  We present an experimental and theoretical study of the low-field dynamics of
current-driven vortex oscillations in nanocontacts based on spin-valve
multilayers. These oscillations appear as low-frequency (250-500 MHz)
excitations in the electrical power spectrum which arise from to variations in
the giant-magnetoresistance. We show that the vortex oscillations, once
nucleated at large fields applied perpendicular to the film plane, persist at
zero applied magnetic fields. Some training effects on the oscillation
frequency and linewidth also observed for small in-plane magnetic fields.
