Emergent spin Hall conductivity in tantalum-rhenium alloys

We investigate the spin Hall conductivity (SHC) of a composition series of a Ta-Re bcc solid solution. At approximately 60 at.% Ta the Ta-Re alloy features a SHC similar to bcc-W, while both endpoints of the compositional series have rather moderate SHC. The intermediate stoichiometries exhibit a substantial enhancement due to Fermi-level tuning through the same band structure feature which gives bcc-W and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>β</a:mi></a:math>-W its large SHC. We provide experimental evidence for the enhancement of the SHC in the alloy via THz emission upon ultrafast laser excitation of Ta-Re/CoFeB bilayers. We demonstrate that a rigid-band model derived from bcc-W and Fermi-level tuning describes the experimental data with a similar accuracy as the coherent-potential approximation alloy calculations of the SHC.