Magnetotransport properties of a layered La1.6Ca1.4Mn2O7 polycrystal

Magnetotransport properties of a two-layered La1.6Ca1.4Mn2O7 polycrystal have been examined as a function of temperature and applied field. It was found that the magnetic transition temperature (T-c) is about 70 K higher than the insulator-metal transition temperature (T-p). Two peaks were observed on both the temperature dependence of the imaginary part of the ac magnetic susceptibility chi"(T) and that of the magnetoresistance MR(T). One is slightly below T-p similar to 107 K and the other is near T-c similar to 170 K. Below 70 K, the MR ratio increases with decreasing temperature. Around and above T-p but below T-c, the magnetization shows some indication of saturation, whereas the MR ratio shows no indication of saturation. The magnetotransport proper-ties can be explained by considering the anisotropy exchange interactions along the a-b plane and the c direction, and the low-temperature MR can be attributed to the effects of the nearly fully spin-polarized carriers' tunneling through the insulating (La, Ca)(2)O-2 layers between the adjacent MnO2 bilayers.