We show analytically why a spin torque oscillator (STO) locks to an alternating current (I-ac), having the same frequency as the STO, at a nontrivial, intrinsic, phase value Delta phi(0). We determine both the stability and the exact phase of the locked state and explain the recently observed phase jump by about 180 degrees at the in-plane-to-out-of-plane precession boundary. Delta phi(0) depends on the STO material parameters and is tunable through both the applied field and the dc current, which provides an additional control of the exact I-V phase in an STO driven radio frequency circuit. (c) 2008 American Institute of Physics.
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