Response to noise of a vortex based spin transfer nano-oscillator

We investigate experimentally and analytically the impact of thermal noise on the sustained gyrotropic mode of vortex magnetization in spin transfer nano-oscillators and its consequence on the linewidth broadening due to the different nonlinear contributions. Performing time domain measurements, we are able to extract separately the phase noise and the amplitude noise at room temperature for several values of dc current and perpendicular field. For a theoretical description, we extend the general model of nonlinear auto-oscillators to the case of vortex core dynamics and provide analytical expressions of the parameters describing the response to noise of the system. From the analysis of our experimental results, we demonstrate the major role of the amplitude-to-phase noise conversion on the linewidth broadening, and propose several solutions to increase even more the spectral coherence of vortex-based spin transfer nano-oscillators.