Nature of electrical conduction in potassium-substituted lanthanum manganites between 10 and 300Â K

The temperature dependence of the electrical resistivity in potassium-substituted LaMnO3 prepared by a novel pyrophoric method has been investigated between 10 and 300 K for H=0 and 0.8 T. K substitution enhances the conductivity of this system. Curie temperature (TC) also increases from 260 to 309K with increasing K content. Above the metal-insulator transition temperature (TMI), the electrical resistivity is well represented by adiabatic polaronic model, while in the ferromagnetic region at low temperatures, the resistivity data show a nearly perfect fit for all the samples to an expression containing the residual resistivity, a two-magnon scattering term and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Based on a scenario that the doped manganites consist of phase-separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K could be described very well. All the K-doped samples clearly reveal the existence of a resistivity minimum, which is strongly influenced by an applied magnetic field (H=0.8T). The results have been analyzed in terms of the model-based on charge carrier tunneling between antiferromagnetically coupled grains, and an excellent agreement between the experimental data and those calculated from the model is obtained for all the samples.