Semiconducting and magnetic properties of La/sub 0.67/Ca/sub 0.33/Mn/sub 1-x/Dy/sub x/O/sub 3/ ceramics

Electrical resistivity measurements of La/sub 0.67/Ca/sub 0.33/Mn/sub 1-x/Dy/sub x/O/sub 3/, x=0 to 0.12, ceramics have been studied using a standard four point probe technique, while the magnetic behaviour has been studied using AC susceptibility techniques. The transport properties show the transition from semiconducting to metallic conductivity at T/sub p/ and paramagnetic-ferromagnetic transitions, T/sub c/, were observed in the /spl chi/-temperature curves for all samples. The existence of metallic conductivity, T/sub p/, and ferromagnetism, T/sub c/, were found to be linearly correlated. This phenomenon of coexistence is due to the double exchange interaction of two electrons in Mn/sup 3+/-O/sup 2-/-Mn/sup 4+/ and Mn/sup 4+/-O/sup 2-/-Mn/sup 3+/ configuration which brings the system below T/sub c/ into a metallic state. Hence, it is observed that the Curie temperature T/sub c/ is closely related to the sharp decrease in electrical resistivity of the samples. However, both transition temperatures shift to lower temperatures as dysprosium doping increases, indicating the loss of ferromagnetic order and transport properties. As for the transport properties, the semiconductor model ln(/spl sigma/)/spl sim/(E/sub g//2kT) was used to explain the conduction mechanism of perovskite manganites above T/sub p/. It was concluded that the total conductivity /spl sigma//sub tot/ consists of the intrinsic and extrinsic components such that /spl sigma//sub tot/=/spl sigma//sub int/+/spl sigma//sub ext/. The energy gap decreases initially from 0.11 eV to a minimum value of 0.06 eV at x=0.03 and increases again to 0.08 eV at the composition of x=0.12 for the extrinsic region.