Spin-polarized current-induced excitations in a coupled magnetic layer system

The influence of dynamic interactions on the spin-polarized current-induced steady state oscillations has been studied numerically for a spin torque oscillator that is composed of a free layer and a synthetic antiferromagnetic (SAF) pinned layer. Two types of dynamical interactions have been considered. The first is the exchange interaction between the two layers that constitute the SAF pinned layer. The second is a mutual spin torque interaction between the free layer and the pinned SAF layer. Taking these interactions into account, the Landau-Lifshitz-Gilbert equation including the spin transfer torque has been solved simultaneously for all three layers in a macrospin approach. Besides the dynamic state diagram for such a coupled spin torque oscillator, three major results have been obtained for the dynamic excitation states: (1) the dynamic states are not in all cases steady state periodic oscillations but can present nonperiodic trajectories reminiscent of chaos, (2) due to the mutual spin torque interaction frequency jumps can occur which are interpreted in terms of resonance excitations and frequency locking between the free layer and SAF magnetizations, and (3) the acoustic-type steady state oscillations of the SAF differ from those of a single layer in that the frequency vs current dependence of the in-plane precession modes can show both redshift as well as blueshift. This is interpreted as a general property of the large-angle nonlinear dynamics of a SAF.