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Frequency modulation experiments were performed on a spin torque vortex oscillator for a wide range of modulation frequencies, up to 10% of the oscillator frequency. A thorough analysis of the intermodulation products shows that the key parameter that describes these experiments is the deviation sensitivity, which is the dynamical frequency-current dependence. It differs significantly from the oscillator tunability discussed so far in the context of spin-transfer oscillators. The essential difference between these two concepts is related to the response time of the vortex oscillator, driven either in quasisteady state or in a transient regime. [ABSTRACT FROM AUTHOR]

Radiofrequency vortex spin-transfer oscillators based on magnetic tunnel junctions with very low-resistance area product were investigated. A high power of excitations has been obtained characterized by a power spectral density containing a very sharp peak at the fundamental frequency and a series of harmonics. The observed behavior is ascribed to the combined effect of spin-transfer torque and Oersted-Ampère field generated by the large applied dc current. We furthermore show that the synchronization of a vortex oscillation by applying an ac bias current is mostly efficient when the external frequency is twice the oscillator fundamental frequency. This result is interpreted in terms of a parametric oscillator. [ABSTRACT FROM AUTHOR]