  Micromagnetic simulations are used to study a spin-torque vortex oscillator
excited by an out-of-plane dc current. The vortex core gyration amplitude is
confined between two orbits due to periodical vortex core polarity reversals.
The upper limit corresponds to the orbit where the vortex core reaches its
critical velocity triggering the first polarity reversal which is immediately
followed by a second one. After this double polarity reversal, the vortex core
is on a smaller orbit that defines the lower limit of the vortex core gyration
amplitude. This double reversal process is a periodic phenomenon and its
frequency as well as the upper and lower limits of the vortex core gyration are
controlled by the input current density while the vortex chirality determines
the onset of this confinement regime. In this non-linear regime, the vortex
core never reaches a stable orbit and thus, it may be of interest for
neuromorphic application, for example as a leaky integrate-and-fire neuron.
