Increase of spin-transfer torque threshold current density in coupled vortex domain walls

We have studied the dependence on domain wall structure of the spin-transfer torque current density threshold for the onset of wall motion in curved, Gd-doped Ni80Fe20 nanowires with no artificial pinning potentials. For single vortex domain walls, both for 10% and 1% Gd doping concentrations, the threshold current density is inversely proportional to the wire width and significantly lower compared to the threshold current density measured for transverse domain walls. On the other hand for high Gd concentrations and large wire widths, double vortex domain walls are formed which require an increase in the threshold current density compared to single vortex domain walls at the same wire width. We suggest that this is due to the coupling of the vortex cores, which are of opposite chirality, and hence will be acted on by opposing forces arising through the spin-transfer torque effect.