1. Max-Planck-Institut für Metallforschung, 70569 Stuttgart, Germany
2. Department of Subatomic and Radiation Physics, Ghent University, 9000 Gent, Belgium
3. Advanced Light Source, LBNL, 94720 Berkeley, California, USA
4. Institut für Festkörperforschung IFF-9 'Elektronische Eigenschaften', Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
5. Fakultät für Physik, Universität Bielefeld, 33615 Bielefeld, Germany
6. Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93040 Regensburg, Germany
7. Present address: Division 'Nano System Technology', ARCS research GmbH, Tech Gate Vienna, Donau-City-Strasse 1, 1220 Vienna, Austria.

Correspondence to: H. Stoll¹ Correspondence and requests for materials should be addressed to H.S. (Email: stoll@mf.mpg.de).

The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials^{1, 2, 3, 4} and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres—the vortex core^{5, 6, 7}. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position^{8, 9, 10}. The sense of gyration is determined by the vortex core polarization¹¹. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy¹². We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

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