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Thesis (Ph. D.)--University of California, San Diego, 2000. / On t.p. "2̳", "x̳", "1̳-̳x̳" are subscripts. Vita. Includes bibliographical references.

Nanoscale materials behave differently than their bulk counterparts due, in part, to the reduced length scales and the increased surface to core atom ratio. As the length scales decrease, the surface atoms become increasingly important as they make up a larger percentage of the total number of atoms. These surface atoms have magnetic properties that differ from the core atoms due to a surface anisotropy that alters the interparticle, intraparticle, and exchange interactions. In this work, we have synthesized three different nanoscale systems that will allow us to explore the physics of the different interactions. Cu/gamma-Fe2O3 core/shell nanoparticles were chosen because the gamma-Fe2O3 cores have vacancies in their B-sites, broken coordination at the surface, and experience superexchange interactions. As a comparison, multiphase undoped and V-doped SiO2/FeCo nanoparticles were chosen as these nanoparticles do not suffer from vacancies or surface disorder and experience both direct exchange interactions from the nanoparticle core and superexchange interactions between the FeCo core and the metal silicate interfacial phase. Finally, Fe nanocrystallites were grown in a Cu matrix as they present no vacancies or surface disorder, and they are single phase. We observed that the interfacial phases that form in these core/shell and nanocrystallite/matrix nanoscale systems alters significantly the physics of the magnetism. The overall magnetic properties, the elemental magnetism, and the atomic magnetism were all observed to be altered by this interfacial phase, along with the interparticle and intraparticle interactions. In addition, the thickness of this interfacial phase, and thus the strength of its affect, was controlled by controlling the thickness of the shells or the amount of intermixing in the case of the nanostructured thin film. / February 2016

Made available in DSpace on 2014-06-11T19:28:28Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-02-28Bitstream added on 2014-06-13T19:16:24Z : No. of bitstreams: 1 bicalho_es_me_jabo.pdf: 503292 bytes, checksum: d2ce2166bc9f68e6b65aa37ca5a4615c (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A emissão de CO2 do solo (FCO2) é influenciada por processos físicos, químicos e biológicos que afetam a produção de CO2 no interior do solo e o seu transporte para a atmosfera, variando no tempo e no espaço em função das condições ambientais e do manejo agrícola da área. O objetivo deste estudo foi investigar a correlação existente entre os padrões de estrutura de variabilidade espacial e temporal de FCO2 e atributos do solo, em área de cana-de-açúcar sob sistema de manejo cana crua, por meio de dimensão fractal (DF), derivada a partir de variogramas isotrópicos e anisotrópicos em diferentes escalas espaciais. A área experimental constituiu-se de uma malha regular de 60 × 60 m contendo 141 pontos espaçados em distâncias mínimas que variaram de 0,5 a 10 m. A emissão de CO2, temperatura e umidade do solo foram avaliadas durante 7 dias, sendo determinados os atributos físico e químicos do solo em amostragem na profundidade de 0,0 a 0,1 m. A média de FCO2 variou de 1,26 a 1,77 μmol m-2 s-1 ao longo dos dias, com dependência temporal na média e longa escalas, em alcances superiores a 20 m. Apesar do comportamento isotrópico observado para FCO2, seus valores de DF, calculados para diferentes direções, evidenciaram maior variabilidade temporal na direção paralela à linha de plantio, indicando influência das práticas de manejo adotadas na área. A variabilidade espacial de FCO2 foi mais bem evidenciada na média (20 a 30 m) e longa (40 a 60 m) escalas, com sua estrutura de variabilidade, caracterizada pelo fractograma, correlacionando-se significativamente com a maioria dos atributos do solo e apresentando comportamento similar à observada para a temperatura do solo e volume total de poros. Além disso, os fractogramas permitiram observar o comportamento da dependência espacial e temporal de FCO2 e... / Soil CO2 emission (FCO2) is influenced by physical, chemical and biological factors that affect the production of CO2 in the soil and its transport to the atmosphere, varying in time and space as a function of environmental conditions and agricultural management. The aim of this study was to investigate the correlation between spatial and temporal variability patterns of FCO2 and soil properties in sugarcane area under green management by using fractal dimension (DF), derived from isotropic and anisotropic variogram at different spatial scales. The experimental area consisted of a regular grid of 60 × 60 m containing 141 points spaced at minimum distances ranging from 0.5 to 10 m. Soil CO2 emission, soil temperature and soil moisture were evaluated over a period of 7 days, and soil physical and chemical properties were determined by sampling at a depth of 0.0 to 0.1 m. The average of FCO2 ranged from 1.26 to 1.77 mol m-2 s-1 throughout the days, with temporal dependence in the medium and large scales, at ranges of more than 20 m. Despite the isotropic behavior observed for FCO2, their DF values, calculated for different directions, showed greater temporal variability in the direction parallel to the row, indicating the influence of area management. Spatial variability of FCO2 was better evidenced in the medium (20 to 30 m) and long (40 to 60 m) scales, with its variability structure, characterized by fractogram, significantly correlated with most soil properties and similar behaving to that observed for the soil temperature and total pore volume. In addition, fractograms allowed to observe the behavior of the spatial and temporal dependence... (Complete abstract click electronic access below)

