  We study the magnetization dynamics in a nanocontact magnetic vortex
oscillators as function of temperature. Low temperature experiments reveal that
the dynamics at low and high currents differ qualitatively. At low currents, we
excite a temperature independent standard oscillation mode, consisting in the
gyrotropic motion of a free layer vortex about the nanocontact. Above a
critical current, a sudden jump of the frequency is observed, concomitant with
a substantial increase of the frequency versus current slope factor. Using
micromagnetic simulation and analytical modeling, we associate this new regime
to the creation of a vortex-antivortex pair in the pinned layer of the spin
valve. The vortex-antivortex distance depends on the Oersted field which favors
a separation, and on the exchange bias field, which favors pair merging. The
pair in the pinned layer provides an additional spin torque altering the
dynamics of the free layer vortex, which can be quantitatively accounted for by
an analytical model.
