Symmetry of spin orbit torque induced by impurities

Spin orbit torque (SOT) arises at the interfaces between ferromagnetic and heavy nonmagnetic metal layers once charge current flows parallel to the interfaces. SOT effect has attracted significant interest due to its potential applications for spin electronics. This effects originates from strong spin orbit coupling (SOC) of heavy 4d/5d elements and its mechanism is attributed either to the Spin Hall Effect (SHE) or Rashba Field Effect (RFE). Theoretical analysis shows that the SHE and RFE mechanisms responsible for correspondingly dominating field like (FLT) T_⊥ = T_⊥ (e×j) ×m and the damping like torques (DLT) T_|| = T_||m×((e×j) ×m) where e, j and m are the unit vectors in the directions of electric field gradient, current and magnetization respectively, as schematically shown on Fig. 1. However, recent experimental studies of SOT in Pt/Co/Ta heterostructures show that amplitudes of both torques have same order of magnitude and T_|| is about two times larger than T_⊥. Moreover, both FLT and DLT amplitudes linearly increase with the thickness of Ta layer indicating a non-interface origin of FLT. These new experimental results once again raise question what is dominating mechanism and if interface plays any significant role in defining how strong is observed SOT effect.