Non-stationary excitation of two localized spin-wave modes in a nano-contact spin torque oscillator

We measure and simulate micromagnetically a framework based upon a nano-contact spin torque oscillator where two distinct localized spin-wave modes can be detected. The resulting frequency spectrum is composed by two peaks, corresponding to the excited modes, which lie below the ferromagnetic resonance frequency, and a low-frequency tail, which we attribute to the nonstationary switching between these modes. By using Fourier, Wavelet and Hilbert-Huang transforms, we investigate the properties of these modes in time, spatial and frequency domains. The existence of an additional localized mode (which was neither predicted by theory nor by our previous numerical and experimental findings) has to be attributed to the large influence of thecurrent-induced Oersted field whose strength, in the present setup, is comparable with that of the external field. As a further consequence, the excited spin-waves, contrarily to what usually assumed, do not possess cylindrical symmetry: the Oersted field induces these modes to be excited at the two opposite sides of the region beneath the nano-contact.