Complete optical response of the magnetic fullerene derivative tetrakis(dimethylamino)ethylene-${\mathrm{C}}_{60}$

We have measured the electrodynamic response of TDAE-${\mathrm{C}}_{60}$ [where TDAE is tetrakis(dimethylamino) ethylene], which undergoes a ferromagnetic phase transition at ${\mathit{T}}_{\mathit{c}}$=16 K. The optical conductivity does not display a clear metallic component, which indicates that TDAE-${\mathrm{C}}_{60}$ is not an itinerant ferromagnet. Moreover, the optical response is temperature independent and thus unaffected by the ferromagnetic phase transition at ${\mathit{T}}_{\mathit{c}}$. We also found several absorption peaks in the far-infrared spectral range, which are associated with the molecular phonon-mode spectrum of ${\mathrm{C}}_{60}$ and TDAE.