Abstract
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 (Tc) is about 70 K higher than the insulator–metal transition temperature (Tp). Two peaks were observed on both the temperature dependence of the imaginary part of the ac magnetic susceptibility χ′′(T ) and that of the magnetoresistance MR(T). One is slightly below Tp∼107 K and the other is near Tc∼170 K. Below 70 K, the MR ratio increases with decreasing temperature. Around and above Tp but below Tc, the magnetization shows some indication of saturation, whereas the MR ratio shows no indication of saturation. The magnetotransport properties 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)2O2 layers between the adjacent MnO2 bilayers.
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