MAGNETIC CONTROL OF SPIN ENTROPY, THERMOELECTRICITY AND ELECTRICAL RESISTIVITY IN SELECTED MANGANITES

Abstract

The Hole-doped Mn-based perovskite oxides, such as Ln1-xAxMnO3 (Ln-trivalent rare-earth cation, A- di/mono-valent cation) show strong coupling between electrical and magnetic properties. They have been extensively studied for colossal magnetoresistance (CMR). Recent studies suggest that the manganites are one of the potential candidates among non-toxic magnetocaloric materials for magnetic refrigeration. However, thermoelectric properties of these oxides are less investigated than the electrical and magnetic properties and correlations between these physical properties are rarely explored. In this thesis, I have investigated the magnetic, magnetocaloric (MCE), magnetoresistance (MR) and magnetothermopower (MTEP) properties in trivalent (Sm), divalent (Sr), and monovalent (Na) doped manganites (Pr1-xSrxMnO3, Pr0.6-xSmxSr0.4MnO3, Pr0.8-xSmxSr0.2MnO3, and Nd1-xNaxMnO3). They have shown wide range of physical properties, such as first and second order magnetic phase transitions, normal and inverse magnetocaloric effects, unusual 4f and 3d spin interactions, charge ordered phase transitions, peak/sharp slope change in thermopower at the magnetic phase transition, colossal magnetoresistance, giant magnetothermopower, filed induced meta-magnetic transitions and large magnetic entropy change, etc.,. In addition, the Universal scaling of magnetic entropy change (?Sm) for second order magnetic transition materials is demonstrated. We show linear correlations among the field dependent magnetization, dc-resistivity, magnetic entropy and thermopower.