DSpace Repository :: Browsing by Author "Mewes, Claudia K. A."

Browsing by Author "Mewes, Claudia K. A."

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Applications of methods beyond density functional theory to the study of correlated electron systems
(University of Alabama Libraries, 2013) Sims, Hunter Robert; Butler, W. H.; University of Alabama Tuscaloosa
The difficulty in accurately treating systems in which electron-electron interactions are the dominant physics has plagued condensed matter physics for decades. Currently, there exist many different computational techniques designed to improve upon density functional theory to varying degrees of accuracy. To date, no unified, parameter-free method exists that is guaranteed to yield the correct answer for all materials. Consequently, proper treatment of such systems often requires a combination of several methods, allowing one to check them against one another when their regions of validity overlap and to expand one's reach when a single method cannot reliably describe all of the physics at work. In this dissertation, I present discussion and, when appropriate, brief derivations of several of the most prominent electronic structure methods currently in use---from the local density approximation through LDA+DMFT. I then present several investigations into the electronic and magnetic structure of materials of potential interest for information technology that also illustrate the current state of affairs in computational condensed matter physics. I explore the intersite exchange interactions in CrO_2 within density functional theory (with and without Hubbard “+U” corrections) and evaluate these results through analytic and numerical means. I study the dependence of the mysterious magnetization of Fe_16 N_2 on crystal and electronic structure and employ a wide range of techniques in an attempt to bring greater rigor and deeper understanding to the widely-varying reports on this material. In conjunction with others' careful experimental analysis, I provide a picture of the band structure of the magnetic insulator NiFe_2 O_4 that reveals a novel hierarchy in its band gaps and suggests applications in spintronics and possibly other areas. Finally, I employ dynamical mean-field theory to study the behavior of impurity states in elemental semiconductors, using H impurities in Ge as a base system.
Broadband Ferromagnetic Resonance Characterization of Anisotropies and Relaxation in Exchange-biased IrMn/CoFe Bilayers
(2017) Mohammadi, Jamileh Beik; Jones, Joshua Michael; Paul, Soumalya; Khodadadi, Behrouz; Mewes, Claudia K. A.; Mewes, Tim; Kaiser, Christian; University of Alabama Tuscaloosa
The magnetization dynamics of exchange-biased IrMn/CoFe bilayers have been investigated using broadband and in-plane angle-dependent ferromagnetic resonance spectroscopy. The interface energy of the exchange bias effect in these bilayers exceeds values previously reported for metallic antiferromagnets. A strong perpendicular magnetic anisotropy and a small in-plane uniaxial anisotropy are also observed in these films. The magnetization relaxation of the bilayers has a strong unidirectional contribution, which is in part caused by two-magnon scattering. However, a detailed analysis of in-plane angle– and thickness-dependent linewidth data strongly suggests the presence of a previously undescribed unidirectional relaxation mechanism.
Computational studies of the catalytic reactions of group ivb and vib transition metal oxide clusters
(University of Alabama Libraries, 2014) Fang, Zongtang; Dixon, David A.; University of Alabama Tuscaloosa
Computational chemistry approaches have been used to study the reactivity of Group IVB and VIB transition metal oxide clusters. The hydrolysis of MCl₄ (M = Zr, Hf) as the initial steps on the way to form zirconia and hafnia nanoparticles has been studied with density functional theory (DFT) and coupled cluster [CCSD(T)]theory. Instead of the direct production of MOCl₂ and HCl or MO₂ and HCl, the hydrolysis reaction starts with the formation of oxychlorohydroxides followed by the release of HCl due to the large endothermicities associated with the direct path to form gas phase MO₂. The formation of MO₂ nanoparticles by the high temperature oxidation method is complicated and is associated with the potential production of a wide range of intermediates. The interaction between H₂O and small (MO₂)n (M = Ti, Zr, Hf, n = 1−4) nanoclusters has been studied for the first step to understand the reaction mechanism of photocatalytic water splitting with the presence of (MO₂)n as catalysts. Both the singlet and the first excited potential energy surfaces (PESs) are studied. The hydrolysis reactions begin with the formation Lewis acid-base adducts followed by proton transfer from