  In order to explain recent experiments reporting a motion of magnetic domain
walls (DW) in nanowires carrying a current, we propose a modification of the
spin transfer torque term in the Landau-Lifchitz-Gilbert equation. We show that
it explains, with reasonable parameters, the measured DW velocities as well as
the variation of DW propagation field under current. We also introduce
coercivity by considering rough wires. This leads to a finite DW propagation
field and finite threshold current for DW propagation, hence we conclude that
threshold currents are extrinsic. Some possible models that support this new
term are discussed.
