Optical Study of the Evolution of Charge and Spin Ordering in the Manganese Perovskite ${\mathrm{Bi}}_{1-\mathit{x}}{\mathrm{Ca}}_{\mathit{x}}{\mathrm{MnO}}_3$ $(\mathit{x}>0.5)$

We report an optical reflectivity study of charge and spin ordering in ${\mathrm{Bi}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ ( $x=0.74\mathrm{}$ and 0.82) single crystals. In the high temperature phase, $T>\mathrm{}{T}_{\mathrm{co}}$, which is characterized by ferromagnetic correlations, the low-energy contribution to the optical conductivity ${\sigma }_1\left(\omega \right)$ is dominated by small polaron dynamics. For $T_{\mathrm{co}}>T>\mathrm{}{T}_{\mathrm{N}}$, 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 $T_{\mathrm{N}}$, the polaron response is completely suppressed, and a complete charge gap develops, coincident with the onset of long-range antiferromagnetic order.