H₂O to the nanclusters. The reactions are highly exothermic with very small activation energies. Thus, H₂O should readily decompose to generate two OH groups on (MO2)n nanoclusters. The generation of H₂ and O₂ starting from the hydroxides formed in the hydrolysis step has been studied with the same computational methods as used for the hydrolysis study. The water splitting reactions prefer to take place on the first excited triplet potential energy surface (PES) due to its requirement of less energy than that on the singlet PES. A low excess potential energy is needed to generate 2H₂ and O₂ from 2H₂O if the endothermicity of the reaction is overcome on the first excited triplet PES using two visible photons. Hydrogen generation occurs via the formation of an M−H containing intermediate and this step can be considered to be a proton coupled, electron transfer (PCET) reactions with one or two electrons being transferred. Oxygen is produced by breaking two weak M−O bonds on the triplet PES. Ethanol (CH3CH2OD) conversions on cyclic (MO3)3 (M = Mo, W) clusters have been studied experimentally with temperature programmed desorption and computationally with both DFT and CCSD(T) methods. The addition of two alcohol molecules is required to match experiment. The reaction begins with the elimination of water with the formation of an intermediate of dialkoxy species for further reaction. The dehydration reaction proceeds through a β hydrogen transfer to a terminal MVI = O atom without the involvement of a redox process. The dehydrogenation reaction is through an α hydrogen transfer to an MoVI = O with redox involved or a WVI avoiding redox. The same computational methods have been used to study the other alcohol species such as methanol, n-propanol and isopropanol. The reactions with single, double and triple alcohols per M3O9 cluster have been studied. The dehydrogenation and dehydration for single alcohol reactions is via a common intermediate of metal hydroalkoxide formed by the dissociation of alcohol. The dehydration is through a β hydrogen transfer to OH group. The lowest energy pathway for dehydrogenation is the same for different alcohols in both single and double alcohol reactions. Three alcohols involved condensation reaction may lower the reaction barrier tremendously by the sacrifice of an alcohol to form a metal hydroalkoxide, a strong gas phase Brønsted acid. This is a Brønsted acid driven reaction different from dehydrogenation and dehydration reactions governed by the Lewis acidity of the metal center and its reducibility.
Dynamic transport measurements of vo2 thin films through the metal-to-insulator transition
(University of Alabama Libraries, 2018) Jones, Joshua Michael; LeClair, Patrick R.; University of Alabama Tuscaloosa
VO2 is a transition metal oxide material well known for its high magnitude metal-to-insulator transition (MIT) with a corresponding change in crystal structure [1]. At room temperature, VO2 is found in an insulating monoclinic phase (P21/c) that upon heating through the transition temperature (Tc, ~341 K in bulk material) changes to a metallic rutile phase (P42/mnm) [2]. The MIT can be activated thermally by heating or cooling through Tc, but has also been shown to be sensitive to electric field [3], infrared radiation [4], pressure [5], and strain [6]. The value of Tc is also highly tunable through doping [7] and growth of strained epitaxial thin films [8]. The massive 3-4 order of magnitude change in electrical resistivity (ρ) has drawn interest for possible device level applications. The transition is characterized by the coexistence of rutile metallic domains and a monoclinic insulating matrix that results in a smooth progression of the DC transport and dielectric properties as the MIT is induced. In this thesis, we present an overview of three novel transport experiments all of which involve epitaxial TiO2/VO2 films grown in a home-built low-pressure chemical vapor deposition system. The first experiment looks at the time evolution of the film resistance and capacitance as it settles for an extended period very near Tc. We report evidence that this settling process is characterized by at least two underlying relaxation processes. The second experiment involves the deposition and ferromagnetic resonance (FMR) characterization of TiO2/VO2/Ru/Py heterostructures. Our analysis indicates enhanced spin pumping into the VO2 layer when in the metallic state that is associated with an increase in the effective Gilbert damping parameter. Finally, we discuss the results of 1/f noise spectroscopy measurements collected on Hall-bar patterned VO2(100) films. We show that the processes governing noise along both crystallographic axes are identical and, in the metallic rutile state, follows a unique R-3 scaling behavior.
