Time-resolved scanning transmission X-ray microscopy was implemented by our group at the Advanced Light Source (ALS), Berkeley, at the Canadian Light Source (CLS), Saskatoon and at BESSY II, Berlin. A sophisticated data acquisition technique allows us to take advantage of the full photon flux in the standard multi-bunch mode of the accelerator for measurements with almost any excitation frequency up to about 12 GHz.

By applying the above measurement technique on micron sized magnetic vortex structures we discovered low-field switching of the vortex core when exciting the gyrotropic eigenmode of the vortex structure with an RF magnetic field burst [1]. A model was suggested [1] and later confirmed experimentally [2], which is based on the creation and annihilation of a vortex – anti-vortex (VA) pair. This VA mechanism for vortex core reversal is now generally accepted in the community.

By excitation with rotating RF magnetic fields the vortex core can be switched in an ‘unidirectional’ way, either to its up or its down position only, depending on the sense of rotation of this external RF field [3].

Vortex structures possess azimuthal spin wave modes with much higher eigenfrequencies compared to the vortex gyromode. We demonstrated by experiments and micromagnetic simulations that unidirectional low-field vortex core reversal is also possible by exciting these spin wave eigenmodes with rotating multi-GHz magnetic fields [4] or with orthogonal field pulse sequences in x and y direction [5]. In that way we achieved vortex core switching times below 100 ps [5] (from the start of the orthogonal pulse sequence to the end of the vortex core reversal).