Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    COMBINATORIAL INVESTIGATION OF RARE-EARTH FREE PERMANENT MAGNETS
    (2015) Fackler, Sean Wu; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The combinatorial high throughput method allows one to rapidly study a large number of samples with systematically changing parameters. We apply this method to study Fe-Co-V alloys as alternatives to rare-earth permanent magnets. Rare-earth permanent magnets derive their unmatched magnetic properties from the hybridization of Fe and Co with the f-orbitals of rare-earth elements, which have strong spin-orbit coupling. It is predicted that Fe and Co may also have strong hybridization with 4d and 5d refractory transition metals with strong spin-orbit coupling. Refractory transition metals like V also have the desirable property of high temperature stability, which is important for permanent magnet applications in traction motors. In this work, we focus on the role of crystal structure, composition, and secondary phases in the origin of competitive permanent magnetic properties of a particular Fe-Co-V alloy. Fe38Co52V10, compositions are known as Vicalloys. Fe-CoV composition spreads were sputtered onto three-inch silicon wafers and patterned into discrete sample pads forming a combinatorial library. We employed highthroughput screening methods using synchrotron X-rays, wavelength dispersive spectroscopy, and magneto-optical Kerr effect (MOKE) to rapidly screen crystal structure, composition, and magnetic properties, respectively. We found that in-plane magnetic coercive fields of our Vicalloy thin films agree with known bulk values (300 G), but found a remarkable eight times increase of the out-of-plane coercive fields (~2,500 G). To explain this, we measured the switching fields between in-plane and out-of-plane thin film directions which revealed that the Kondorsky model of 180° domain wall reversal was responsible for Vicalloy’s enhanced out-of-plane coercive field and possibly its permanent magnetic properties. The Kondorsky model suggests that domain-wall pinning is the origin of Vicalloy’s permanent magnetic properties, in contrast to strain, shape, or crystalline anisotropy mechanisms suggested in the literature. We also studied the thickness dependence of an Fe70Co30- V thin film library to consider the unique effects of our thin film libraries which are not found in bulk samples. We present results of data mining of synchrotron X-ray diffraction data using non-negative matrix factorization (NMF). NMF can automatically identify pure crystal phases that make up an unknown phase mixture. We found a strong correlation between magnetic properties and crystal phase quantity using this valuable visualization. In addition to the combinatorial study, this dissertation includes a study of strain controlled properties of magnetic thin films for future applications in random access memories. We investigated the local coupling between dense magnetic stripe domains in transcritical Permalloy (tPy) thin films and ferroelectric domains of BaTiO3 single crystals in a tPy/BaTiO3 heterostructure. Two distinct changes in the magnetic stripe domains of tPy were observed from the magnetic force microscopy images after cooling the heterostructure from above the ferroelectric Curie temperature of BaTiO3 (120°C) to room temperature. First, an abrupt break in the magnetic stripe domain direction was found at the ferroelectric a-c-domain boundaries due to an induced change in in-plane magnetic anisotropy. Second, the magnetic stripe domain period increased when coupled to a ferroelectric a-domain due to a change in out-of-plane magnetic anisotropy. Micromagnetic simulations reveal that local magnetic anisotropy energy from inverse magnetostriction is conserved between in-plane and out-of-plane components.
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    ENGINEERING OF SELF-ASSEMBLED MULTIFERROIC NANOSTRUCTURES IN PbTiO3-CoFe2O4 THIN FILMS
    (2006-01-31) Li, jianhua; Roytburd, Alexander L; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Multiferroic materials which display a coexistence of ferroelectric and ferromagnetic properties attract considerable interest for their potential for novel device applications as well as for the interesting physics and materials science underlying their functional responses. In multiferroic composite, electromagnetic coupling facilitate elastic interaction between ferroelectric and ferromagnetic components via piezoeffect and magnetostriction. The major goal of our research is designing the transverse epitaxial multiferroic nanostructures with controlled morphologies. The PbTiO3-CoFe2O4 system was selected for this study because of the (i) large spontaneous strain associated with the ferroelectric phase transition in PbTiO3 (6.5%), which should create strong elastic interactions between the two phases accompanying the piezoelectric effect, and (ii) large magnetostriction of ferrimagnetic CoFe2O4. We successfully fabricate self-assembling multiferroic nanostucture films of CoFe2O4-PbTiO3 by PLD deposition on SrTiO3 substrates of different orientations. X-ray and TEM characterization show that all films have columnar architecture and 3D epitaxial relationships between phases and each phase and substrates. The morphology of nanostructures has been controlled by changing orientation of a substrate. It has been shown that it is possible to obtain the ferromagnetic (CoFe2O4) rods with a diameter about 10-20 nm in the ferroelectric PbTiO3 matrix in (001) films of composition 0.67PbTiO3-0.33CoFe2O4, and vise versa: ferroelectric rods in ferrimagnetic matrix in (111) films of composition 0.33PbTiO3-0.67CoFe2O4. The lamellate morphology with a specific crystallographic orientation of lamellae corresponding to {111} planes has been obtained in (110) films. The measurements of lattice parameters of the constitutive phases at different temperature allows us to determine the level of internal stresses due to misfit between phases. The measurements of piezo- and magnetic responses of the films prove that the films are ferroelectric and ferromagnetic simultaneously. The piezo- and magnetic responses are considerable suppressed due to mutual constraints between phases. This suppression indicates the strong elastic interactions between the phases which allows us to suggest the strong electro-magnetic coupling in the films. Combining theoretical and experimental studies of self-assembled multiferroic nanostructures in epitaxial films has revealed that the elastic interactions caused by epitaxial stresses play the dominate role in defining the morphology of the nanostructures and their magnetic and electric responses.