Electron tunneling in the tight-binding approximation
(University of Alabama Libraries, 2016) Mackey, Frederick D.; Butler, W. H.; University of Alabama Tuscaloosa
In this thesis, we treat tunneling similar to a scattering problem in which an incident wave on a barrier is partially transmitted and partially reflected. The transmission probability will be related to the conductance using a model due to Landauer. Previously tunneling has been treated using a simple barrier model, which assumes the electron dispersion is that of free electrons. In this model it is not possible to investigate tunneling in the gap between a valence band and a conduction band. We shall remedy this limitation by using the tight-binding model to generate a barrier with a gap separating a valence band and a conduction band. To do this, we constructed a model consisting of semi-infinite chains of A atoms on either side of a semi-infinite chain of B-C molecules. The B-C chain has a gap extending between the onsite energy for the B atom and the onsite energy for the C atom. Tunneling through the gap has been calculated and plotted. We present exact closed form solutions for the following tunneling systems: (i) A-B interface, (ii) A-(B-C) interface, (iii) A-B-A tunnel barrier, (iv) A-(B-C) interface with the orbitals on B having s-symmetry and those on C having p-symmetry, (v) A-(B-C)-A tunnel barrier.
Evaluation of Pb0.95La0.05Zr0.54Ti0.46O3 thin film based capacitors for photovoltaic applications: using top electrodes of varying work function
(University of Alabama Libraries, 2018) Batra, Vaishali; Kotru, Sushma; University of Alabama Tuscaloosa
Ferroelectricity and optical transparency are the two properties of lanthanum doped lead zirconate titanate (PLZT) that are utilized to explore its use for photovoltaic (PV) applications. A ferroelectric material sandwiched between two electrodes forms a MFM (metal/ferroelectric/metal) capacitor structure and can generate photocurrent and photovoltage under illumination. The PV effect in these structures is a combination of bulk PV effect and metal/ferroelectric interface effect. Both the material and the electrode properties are critical to improve the PV parameters obtained from these structures. A careful selection of electrode material can enhance the interface photovoltaic effect by modifying the Schottky barrier formed at the interface between electrode and ferroelectric material. This work investigates the effect of top electrodes of varying work functions on the photovoltaic properties of ferroelectric Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) thin films. PLZT thin films were prepared using a chemical solution deposition method and post annealed in the temperature range of 550-750 ˚C. A variety of characterization methods including x-ray diffraction, Raman, UV-visible and x-ray spectroscopy were used to understand the material properties of the grown films. A detailed analysis of ferroelectric measurements of the films was carried out to study their electrical behavior. Films annealed at annealing temperature (Ta) of 750 ˚C exhibit perovskite peaks, and higher chemical valence states and polarization compared to films annealed at lower temperatures. Thus, Ta of 750 ˚C was chosen to design capacitors studied in this work. Metal/PLZT/Pt (MFM) symmetric and asymmetric capacitor structures with various metals (Pt, Au, and Al) as top electrodes were fabricated. The asymmetric structure designed with Au top and Pt bottom electrode showed an improvement in the PV parameters, while retaining the ferroelectric properties. On the other hand, the symmetric structure designed with Pt top and bottom electrode showed lower PV parameters, but higher polarization. This work further investigated the ferroelectric and photovoltaic properties of PLZT thin film capacitors in two different electrode configurations, coplanar and interplanar, using Au electrodes. A simulation PV model was designed for the symmetric structure, which was helpful in understanding the charge transport properties. This work was extended to design a simulation model for asymmetric structure using ITO as top and Pt as bottom electrode. This work suggests that asymmetric structure results in higher charge transport, which results in higher PV parameters of PLZT thin film based capacitors.
Exploring the magnetic phases in dysprosium by neutron scattering techniques
(University of Alabama Libraries, 2014) Yu, Jian; Mankey, Gary J.; University of Alabama Tuscaloosa
With one of the highest intrinsic magnetic moments (10.6 μ_B) among the heavy rare-earth elements, dysprosium (Dy) exhibits a rich magnetic phase diagram, including a few modulated magnetic phases. Aided by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the magnetic modulations propagate coherently over a long range, even with intervening non-magnetic layers. Neutron diffraction experiments were performed to determine the microscopic magnetic origin of the field induced phases in bulk Dy as a function of temperature, covering regions of the well-known ferromagnetic, helical antiferromagnetic, fan phases and several possible new phases suggested by previous studies. A short range ordered (SRO) fan phase was identified as the intermediate state between ferromagnetism and long range ordered (LRO) fan. The temperature range of a coexisting helix/fan phase was also determined. The magnetic phase diagram of Dy was thus refined to include the newly determined magnetic structures and the associated phase boundaries. Based on the period of the magnetic modulation and the average magnetization, the evolution of the spin arrangement upon heating was derived quantitatively for the modulated magnetic phases. To gauge the effect of nanostructuring on the magnetic phases with nonmagnetic Y layers, epitaxial Dy/Y superlattices of various layer thicknesses and repeats were fabricated by magnetron sputtering under carefully controlled conditions. X-ray characterizations confirmed that the crystallographic and interfacial qualities of the superlattices are comparable to those grown by MBE in previous studies. The macroscopic magnetization was characterized by magnetometry, whereas the microscopic magnetic structures were extracted from neutron diffraction and polarized neutron reflectometry (PNR) measurements. The ordering of helical modulation is sensitive to the interfacial roughness of the multilayer as well as the cooling histories. Off-specular PNR was applied the first time to characterize the helical domain structures in Dy/Y multilayers. The lateral correlation length in the helical magnetic structure was in the order of 100 nm.
