Effect of multielement doping on low-field magnetotransport in La_(0.7-x)Mm_xCa_(0.3)MnO_3 (0.0≤x≤0.45) manganite

摘要

We report the synthesis, structure and low-field magnetotransport properties of Mischmetal (Mm)-doped La_(0.7-x)Mm_xCa_(0.3)MnO_3 (0≤x≤0.45) manganite. Mischmetal-Mm-is a natural mixture of rare earth elements La, Ce, Pr and Nd with ～28%, 50%, 6% and 16% composition, respectively. All the samples crystallize in orthorhombic structure. Increasing x (Mm), corresponding to decreasing the La-site average ionic radii () hence increasing the size mismatch (i.e. variance σ~2), results in strong suppression of ferromagnetism (T_C) and the associated metallicity (T_(IM)). It may be pointed out that Mm (La, Ce, Pr and Nd) substitution has been done to create two effects. First, creation of multivalence of Mn (2+, 3+ and 4+) via Ce substitution and second to create higher degree of disorder due to size difference brought in not only by Ce but also by Pr and Nd. Evidences and arguments based on XPS analysis suggest that multivalent ions La, Mm and Ca, and the resulting presence of Mn~(2+), Mn~(3+) and Mn~(4+), causes the simultaneous operation of ferromagnetism-double exchange (Mn~(2+)/Mn~(3+) and Mn~(3+)/Mn~(4+)) and antiferromagnetic-superexchange (Mn~(3+)/Mn~(3+) and Mn~(2+)/Mn~(2+)) interaction. In addition, Mm doping also creates inhomogenities at La-as well as Mn-site due to size and valency difference. A curiously huge magnetoresistance as high as ～63% for x = 0.35, under a moderate magnetic field of ～10 kOe has been observed and even at low magnetic field of ～3 kOe MR is ～30%. The competing double exchange and superexchange coupled with inhomogenities are the most likely cause for the occurrence of large ～63% CMR in the Mm-doped LCMO.