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Structure-property relationships of two crystal structures were investigated using computational methodologies in two different length scales:electronic and atomistic length scales. Electronic structure calculations were performed using density functional theory (DFT) with generalized gradient approximation (GGA), GGA+U (U is “on-site” electronelectron repulsion) and hybrid functional forms. Atomistic calculations were performed utilizing the semi-empirical interatomic formulation, Modified Embedded Atom Method (MEAM). Classical molecular dynamics simulations were performed on the atomistic length scale in order to investigate thermal properties. In the first study, structural, elastic and thermal properties of cementite (Fe3C) were investigated using a Modified Embedded Atom Method (MEAM) potential for iron-carbon (Fe-C) alloys. Previously developed Fe and C single element potentials were used to develop a Fe-C alloy MEAM potential, using a statistically-based optimization scheme to reproduce structural and elastic properties of cementite, the interstitial energies of C in bcc Fe as well as heat of formation of Fe-C alloys in L12 and B1 structures. The stability of cementite at high temperatures was investigated by molecular dynamics simulations. The nine single crystal elastic constants for cementite were obtained by computing total energies for strained cells. Polycrystalline elastic moduli for cementite were calculated from the single crystal elastic constants of cementite. The formation energies of (001), (010), and (100) surfaces of cementite were also calculated. The melting temperature and the variation of both the specific heat and volume with respect to temperature were investigated by performing a two-phase (solid/liquid) molecular dynamics simulation of cementite. The predictions of the potential are in good agreement with first-principles calculations and experiments. In the second study the site occupancy and magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite (SrFe12-x(Zn0.5Sn0.5)xO19 with x = 1) were investigated using firstprinciples total-energy calculations. We find that in the ground-state configuration Zn-Sn ions preferentially occupy 4f1 and 4f2 sites unlike the model previously suggested by Ghasemi et al. where Zn-Sn ions occupy 2b and 4f2 sites. Our model predicts a rapid increase in saturation magnetic moment (Ms) as well as decrease in magnetic anisotropy compared to the pure M-type Sr-hexaferrite, which is consistent with experimental observations.

Ph.D. / Electrical Engineering / A novel combinatory on-line technique coupling Electrochemistry (EC) with Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) for in-situ quantitative determination of the corrosion mechanism in magnetic thin film structures has been developed in this research. Detailed construction of a system and a comprehensive methodology was described in this dissertation. Uniformly coated multi-layer magnetic thin film samples with multi-elemental alloys containing CoCrPtB and CrMo/Cr on the Ni/P substrate were used in this research for demonstrations. In-situ quantifications conducted in a series of experiments revealed that elemental dissolution was a predominant mechanism during corrosion courses of metallic thin film materials. At the microscopic scale, using results from depletion rate determination as well as cross-sectional analyses of microstructures and magnetic features, elemental passivity was observed to occur, depending on corrosion conditions. Without external influences, surface topographic measurements indicated that passive film could be produced at the macro-scale. The dependence of the dissolution rate of each metallic ion of alloys on electrolyte concentration, potential bias, scanning rate, and corrosion duration suggested that the most critical influential factor in corrosion mechanisms was epitaxial microstructures with strongly-oriented arrangements of grains and grain boundaries. Through the use of cross-sectional microstructural analysis, including high resolution TEM micrography, electron FFT diffraction, and nano-probe with EDS profiling, variations of elemental spatial distributions at grains and grain boundaries due to the corrosion phenomena were discovered, which provided a comprehensive understanding of occurrences of micro-corrosion in thin film structures. Because of the unique magnetic property of magnetic thin films, extensive studies of field strengths from the surface were also performed in this research. Important magnetization variations were noticed when cross-sectional images were obtained. Finally, models of corrosion kinetics in the multiple layers of magnetic thin film structures were proposed.