Unusual magnetic and transport properties of the CMR Pr1−x Ca x MnO3−y system
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The substituted nonstoichiometric perovskite Pr1−x Ca x MnO3−y compounds have been synthesized by a standard combustion technique, which show uniphase solid solutions. The all samples of the Pr1−x Ca x MnO3−y system show an orthorhombic crystal system and the cell volumes are decreased with increasing the larger amounts of substituted atoms or the increasing x values. The mixed valence of Mn ions is identified by the XAS (XANES/EXAFS) spectroscopy and the amounts of Mn4+ ions are determined by an iodometric titration method. Nonstoichiometric chemical formulas of the Pr1−x Ca x Mn 1−τ 3+ Mn τ 4+ O3−y compounds have been obviously formulated. Magnetic properties are investigated by SQUID and thus the Pr1−x Ca x MnO3−y (x = 0.4, 0.6, and 0.8) compounds show the transition from antiferromagnetic state to paramagnetic state. The Pr1−x Ca x MnO3−y (x = 0.0, 0.2, and 1.0) compounds show the transition from ferromagnetic state to paramagnetic state. The facts that Mn4+ contents play important roles in the magnetic ordering have been found out. The transport properties have been studied by the DC electrical conductivity measurement under magnetic fields of 0 G and 3 kG. Maximum and minimum MR ratios are 1016% of the Pr0.6Ca0.4MnO2.846, and −77.5% of the PrMnO3.021 compound, respectively.
KeywordsRietveld Refinement Effective Magnetic Moment Orthorhombic Crystal System Positive Magnetoresistance Pnma Space Group
This work was supported by Korea Research Foundation Grant (KRF-2000-015-DP0200). The X-ray absorption experiments at PLS were supported in part by MOST and POSCO.
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