Optical study of the evolution of charge and spin ordering in the manganese perovskite Bi1-xCaxMnO3 (x>0.5)

We report an optical reflectivity study of charge and spin ordering in Bi1-xCaxMnO3 (x = 0.74 and 0.82) single crystals. In die high temperature phase, T > T-co, which is characterized by ferromagnetic correlations, the low-energy contribution to the optical conductivity sigma(1)(omega) is dominated by small polaron dynamics. For T-co > T > TN, the presence of both small polaron and charge-gap-like features signifies "phase separation" behavior in which domains of ferromagnetic and antiferromagnetic order coexist. Below T-N, the polaron response is completely suppressed, and a complete charge gap develops, coincident with the onset of long-range antiferromagnetic order. [S003I-9007(98)07698-4].