Magnetotransport and thermoelectric power of La2/3Ba1/3Mn1-xSbxO3 (x=0-0.05) manganite perovskites

The effect of Sb⁺⁵-doping on the magnetotransport and thermoelectric power of La_2/3Ba_1/3Mn_1-xSb_xO₃ (x=0-0.05) perovskite manganites is reported here. Two insulator-metal (I-M) transitions have been observed in the electrical resistivity-temperature ρ (T) behavior of the undoped sample La_2/3Ba_1/3MnO₃. Both the transitions (at T_{P 1} and T_{P 2}) shift to lower temperatures with doping but to a different extent. T_{P 1} decreases faster while T_{P 2} remains almost invariant up to 3% of doping and then decreases. With increasing Sb⁺⁵ content, the intrinsic magnetoresistance (MR at T_{P 1}) gets suppressed whereas the extrinsic magnetoresistance at lower temperatures gets enhanced. The thermoelectric behavior S (T) of the pristine sample shows a peak at T_{P 1} while for the doped samples the peak gets suppressed. All the samples exhibit a crossover in their S (T) behavior from positive to negative at a temperature T^*, indicating that the dominant carrier in these compounds changes from hole to electron above T^*. Transport behavior above T_{P 1} (the paramagnetic insulating region) is explained on the basis of the small polaron hopping model while the electron-magnon scattering process has been invoked to explain the thermoelectric power and electrical resistivity behavior in the ferromagnetic regime.