  In doped manganites, the strong electron-phonon coupling due to the
Jahn-Teller effect localizes the conduction-band electrons as polarons. This
results in polarons are carriers responsible for transport and ferromagnetic
ordering rather than the bare eg electrons, and sequentially polaron exchange
model is emerged for describing ferromagnetic ordering. In
Pr0.7(Sr1-xCax)0.3MnO3(x=0.3-0.6) epitaxial thin films, for higher-temperature
paramagnetic state and lower-temperature ferromagnetic state, both the
temperature dependent transports present behaviors of small polaron; for
paramagnetic-ferromagnetic transition, the experimental data of Curie
temperature are well described by an energy balance expression induced by
polaron exchange model. These results demonstrate that the polaron models are
proper ways to describe the strongly correlated electrons in the doped
manganites.
