Polaronic effects on the resistivity of manganite thin films

The high-temperature resistivity of La0.7Ca0.3MnO₃ and La0.7Ba0.3MnO₃ thin films on various substrates LaAlO₃,SrTiO₃, and Si was investigated. We show that the intrinsic resistivity of high-quality epitaxial films in the paramagnetic phase has a thermally activated form, whereas the extrinsic resistivity found in polycrystalline films follows a variable range hopping law. The small activation energies measured are not compatible with the formation of a band gap; instead they indicate polaron hopping. The polaron mobility is drastically enhanced by the ferromagnetic ordering. We show that the reduction of the polaron activation energy below the Curie temperature is linear in the magnetization. The shift of the metal-insulator transition temperature depends sublinearly on the applied field.