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Spin wave mediated vortex core reversal has been investigated bytime-resolved scanning transmission X-ray microscopy (STXM). Movies showing thedevelopment of the spin wave and vortex core magnetization dynamics duringunidirectional vortex core reversal could be taken in Permalloy discs, 1.6 \mum in diameter and 50 nm thick, during excitation with rotating ac field burstsof one period duration at 4.5 GHz and with amplitudes up to 4 mT.Unidirectional switching is achieved by taking advantage of an asymmetry for CWor CCW excitation caused by the gyrofield. The differences in the magnetizationdynamics due to this asymmetry could be imaged during continuous excitationwith multi-GHz rotating fields. All our experimental results are in goodagreement with micromagnetic simulations.  In addition, for the sample geometry given above, simulations reveal a lowerlimit of about 200 ps for the time of unidirectional vortex core switching,which cannot be overcome by shortening the excitation length or by increasingthe excitation amplitude. We explain this limitation by the finite time neededfor an energy transfer of the global excitation towards the center of thesample. Thus smaller samples will allow for much shorter vortex core reversaltimes.

200 ps Vortex Core Reversal by Azimuthal Spin Waves