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The present work provides the first detailed simulation study aimed at enhancing the synchronization of Spin Torque Oscillators (STO). Unavoidable process variations will inevitably lead to variations in the STO parameters, most noticeably in the anisotropy field Hk, which translates into variations in the oscillator frequency. We have carried out detailed LLG simulations of pairs of serially connected STOs, in parallel with a resistive load. To study the impact of realistic process variations on STO synchronization we let Hk of one of STOs vary over a wide range with respect to the other. The simulation also provides for a time delay tau to account for a distributed RC constant of a realistic circuit. We construct a phase diagram of the STO synchronization as a function of Hk, dc current (Idc) , and tau. The phase diagram turns out to be extremely 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% Hk variation, the addition of a small time delay dramatically improves its robustness and allow as much as 145% Hk variation over the entire frequency range. By tuning the circuit I-V phase shift, the synchronization of the oscillators is much enhanced and the sensitivity to imperfections in the fabrication process is reduced by more than two orders of magnitude, hence relaxing the severe demands on limited device variability and opening the way to high integration level, low-power and low-cost, current-tunable spin torque oscillator applications. |