Three-dimensional (3D) topological insulators (TIs) with large spin Hall conductivity have emerged as potential candidates for spintronic applications. Here, we report spin to charge conversion in bilayers of amorphous ferromagnet (FM) ${\mathrm{Fe}}_{78}{\mathrm{Ga}}_{13}{\mathrm{B}}_9$ (FeGaB) and 3D TI ${\mathrm{Bi}}_{85}{\mathrm{Sb}}_{15}$ (BiSb) activated by two complementary techniques: spin pumping and ultrafast spin-current injection. DC magnetization measurements establish the soft magnetic character of FeGaB films, which remains unaltered in the heterostructures of FeGaB-BiSb. Broadband ferromagnetic resonance (FMR) studies reveal enhanced damping of precessing magnetization and large value of spin mixing conductance $(5.03\times {10}^{19}{\mathrm{m}}^{\text{–}2})$ as the spin angular momentum leaks into the TI layer. Magnetic field controlled bipolar DC voltage generated across the TI layer by inverse spin Hall effect is analyzed to extract the values of spin Hall angle and spin diffusion length of BiSb. The spin pumping parameters derived from the measurements of the femtosecond light-pulse-induced terahertz emission are consistent with the result of FMR. The Kubo-Bastin formula and tight-binding model calculations shed light on the thickness-dependent spin-Hall conductivity of the TI films, with predictions that are in remarkable agreement with the experimental data. Our results suggest that room temperature deposited amorphous and polycrystalline heterostructures provide a promising platform for creating novel spin orbit torque devices.

• Received 27 September 2021
• Accepted 22 November 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.124410