Spin-Transfer-Torque-Assisted Domain-Wall Creep in a Co/Pt Multilayer Wire

We have studied field-and current-driven domain-wall (DW) creep motion in a perpendicularly magnetized Co/Pt multilayer wire by real-time Kerr microscopy. The application of a dc current of density of less than or similar to 10(7) A/cm(2) assisted only the DW creeping under field in the same direction as the electron flow, a signature of spin-transfer torque effects. We develop a model dealing with both bidirectional spin-transfer effects and Joule heating, with the same dynamical exponent mu = 1/4 for both field-and current-driven creep, and use it to quantify the spin-transfer efficiency as 3.6 +/- 0.6 Oe cm(2)/MA in our wires, confirming the significant nonadiabatic contribution to the spin torque.