Electro-magnetic transport behavior of La0.7Ca0.3MnO3/SnO2 composites

The composites of (1 − x)La0.7Ca0.3MnO3 (LCMO) + xSnO2 (x = 0.01, 0.05, 0.10, 0.30, 0.50, 0.60, 0.65 and 0.70) were synthesized by conventional solid-state reaction method. The results of X-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicate that SnO2 and LCMO coexist in the composites and SnO2 mainly segregates at the grain boundaries of LCMO, which are in accordance with the results of the magnetic measurements. The detailed electrical characterizations for all the samples showed that a new metal-insulator (M-I) transition temperature (TP2) appeared at a lower temperature compared with the intrinsic metal-insulator (M-I) transition temperature (TP1) when x < 0.50 (TP1 > TP2). When x > 0.50, TP1 disappeared, leaving only TP2. The resistivity percolation threshold of the composites occurred at x = 0.60. Corresponding to the two M-I transition peaks, the curves of magnetoresistance against temperature also showed two peaks for all composites. These phenomena can be explained by the segregation of a new phase related to SnO2 at the grain boundaries or surfaces of the LCMO grains.