  (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ topological insulators (TIs) are gathering
increasing attention owing to their large charge-to-spin conversion efficiency
and the ensuing spin-orbit torques (SOTs) that can be used to manipulate the
magnetization of a ferromagnet (FM). The origin of the torques, however,
remains elusive, while the implications of hybridized states and the strong
material intermixing at the TI/FM interface are essentially unexplored. By
combining interface chemical analysis and spin-transfer ferromagnetic resonance
(ST-FMR) measurements, we demonstrate that intermixing plays a critical role in
the generation of SOTs. By inserting a suitable normal metal spacer, material
intermixing is reduced and the TI properties at the interface are largely
improved, resulting in strong variations in the nature of the SOTs. A dramatic
enhancement of a field-like torque, opposing and surpassing the Oersted-field
torque, is observed, which can be attributed to the non-equilibrium spin
density in Rashba-split surface bands and to the suppression of spin memory
loss.
