  Due to their nonlinear properties, spin transfer nano-oscillators can easily
adapt their frequency to external stimuli. This makes them interesting model
systems to study the effects of synchronization and brings some opportunities
to improve their microwave characteristics in view of their applications in
information and communication technologies and to design innovative computing
architectures. So far, mutual synchronization of spin transfer nano-oscillators
through propagating spin-waves and exchange coupling in a common magnetic layer
has been demonstrated. Here we show that the dipolar interaction is also an
efficient mechanism to synchronize neighbouring oscillators. We experimentally
study a pair of vortex-based spin-transfer nano-oscillators, in which mutual
synchronization can be achieved despite a significant frequency mismatch
between oscillators. Importantly, the coupling efficiency is controlled by the
magnetic configuration of the vortices, as confirmed by an analytical model
highlighting the physics at play in the synchronization process as well as by
micromagnetic simulations.