Fabrication and ferromagnetic resonance study of epitaxial spinel ferrite films for microwave device applications
(University of Alabama Libraries, 2014) Pachauri, Neha; LeClair, Patrick R.; Gupta, Arunava; University of Alabama Tuscaloosa
Single crystalline nickel ferrite and lithium ferrite thin films have attracted a lot of research attention recently, because of their unique physical properties for practical applications in next generation technologies, such as monolithic microwave integrated circuits (MMIC) and multiferroic heterostructures. The properties of these materials are closely related to the specific growth method and can be tailored by factors like surface morphology, microstructure and chemical composition. Different thin film growth techniques have been investigated in the past few decades for the fabrication of single crystalline thin films of both these spinel ferrites. However, the difficulty to attain high quality, homogeneous epitaxial films with limited surface and bulk defects and low microwave loss still remains a challenging task. Moreover, there have been very limited reports on the detailed ferromagnetic resonance (FMR) studies of these single crystalline nickel and lithium ferrite thin films, which is an essential aspect to understand the relaxation in magnetization precession (microwave damping) in these materials. In this dissertation work, fabrication and study of structural, magnetic and FMR properties of single crystalline lithium ferrite (LiFe5O8) and nickel ferrite (NiFe2O4) films by direct liquid injection chemical vapor deposition (DLI-CVD) are studied in detail. The growth conditions, which play a crucial role in attaining the desired film morphology and stoichiometry, are optimized to achieve epitaxial, single crystalline lithium ferrite films having low ferromagnetic resonance linewidth coupled with excellent magnetic properties. A detailed ferromagnetic resonance (FMR) study has been done to identify as well as quantify the magnetic relaxation mechanisms in the `as-grown' nickel ferrite films. The broadband frequency, angle and temperature dependent measurements reveal the existence of two-magnon scattering as the active relaxation mechanism for the films.
Investigation of magnetic relaxation mechanisms and dynamic magnetic properties in thin films using ferromagnetic resonance (fmr) technique
(University of Alabama Libraries, 2016) Khodadadi, Behrouz; Mewes, Tim; University of Alabama Tuscaloosa
Investigating the damping processes and the behavior of dynamic magnetic properties in ferromagnetic thin films has been an important key towards design and fabrication of different microwave and magnetic recording devices. This thesis discusses the dynamic magnetic properties and also the physics behind different relaxation mechanisms in ferromagnetic thin films using comprehensive experimental investigations by means of broadband ferromagnetic resonance (FMR) technique. In chapter one the basics of ferromagnetic resonance technique and the experimental features of the FMR setup used in this study are discussed, also the FMR data analysis is explained. Chapter two is devoted to the study of the interfacial perpendicular magnetic anisotropy (PMA) and damping parameter in Co2FeAl thin films. In chapter three broadband temperature dependent FMR measurements were carried out on Ni80Fe20/Gd thin films to investigate the behavior of ferromagnetic relaxation, and gyromagnetic ratio as the system goes through the Curie temperature of Gd. In chapter four, the ferromagnetic relaxation mechanisms in ferrites are discussed. The low loss Nikel Ferrite and Lithium ferrite single crystal, thin and ultra-thin films were characterized by detailed FMR measurements to investigate the effect of microstructural defects on the magnetization relaxation. A comprehensive study on the interlayer exchange coupling strength in Co90Fe10/Ru/ Co90Fe10 multilayers is the subject of chapter five, in which the mutual spin pumping is discussed as a recently discovered channel for relaxation in exchange coupled multilayers.
