magnetoresistance generated by twin-free and amorphous Bi0.9Sb0.1 topological insulator films

Abstract

Topological insulators have attracted great interest as generators of spin-orbit torques (SOTs) in spintronic devices. Bi\textsubscript{1-x}Sb\textsubscript{x} is a prominent topological insulator that has a high charge-to-spin conversion efficiency. However, the origin and magnitude of the SOTs induced by current-injection in Bi\textsubscript{1-x}Sb\textsubscript{x} remain controversial. Here we report the investigation of the SOTs and spin Hall magnetoresistance resulting from charge-to-spin conversion in twin-free epitaxial layers of Bi\textsubscript{0.9}Sb\textsubscript{0.1}(0001) coupled to FeCo, and compare it with that of amorphous Bi\textsubscript{0.9}Sb\textsubscript{0.1}. We find a large charge-to-spin conversion efficiency of 1 in the first case and less than 0.1 in the second, confirming crystalline Bi\textsubscript{0.9}Sb\textsubscript{0.1} as a strong spin injector material. The SOTs and spin Hall magnetoresistance are independent of the direction of the electric current, indicating that charge-to-spin conversion in single-crystal Bi\textsubscript{0.9}Sb\textsubscript{0.1}(0001) is isotropic despite the strong anisotropy of the topological surface states. Further, we find that the damping-like SOT has a non-monotonic temperature dependence with a minimum at 20~K. By correlating the SOT with resistivity and weak antilocalization measurements, we conclude that charge-spin conversion occurs via thermally-excited holes from the bulk states above 20~K, and conduction through the isotropic surface states with increasing spin polarization due to decreasing electron-electron scattering below 20~K.

arXiv commentSupporting information at https://doi.org/10.1002/adma.202304905

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• Estimation of spin-orbit torques in the presence of current-induced magnon creation and annihilation
• Epitaxial growth and characterization of Bi${}_{1-x}$Sb${}_x$ thin films on (0001) sapphire substrates
• Spin Hall conductivity in Bi${}_{1-x}$Sb${}_x$ as an experimental test of bulk-boundary correspondence
• Spin Hall magnetoresistance in Pt/(Ga,Mn)N devices
• Charge-to-spin conversion in epitaxial and polycrystalline Bi and Bi/Ag layers