Nonlinear Electrical And Magnetotransport Properties Of ZnO/Perovskite Manganite Ceramic Composites
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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.
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