Nonlinear power-dependent effects in exchange-coupled magnetic bilayers

We investigate the dynamics of exchange-coupled ferromagnetic films in the nonlinear limit and find a number of interesting features. The introduction of asymmetries in the magnetic driving field can produce a power-dependent localization of the magnetization oscillations, in one film or the other, by way of the optic mode frequency shifting in the large field limit. A nonlinear mixing of the acoustic and optic modes in the antiferromagnetically coupled system leads to quasiperiodic and multiperiodic composite modes. Numerical ferromagnetic resonance experiments will show additional absorption peaks (in the 20–50 GHz range) which have a rapid increase in strength with input power. The results are calculated through linearization techniques and numerical solutions of the Landau-Lifshitz-Gilbert equation. The nonlinear (and linear) behavior is illustrated through Poincaré section bifurcation diagrams.