Crossover of polaron conductivity and the inhomogeneous state of lanthanum manganites in the magnetic phase transition region

The transport properties of lanthanum manganites over a wide range of temperatures below the magnetic phase transition point are discussed within the model of a two-phase composite whose phases differ in the magnetic order and charge carrier concentration. The volume ratio of the phases depends on the temperature and the magnetic field. The magnetoelastic polarons are charge carriers in both phases, and the metal-dielectric transition occurs as a percolation transition accompanied by the crossover of the polaron conductivity. The results obtained by numerical simulation of the resistivity, magnetoresistance, and thermopower are compared with the experimental data for La0.7Mn1.3O3−δ thin films. The theoretical and experimental data are in good agreement.