  We demonstrate that the spectroscopic g-factor can be determined with high
precision and accuracy by broadband ferromagnetic resonance measurements and
applying an asymptotic analysis to the data. Spectroscopic data used to
determine the g-factor is always obtained over a finite range of frequencies,
which can result in significant errors in the fitted values of the
spectroscopic g-factor. We show that by applying an asymptotic analysis to
broadband datasets, precise values of the intrinsic g-factor can be determined
with errors well below 1 %, even when the exact form of the Kittel equation
(which describes the relationship between the frequency and resonance field) is
unknown. We demonstrate this methodology with measured data obtained for
sputtered Ni80Fe20 ("Permalloy") thin films of varied thicknesses, where we
determine the bulk g-factor value to be 2.109 +/- 0.003. Such an approach is
further validated by application to simulated data that includes both noise and
an anisotropy that is not included in the Kittel equation that was used in the
analysis. Finally, we show a correlation of thickness and interface structure
to the magnitude of the asymptotic behavior, which provide insight into
additional mechanisms that may induce deviations from the Kittel equation.
