Size-dependent frequency bands in the ferromagnetic resonance of a Fe-nanocube

Using full micromagnetic simulations we calculate the spectra of ferromagnetic resonance (FMR) for an iron (core-shell) nanocube and show that the FMR characteristics are strongly size dependent. For instance, for a 40 nm it is found that, in contrast to a macrospin picture, the spectrum of the iron nanocube possesses two bands centered around 0.4 T and ≈0.1 T. The peaks originate from the surface anisotropy induced by the strong demagnetizing fields (DMFs) of iron. Further simulations reveal that for ≈20 nm nanocubes the macrospin model becomes viable. Above 40 nm we find a broad band for FMR absorption. Our results point to possible interpretations of existing FMR experimental observations for the system studied here.