Nature of electrical conduction in potassium-substituted lanthanum manganites between 10 and 300 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 (T-Ml), 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 similar to 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 = 0.8 T). 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. (c) 2004 Elsevier B.V. All rights reserved.