Angular dependence and symmetry of Rashba spin torque in ferromagnetic heterostructures

In a ferromagnetic heterostructure, the interplay between Rashba spin-orbit coupling and exchange splitting gives rise to a current-driven spin torque. In a realistic device setup, we investigate the Rashba spin torque in the diffusive regime and report two major findings: (i) a nonvanishing torque exists at the edges of the device even when the magnetization and effective Rashba field are aligned; (ii) anisotropic spin relaxation rates driven by the Rashba spin-orbit coupling assign the spin torque a general expression $T=T∥y(θ)m×(ŷ×m)+T⊥y(θ)ŷ×m+T∥z(θ)m×(ẑ×m)+T⊥z(θ)ẑ×m$⁠, where the coefficients $T∥,⊥y,z$ depend on the magnetization direction. Our results agree with recent experiments.