Simple model of current-induced spin torque in domain walls

The effective spin pressure induced by an electric current on a domain wall in a ferromagnet is determined using a simple classical model, which allows us to extend previous theories to arbitrary domain-wall widths. In particular, the role of spatially nonuniform components of the torques are analyzed in detail. We find that in the steady state, the main effect of the current is a distortion of the wall, which should enhance depinning. We also discuss the nonadiabatic part of the torque and find that this term, responsible for the pressure on the wall, depends on the nature of spin-flip scattering events.