Effect of oxygen reduction on the optical conductivity ofLa0.75Ca0.25MnO3

Reflectivity measurements are used to obtain the optical conductivity of three ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_{3\ensuremath{-}y}$ samples annealed under increasingly strong reducing conditions. All three samples are nearly saturated ferromagnets although their Curie temperatures fall progressively from ${T}_{c}=221\mathrm{K}$ to ${T}_{c}=187\mathrm{K}$ with maximum de-oxygenation. The presence of Drude absorption indicates that the unreduced and the slightly reduced samples are electrical conductors below ${T}_{c}.$ However, the most reduced sample manifests an insulatinglike optical conductivity at all temperatures. Thus, ferromagnetism and insulating behavior coexist in the most-reduced sample. This result contradicts the notion that mobile charge carriers of an electrically conducting manganite induce its ferromagnetism. Beyond Drude absorption, midinfrared and near-infrared absorptions are also observed. A midinfrared absorption band is present in all samples. The energy at its peak falls markedly with decreasing temperature. Moreover, near-infrared absorption is found in the nonconducting sample and in the high-temperature insulating phase of the conducting samples. The implications of these findings are discussed in terms of models for properties of doped manganites.