Phonon anomalies and charge dynamics in Fe 1-x Cu x Cr 2 S 4 single crystals

A detailed investigation of phonon excitations and charge carrier dynamics in single crystals of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ $(x=0,0.2,0.4,0.5)$ has been performed by using infrared spectroscopy. In $\mathrm{Fe}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ the phonon eigenmodes are strongly affected by the onset of magnetic order. Despite enhanced screening effects, a continuous evolution of the phonon excitations can be observed in the doped compounds with $x=0.2$ (metallic) and $x=0.4,0.5$ (bad metals), but the effect of magnetic ordering on the phonons is strongly reduced compared to $x=0$. The Drude-type charge-carrier contribution to the optical conductivity in the doped samples indicates that the colossal magnetoresistance effect results from the suppression of spin-disorder scattering.