Fabrication And Magnetotransport Properties Of (La, A) MnO₃/MgO Nanocomposites And Synthesis And Microstructural Investigation Of Ba₄NdMn₃O₁₂

Since discovery of colossal magnetoresistance (CMR) effect in the perovskite manganites, researchers have expressed great interest in these manganite materials due to their potential applications in magnetic sensors, magnetic storage devices and spin transistors, etc. In order to enhance the low-field magnetoresistance (LFMR), (La,A)MnO₃/MgO nanocomposites have been prepared and their magnetotransport properties were studied systematically. In addition, a new manganite Ba₄NdMn₃O₁₂ has been synthesized and microstructures of this manganite have been investigated carefully. The main points are summarized as follows:1. La_2/3A_1/3MnO₃ (A=Ca,Sr,Ba) nanopowders have been prepared by a polyacrylamide gel route. X-ray diffraction (XRD) analysis showed that the powders are single-phased without any traces of impurities. Scanning electron microscope (SEM) observation revealed that the powders have a small and uniform particle size (the average particle size slightly varies between the samples), and the particles are almost spherical and polydisperse without any agglomeration or adhesion.2. (1-x)La_2/3Ca_1/3MnO₃(LCMO)/xMgO(x=0.1,0.2,0.3,0.4) composite nanopowders with core-shell structure have been prepared via the polyacrylamide gel method. With inceasing MgO content, the lattice of LCMO was revealed from XRD results to expand. This phenomenon can be explained as the result of the tensile strain of LCMO originating from the lattice mismatch between LCMO and MgO. The LCMO phase was also found to experience a structure phase transition from Pnma orthorhombic (x≤0.2) to rhombohedral structure (x≥0.3); this transition leads to a stress release for the LCMO phase. With inceasing MgO content, the Curie temperature T_c was observed to decrease for the orthorhombic phase (x≤0.2), whereas for the rhombohedral phase (x≥0.3) the Tc increased to a value similar to that of the pure LCMO phase. The T_c change can be attributed to the strain of LCMO and the stress-induced structure transition.In addition,the low-field magnetoresistance was found to be enhanced significantly with the increase of MgO content. These results have demonstrated that the magnetotransport properties of manganite-based composites can be manipulated by introducing an appropriate insulating phase.3. A new manganite Ba₄NdMn₃O₁₂ has been synthesized by traditional solid-state reaction method, and was determined to be a 12R perovskite-type structure by the combinative analysis of XRD, electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM). Two types of important polytypic faults were observed in the Ba₄NdMn₃O₁₂ compound, and their properties and formation mechanism were carefully studied.