  Time-resolved X-ray microscopy is used to image the influence of alternating
high-density currents on the magnetization dynamics of ferromagnetic vortices.
Spin-torque induced vortex gyration is observed in micrometer-sized permalloy
squares. The phases of the gyration in structures with different chirality are
compared to an analytical model and micromagnetic simulations, considering both
alternating spinpolarized currents and the current's Oersted field. In our case
the driving force due to spin-transfer torque is about 70% of the total
excitation while the remainder originates from the current's Oersted field.
This finding has implications to magnetic storage devices using spin-torque
driven magnetization switching and domain-wall motion.
