  When fabricating magnetic memories, one of the main challenges is to maintain
the bit stability while downscaling. Indeed, for magnetic volumes of a few
thousand nm3, the energy barrier between magnetic configurations becomes
comparable to the thermal energy at room temperature. Then, switches of the
magnetization spontaneously occur. These volatile, superparamagnetic
nanomagnets are generally considered useless. But what if we could use them as
low power computational building blocks? Remarkably, they can oscillate without
the need of any external dc drive, and despite their stochastic nature, they
can beat in unison with an external periodic signal. Here we show that the
phase locking of superparamagnetic tunnel junctions can be induced and
suppressed by electrical noise injection. We develop a comprehensive model
giving the conditions for synchronization, and predict that it can be achieved
with a total energy cost lower than 10-^13 J. Our results open the path to
ultra-low power computation based on the controlled synchronization of
oscillators.
