Tunable intrinsic phase shift between a spin torque nano-oscillator and an AC current

We present magnetodynamic simulations, based on the Landau-Lifshitz-Gilbert-Slonczewski equations, of the interaction between a spin torque oscillator (STO) and an ac current (I
_ac
). To avoide any extrinsic phase shift we inject the ac current at the intrinsic frequency (f
_STO
) of the STO. We nevertheless find an unexpected intrinsic preferred phase shift Δϕ
₀
between the STO and I
_ac
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 (Δϕ
₀
=87-94°). In this regime Δϕ
₀
increases somewhat with the dc current. However, as the precession changes into the Out-Of-Plane (OOP) mode, Δϕ
₀
exhibits a dramatic jump by about 180°, i.e. the STO resistance now precedes I
_ac
about a quarter of a wave length (|Δϕ
₀
|=86°). Δϕ
₀
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, the ac current mixes the two oscillation modes and both chaotic and periodic mixing is observed. We argue that the intrinsic Δϕ
₀
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.