| The composites of (1?x)La0.7Ca0.3MnO3(LCMO)+xSnO2 (x = 0.01,0.05,0.10,0.30,0.50, 0.60,0.65,0.70) were synthesized by conventional solid-state reaction method.The results of X-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicate that the SnO2 and LCMO phase are coexist in the composites and the SnO2 phase mainly segregates at the grain boundaries of LCMO,which is accordant with the results of magnetic measurement. 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.When x > 0.50,TP1 disappeared,leaving only TP2.The percolation threshold occurred at x = 0.60.Corresponding to two M-I transition peaks,the curves of magnetoresistance against temperature also appeared two peaks for all composites.These phenomena can be explained by the segregation of a new phase related to SnO2 at grain boundaries or surfaces of the LCMO grains.Otherwise Ferromagnet-metal-type composites, LCMO-Ag, have been fabricated by both electroless plating technique and Solid-state reaction. The contents of Ag were controlled through changing the plating time. After sintered with different conditions, the samples were characterized by X-ray powder diffraction (XRD), which shows that the element of Ag exits with different form in different conditions. Temperature dependence resistivity and magnetoresistance in the applied field of 3kOe were measured. Experiment results show that Ag existed at the grain boundary of LCMO increases intrinsic insulator/metal transition temperature (TP) and greatly enhance the intrinsic magnetoresitance of LCMO. These phenomena can be explained by considering that the segregation of new phases of Ag at grain boundaries or surfaces of grains have a modification of grain boundaries. |
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