Analyses structurales par microscopie électronique d'hexaferrites magnétiques Ca2+xFe16-xO26-(x/2)

Abstract : This thesis reports on the synthesis and the characterization of hexaferrite compounds in the Ca-Fe-O system. This work has allowed to isolate four polycrystalline compounds presenting the chemical formula (Ca4Fe5O13)1-x(Fe9O12)1+x (x= 0.334; 0.301 and 0.128) and (Ca4Fe5O13)(Fe4O4). Their crystalline structure has been determined using the precession electron diffraction tomography and has been validated through high resolution imaging microscopy (HREM/HAADF). X-ray diffraction and neutron diffraction studies on polycrystalline samples have confirmed the different structural models. Fine analysis of intergrowth defects in HAADF imaging revealed significant deviations in composition with respect to the ideal composition (Ca4Fe5O13)(Fe9O12) at the origin of the three observed polymorphs. In addition to the studies on micron-sized crystals in the 80s, obtaining polycrystralline samples allowed the measurement of their physical properties. Despite the complexity of these structures and the presence of extensive defects, the Mössbauer spectroscopy has highlighted a unique oxidation degree of iron (+3) and confirmed as well the various magnetic transitions initially detected by magnetization measurements, as well as their evolution versus the x deviation value. Electrical resistivity and Seebeck coefficient measurements were performed on the samples.
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Laurine Monnier. Analyses structurales par microscopie électronique d'hexaferrites magnétiques Ca2+xFe16-xO26-(x/2). Matériaux. Normandie Université, 2018. Français. ⟨NNT : 2018NORMC224⟩. ⟨tel-02094796⟩

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