  Superparamagnetic tunnel junctions are nanostructures that auto-oscillate
stochastically under the effect of thermal noise. Recent works showed that
despite their stochasticity, such junctions possess a capability to synchronize
to subthreshold voltage drives, in a way that can be enhanced or controlled by
adding noise. In this work, we investigate a system composed of two
electrically coupled junctions, connected in series to a periodic voltage
source. We make use of numerical simulations and of an analytical model to
demonstrate that both junctions can be phase-locked to the drive, in phase or
in anti-phase. This synchronization phenomenon can be controlled by both
thermal and electrical noises, although the two types of noises induce
qualitatively different behaviors. Namely, thermal noise can stabilize a regime
where one junction is phase-locked to the drive voltage while the other is
blocked in one state. On the contrary, electrical noise causes the junctions to
have highly correlated behaviors and thus cannot induce the latter. These
results open the way for the design of superparamagnetic tunnel junctions that
can perform computation through synchronization, and which harvest the largest
part of their energy consumption from thermal noise.
