Synthesis and magnetotransport properties of Mm-doped La0.7Ca0.3MnO3

We have synthesized Mm (Mischmetal)-doped polycrystalline La0.7Ca0.3MnO3 manganite by standard solid state reaction route and explored its structure and magnetotransport properties. Mm is a natural mixture of rare earth elements La, Ce, Pr and Nd. All Mm-doped samples crystallize in orthorhombic unit cell. At lower Mm doping the samples are single phase and at higher doping levels small amount of unreacted CeO2 is observed. The lattice parameter and unit cell volume decrease with increasing Mm content due to decrease in average A-site radii. The decrease in A-site radii also causes reduction in ferromagnetism and metallicity. All samples possess significant MR in the vicinity of TC, which increases on Mm doping. For Mm = 0.35 a huge magnetoresistance as high as ∼30% is observed at a low magnetic field of ∼3 kOe and this nearly doubles to ∼63% at ∼10 kOe. The improved MR in these materials is due to the existence of multivalent ions La, Mm and Ca, and the resulting presence of Mn2+, Mn3+ and Mn4+. This results in simultaneous operation of two competing interactions embodying FM double exchange between Mn2+–Mn3+ and Mn3+–Mn4+, and AFM superexchange between Mn2+–Mn2+ and Mn3+–Mn3+. In addition, the substitution of Mm also creates disorder at La-site as well as Mn-site. Thus, the simultaneous presence of two competing interactions and inhomogeneities induced disorder are prominent in enhancing the MR in the present case.