Vortex-core reversal by rotating currents

The investigation of the interaction between a spin-polarized current and the magnetization of a ferromagnet is of great interest. One concept for data storage is to use the current- induced switching of the vortex-core polarization, i.e. the out- of-plane component of the magnetization in the center of a micronsized permalloy element. It has been shown both theoretically [1] and experimentally [2] that the polarization can be switched selectively by resonant field excitation. We carried out time-resolved scanning transmission X-ray microscopy while exciting the vortex core with rotating currents of varying frequency, amplitude and rotation sense. We observed vortex core switching, and by analyzing the gyration phase with respect to the exciting current we derive whether the Oersted-field or the spin torque mainly contributes to the excitation and causes the switching process. [1] S. K. Kim et al., Appl. Phys. Lett. \textbf{92}, 022509 (2008). [2] M. Curcic et al., Phys. Rev. Lett. \textbf{101}, 197204 (2008).