  The effect of damping-like spin-orbit torque (DL SOT) on magnetic domain
walls (DWs) in in-plane magnetised soft tracks is studied analytically and with
micromagnetic simulations. We find that DL SOT drives vortex DWs, whereas
transverse DWs, the other typical DW structure in soft tracks, propagate only
if the Dzyaloshinskii-Moriya interaction (DMI) is present. The SOT drive can
add to, and be more efficient than, spin-transfer torque (STT), and so may
greatly benefit applications that require in-plane DWs. Our analysis based on
the Thiele equation shows that the driving force arises from a cycloidal
distortion of the DW structure caused by geometrical confinement or DMI. This
distortion is higher, and the SOT more efficient, in narrower, thinner tracks.
These results show that the effects of SOT cannot be understood by simply
considering the effective field at the center of the structure, an ill-founded
but often-used estimation. We also show that the relative magnitude of STT and
DL SOT can be determined by comparing the motion of different vortex DW
structures in the same track.
