Second harmonic injection locking of coupled spin torque vortex oscillators with an individual phase access.

The synchronisation of magnetic tunnel junctions in the high frequency domain has attracted significant interest in the context of novel computation paradigms, specifically neuromorphic spintronics and probabilistic computing. In this work, a design for the coupling and synchronization of spin torque vortex oscillators (STVOs) is implemented. The geometry comprises the fabrication of adjacent pairs of STVO nanopillars (MgO-based magnetic tunnel junctions), with an edge-to-edge distance down to 100 nm, together with individual top contacts that allow an independent electrical access to each device. In this geometry, the magneto-dipolar coupling promotes the synchronization of the two oscillators, at the same time as the access to the frequency and phase of each individual oscillator is possible. Both frequency and time domain measurements confirm a successful synchronization, with the coupling being controlled by the relative DC bias in each oscillator. As a proof-of-concept towards an oscillator-based Ising machine, it is also shown that the second harmonic injection locking of an STVO can be controlled by tuning the magneto-dipolar coupling to its correspondent STVO pair. These results represent a step forward for the implementation of magneto-dipolar coupled magnetic tunnel junctions, specifically in the field of unconventional computing hardware. Spin torque nano-oscillators (STNOs) have been proposed as the building blocks of Ising machines for solving combinatorial optimization problems. In this study, the authors experimentally demonstrate how the short-range magnetic coupling between a pair of STNOs controls their ability to lock with an external second harmonic signal. [ABSTRACT FROM AUTHOR]