Multiple synchronization attractors of serially connected spin-torque nanooscillators

Spin-torque nanooscillators (STNOs), which have both the common properties of nanosized oscillators (small size, tunable operating frequency) and some particular ones (wide operating range, easy on-chip integration, etc.), have received a great deal of attention due to their high potential in applications. Yet synchronization of serially connected STNOs has been considered essential for applications. In this paper, we present findings concerning the following properties of synchronized serially connected STNOs: (i) multiple synchronization attractors coexist, and the attracting basins are entangled in a complicated manner; (ii) these attractors have different synchronized frequencies and output powers; and (iii) switching among these attractors can be induced by a small noise, which causes a resonance peak in the power spectra to vanish. These characteristics can be understood using saddle-node bifurcations and have direct impact on laboratory experiments and the potential applications of STNO-based devices.