Relationship of Polaron Exchange with Ferromagnetic and Insulator--Metal Transitions in Doped Manganites

We report the experiment results and data analyses based on a polaron exchange model for La_0.7Ca_0.3MnO₃ and Pr_0.7(Sr_1-xCa_x)_0.3MnO₃ epitaxial thin films. In the polaron exchange model with an energy balance condition, critical temperature of T_C for stable ferromagnetic (FM) ordering should depend on ∆ E as k_BT_C =E₀ exp(-ΔE/k_BT_C), where ∆ E denotes the potential barrier for the exchange polarons to overcome. Using the small polaron hopping model, the resistivity peak temperature T_P is a function of the hopping energy E_hop. The dependence of T_P on E_hop is similar to the dependence of T_C on ∆ E, which reveals that the polaron exchange relates to FM and insulator--metal transitions. The result indicates that the polaron exchange model is a simple way for describing the FM ordering, and is very helpful for understanding of complex doped manganites.