Stochastic resonance in noise-induced transitions between self-oscillations and equilibria in spin-valve nanomagnets

The thermally induced synchronization between magnetization transitions and "weak" ac excitations is studied for spin-transfer oscillators. A theoretical approach, based on the separation of time scales, is developed to investigate this physical phenomenon. By applying the appropriate averaging technique to the Fokker-Planck equation associated with the stochastic magnetization dynamics, a stochastic differential equation for the "slow" (energy) variable is derived. This equation is used to analyze intrawell thermal transitions between magnetization equilibria and self-oscillations. It is demonstrated that the thermally induced synchronization between magnetization transitions and ac excitation can be viewed as a stochastic resonance effect. It is shown that this effect occurs in spin-transfer nano-oscillators both in the classical case of subthreshold ac excitation as well as in the suprathreshold case. The theoretical predictions are in very good agreement with simulations of the Landau-Lifshitz-Slonczewski dynamics.