Mutual synchronization of spin-torque oscillators within a ring array.

An array of spin torque nano-oscillators (STNOs), coupled by dipolar interaction and arranged on a ring, has been studied numerically and analytically. The phase patterns and locking ranges are extracted as a function of the number N, their separation, and the current density mismatch between selected subgroups of STNOs. If [Formula: see text] for identical current densities through all STNOs, two degenerated modes are identified an in-phase mode (all STNOs have the same phase) and a splay mode (the phase makes a 2[Formula: see text] turn along the ring). When inducing a current density mismatch between two subgroups, additional phase shifts occur. The locking range (maximum current density mismatch) of the in-phase mode is larger than the one for the splay mode and depends on the number N of STNOs on the ring as well as on the separation. These results can be used for the development of magnetic devices that are based on STNO arrays.

Injection locking at 2f of spin torque oscillators under influence of thermal noise.

Integration of Spin Torque Nano-Oscillators STNO's in conventional microwave circuits means that the devices have to meet certain specifications. One of the most important criteria is the phase noise, being the key parameter to evaluate the performance and define possible applications. Phase locking several oscillators together has been suggested as a possible means to decrease phase noise and consequently, the linewidth. In this work we present experiments, numerical simulations and an analytic model to describe the effects of thermal noise in the injection locking of a tunnel junction based STNO. The analytics show the relation of the intrinsic parameters of the STNO with the phase noise level, opening the path to tailor the spectral characteristics by the magnetic configuration. Experiments and simulations demonstrate that in the in-plane magnetized structure, while the frequency is locked, much higher reference currents are needed to reduce the noise by phase locking. Moreover, our analysis shows that it is possible to control the phase noise by the reference microwave current (IRF) and that it can be further reduced by increasing the bias current (IDC) of the oscillator, keeping the reference current in feasible limits for applications.