Browsing by Author "Keshavarz, Sahar"

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    Bulk Single Crystal-Like Structural and Magnetic Characteristics of Epitaxial Spinel Ferrite Thin Films with Elimination of Antiphase Boundaries
    (Wiley-VCH, 2017) Singh, Amit V.; Khodadadi, Behrouz; Mohammadi, Jamileh Beik; Keshavarz, Sahar; Mewes, Tim; Negi, Devendra Singh; Datta, Ranjan; Galazka, Zbigniew; Uecker, Reinhard; Gupta, Arunava; University of Alabama Tuscaloosa; Department of Science & Technology (India); Jawaharlal Nehru Center for Advanced Scientific Research (JNCASR); Leibniz Institut fur Kristallzuchtung (IKZ)
    Spinel ferrite NiFe2O4 thin films have been grown on three isostructural substrates, MgAl2O4, MgGa2O4, and CoGa2O4 using pulsed laser deposition. These substrates have lattice mismatches of 3.1%, 0.8%, and 0.2%, respectively, with NiFe2O4. As expected, the films grown on MgAl2O4 substrate show the presence of the antiphase boundary defects. However, no antiphase boundaries (APBs) are observed for films grown on near-lattice-matched substrates MgGa2O4 and CoGa2O4. This demonstrates that by using isostructural and lattice-matched substrates, the formation of APBs can be avoided in NiFe2O4 thin films. Consequently, static and dynamic magnetic properties comparable with the bulk can be realized. Initial results indicate similar improvements in film quality and magnetic properties due to the elimination of APBs in other members of the spinel ferrite family, such as Fe3O4 and CoFe2O4, which have similar crystallographic structure and lattice constants as NiFe2O4.
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    Synthesis and characterization of novel electronic and magnetic systems
    (University of Alabama Libraries, 2016) Keshavarz, Sahar; LeClair, Patrick R.; University of Alabama Tuscaloosa
    Fabrication and characterization of novel electronic and magnetic materials are pursued under two main subjects during this dissertation: Part I deals with the low frequency noise spectroscopy of vanadium dioxide (VO2) thin films in the vicinity of the first-order metal-insulator transition (MIT). High quality epitaxial thin films of vanadium dioxide were deposited on TiO2 substrates using chemical vapor deposition methods and characterized for our investigations. The design and characterization of our home-built “cross-spectrum” analyzer, which was used for the fluctuation spectroscopy of VO2 films, is explained. 1/f noise analysis of vanadium dioxide across the MIT shows the significant increase of noise amplitude, which can be explained by “Local interference” model. Energy distribution investigations in this material showed the signature of the percolation model. Critical exponent studies are suggestive of the p-noise model as the mechanisms governing the electronic processes in VO2. Evidence of anisotropic electrical transport along different crystallographic axes of VO2 is presented by the conductance, Hall measurements and noise spectroscopy of the patterned VO2 films. Part II of this dissertation presents the systematic, theoretical, experimental and analytical search for novel half-metallic Heusler compounds. Experimental and analytical methods to synthesize and characterize these new materials in polycrystalline bulk form are thoroughly discussed. As a result of our investigation a new Heusler material with the composition of Fe1.5TiSb was synthesized and was recognized as the most stable compound in FexTiSb systems. The theoretical investigations of this system are suggestive of an alternative L21/C1b layered structure for this new phase of material with weak magnetism and metallic characteristics. Our investigations for finding half-metallic Heusler Fe2MnGe led to synthesizing this compound with the hexagonal DO19 structure. This material has a high magnetic moment and uniaxial crystalline anisotropy, which is a step toward finding the half-metallic hexagonal Heusler compounds with applications in perpendicular media and CPP-GMR.