This thesis deals with the investigations on the nonlinear electrical and manganetotransport properties of polycrystalline multi-phase ceramic composites of Zno/pervoskite manganite. Multifunctional properties are studied such as the enhanced low-field magnetoresistance(LFMR). magnetically tuneable low-voltage nonlinear current-voltage (I-V) characteristics with larger nonlinearity coefficients suitable for semiconducting and magnetoelectric devices. A brief introduction on the structure-property correlations, electronic and magnetic structures, nonlinear electrical conduction, phase separation, grain size and grain boundary effects on transport properties of manganites are presented. The nonlinear current-voltage characteristics of ZnO based varistors are also summarized. The thesis describes the synthesis of the ceramics and the methodology of different techniques utilized in characterizing the samples. The phase conversions in calcium manganite with changing Ca/Mn ratios as well as the oxygen non-stoichiometry and their influence on electrical transport properties were studied. The realization of low-voltage varistors prepared from ZnO+ CaMnO3 ceramic composites was described. An energy band model consisting of n-p-n heterojunctions of n-ZnO1-γ:Mn/p-CMZO/n-ZnO1 γ:Mn has been proposed in order to explain the large nonlinearity coefficients obtained at low field-strengths of 1.8 to 12 V/mm. The detailed investigationos on the structural identification and physico-chemical analyses of Ca4Mn7Zn3O21-δ(CMZO) phases having the beta-alumina or magnetoplumbite-type structures were carried out. The thesis also embodies the magnetically tuneable nonlinear I-V characteristics and the magnetotransport properties of ZnO/La(Sr)MnO3 and ZnO/La(Ca,Sr)MnO3 ceramic composites. The present investigations demonstrate that the ferromagnetic insulating (FMI) La06 Sr04Mn1-yZnyO3(y = 3 to 8 at.%) when present as minor phase in ZnO1- γ:Mn ceramics enables in attaining magnetically tunealbe nonlinear I-V characteristics. Wherein, the dominant ZnO1- γ:Mn phase remains paramagnetic. The results also indicate that the prevalence of ferromagnetism in ZnO1-γ:Mn is not significant for realizing magnetically tuneable I-V curves. The controversial results related to the existence of ferromagnetism in ZnO(doped)leading to diluted magnetic semiconductors(DMS) have been investigated. Another novel aspect of the present work is the low-field magnetoresistive(LFMR) property of ZnO/La(Sr)MnO3 and ZnO/La(Ca.Sr)MnO3 ceramic composites which been explained on the basis of spin-polarised tunneling across the intergrain regions. The influence of Zn2+ as a diamagnetic substitutent in modifying the crystallographic phase content, electrical transport and magnetic properties of Lao6Sro4MnO3 were studied in detail. The results point towards the fact the large decrease of Tc and Ms at lower Zn contents(≤ 8 at.%)is due to the dominant role played by the excess oxygen vacancy (Vo) as an electron donor in p-type Lao6Sro4Mn1-yZnyO3-δ rather than the charge compensatively predictable values. The modifications of electronic and magnetotransport properties were carried out on Lao6Sro4MnO3 substituted with diamagnetic ions such as Mg2+ - Al3+ - Ti4+ - Nb5+ - Mo6+ or W6+ at Mn-sublattice. The TEM studies including HREM results point to the fact the large ΔT(= Tc-TM-1)is accountable in terms of charge conduction within the electronically heterogeneous phase mixtures of charge ordered insulating (CO1) bi-stripes prevailing within the charge disordered FMI phases.

La réfrigération magnétique attire beaucoup d’attention ces dernières années parce qu’elle est considérée comme une technologie respectueuse de l’environnement et énergétiquement économique. Aujourd’hui, cette technologie avancée est encore en phase de recherche que des dispositifs de réfrigérations magnétiques soient déjà opérationnels. Ce travail de thèse consiste à étudier la potentialité de résistance à la corrosion de différents types de matériaux magnétocaloriques (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga et Pr0.66Sr0.34MnO3) en contact avec un fluide caloporteur. Afin de comprendre les propriétés magnétocaloriques des matériaux, nos recherches se sont aussi focalisées sur les relations entre la transition magnéto-structurales d’alliages Heusler Ni2Mn0.75Cu0.25Ga et (i) la distribution cationique au sein de la structure cristalline et/ou (ii) la microstructure. Finalement, le diagramme de phase magnétique et nucléaire en lien avec les effets magnétocalorique obtenu grâce à la diffraction de neutrons et de pérovskite Pr1-xSrxMnO3 (0.25≤x≤0.45) est également présenté. / Magnetic refrigeration has gained lot of importance and attention as they are highlighted to be environmental friendly, energy efficient. Presently, though at research stage, the magnetic refrigerators are pushed towards realization in domestic application with extensive work on materials and with few working models. One critical issue, the potential resistance to corrosion in case of different class of magnetocaloric materials (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga and Pr0.66Sr0.34MnO3) against the heat transport fluid is addressed. To better understand and improve the observed magnetocaloric properties in Heulser alloys Ni2Mn0.75Cu0.25Ga and to elaborate the same with the magneto-structural relation, studies on (i) cation distribution with in crystal structure and/or (ii) microstructural dependence are presented. Nuclear and magnetic phase diagram based on detailed neutron diffraction and magnetism studies for magnetocaloric perovskite oxide Pr1-xSrxMnO3 (0.25≤x≤0.45) is also presented