Domain wall motion in nanopillar spin-valves with perpendicular anisotropy driven by spin-transfer torques

Using transport measurements and micromagnetic simulations we have investigated the domain wall motion driven by spin-transfer torques in all-perpendicular hexagonal nanopillar spin-valves. In particular, we probe domain walls nucleated in the free layer of the spin-valves, which are then pinned in the devices. We have determined both the field-current state diagrams for the domain-wall state and the thermally activated dynamics of the nucleation and depinning processes. We show that the nucleation process is well-described by a modified Neel-Brown model taking into account the spin-transfer torque, whereas the depinning process is independent of the current. This is confirmed by an analytical calculation which shows that spin-torques have no effect on the Arrhenius escape rate associated with thermally activated domain wall depinning in this geometry. Furthermore, micromagnetic simulations indicate that spin-transfer only weakly affects the domain wall motion, but instead modifies the inner domain wall structure.