Domain wall motion induced by spin polarized currents in ferromagnetic ring structures

We present an experimental study of the influence of spin-polarized currents on the displacement of domain walls in submicrometer permalloy ring structures. Using magnetoresistance (MR) measurements with multiple nonmagnetic contacts, we can sense the displacement of a domain wall and, by injecting large dc current densities ${\text{(10}}^{11}\hspace{0.5em}{\mathrm{A/m}}^2\mathrm{),}$ we can increase or decrease the magnetic field needed to move a single domain wall, depending on the direction of the current with respect to the applied field direction. Using rings with and without notches and by measuring the MR with the magnetic field applied along different directions, we show that we can exclude the possibility that the dominating effect is a classical Oersted field. We conclude that our observations can be explained by a directional spin torque effect.