Current-induced spin torques in III-V ferromagnetic semiconductors

We formulate a theory of current-induced spin torques in inhomogeneous III-V ferromagnetic semiconductors. The carrier spin 3/2 and large spin-orbit interaction, leading to spin nonconservation, introduce significant conceptual differences from spin torques in ferromagnetic metals. We determine the spin density in an electric field in the weak momentum scattering regime, demonstrating that the torque on the magnetization is intimately related to spin precession under the action of both the spin-orbit interaction and the exchange field characteristic of ferromagnetism. The spin polarization excited by the electric field is smaller than in ferromagnetic metals and, due to lack of angular-momentum conservation, cannot be expressed in a simple closed vectorial form. Remarkably, scalar and spin-dependent scatterings do not affect the result. We use our results to estimate the velocity of current-driven domain walls.