| Perovskite manganite RE1-xTxMnO3 (RE for Rare earth element, T for alkaline-earth element) with ABO3 structure, has attracted much attention, because of its colossal magnetoresistance effect and broad application prospects for the past 10 years. The double exchange mechanism proposed by Zener can be used to explaine qualitatively the electromagnetic and magnetic properties of such compounds.In this paper, we prepared the two series samples with nominal components La1-xMnO3 and La0.60Sr0.40-xCuxMnO3, by the sol-gel method. On the basis of experimental and theoretical studies, phase structures of the samples were explained satisfactorily. In addition, we prepared La0.6Sr0.25Na0.15MnO3/La0.6Sr0.35Na0.05MnO3 heterostructure samples and studied their magnetoresistance effect.1. The single-phase self-doped perovskite manganites La1-xMnO3-1.5x. We prepared the self-doped perovskite manganite with nominal component La1-xMnO3, by the sol - gel method. The sample powders for XRD measurements and magnetic study were calcined finally at 800°C. We measured X-ray diffraction spectra, and found that all samples were the single- phase perovskite structure. Assuming that there were no vacancies, the lacking cations at A site were filled by Mn2+ ions, and all the samples possessed standard ABO3 structure, we calculated the ion ratios of at A, B and O sites. The ion ratios were used to the Rietveld fitting for XRD spectra, obtained error parameters Rp, Rwp and s were acceptable. The magnetic properties of samples were measured by Lake Shore M7310 Vibrating Sample Magnetometer (VSM). It was found that the dependences of the Curie temperature TC on the content of Mn4+ ions at B site, are similar to those of the typical perovskite La1-xCaxMnO3. Thus, the author concludes that, there are less vacancies in these self-doped perovskite materials with nominal component La1-xMnO3, but there are Mn2+ ions in A site of the samples. The samples formed standard ABO3 structure, in which the ionic ratio of A, B and O site is approximately 1:1:3. Therefore, it provides a new experimental evidence for the ion content calculation method propsoed by us, for the self-doped perovskite materials.2. La0.6Sr0.4-xCux-yMn1-yO3-3y/(Cu1.5Mn1.5O4)2y/3 composites. We prepared the composites with nominal component La0.6Sr0.4-xCuxMnO3, by the sol-gel method. The sample powders for XRD measurements and magnetic study were calcined finally at 800°C. We measured X-ray diffraction spectra, and found that all samples were the single- phase perovskite structure. We found that such material were the composite La0.6Sr0.4-xCux-yMn1-yO3-3y/Cu1.5Mn1.5O4. Assuming that there were no vacancies, the ionic ratio of A, B and O site in the perovskite phase is 1:1:3. We calculated the ion content of perovskite phase. The ion ratios were used to the Rietveld fitting for XRD spectra, obtained error parameters Rp, Rwp and s were acceptable. Therefore, these materials are composite La0.6Sr0.4-xCux-yMn1-yO3-3y/(Cu1.5 Mn1.5O4)2y/3. The dependences of the Curie temperature TC these materials on the content of Mn4+ ions at B site, are similar to those of the typical perovskite manganites La1-xSrxMnO3, which can be explained by the double exchange mechanism of Zener. However, the Curie temperature of the samples with Mn4+ content falled rapidly than that of La1-xSrxMnO3, which may be caused by that Sr2+ in the A-site were replaced by Cu2+, and the average radius in the A-site decreased rapidly.3. Magnetoresistance effect of heterostructure material La0.6Sr0.25Na0.15MnO3/La0.6 Sr0.35Na0.05MnO3.The doped manganese oxide powders La0.6Sr0.15Na0.25MnO3 and La0.6Sr0.05Na0.35MnO3. were prepared by sol-gel method. The powders were pressed into the bulks in the pressure of 754 MPa, then, sintered at 1400℃for 12 hours. The heterostructure materials La0.6Sr0.25Na0.15MnO3/La0.6Sr0.35Na0.05MnO3 were prepared by joint the two bulks with soldering tin, the resistance of the two bulks are aproximate same each other by adjusting their size ratio. There are two peaks on the curves of the magnetoresistance versus the temperature for the heterostructure materials, the magnetoresistance magnitude in the temperature region between the two peaks is higher than anyone of the two bulks. Therefore, a suggestion was obtained: heterostructure sample appears in the different temperature. From this we may obtain the enlightenment: If we select a series of material whose magnetoresistance peaks are in the different temperature regions, we can make the heterostructure multilayer films, and may make its magnetoresistance be stable aproximately in a wider temperature region, and possess a magnetic field sensitivity, by adjusting every film thickness. Thus, the heterostructure materils may be as the better magnetoresistance magnetic field sensor.
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