Effect of oxygen reduction on the optical conductivity of La0.75Ca0.25MnO3

Reflectivity measurements are used to obtain the optical conductivity of three La0.75Ca0.25MnO3-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 K to T-c=187 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.