The passage of spin-polarized currents through magnetic nanocontacts can lead to the excitation of self-sustained
vortex oscillations in the free layer of a spin-valve stack. These oscillations involve the large amplitude translational
motion of the vortex core about the contact region, with oscillation frequencies typically in the 200-500
MHz range. Here, we present a detailed experimental study of such current-driven vortex oscillations. In particular,
we show that the oscillation mode is possible in zero applied magnetic field and is only stable within a
range of in-plane applied fields.