Ferromagnetic resonance of 2D array of magnetic nanocaps

Nanostructured Co and permalloy films are fabricated on top of a polymethyl methacrylate (PMMA) colloidal crystals by magnetron sputtering. The influence of the geometry of the samples on the ferromagnetic resonance spectra is studied. A number of spin-wave resonances are found in the nanostructured system, while only a single resonance is observed in the spatially uniform flat film. The number of the observed resonance peaks increases with both the period of the colloidal crystal (120 ÷ 340 nm) and the film thickness (20 ÷ 90 nm). The micromagnetic simulations of the system show that the excited spin-wave modes are nonreciprocal because of nonzero toroidal momentum of the magnetization of the system.