Influence of A-site cation mismatch on structural, magnetic and electrical properties of lanthanum manganites

With a view to understand the influence of cation mismatch on magnetic as well as electrical behavior of colossal magnetoresistive materials, a systematic investigation of La0.67D0.33MnO3 (D = Ca, Sr, Pb and Ba) manganite perovskites has been undertaken. The materials were prepared by sol-gel route sintering at 900 oC and after usual structural characterization by X-ray diffraction (XRD), their metal-insulator transition (T P) as well as magnetic transition (T C) temperatures were determined. The XRD data have been analyzed by Rietveld refinement technique. It has been concluded that apart from A-site cation, the size variance factor (σ 2) also influences T P and T C values. The extrinsic nature of magnetoresistance in ferromagnetic metallic region is explained on the basis of inter-grain spin polarized tunneling effect. A systematic analysis of electrical resistivity data were carried out using theoretical models to understand the conduction mechanism. It has been concluded that the electrical resistivity data in the ferromagnetic (metallic) regime (T < T P) may be explained by domain or grain boundary and single magnon scattering processes, while the adiabatic small polaron and variable range hopping models may be used to explain the resistivity data of high temperature paramagnetic insulating region (T > T P)