The Study Of Electrical And Magnetic Transport Of Ce Doped Manganese

The perovskite manganites possess great applying potential and relate to many basic physical mechanism because of colossal magnetoresistance (CMR) effect.In this dissertation, the effect of the doping in A site with Ce on the electrical transport and magnetoresistance effect of La_1-xCe_xMnO₃ is carried out. The main investigative results are as follows:(1) The different magnetoresistance material is briefly reviewed. The advance in the study of perovskite manganites has been summarized. More attentions have been paid to the doping effect.(2) Polycrystalline samples of nominal composition La_1-xCe_xMnO₃ were prepared with solid-state reaction method. We study the effect on the electrical transport and magnetoresistance effect with different sintering thelonogy. We find, similarly to the classic polycrystalline La_1-xCa_xMnO₃ and La_1-xSr_xMnO₃ , La_1-xCe_xMnO₃ system also would be transited into metal state from insulator with the temperature decreasing;And the longer time and the higher temperature the samles are sinered ,then the higher temperature of insulator-metal transition and the less resistivity. The high temperature region can be fitted with the expression for the adiabatic small polaron conduction mechanism.(3)After we study the series of La_1-xCe_xMnO₃ samples ,we find that the resistivity decreases with increase of Ce content. The decrease can be accounted for by considering the valence fluctuations in the Mn site arising due to the fluctuation in the Ce sites. With the Ce content, the concentration of both Mn4+and Mn2+ increases. High concentration of Ce⁴⁺ in the A site can lead to large oxygen excess (La_1-xCe_xMnO_3+δ) conditions in all the La–Ce compositions. In such oxygen excess conditions, Mn ions can stabilize in +4 or +3 oxidation states in addition to the presence of Mn2+ ions depending upon the extent of nonstoichiometry. Mn ions are present in three different oxidation states and, in each La–Ce compositions, Mn³⁺–Mn⁴⁺ pairs increase with Ce content over Mn²⁺–Mn³⁺ pairs. Therefore double exchange (Mn3+-O-Mn4+)dominates over other super-exchange (Mn3+-O-Mn3+, Mn2+-O-Mn3+) mechanisms resulting in M–I transition and shift of Tc to higher temperatures. Thus there will be competition between double exchange and super-exchange resulting in high resistivity and the shift in ferromagnetic transition temperature to higher temperature with Ce addition is suppressed.(4) Within the low temperature region,commonly under 120K,all the samples show a tendency of CO-ordering phase.(5) In the Ca1-xCexMnO3 (0