Long-timescale fluctuations in zero-field magnetic vortex oscillations driven by dc spin-polarized current

We report time and frequency domain studies of spin-torque-driven vortex self-oscillations at zero magnetic field. We observe two types of abrupt fluctuations in the frequency and amplitude, with very long random mean lifetimes (∼ 102 to ∼ 104 oscillation cycles). First, we observe fluctuations between two center frequencies separated by 10s of MHz that we identify with switching between quasiuniform and vortex states of the magnetic polarizing layer. Second, we resolve much smaller, discrete frequency fluctuations that lead to a fine structure of the oscillations. We find that this fine structure plays a key role in determining the long-time average linewidths and we suggest a possible physical origin.