The theoretical prediction and experimental confirmation of current-driven steady-state precessional motion of magnetization have opened up for new spin torque based microwave applications using nanosized magnetic devices. Recently, it was experimentally demonstrated that a Spin Torque Oscillator (STO) can phase lock to an injected ac current. In the locked state the external ac current can both pull and push the STO frequency and provides a means for frequency mixing and circuit phase control. However, a detailed study of the exact nature of the phase-locked state is still missing.
Using dynamical simulations of the Landau-Lifshitz-Gilbert equation, we study the detailed interaction between an STO and an external ac current (Iac) having the exact same frequency as the STO (no frequency pull/push). We observe an unexpected intrinsic phase shift Äö0 between the STO and the applied ac current, as well as a novel mixed-mode oscillation regime at the boundary of in-plane and out-of-plane STO precession. In this region, subharmonic terms appear in the output spectrum, and it is found that both periodic and chaotic precession can be induced by varying the dc and ac current amplitude.
In the in-plane (IP) precession state, Äö0 varies continuously from about 88° at the onset of oscillation to about 93° just before the STO switches over to out-of-plane (OOP) precession. As the precession changes to the OOP mode Äö0 jumps about 180° and instead of lagging Iac, the STO resistance now precedes Iac by 86°. It is noteworthy that for moderate Iac, Äö0 is not a function of Iac. We varied Iac0 from 1% to 20% of Idc without any change in the preferred phase shift. The phase shift is however dependent on the magnetic parameters of the STO as well as the applied field. Our study hence also gives an indication of the typical variation one can expect from material and processing imperfections (typically variations in Hk), as well as the tuning range when changing the applied field. It also opens up for the possibility of detailed phase control in STO circuits.
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