Current-induced resonant motion of a magnetic vortex core: Effect of nonadiabatic spin torque

The current-induced resonant excitation of a magnetic vortex core is investigated by means of analytical and micromagnetic calculations. We find that the radius and phase shift of the resonant motion are not correctly described by the analytical equations because of the dynamic distortion of a vortex core. In contrast, the initial tilting angle of a vortex core is free from the distortion and determined by the nonadiabaticity of the spin torque. It is insensitive to experimentally uncontrollable current-induced in-plane Oersted field and thus allows a direct comparison with experimental results. We propose that a time-resolved imaging of the very initial trajectory of a core is a plausible way to experimentally estimate the nonadiabaticity.