  The influence of the presence of multiple resonances in ferromagnetic
resonance spectra on extracted material parameters is investigated using
numerical simulations. Our results show that the systematic error of assuming
an incorrect number of resonances for a material can lead to the extraction of
material parameters that significantly deviate from any of the true material
parameters. When noise is present in experimental ferromagnetic resonance
spectra increasing the frequency range of the broadband characterization can
significantly reduce the error-margins when the data is analyzed assuming the
correct number of resonances present in the material. For the cases
investigated in this study it was found that analyzing the data using a single
resonance results in extracted gyromagnetic ratios and effective magnetization
parameters that are consistent with the weighted average of the true material
parameters. We further provide a cautionary example regarding the extraction of
the inhomogeneous linewidth broadening and damping parameters of materials that
contain an unknown number of resonances.
