Ferromagnetic coupled modes in continuous/granular multilayers: Model and experiments

We have developed an adapted model in order to describe the ferromagnetic resonance (FMR) spectra in a trilayer system in which two continuous ferromagnetic films are coupled by a granular magnetic spacer. The model allowed us to study the influence that different parameters (e.g., the Fe volume concentration and the thickness of the granular spacer, the exchange coupling field between layers, the microwave frequency, etc.) have on the overall line shape of the spectra. We present the general results predicted by the model and compare them with FMR experimental measurements made on a particular trilayer {Fe [Fe(x)- SiO2 (1-x)](t) Ni80 Fe20} formed by two continuous ferromagnetic layers, Fe and Permalloy (Ni80 Fe20), separated by a granular film of Fe- SiO2, in which we changed the Fe volume concentration x (0.45<x<0.85), and the thickness t (t=1,2,4,9, and 18 nm) of the granular spacer. Room-temperature FMR measurements were made at the Q (ν=34 GHz) and X bands (ν=9.5 GHz) with the external field applied parallel to the film plane. Two well-resolved absorption modes, one at low fields and another at higher fields, were generally observed. From the dependence of the resonance field and the relative intensity of these modes on x and t it was possible to deduce that the granular layer strongly interacts with the Fe layer, whereas the Permalloy layer is only weakly coupled with the rest of the layers.