  In systems with strong spin-orbit coupling, the relationship between
spin-transfer torque and the divergence of the spin current is generalized to a
relation between spin transfer torques, total angular momentum current, and
mechanical torques. In ferromagnetic semiconductors, where the spin-orbit
coupling is large, these considerations modify the behavior of the spin
transfer torques. One example is a persistent spin transfer torque in a spin
valve: the spin transfer torque does not decay away from the interface, but
approaches a constant value. A second example is a mechanical torque at single
ferromagnetic-nonmagnetic interface.
