  A theoretical study on how synchronization and resonance-like phenomena in
superparamagnetic tunnel junctions can be driven by spin-transfer torques is
presented. We examine the magnetization of a superparamagnetic free layer that
reverses randomly between two well-defined orientations due to thermal
fluctuations, acting as a stochastic oscillator. When subject to an external ac
forcing this system can present stochastic resonance and noise-enhanced
synchronization. We focus on the roles of the mutually perpendicular
damping-like and field-like torques, showing that the response of the system is
very different at low and high-frequencies. We also demonstrate that the
field-like torque can increase the efficiency of the current-driven forcing,
specially at sub-threshold electric currents. These results can be useful for
possible low-power, more energy efficient, applications.
