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 $approx 0.1~$T. The peaks originate from the surface anisotropy induced by the strong demagnetizing fields (DMFs) of iron. Further simulations reveal that for $approx 20~$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.
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