  This paper predicts the possibility to achieve synchronization (via injection
locking to a microwave current) of spin-transfer torque oscillators based on
hybrid spin-valves composed by two free layers and two perpendicular polarizers
at zero bias field. The locking regions are attained for microwave frequency
near 0.5f0, f0, and 2f0 where f0 is the input oscillator frequency. Those
properties make this system promising for applications, such as high-speed
frequency dividers and multipliers, and phase-locked-loop demodulators.
