  This paper shows that the presence of two dynamical regimes, characterized by
different precessional-axis, is the origin of the non-monotonic behavior of the
output integrated power for large-amplitude magnetization precession driven by
spin-polarized current in nanoscale exchange biased spin-valves. In particular,
at the transition current between those two regimes exists an abruptly loss in
the integrated output power. After the introduction of a time-frequency
analysis of magnetization dynamics based on the wavelet transform, we performed
a numerical experiment by means of micromagnetic simulations. Our results
predicted that, together with a modulation of the frequency of the main excited
mode of the magnetization precession, at high non-linear dynamical regime the
instantaneous output power of the spin-torque oscillator can disappear and then
reappear at nanosecond scale.