Low Gilbert Damping in Co2FeSi and Fe2CoSi Films
(2016) Sterwerf, Christian; Paul, Soumalya; Khodadadi, Behrouz; Meinert, Markus; Schmalhorst, Jan-Michael; Buchmeier, Mathias; Mewes, Claudia K. A.; Mewes, Tim; Reiss, Günter; University of Alabama Tuscaloosa
Magnetic anisotropies and dynamic magnetic properties in multilayered thin films
(University of Alabama Libraries, 2017) Beik Mohammadi, Jamileh; Mewes, Tim; Mewes, Claudia K. A.; University of Alabama Tuscaloosa
Theoretical and experimental research on magnetic materials and magnetic devices intend to investigate novel materials and structures which can be used for the next generation spintronic devices. Moreover, it is essential to conduct fundamental research on new phenomena aiming for new generation of devices that can be faster, smaller, cheaper, and more reliable. Magnetic anisotropies have been widely used in spintronic devices. From the unidirectional exchange bias anisotropy that is used in magnetic read heads and giant magnetic resonance (GMR) sensors, to the interfacial perpendicular magnetic anisotropy (PMA) that is essential for magnetic tunnel junctions (MTJs). In the first chapter of this dissertation a short introduction to magnetization dynamics including experimental techniques is given. In the second chapter, the exchange bias anisotropy and the interfacial origin of relaxation in Ru/IrMn/CoFe/Ru exchange biased systems is discussed and investigated. The interfacial perpendicular anisotropy is observed in these systems and can be quantified using FMR technique. Such anisotropy can exist in a thin ferromagnetic film (such as NiFe) that is in proximity to a metallic (e.g.Ru) or insulating (e.g. SiO2) non-magnetic layer, which is the topic of the third chapter of this dissertation. In addition, experimental results confirm that spatial fluctuations of the uniaxial perpendicular anisotropy can push the easy axis of the magnetization in an orientation that is neither perpendicular to the film nor normal to it. The effect of the lateral fluctuation of the uniaxial anisotropy on the magnetic energy landscape and the magnetization dynamics of thin magnetic layers is reported in chapter four using micromagnetic simulations.
Multiscale simulation of boron-doped nanocarbons in electrochemical applications
(University of Alabama Libraries, 2014) Zhang, Zhongtao; Turner, C. Heath; University of Alabama Tuscaloosa
The stability, properties, and dispersion of novel organometallic/doped nanocarbon complexes for electrochemical application are investigated in this work with density functional theory (DFT) and molecular dynamics (MD) simulations. We suggest that electrochemical active centers like cyclopentadiene (Cp) transition metal (TM) complexes can be stabilized on boron-doped nanocarbons to create stable and high-performance support materials. We present a systematic study of the geometries, energetics, and electronic properties of CpTM (where TM=Fe, Ni, Co, Cr, Cu) complexes adsorbed on both pristine and boron-doped carbon nanotubes (CNTs) and graphene supports using DFT calculations. Significant stabilization of CpTM on boron-doped CNTs (B-CNTs) and graphenes are found, which surpasses the binding energies (BEs) of the isolated TM atoms by about 2 eV. To evaluate the redox activity (CpFe) on B-doped nanocarbon supports, we calculate the redox potentials of CpFe/B-doped, N-doped and pristine graphene complexes with different doping patterns and concentrations with DFT calculations, combined with a conductor-like polarizable continuum model (CPCM) solvation model. The CpFe/B-doped graphene complexes show potential to be a ferrocene substitute for ferrocene-mediated electrochemical process, such as bio-sensing and dye-sensitized solar cells. The dispersion of B-doped nanocarbons is also investigated in our work. Molecular dynamics (MD) simulations, parameterized by DFT-calculated partial charges are used to investigate the water-induced interactions, the hydration, and the debundling behavior of B-CNTs with varying diameters and B-doping patterns within aqueous solutions. By evaluating the potential of mean force (PMF) of one, two, and three solvated B-CNTs, we demonstrate that the water-induced interactions between B-CNTs extend over prolonged distances, and the B-CNTs are shown to be more reagglomeration resistant. In addition, the hydration behavior of the B-CNTs can be understood by evaluating the water density profiles and hydrogen bonds during the solvation. These results provide guidelines for separating and dispersing B-doped nanocarbons in aqueous environments. Overall, our simulations predict that the CpTM/B-doped nanocarbon complexes are potential candidates for multiple electrochemical applications with significant stability, comparable redox performance to ferrocene, and enhanced dispersibility.
