  Multiple neuromorphic applications require the tuning of two or more devices
to a common signal. Various types of neuromorphic computation can be realized
using spintronic oscillators, where the DC current induces magnetization
precession, which turns into an AC voltage generator. However, in spintronics,
synchronization of two oscillators using a DC signal is still a challenging
problem because it requires a certain degree of similarity between devices that
are to be synchronized, which may be difficult to achieve due to device
parameter distribution during the fabrication process. In this work, we present
experimental results on the mechanisms of synchronization of spin-torque
oscillators. Devices are based on magnetic tunnel junction with a
perpendicularly magnetized free layer and take advantage of a uniform
magnetization precision in the presence of the magnetic field and a DC bias. By
using an external microwave source, we show the optimal condition for the
synchronization of the magnetic tunnel junctions. Finally, we present results
on the in-series connection of two junctions and discuss the possible path
towards improving oscillation power and linewidth. In addition, using numerical
simulations of the coupled oscillators model, we aim to reproduce the
conditions of the experiments and determine the tolerance for achieving
synchronization.
