We report on a preferred phase shift ΔΦ0 between a spin torque oscillator (STO) and an ac current (I ac ) injected at the intrinsic frequency (fSTO) of the STO. In the in-plane precession mode (IP) the STO adjusts to a state where its resistance (or voltage) lags I ac about a quarter of a wave length (ΔΦ0 = 87°−94°). In the IP mode ΔΦ0 increases somewhat with the dc current. As the precession changes into the Out-Of-Plane (OOP) mode, ΔΦ0 jumps by about 180°, i.e. the STO resistance now precedes I ac by about a quarter of a wave length (|ΔΦ0| = 86°). At the IP/OOP boundary, the ac current mixes the two oscillation modes and both periodic and chaotic oscillations are observed. As a consequence of mixing, subharmonic terms appear in the STO signal. ΔΦ0 can furthermore be tuned by changing one or more of the anisotropy field, the demagnetizing field or the applied field. At the IP/OOP boundary, I ac mixes the two oscillation modes. The intrinsic ΔΦ0 will impact any circuit design based on STO technology and will e.g. have direct consequences for phase locking in networks of serially connected STOs.
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