University of Khartoum

Structural and Magnetic Studies of Pure and Doped Micro- and Nano-crystalline Materials with Perovskite- and Spinel Structures

Structural and Magnetic Studies of Pure and Doped Micro- and Nano-crystalline Materials with Perovskite- and Spinel Structures

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Title: Structural and Magnetic Studies of Pure and Doped Micro- and Nano-crystalline Materials with Perovskite- and Spinel Structures
Author: Mohamed, Nodar Osman Khalifa
Abstract: Thestructural and magnetic properties of mechanosynthesized single-phased perovskite-related nanocrystalline particles pure and Ti4+-doped YFeO3 and TbFeO3 are reported in this thesis. We also report on the structural and magnetic properties of conventionally prepared single-phased spinel-related pure and Mn4+-doped Li0.5Fe2.5O4 microcrystalline particles of the composition Li0.5MnxFe2.5-yO4 (y =4x/3, x= 0.00, 0.10, 0.15, 0.20, 0.25, 0.30).The perovskite samples were mechanosynthesized in the air using a planetary ball mill machine. Ti4+-doped -Fe2O3 was used as a precursor to produce Ti4+-doped YFeO3 and TbFeO3 rather than using stoichiometric amounts of -Fe2O3 and TiO2. The average particles size determined from Transmission Electron Microscopy (TEM) and Scanning Electron Microscope (SEM). Fourier Transform Infrared Spectroscopy (FT-IR) confirms the formation of the expected structures. The Mössbauer spectra of all nanocrystalline perovskite samples show the presence of a ~20 % superparamagnetic component with blocking temperatures less than the 78 K. Rietveld refinement of the patterns as indicated by X-ray diffraction (XRD) and interatomic potential (atomistic) simulations with General Utility Lattice Program (GULP) show that the Ti4+ dopant ions to exclusively occupy octahedral B-sites in the doped perovskite materials. The magnetizations of the doped YFeO3 and TbFeO3 nanoparticles were found to be less than those of the pure bulk pristine materials. This is indicative of the presence collective magnetic excitations as well as reduced super-exchange magnetic interactions between the Fe3+ ions due to the presence of the non-magnetic Ti4+ ions. Additionally, the Ti4+-doped-YFeO3 and TbFeO3 nanocrystalline particles illustrate present of week ferromagnetism that could be attributed to spin canting and/or thermal spin reversal. In the spinel sample a series of Mn4+ doped- Li0.5MnxFe2.5-yO4 was conventially prepared. The Rietveld structural analysis of the Li0.5MnxFe2.5-yO4 (y =4x/3, x= 0.00, 0.10, 0.15, 0.20, 0.25, 0.30) indicate a slight increase in the lattice parameter with increasing Mn4+ content. The average of particle size was found to be depending on the Mn4+ concentrations. Rietveld structural and Mössbauer analyses indicated the Mn4+ dopant ions to substitute for Fe3+ at the octahedral B-sites of the spinel-related structure. These results as consistent with those inferred from atomistic simulations. The magnetic properties obtained from Mössbauer and VSM techniques, respectively, show the hyperfine magnetic fields at the Fe nuclear sites and the macroscopic magnetization to decrease with increasing Mn4+ concentrations.
URI: http://khartoumspace.uofk.edu/123456789/26117


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