Enhanced low-field magnetoresistance in La0.7Ca0.3MnO3 + x(ZnO) composites

La0.7Ca0.3MnO3+x(ZnO) composites with x = 0, 2, 3, 4, 6, 12, 20, 35 and 55% (the molar ratio of ZnO to LCMO matrix) were obtained by mixed LCMO and ZnO powders and sintered at 1000 deg C for 3 h. The structure, morphologies, electro-magnetic transport behavior and magnetic properties of the composites were systematically studied. The results of X-ray diffraction and scanning electronic microscopy indicated no new phase appeared in the composites except LCMO and ZnO phases, and ZnO segregated mostly at the grain boundaries of LCMO. Compared with pure La0.7Ca0.3MnO3, we have observed two peaks in the curve of the temperature dependence on the resistivity. One is the insulator-metal (I-M) transition temperature (TP1), which kept almost the same for all the samples and was observed and reported generally at high temperature in LCMO. The other is a new I-M transition temperature (TP2), which occurred at low temperature for the doped samples (x > = 3%) and decreased with increasing ZnO concentration. TP1 and TP2 of the composites are corresponding to the magnetic transition temperature TC1 and TC2, respectively. According to the magnetic properties of the composites, the magnetic transition temperature TC1 of the samples is the same. Enhanced low-field magnetoresistance was obtained in the composite. We have observed the max MR (43%) for the sample x = 35% under the field 3000 Oe at 110 K. These phenomena can be explained using the double-exchange (DE) mechanism, the grain boundary effect and the intrinsic transport properties together.