Unusual magnetic and transport properties of the CMR Pr 1− x Ca x MnO 3− y system

The substituted nonstoichiometric perovskite Pr 1− x Ca x MnO 3− y compounds have been synthesized by a standard combustion technique, which show uniphase solid solutions. The all samples of the Pr 1− x Ca x MnO 3− 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 Mn 4+ ions are determined by an iodometric titration method. Nonstoichiometric chemical formulas of the Pr 1− x Ca x Mn 1−τ 3+ Mn τ 4+ O 3− y compounds have been obviously formulated. Magnetic properties are investigated by SQUID and thus the Pr 1− x Ca x MnO 3− y ( x = 0.4, 0.6, and 0.8) compounds show the transition from antiferromagnetic state to paramagnetic state. The Pr 1− x Ca x MnO 3− y ( x = 0.0, 0.2, and 1.0) compounds show the transition from ferromagnetic state to paramagnetic state. The facts that Mn 4+ 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 Pr 0.6 Ca 0.4 MnO 2.846 , and −77.5% of the PrMnO 3.021 compound, respectively.