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

Abstract

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.