  We discuss the coherence of magnetic oscillations in a magnetic tunnel
junction-based spin-torque oscillator as a function of external field angle.
Time-frequency analysis shows mode-hopping between distinct oscillator modes,
which arises from linear and nonlinear couplings in the Landau-Lifshitz-Gilbert
equation, analogous to mode-hopping observed in semiconductor ring lasers.
These couplings and therefore mode-hopping are minimized near the current
threshold for antiparallel (AP) alignment of free layer with reference layer
magnetization. Away from the AP alignment, mode-hopping limits oscillator
coherence.
