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

Abstract

We report on a preferred phase shift Δ φ0 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 [Δ φ0 = (87-94) °]. In the IP mode Δ φ0 increases somewhat with the direct current. As the precession changes into the out-of-plane (OOP) mode, Δ φ0 jumps by about 180°, i.e., the STO resistance now precedes Iac about a quarter of a wavelength (Δ φ0 =-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 Δ φ0 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. © 2007 American Institute of Physics.