  We observe the dynamics of a single magnetic vortex in the presence of a
random array of pinning sites. At low excitation amplitudes, the vortex core
gyrates about its equilibrium position with a frequency that is characteristic
of a single pinning site. At high amplitudes, the frequency of gyration is
determined by the magnetostatic energy of the entire vortex, which is confined
in a micron-scale disk. We observe a sharp transition between these two
amplitude regimes that is due to depinning of the vortex core from a local
defect. The distribution of pinning sites is determined by mapping fluctuations
in the frequency as the vortex core is displaced by a static in-plane magnetic
field.
