Structural, magnetic and electrical properties of self-doped La0.8Na0.2−x□xMnO3 manganites

Polycrystalline compounds La0.8Na0.2−x□xMnO3 were prepared by the solid-state reaction with 0.00≤x  ≤0.15. Structural, magnetic and electrical measurements were investigated. The XRD data have been analyzed by Rietveld refinement technique which reveals that all samples are crystallized in a rhombohedral structure with R3¯c space group. Magnetic measurement versus temperature shows that all samples exhibit a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase when increasing temperature. The Curie temperature (TC) decrease from 340 K for x=0.00 samples to 260 K for x=0.15 one. Hysteresis cycles confirm the ferromagnetic character at low temperature with a decrease of the remanent magnetization and the coercive field when the vacancy rate increases. Moreover, the temperature dependence of electrical resistivity shows a metal–insulator transition at Tρ for all samples. In addition, Tρ decreases with vacancy content in accordance with TC. Thus, the conduction mechanism was explained by the adiabatic small polaron hopping (ASPH) in the insulating region and by the competition between the small-polaron and spin-wave scattering and the electron–magnon scattering mechanisms. Finally, the minimum of resistivity at very low temperature range is explained by the Kondo-like scattering model.