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

## TL;DR

This study uses micromagnetic simulations to show that ferromagnetic resonance spectra of Fe nanocubes are highly dependent on size, revealing distinct spectral features and the limits of macrospin models at different scales.

## Contribution

The paper demonstrates size-dependent FMR spectral features in Fe nanocubes and identifies the size threshold where macrospin models become applicable.

## Key findings

- FMR spectra show two bands at 0.4 T and 0.1 T for 40 nm nanocubes.
- Macrospin model is valid for approximately 20 nm nanocubes.
- Above 40 nm, a broad FMR absorption band appears.

## Abstract

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.