Unusual magnetic and transport properties of the CMR Pr1–xCaxMnO3–y system

The substituted nonstoichiometric perovskite Pr1–xCaxMnO3–y compounds have been synthesized by a standard combustion technique, which show uniphase solid solutions. The all samples of the Pr1–x CaxMnO3–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–xCaxMn1–s 3+ Mns 4+O3–y compounds have been obviously formulated. Magnetic properties are investigated by SQUID and thus the Pr1–xCaxMnO3–y (x = 0.4, 0.6, and 0.8) compounds show the transition from antiferromagnetic state to paramagnetic state. The Pr1–xCaxMnO3–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.