Linewidth Variation of the Higher Harmonics in Spin-Torque Vortex Oscillators

We have studied microwave emission due to vortex gyration in pseudo-spin-valve nanopillar devices. Subgigahertz emission spectra with a large number of higher harmonics are found with linewidth of the fundamental peak as low as ~500 kHz. Several distinct gyration modes exist whose peak frequencies are separated by a few tens of megahertz, likely corresponding to magnetic oscillations with different metastable vortex core trajectories. In a single spectral measurement, however, only one of the distinct modes appears, indicating that the excited mode is stable. The linewidth of higher harmonics, in many cases, increases linearly with the harmonic number n, indicating that the frequency-amplitude coupling is significant. These results show the strong nonisochronous characteristics of spin-torque vortex oscillators.