Influence of polyparaphenylene on the magnetotransport of manganite/polymer composites

The ferromagnetic, polycrystalline manganese oxide La0.7Sr0.3MnO3 (LSMO) and the conjugated polymer polyparaphenylene (PPP) were prepared by conventional solid-state reaction processing in air and synthesis by cationic polarization of benzene, respectively. The so-obtained compounds were individually characterized after their electrical, magnetic, structural or thermal properties by using conventional analysis methods. By mixing the pre-prepared LSMO and PPP powders, [LSMO]1−x[PPP]x (x stands for the weigh fraction of PPP) two-phase manganite/polymer granular composites were obtained and their microstructural, transport and magnetic features carefully investigated. The resistance of composites calcined at 400 °C for 1 h in air atmosphere increased with x on the whole temperature range. In turn, these samples displayed a pronounced low-field magnetoresistance (LFMR) effect within a wide temperature range. Spin-polarized tunneling of conduction electrons across interfaces or grain boundaries was invoked as the dominant mechanism controlling this effect. The addition of the polymer may modify the LSMO grain surfaces adjusting the conduction carrier's spin-dependent tunneling distance, which sensitively depends on the nature of the interface or on interface-related disorder. Concretely, the PPP should provoke spin disorder at the LSMO interface.