Origin of Low Gilbert Damping in Half Metals
(2009) Liu, Chunsheng; Mewes, Claudia K. A.; Chshiev, Mairbek; Mewes, Tim; Butler, William H.; University of Alabama Tuscaloosa
Using a combination of first-principles calculations and an extended Hückel tight binding model this letter reports on the origin of the low Gilbert damping in half metals. This approach enables the prediction of the lower limit for the magnetization relaxation in a wide variety of material systems relevant for future spintronic applications. For the two model systems Co2MnGe and Co2MnSi minimal damping parameters of 1.9x10−4 and 0.6x10−4 are predicted.
Photo-Electroswitchable Arylaminoazobenzenes
(University of Alabama Libraries, 2021) Keane, Katie Strickland; Blackstock, Silas C.; University of Alabama Tuscaloosa
Azobenzenes consist of two phenyl rings linked by an azo unit (N=N), existing in E or Z isomeric forms. Their ability to reversibly transform between 'extended' thermodynamically favored E and higher energy 'contracted' Z isomeric forms upon photo-stimulation make them useful molecular 'flexors.' E→Z switching is rapidly achieved using light, which has popularized the use of azobenzenes in a variety of chemical systems to gain nano mechanical photoswitchable characteristics. In most cases, Z→E isomerization occurs slowly thermally or, when possible by photoisomerization, though slower than photo E→Z conversion and typically incomplete.To address the Z→E isomerization limits (slow or incomplete), we have developed using electron removal as a new azobenzene switching mechanism for amino-substituted azobenzenes and investigated the prospect of switching multiple azo linkages with a single electron loss event. Blackstock and coworkers have covalently attached a redox aryl amine to the azobenzene moiety allowing for rapid, catalytic, and compete Z→E isomerization upon oxidation. The oxidized redox auxiliary dramatically reduces the Z→E isomerization energy barrier by factors of at least 105. Once initiated, the aryl amine radical cation is chemically stable and persistent enough to exchange electrons with a neutral Z isomer amine, generating an efficient electron-transfer chain reaction for Z→E isomerization. The synthesis, photo- and thermoisomerization, and lifetime effects of linking multiple azobenzenes to a single arylamine redox center are investigated for four tertiary amine derivatives: 4-methoxy-4'-(N,N-dianisyl)-aminoazobenzene (11), N,N-bis(azobenzene)-p-anisidine (20), N,N-bis(2,2',6,6'-tetrafluoroazobenzene)-p-anisidine (21), and N,N,N-tris(azobenzene)amine (29). Blue light irradiation of these azobenzene systems yields an equilibrium of Z-enriched isomers as a photostationary state (PSS). Dynamic UV-vis and NMR spectroscopy are used to measure PSS compositions and thermal dynamics of these mixtures. Ortho-fluorination is employed to increase Z isomer lifetime from hours (20) to weeks (21), resulting in an extended switching time domain for the dual-flexor system. Electron loss from a single arylamine efficiently catalyzes the Z→E isomerization of up to three connected azobenzene units, resulting in rapid, large geometry changes for these conglomerate structures. Stimulated, reversible flexing is thus demonstrated using electronic excitation and electron transfer. Incorporating a photosensitizer (methylene blue) allows for a dual photo, photo-electron transfer Z,E switching mechanism, which can be easily cycled with light. Red light excites methylene blue, which in turn oxidizes the redox amine to achieve rapid, complete Z→E conversion. Thus, blue and red light irradiation in tandem is shown to generate an E→Z→E switching cycle for three of the systems (11, 20, and 29).
