Enhancement of temperature coefficient of resistance (TCR) and magnetoresistance (MR) of La0.67-xRExCa0.33MnO3 (x=0, 0.1; RE = Gd, Nd, Sm) system via rare-earth substitution

We investigated the influence of 10% substitution of rare-earth (RE) elements on the crystallographic, transport, and magnetic properties of La0.67-xRExCa0.33MnO3(RE = Nd, Sm, and Gd, x = 0.0, 0.1) manganite perovskite compounds. The bulk polycrystalline samples were synthesized using solid-state reaction method. The phase purity and crystal structure of studied samples were confirmed by room temperature X-ray diffraction followed by the Rietveld refinement analysis. A high temperature insulator to low temperature metal phase transition is observed in electrical transport measurement. We observed an enhancement in the temperature coefficient of resistance (TCR) and magnetoresistance (MR) by partially substituting La with RE ions. The maximum TCR 22% and MR 96% are observed in Gd doped sample. The magnetic transition temperature, T-c, decreases from similar to 254 K for the pristine sample to about similar to 165 K for the Gd-doped sample. Our analysis of electrical and thermal transport data shows that the Small Polaron Hopping (SPH) is predominant at high temperatures conduction mechanism, whereas at low temperatures mechanism is dominated by electron-magnon scattering. The high temperature insulator paramagnetic phase to low temperature metallic ferromagnetic phase transitions are also observed in thermal conductivity and specific heat.