Materials Science & Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2792
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Item 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.Item Combinatorial Investigation of Magnetostrictive Materials(2007-08-24) Hattrick-Simpers, Jason Ryan; Takeuchi, Ichiro; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combinatorial materials synthesis is a research methodology, which allows one to study a large number of compositionally varying samples simultaneously. We apply this technique in the search for novel multifunctional materials. The work presented here will discuss the combinatorial investigation of novel magnetostrictive materials. In particular, binary Fe-Ga and the ternary Fe-Ga-Al, Fe-Ga-Pd systems are studied. Magnetron co-sputtered composition spread samples of the alloys have been fabricated to study composition dependent trends in magnetostriction. Magnetostriction measurements on all systems studied here have been carried out by optically measuring the deflection of micro-machined cantilever arrays. Measurements of the magnetostriction on binary Fe-Ga thin-films show similar compositional trends as had been reported in bulk systems. The maximum value of magnetostriction observed is 220 ppm, which is comparable to bulk values. A previously unreported minor maximum in magnetostriction as a function of composition has been found for Ga contents of about 4 at%. It is believed that the origin of this minor maximum is related to a peak in the magnetic moment of Fe atoms in Fe-Ga alloys at this composition. We have mapped the Fe-Ga-Pd and Fe-Ga-Al ternary systems. Large regions of the phase diagrams have been mapped out in a single experiment, and the observed magnetostrictive dependence on Ga content matches trends seen in bulk. It was found that the trend of magnetostriction deviated from that of bulk with the inclusion of as little as 1 at% Pd. The addition of up to 10 at % Al to Fe70Ga30 was possible without severe degradation of its magnetostriction.