  Current-induced domain wall (DW) motion has drawn great attention in the last
decades as the key operational principle of emerging magnetic memory devices.
As the major driving force of the current-induced DW motion, the spin-orbit
torque (SOT) on chiral DWs has been proposed and extensively studied nowadays.
However, we demonstrate here that there exists another driving force, which is
larger than the SOT in ultra-thin Co films. Moreover, the direction of the
present force is found to be opposite to the prediction of the spin-transfer
torque (STT), resulting in the DW motion along the current direction. The
symmetry of the force and its peculiar dependence on the DW structure suggest
that the present force is, most likely, attributed to considerable enhancement
of the nonadiabatic STT with a negative spin polarization in atomically thin Co
layers. These findings open a new pathway to enhance the overall spin torque
efficiency-the critical parameter in emerging spintronic devices.
