  Skyrmions can be driven by spin-orbit torques as a result of the spin Hall
effect. Here we model an additional contribution in ultra-thin multilayers,
arising from the spin accumulation at heavy metal / ferromagnetic interfaces
and observe the effects on a large range of skyrmion diameters. The combination
of the interfacial spin-transfer torque and the spin-orbit torque results in
skyrmion motion which helps to explain the observation of small skyrmion Hall
angles for skyrmion diameters less than 100 nm. We show that this additional
term has a significant effect on the skyrmion dynamics and leads to rapidly
decreasing skyrmion Hall angles for small skyrmion diameters, as well as a
skyrmion Hall angle versus skyrmion velocity dependence nearly independent of
the surface roughness characteristics. Also, the effect of various disordered
energy landscapes, in the form of surface roughness, on the skyrmion Hall angle
and velocity is shown to be largely drive-dependent. Our results show good
agreement with those found in experiments thus concluding that the interfacial
spin-transfer torque should be included in micromagnetics simulations for the
reproduction of experimental results.
