  Spin wave mediated vortex core reversal has been investigated by
time-resolved scanning transmission X-ray microscopy (STXM). Movies showing the
development of the spin wave and vortex core magnetization dynamics during
unidirectional vortex core reversal could be taken in Permalloy discs, 1.6 \mu
m in diameter and 50 nm thick, during excitation with rotating ac field bursts
of 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 CW
or CCW excitation caused by the gyrofield. The differences in the magnetization
dynamics due to this asymmetry could be imaged during continuous excitation
with multi-GHz rotating fields. All our experimental results are in good
agreement with micromagnetic simulations.
  In addition, for the sample geometry given above, simulations reveal a lower
limit of about 200 ps for the time of unidirectional vortex core switching,
which cannot be overcome by shortening the excitation length or by increasing
the excitation amplitude. We explain this limitation by the finite time needed
for an energy transfer of the global excitation towards the center of the
sample. Thus smaller samples will allow for much shorter vortex core reversal
times.
