  Magnetic platelets with a vortex configuration are attracting considerable
attention. The discovery that excitation with small in-plane magnetic fields or
spin polarised currents can switch the polarisation of the vortex core did not
only open the possibility of using such systems in magnetic memories, but also
initiated the fundamental investigation of the core switching mechanism itself.
Micromagnetic models predict that the switching is mediated by a
vortex-antivortex pair, nucleated in a dynamically induced vortex core
deformation. In the same theoretical framework, a critical core velocity is
predicted, above which switching occurs. Although these models are extensively
studied and generally accepted, experimental support has been lacking until
now. In this work, we have used high-resolution time-resolved X-ray microscopy
to study the detailed dynamics in vortex structures. We could reveal the
dynamic vortex core deformation preceding the core switching. Also, the
threshold velocity could be measured, giving quantitative comparison with
micromagnetic models.
