Optical study of the evolution of charge and spin ordering in the manganese perovskite Bi_1-xCa_xMn₃ (x > 0.5)

We report an optical reflectivity study of charge and spin ordering in Bi_1-xCa_xMn₃ (x = 0.74 and 0.82) single crystals. In the high temperature phase, T < T_∞co, which is characterized by ferromagnetic correlations, the low-energy contribution to the optical conductivity σ₁(ω) is dominated by small polaron dynamics. For T_co < T < T_N, 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.