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 R_1-xE_xMnO_3-δ, i.e., oxygen deficient La_0.7Ca_0.3MnO_3-δ. 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 p(T), the magnetoresistance, and the thermopower of two nondeficient compounds, i.e., La_0.7Ca_0.3Mn_1-yFe_yO₃, 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).