Effect of thermal fluctuations on the spin self-oscillations in a microwave nanosized oscillator

Reduced equations are derived for the slow evolution of the phase and amplitude of spin self-oscillations arising in a free magnetic layer of a nanosized microwave oscillator under the action of an over-threshold spin-polarized current. These equations are used to calculate the spectral intensity of the fundamental-harmonic signal of a spin self-oscillator allowing for thermal noise in the spin subsystem. It is shown that the line width as a function of the current is determined by a bifurcation-type variation in the spin state near the self-oscillation threshold. The self-oscillation nonisochronism makes the spectral line profile asymmetric. The line-profile asymmetry can change as the cycle-appearance point is approached.