Preparation and evaluation of amorphous metal oxide magnetic thin films and nanocomposites for spintronics

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(Thesis) Ph.D.
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In this work, photochemical metalorganic deposition (PMOD) using thin films of metal 2-ethylhexanoates (Mo and Fe) was used to deposit patterned thin films of a-MoO3 and a-Fe2O3 (where a stands for amorphous) in a positive tone. The mechanism of positive tone PMOD of a-Fe2O3 was examined by using spectroscopic ellipsometr y, which allowed the determination that the development preferentially removed the amorphous iron oxide from the film. The magnetic properties of dense assemblies of spinel ferrite superparamagnetic nanocrystals were studied. This was done for nanocomposite thin films prepared by PMOD containing nanocrystals of one (gamma-Fe2O3 in a-MnO) or two chemical identities (gamma-Fe2O3+CoFe2O4 in a-Al2O3 and a-TiO2), and bulk binary mixtures of nanocrystalline powders (gamma-Fe2O3+CoFe2O4). Based on the magnetic properties of these nanocomposite thin films and powders, a model in which strong, interparticle magnetic interactions affect the magnetic properties is presented. To gain an understanding of the local atomic structure of a-Fe2O3, X-ray absorption spectroscopy was used to determine the oxidation state and coordination environment of the iron atoms in this amorphous material. It was found the iron is in the +3 oxidation state, and occupied an octahedral environment. Similarl y, it was found that octahedral Co2+ and Fe3+ are present in a -CoFe2O4 and that octahedral Cr3+ is present in a-Cr2O3. The local structure in these amorphous metal oxides a-MOx thin films is also discussed based on an analysis of the extended x-ray absorption fine structure (EXAFS). This represents the first determination of the local structure in a-MOx prepared by PMOD. The magnetic properties of PMOD-prepared a-Fe2O3, a-CoFe2O4, and a-Cr2O3thin films were investigated. This work shows that these a-MOx thin films exhibit ferromagnetic properties at room temperature. Prior to this work, no magnetic amorphous iron oxide had previously been reported. Iron(III) 2-ethylhexanoate was used as a precursor for direct-write electron beam induced lithography. The deposition of sub-100 nm iron oxide features was shown. Sub-40 nm nanowires with low line edge and width roughnesses (< 3 nm) could be patterned. TEM and AFM inspection also identified free-standing, hollow nanotubes.
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