  We have conducted experiments to probe how the dynamics of nanocontact vortex
oscillators can be modulated by an external signal. We explore the
phase-locking properties in both the commensurate and chaotic regimes, where
chaos appears to impede phase-locking while a more standard behavior is seen in
the commensurate phase. These different regimes correspond to how the
periodicity of the vortex core reversal relates to the frequency of core
gyration around the nanocontact; a commensurate phase appears when the reversal
rate is an integer fraction of the gyration frequency, while a chaotic state
appears when this ratio is irrational. External modulation where the power
spectral density exhibits rich features, appears due to the modulation between
the external source frequency, gyration frequency, and core reversal frequency.
We explain these features with first- or second-order modulation between the
three frequencies. Phase-locking is also visible between the external source
frequency and internal vortex modes (gyration and core reversal modes).
