  We consider theoretically domain wall motion driven by spin-orbit and spin
Hall torques. We find that it is possible to achieve universal absence of
Walker breakdown for all spin-orbit torques using experimentally relevant
spin-orbit coupling strengths. For spin-orbit torques other than the pure
Rashba spin-orbit torque, this gives a linear current-velocity relation instead
of a saturation of the velocity at high current densities. The effect is very
robust and is found in both soft and hard magnetic materials, as well as in the
presence of the Dzyaloshinskii-Moriya interaction and in coupled domain walls
in synthetic antiferromagnets, where it leads to very high domain wall
velocities. Moreover, recent experiments have demonstrated that the switching
of a synthetic antiferromagnet does not obey the usual spin Hall
angle-dependence, but that domain expansion and contraction can be selectively
controlled toggling only the applied in-plane magnetic field magnitude and not
its sign. We show for the first time that the combination of spin Hall torques
and interlayer exchange coupling produces the necessary relative velocities for
this switching to occur.
