Coupling-induced bistability in self-oscillating regimes of two coupled identical Spin-Torque Nano-oscillators

We consider a system of two coupled identical Spin Torque Nano-Oscillators (STNOs) with cylindrical geometry. The STNOs have free layers in the vortex magnetization configuration and fixed layer with magnetization in the out-of-plane direction. The magnetization dynamics in the two STNOs are analyzed by using a Thiele-like equation governing the dynamics of the vortex core positions. The coupling is assumed to be linear and symmetric and this enables to characterize it by a single complex parameter. We consider numerical simulations of the model as a function of the coupling strength k and phase phi(k). Depending on their values, different motions of the vortex cores are observed, e.g. periodic, and quasi-periodic. Each type of dynamics can be classified according to the symmetry property shown by the trajectories. In this respect, the transition between different types of motion corresponds to a symmetry-breaking process and we describe it as a bifurcation process. Fixing the value of the coupling constant phase, we found that there is a range of coupling constant strength in which the periodic and quasi-periodic motions coexist and both are stable. This bistability is an interesting phenomenon in connection to research on neuromorphic circuits based on STNOs.