  It is well known that skyrmions can be driven using spin-orbit torques due to
the spin-Hall effect. Here we show an additional contribution in multilayered
stacks arises from vertical spin currents due to inter-layer diffusion of a
spin accumulation generated at a skyrmion. This additional interfacial spin
torque is similar in form to the in-plane spin transfer torque, but is
significantly enhanced in ultra-thin films and acts in the opposite direction
to the electron flow. The combination of this diffusive spin torque and the
spin-orbit torque results in skyrmion motion which helps to explain the
observation of small skyrmion Hall angles even with moderate magnetisation
damping values. Further, the effect of material imperfections on threshold
currents and skyrmion Hall angle is also investigated. Topographical surface
roughness, as small as a single monolayer variation, is shown to be an
important contributing factor in ultra-thin films, resulting in good agreement
with experimental observations.
