Phase-locked spin torque oscillators: Impact of device variability and time delay

We have carried out detailed Landau-Lifshitz-Gilbert simulations of pairs of serially connected spin torque oscillators (STOs) in parallel with a resistive load. To study the impact of realistic process variations on STO synchronization we let the two STOs have different in-plane anisotropy fields (H-k). The simulation also provides for a time delay (tau). We construct a phase diagram of the STO synchronization as a function of H-k and direct current (I-dc) at different tau. The phase diagram turns out to be quite rich with different types of synchronized precession modes. While the synchronized state is originally very sensitive to STO process variations and can only sustain up to 4% H-k variation, the addition of a small time delay dramatically improves its robustness and allows as much as 145% H-k variation in the entire out-of-plane precession regime. (C) 2007 American Institute of Physics.