  We address the nature of spin-orbit torques at the magnetic surfaces of
topological insulators, using both Kubo formula and Keldysh formalism. Through
the analysis of the derived spin-charge equations, we find that the so-called
Dirac torques possess a different symmetry compared to their Rashba
counterpart, as well as a high anisotropy as a function of the magnetization
direction. In particular, the damping torque vanishes when the magnetization
lies in the plane of the topological insulator surface. This peculiarity has
important consequences in terms of magnetization dynamics and switching, as
demonstrated numerically via a macrospin model.
