Magnetotransport, magnetization and thermoelectric power of Pr2/3Ba1/3MnO3 : PdO composite manganites

Abstract

Electrical, magnetotransport, magnetic and thermoelectric power (TEP) studies carried out on the (1 − x)Pr2/3Ba1/3MnO3 + xPdO composite system are reported here. The electrical resistivity of the pristine sample Pr2/3Ba1/3MnO3 (PBMO) exhibits two insulator–metal (I–M) transitions (TP1 and TP2). The higher temperature transition (TP1) becomes sharper and the lower temperature transition (TP2) disappears with higher content of PdO (>10 mole%). The intrinsic magnetoresistance (MR) becomes enhanced whereas the extrinsic MR decreases monotonically. The observed MR behaviour of the composite system is attributed to the decrease in the electrical resistivity (due to PdO decomposition to metallic Pd), the opening of new conducting channels and the decrease of spin dependent scattering at the grain boundaries due to the disappearance of the barrier formed at the grain boundary from the canting of spins, defects, etc. Electrical resistivity above TP1 follows the non-adiabatic small polaron hopping model whereas the ferromagnetic metallic region is governed by the electron–magnon scattering process. The Curie temperature (TC) of the composite system remains unchanged; however, magnetization at low temperatures decreases with PdO addition. TEP exhibits an I–M transition near TP1 in all the manganites. The TEP crossover observed at temperature T* suggests that the dominating carriers in these compounds change from electrons to holes below T*.