Hysteretic synchronization of nonlinear spin-torque oscillators

We report the observation of hysteretic synchronization of point-contact spin-torque nano-oscillators by a microwave magnetic field. The hysteresis is asymmetric with respect to the frequency detuning of the driving signal and appears in the region of a strong dependence of the oscillation frequency on the bias current. Theoretical analysis shows that hysteretic synchronization occurs when the width of the synchronization range, enhanced by the oscillator's nonlinearity, becomes comparable to the dissipation rate, while the observed asymmetry is a consequence of the nonlinear dependence of frequency on the bias current. Hysteretic synchronization is a general property of strongly nonlinear oscillators, and can therefore be expected to affect the dynamics of a variety of physical systems.