Electrical Transport and Magnetoresistance of La_0.67Ca_0.33MnO₃: Ag_x(x = 0, 0.1, 0.2, 0.3, 0.4) Composites

The structural, magnetic and magnetotransport properties of $La_0.67Ca_0.33MnO_3$: $Ag_x$ (x = 0, 0.1, 0.2, 0.3 and 0.4) composites were investigated systematically. X-ray and EDX analysis indicated that Ag is not substituted into the main $La_0.67Ca_0.33MnO_3$ phase and remains an additive to the second phase at the grain boundary. The Curie temperature first decreased from 269 K for x = 0 to 257 K for x = 0.1 and then remained nearly unchanged with increasing Ag content. For the x > 0.1 samples, a second transition temperature ($T_MI2$) was observed in the resistance curves. At temperatures below 150 K, a significant enhancement in MR was observed while high temperature MR decreased with increasing Ag content. The maximum MR was observed to be 55% in the x = 0.4 sample at 10 K and a 6T magnetic field, this value is larger than that of pure $La_0.67Ca_0.33MnO_3$ (53% at 265 K and 6 T). In addition, at low fields (H < 1T), a sharp increase in the MR was observed.