Electric, magnetic, and thermo-electric properties of Cr doped La_0.8Ca_0.2Mn_1-xCr_xMnO₃ manganites

A detailed study of the structural, magnetic, magneto-transport and thermoelectric properties of polycrystalline La_0.8Ca_0.2Mn_1-xCr_xMnO₃ (0<x<0.07) compounds is presented in the present communication. XRD studies confirm that the samples are pure and single phased. We have observed from electrical and magnetic studies that all the samples are exhibiting metal-insulator transition temperature, T_MI and ferromagnetic-paramagnetic (FM-PM) transition temperature, T_C. With increasing Cr-content, both T_MI and T_C are observed to decrease. The electrical resistivity data is analyzed using different theoretical models at various regions viz., metallic, insulating and percolation region. Analysis in the metallic region (T<T_MI) suggests that the electron-electron scattering in predominant in the region whereas the insulating region (T>T_MI) is well described using SPH model. However, the resistivity data in the whole temperature range is analyzed using a phenomenological model based on phase segregation of ferromagnetic metallic and paramagnetic insulating regions. Thermoelectric power, S measurements were performed to understand the conduction mechanism and to ascertain the types of charge carrier responsible for conduction. It is observed that pristine as well as Cr-doped compounds show positive value of S which demonstrates that the charge carriers are holes.