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

Reflectivity measurements are used to obtain the optical conductivity of three La0.75Ca0.25MnO32y samples annealed under increasingly strong reducing conditions. All three samples are nearly saturated ferromagnets although their Curie temperatures fall progressively from Tc5221 K to Tc5187 K with maximum deoxygenation. The presence of Drude absorption indicates that the unreduced and the slightly reduced samples are electrical conductors below Tc . 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.