Role of B diffusion in the interfacial Dzyaloshinskii-Moriya interaction in Ta / Co₂₀ Fe₆₀B₂₀/MgO nanowires

We report on current-induced domain wall motion in Ta/Co20Fe60B20/MgO nanowires. Domain walls are observed to move against the electron flow when no magnetic field is applied, while a field along the nanowires strongly affects the domain wall motion velocity. A symmetric effect is observed for up-down and down-up domain walls. This indicates the presence of right-handed domain walls, due to a Dzyaloshinskii-Moriya interaction (DMI) with a DMI coefficient D=+0.06mJ/m2. The positive DMI coefficient is interpreted to be a consequence of B diffusion into the Ta buffer layer during annealing, which was observed by chemical depth profiling measurements. The experimental results are compared to one-dimensional model simulations including the effects of pinning. This modeling allows us to reproduce the experimental outcomes and reliably extract a spin-Hall angle θSH=-0.11 for Ta in the nanowires, showing the importance of an analysis that goes beyond the model for perfect nanowires.