  A vortex gyrating in a magnetic disk has two regimes of motion in the
presence of disorder. At large gyration amplitudes, the vortex core moves
quasi-freely through the disorder potential. As the amplitude decreases, the
core can become pinned at a particular point in the potential and precess with
a significantly increased frequency. In the pinned regime, the amplitude of the
gyration decreases more rapidly than it does at larger precession amplitudes in
the quasi-free regime. In part, this decreased decay time is due to an increase
in the effective damping constant and in part due to geometric distortion of
the vortex. A simple model with a single pinning potential illustrates these
two contributions.
