  Heterostructures utilizing topological insulators exhibit a remarkable
spin-torque efficiency. However, the exact origin of the strong torque, in
particular whether it stems from the spin-momentum locking of the topological
surface states or rather from spin-Hall physics of the topological-insulator
bulk remains unclear. Here, we explore a mechanism of spin-torque generation
purely based on the topological surface states. We consider
topological-insulator-based bilayers involving ferromagnetic metal (TI/FM) and
magnetically doped topological insulators (TI/mdTI), respectively. By ascribing
the key theoretical differences between the two setups to location and number
of active surface states, we describe both setups within the same framework of
spin diffusion of the non-equilibrium spin density of the topological surface
states. For the TI/FM bilayer, we find large spin-torque efficiencies of
roughly equal magnitude for both in-plane and out-of-plane spin torques. For
the TI/mdTI bilayer, we elucidate the dominance of the spin-transfer-like
torque. However, we cannot explain the orders of magnitude enhancement
reported. Nevertheless, our model gives an intuitive picture of spin-torque
generation in topological-insulator-based bilayers and provides theoretical
constraints on spin-torque generation due to topological surface states.
