A-site Doping Effect On Electrical Transport And Magnetoresistance Of Perovskite-type Manganese Oxides

The discovery of colossal magnetoresistance (CMR) effect results in the wide concerning on perovskite-type manganese oxides. It is not only because of CMR has tremendous potential application on sensor, but also because it is a strong correlate electronic system, related to a series of basic questions in condensed matter physics such as the strong correlation and multiple body systems, metal-insulator transition . So it is very important to the study of basic physics research or material research. This paper mainly studies resistivity effort and electromagnetic transport properties on rare earth doped perovskite-type manganese oxides RE1-xAExMnO3.Our experimental samples are made by glycine-nitrate synthesis method, which have the advantages such as the reaction is more quickly, the purity of product is higher and component segregation is small, the power is small and uniform.1, La_2/3Ca_xSr_1/3-xMnO₃ (x=0.1, 0.15, 0.2) series samples: study shows that with the increase of Ca crystal structure of the sample did not change significantly, However, the resistance increases rapidly. Each sample has two resistivity - temperature change, we think this is because the different Ca and Sr abundance in different regions of the simples. Low-temperature magnetoresistance effect will do not change, the CMR effort drop with the rise of temperature drop. At about 250K, drop to a stable data about 5%. At room temperature there is a levy CMR effect which is around 8%. The temperature of CMR slowly decline with the increase of Ca. The results of metal conductivity fitting show that the low temperature of the electron-electron Coulomb interaction.2. On the basis of below reserch we study the change of La_2/3Ca_xSr_1/3-xMnO₃ with 5%La absent. Research shows that the lack of 5%La make some impact to La_2/3Ca_xSr_1/3-xMnO₃: XRD showd that the lattice size become smaller; Raman analysis of the samples showed Mn-O-Mn bond angle has changed; resistivity is obviously lower than La_2/3Ca_xSr_1/3-xMnO₃ systems. We believe the reason could be the proportion of Mn3 + and Mn4+ of become different and in the substance composition and the ratio of LSMO and LCMO also changed. Samples' CMR effect become lower than La_2/3Ca_xSr_1/3-xMnO₃ .3. To study the effect of Ag-added samples, La2/3Ca0.15Sr1/3-0.15MnO3 selected as a representative study. Experimental results show that Ag were not in lattice, but as a single element dispersed in the samples, and left many stomata, made the resistivity of the sample increasing. Ag doping enhanced the CMR effect at low temperature.4. In this chapter we synthesis La_2/3Ba_xSr_1/3-xMnO₃ series of samples (x = 0.1, x = 0.15, x = 0.2) by glycine-nitrate process. All the samples are orthorhombic perovskite structure, with Ba increasing, lattice volume increased slightly, because of Ba2 + radius is bigger than Sr2 +. The resistivity of Ba-doped sample is much more times than La_2/3Ca_xSr_1/3-xMnO₃. From La_2/3Ca_xSr_1/3-xMnO₃ to La_2/3Sr_1/3MnO₃ until La_2/3Ba_xSr_1/3-xMnO₃ resistance rate was reduced first and rose then; view from magnet La_2/3Sr_1/3MnO₃ has the highest Tc, which shows double-exchange mechanism is stronger in these simples. From the two aspects we can see our La_2/3Ba_xSr_1/3-xMnO₃ simples have a common characteristic, the essential magnetoresistance effect is very weak, we can hardly find its peak in the CMR graph. Throughout the test temperature range, the CMR effect basically has linear downward trend. Low-temperature magnetoresistance effect should be slightly higher than the samples in third chapter. We fit metal and semiconductor conductivity of La_2/3Ba_xSr_1/3-xMnO₃ with the existing mechanism, and found that low-temperature conductivity more belong to electron-electron Coulomb interaction, and the high-temperature semiconductor conductivty become small polaron model.