Relating processing-microstructure-mechanical properties of inconel 718 fabricated by selective laser melting
(University of Alabama Libraries, 2018) Holland, Sharniece; Li, Lin; University of Alabama Tuscaloosa
Selective laser melting (SLM), uses a computer controlled scanning beam to selectively melt pre-spread powders in a layer by layer fashion. SLM has become a powerful tool to fabricate Inconel 718 due to the ability to obtain a desired microstructure and mechanical properties such as hardness and tensile strength. The combination of rapid melting and solidification in the SLM process, produces a very unique microstructures, which results in different material properties in comparison to conventional routes. Although Inconel 718 has been thoroughly studied, an in-depth analysis of grain boundary (GB) network structures and multiscale correlation of microstructure and mechanical behaviors is lacking. This work aims to relate the processing-microstructure-property relationship of Inconel 718 fabricated by SLM using experimental characterization, computational simulation, and theoretical analysis. These investigations reveal: 1. The as-SLM Inconel 718 exhibits spatial heterogeneity with a typical fast solidification structure with melting pool, columnar and equiaxial grains. In the melt pools, finer dendritic structure and a cellular structure, reflecting micro-segregation due to the laser induced localized thermal history. Upon heat treatment, the unique SLM structures are completely removed. The HT sample displays a fully recrystallized grain structure with appearance of twin boundaries and precipitates along the GBs and within the grain interiors. 2. Effective properties are influenced by the character and connectivity. The GBs diffusivity for both random and crystallographically consistent networks exhibits two types of behaviors, based on the contrast of local diffusivity of individual GBs. The as- SLM samples are occupied by one large connected general cluster reflecting the dominance of J0 triple junctions. While the HT sample multiple special clusters reflecting the interweaving of general and special boundaries. 3. The Vickers hardness of the as-SLM sample is ~ 3.14 GPa and the measured wear rate is ~ 2.16 ×10-6 mm2 while the HT sample had a significant increase in the Vickers hardness to ~ 4.49 GPa, and a reduction of wear rate to 1.75×10-6 mm2. Such enhancement of the mechanical performance can be contributed to the improvement of part quality via lowering the porosity, the removal of residual thermal stress, and the precipitation of nanoscale strengthening phases.
Switching Distributions for Perpendicular Spin-Torque Devices within the Macrospin Approximation
(2012) Butler, W. H.; Mewes, Tim; Mewes, Claudia K. A.; Visscher, P. B.; Rippard, William H.; Russek, Stephen E.; Heindl, Ranko; University of Alabama Tuscaloosa
We model “soft” error rates for writing (WSER) and for reading (RSER) for perpendicular spintorque memory devices by solving the Fokker-Planck equation for the probability distribution of the angle that the free layer magnetization makes with the normal to the plane of the film. We obtain: (1) an exact, closed form, analytical expression for the zero-temperature switching time as a function of initial angle; (2) an approximate analytical expression for the exponential decay of the WSER as a function of the time the current is applied; (3) comparison of the approximate analytical expression for the WSER to numerical solutions of the Fokker-Planck equation; (4) an approximate analytical expression for the linear increase in RSER with current applied for reading; (5) comparison of the approximate analytical formula for the RSER to the numerical solution of the Fokker-Planck equation; and (6) confirmation of the accuracy of the FokkerPlanck solutions by comparison with results of direct simulation using the single-macrospin Landau-Lifshitz-Gilbert (LLG) equations with a random fluctuating field in the short-time regime for which the latter is practical.
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.
Theory based design and optimization of materials for spintronics applications
(University of Alabama Libraries, 2012) Xu, Tianyi; Butler, W. H.; University of Alabama Tuscaloosa
The Spintronics industry has developed rapidly in the past decade. Finding the right material is very important for Spintronics applications, which requires good understanding of the physics behind specific phenomena. In this dissertation, we will focus on two types of perpendicular transport phenomena, the current-perpendicular-to-plane giant-magneto-resistance (CPP-GMR) phenomenon and the tunneling phenomenon in the magnetic tunnel junctions. The Valet-Fert model is a very useful semi-classical approach for understanding the transport and spin-flip process in CPP-GMR. We will present a finite element based implementation for the Valet-Fert model which enables a practical way to calculate the electron transport in real CPP-GMR spin valves. It is very important to find high spin polarized materials for CPP-GMR spin valves. The half-metal, due to its full spin polarization, is of interest. We will propose a rational way to find half-metals based on the gap theorem. Then we will focus on the high-MR TMR phenomenon. The tunneling theory of electron transport in mesoscopic systems will be covered. Then we will calculate the transport properties of certain junctions with the help of Green's function under the Landauer-Büttiker formalism, also known as the scattering formalism. The damping constant determines the switching rate of a device. We can calculate it using a method based on the Extended Hückel Tight-Binding theory (EHTB). The symmetry filtering effect is very helpful for finding materials for TMR junctions. Based upon which, we find a good candidate material, MnAl, for TMR applications.

Browsing by Author "Mewes, Claudia K. A."

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