Gilbert damping up to 300 GHz in Ni81Fe19

High magnetic fields ($>$10 T) at central facilities, combined with frequency sources approaching 1 THz, enable studies of ferromagnetic resonance (FMR) in a new frequency range. To date, the upper frequency limit of FMR studies of the technologically important alloy Ni$_{81}$Fe$_{19}$ (Py) has been 70 Ghz. No experiments have addressed whether the relatively high levels of Gilbert damping $\alpha$ in this alloy (6-9$\times10^{-3}$), described by $\Delta H=\Delta H_0 +\alpha\omega/\gamma$, where $\Delta H$ is the field-swept linewidth, $\Delta H_0$ is the inhomogeneous broadening, and $\omega/2\pi$ is the microwave frequency, represent a low-frequency limit of richer behavior near THz frequencies. In this work, we compare perpendicular FMR measurements of Ni$_{80}$Fe$_{20}$(5nm) between 4-24 GHz, using a laboratory electromagnet and coplanar waveguide, with measurements at 331.2 GHz and 4-295 K using a 16 T superconducting magnet. We find a consistent Gilbert-type damping between the low- and high-frequency data, with the former predicting a linewidth of $193.4\pm9.5$ mT from the extrapolation and the latter resulting $189.8\pm6.2$ mT.