  Scanning transmission x-ray microscopy is employed to investigate
experimentally the reversal of the magnetic vortex core polarity in cylindrical
Ni81Fe19 nanodisks triggered by two orthogonal monopolar magnetic field pulses
with peak amplitude $B_0$, pulse length ${\tau}$=60 ps and delay time
${\Delta}$t in the range from -400 ps to +400 ps between the two pulses. The
two pulses are oriented in-plane in the x- and y-direction. We have
experimentally studied vortex core reversal as function of $B_0$ and
${\Delta}$t. The resulting phase diagram shows large regions of unidirectional
vortex core switching where the switching threshold is modulated due to
resonant amplification of azimuthal spin waves. The switching behavior changes
dramatically depending on whether the first pulse is applied in the x- or the
y-direction. This asymmetry can be reproduced by three-dimensional
micromagnetic simulations but not by two-dimensional simulations. This behavior
demonstrates that in contrast to previous experiments on vortex core reversal
the three-dimensionality in the dynamics is essential here.
