Positive and negative magnetoresistance in mixed superexchange -: Double exchange systems

The occurrence of a large positive and negative magnetoresistance (+/-MR) in mixed magnetically coupled and weakly lattice-coupled spin systems is discussed and compared with an experimentally observed example of GMR in manganites R1-xExMnO3-delta, i.e., oxygen deficient La0.7Ca0.3MnO3-delta. We indeed find even some of the more capricious features of the MR reproduced by experiment and the condition that the metal-insulator (T-mi) and ferromagnetic-paramagnetic transition (T-c) do not coincide. In addition, the resistivity rho (T), the magnetoresistance, and the thermopower of two nondeficient compounds, i.e., La0.7Ca0.1Mn1-yFeyO3, y = 0, 0.02, are presented. In these cases, we also find a split of T-c and T-mi plus a correlation of the magnetoresistance (MR) and the thermoelectric power S(T).