Intrinsic phase shift between a spin torque oscillator and an alternating current

We report on a preferred phase shift $Δϕ0$ between a spin torque oscillator (STO) and an alternating current $(Iac)$ injected at the intrinsic frequency $(fSTO)$ of the STO. In the in-plane (IP) precession mode the STO adjusts to a state where its resistance (or voltage) lags $Iac$ about a quarter of a wavelength $[Δϕ0=(87–94)°]$⁠. In the IP mode $Δϕ0$ increases somewhat with the direct current. As the precession changes into the out-of-plane (OOP) mode, $Δϕ0$ jumps by about 180°, i.e., the STO resistance now precedes $Iac$ about a quarter of a wavelength $(Δϕ0=–86°)$⁠. At the IP/OOP boundary, the alternating 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. 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.