Phase Locking of Spin Transfer Nano-Oscillators Using Common Microwave Sources

In this paper, we study a typical nonlinear phenomenon of phase locking or synchronization in spin-torque nano oscillators (STNOs). To start with, the oscillators are considered as uncoupled but interlinked through either a common microwave current or a microwave field. We identify the phase locking of an array of STNOs (first for two and then for 100 oscillators) by means of injection locking which represents locking the oscillations to a common alternating spin current or a common microwave magnetic field. We characterize the locking of STNOs through both the first and second harmonic lockings in an array. We find that the second harmonic lockings takes lesser value of microwave current and field than the first-harmonic lockings. Our results also show that oscillating microwave current can induce integer harmonic locking while microwave field can induce both integer and several fractional harmonic lockings. We also extend our analysis to study the locking behavior of two STNOs by introducing time delay feedback and coupling through a current injection and bring out the associated locking characteristics. Finally, we have also analyzed the stability of synchronization of an identical array of STNOs with current coupling by using master stability function formalism.