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 Deltaphi₀ 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 (Deltaphi₀=87-94deg). In this regime Deltaphi₀ increases somewhat with the dc current. However, as the precession changes into the out-of-plane (OOP) mode, Deltaphi₀ exhibits a dramatic jump by about 180deg, i.e. the STO resistance now precedes I_ac about a quarter of a wave length (|Deltaphi₀|=86deg). Deltaphi₀ 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 Deltaphi₀ 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.