  We present an experimental study of phase-locking in a stochastic magnetic
oscillator. The system comprises a magnetic tunnel junction with a
superparamagnetic free layer, whose magnetization dynamics is driven with spin
torques through an external periodic driving current. We show that
synchronization of this stochastic oscillator to the input current is possible
for current densities below $3 \times 10^6$ A/cm$^2$, and occurs for input
frequencies lower than the natural mean frequency of the stochastic oscillator.
We show that such injection-locking is robust and leads to a drastic reduction
in the phase diffusion of the stochastic oscillator, despite the presence of a
frequency mismatch between the oscillator and the excitation.
