Structural and magnetotransport studies of magnetic ion doping for monovalent-doped LaMnO 3 manganites
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In this paper, we report the structural, electrical, and magnetic properties of polycrystalline La0.85–xSmxNa0.15MnO3 (x 5 0.05, 0.1, 0.15) manganites. Rietveld refinement of x-ray data infers that doped manganite compounds possess a rhombohedrally distorted structure (space group R3C). Both lattice parameter and unit cell volume decrease and a systematic change in both Mn–O–Mn bond angle and tolerance factor is observed with Sm content. Resistivity measurements discern metal–insulator transition (TP). For x 5 0.15 sample, a double metal–insulator transition with a single ferromagnetic transition is depicted. All samples exhibit extrinsic magnetoresistance (MR) effect. A large value of MR of 65% (253 K, 8 T) is associated with grain and grain boundary formation. The highest low-field MR of 23% (12 K, 2 T) and 35.2% (23 K, 2 T) for x 5 0.05 and 0.1 is observed. The electronic and magnetic inhomogeneities induced by Sm and nonmagnetic metal Na phases account for MR properties.
I. INTRODUCTION
Recent research has been motivated to understand and exploit the large negative magnetoresistance (MR) effects which appear near and below the Curie temperature. The manganites are one of those kinds of materials that fulfil the above need. The rich electronic phase diagrams of manganites reflect the balance of interactions which determine the electronic ground state. In keeping with the literature data, the highest magnetoresistivity values are observed in the temperature regions for ferromagnetic ordering in manganite.1 The research efforts during the past have pointed out the role played by different oxidation states of manganese ions in La-based doped manganites for larger MR near room temperature at low field. Aliovalent-doped manganites are one of the preferred doping ions because the concentration of Mn41 ions induced by this substitution is twice of that obtained in systems with the divalent substitutions. Accordingly, it covers the region of ferromagnetism and metallic conductivity for 20–40% of Mn41. It is worth to mention that in a definite narrow range, these doped manganites exhibit unusual magnetotransport properties such as ferromagnetic to paramagnetic transition, magnetocaloric effect, and colossal magnetoresistance (CMR). In particular, it was reported that La 1–xA xMnO3 (A 5 Na, K) compounds crystallized in a rhombohedrally distorted perovskite structure exhibit sharp ferromagnetic transition as well as a negative MR about room a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.103 J. Mater. Res., Vol. 29, No. 10, May 28, 2014
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temperature.2,3 Hence, the study of alkali-metal (Na1, K1, Rb1) doped perovskite manganites would offer significant complementary understanding of the CMR effects in these compounds. CMR property has effective correlation with temperature and magnetic field, thus its applications are limited. In view of the above relevance of temperature and field, it will be quite useful to decreasing t
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