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After nearly 15 years of research effort, High Temperature Superconductors (HTS) are finding a wide range of practical applications. A clear understanding of the factors controlling the current carrying capacity of these materials is a prerequisite to their successful technological development. The critical current density (Jc) in HTS is directly dependent on the structure and pinning of the Flux Line Lattice (FLL) in these materials. This thesis presents an investigation of the Jc anisotropy in HTS. The use of thin films grown on off c-axis (vicinal) substrates allowed the effect of current directions outside the cuprate planes to be studied. With this experimental geometry Berghuis, et al. (Phys. Rev. Lett. 79,12, pg. 2332) observed a striking flux channelling effect in vicinal YBa2Cu3O7-δ (YBCO) films. By confirming, and extending, this observation, it is demonstrated that this is an intrinsic effect. The results obtained, appear to fit well with the predictions of a field angle dependent cross-over from a three dimensional rectilinear FLL to a kinked lattice of strings and pancakes. The pinning force density for movement of strings inside the cuprate planes is considerably less than that on vortex pancake elements. When the FLL is entirely string-like this reduced pinning leads to the observed channelling minima. It is observed that anti-phase boundaries enhance the Jc in vicinal YBCO films by strongly pinning vortex strings. The effect on the FLL structure cross-over of increasing anisotropy has been elucidated using de-oxygenated vicinal YBCO films. Intriguingly, the counter intuitive prediction that the range of applied field angle for which the kinked lattice is fully developed reduces with increasing anisotropy, appears to be confirmed. Although vortex channelling cannot be observed in c-axis YBCO films, the pinning force density for vortex string channelling has been extracted by observing string dragging. By studying the effect of rotating the applied field at a constant angle to the cuprate planes, it is possible to observe the cross-over into the string pancake regime in c-axis films. In the 3D region, the observed behaviour is well explained by the anisotropic Ginzburg-Landau model. Measurements were also made on thin films of the much more anisotropic Bi 2Sr2CaCu2O8+x material, grown on vicinal substrates. The absence of any flux channelling effect and clear adherence to the expected Kes-Law behaviour in the observed Jc characteristics does not provide evidence for the existence of the predicted 'crossing lattice' in Bi 2Sr2CaCu2O8+x.

Les molécules-aimants sont une classe d’objets aux propriétés très singulières, et dont les applications envisagées, certes sans doute lointaines, sont multiples. Dans le cadre du stockage de l’information, la principale propriété de ces molécules est le blocage de leur aimantation qui leur confère les propriétés de mémoire au niveau de la molécule unique. Théoriquement l’existence d’une aimantation spontanée au niveau d’une molécule de spin S ne peut exister que s’il y a une levée de dégénérescence à champ nul (ZFS) qui sépare les états de spin ±mS. Dans ce contexte, les molécules-aimants peuvent être divisées en deux classes : (i) les complexes mononucléaires dont la majorité comprennent, en général, des ions paramagnétiques présentant une forte anisotropie axiale qui à leurs tours sont responsables de la relaxation lente de l’aimantation dans le système, et (ii) les complexes polynucléaires, où les ions métalliques (métaux de transition et/ou lanthanides) communiquent entre eux par une interaction d’échange J ou par interaction dipolaire. Ce type de complexes peut à son tour diviser en deux parties : HomoBinucléaires et HétéroBinucléaires. Différents complexes de Co (II) et de Ni (II) sont synthétisés et étudiés chimiquement et magnétiquement avec ce type de ligand qui est capable d’accepter deux métaux dans deux sites de coordination liés par des différents ligands pontants (Cl-, Br-, NO3-, N3-, imidazole, etc..). Les mesures de susceptibilité magnétique de certains complexes montrent une interaction antiferromagnétique avec une anisotropie axiale de type Ising (D < 0) et autre de type planaire (D > 0). Cela est dû de la nature des ions métalliques et les géométries des complexes pentacoordinés (bipyramide trigonale et pyramide à base carrée). / For molecular magnetic materials to be used in applications, they must retain their magnetization at reasonable temperatures, which can be achieved with high-energy barriers for magnetization reversal and slow relaxation of the magnetization. In the field of Single-Molecule Magnets (SMMs), over the last decade, the main focus has shifted from large spin complexes to highly anisotropic systems which have displayed high energy barriers.1 Here, we used a cryptand ligand that forces a trigonal bipyramidal arrangement to obtain a large (in absolute value) negative zero field splitting parameter D.2 In order to use these molecules as models for entangled quantum bits (qbits), the cryptand ligand was chosen to have two coordination sites allowing the design of chemically stable binuclear complexes with tunable antiferromagnetic coupling. We prepared pentacoordinate complexes of Co(II) and Ni(II) with different bridging ligands (Cl-, Br-, NO3-, N3-, Im-, etc.). Magnetic measurements indicated an Ising type anisotropy and weak antiferromagnetic coupling as expected from the nature of the bridging ligands (N3- and Im-) and the trigonal bipyramidal geometry around the metal ions.

This thesis explores finite-size effects on the spin-wave excitations of one-dimensional ferromagnetic and antiferromagnetic systems. Specifically, it presents a theoretical study of the scattering function, the physical observable in inelastic neutron and photon scattering experiments, under the influence of extra magnetic anisotropy energy localized at the system’s boundaries. The method for calculating spin wave scattering functions in bulk is adapted to the finite system case, enabling explicit numerical calculations with edge effects. Our results show a significant broadening of the scattering peaks of low energy spin waves in ferromagnetic and antiferromagnetic systems. We show that broadening is due to the emergence of spin excitations localized at the edge of the system, the so-called edge modes. / Graduate

Spatial characterization of surface microtopography is important in understanding the overland flow generation and the spatial distribution of surface runoff. In this study, fractal parameters (i.e., fractal dimension D and crossover length l) and three hydrotopographic parameters, random roughness (RR) index, maximum depression storage (MDS), and the number of connected areas (NCA), have been applied to characterize the spatial complexity of microtopography. Clear and meaningful relationships have been established between these parameters. The RR was calculated as the standard deviation of the processed elevation, and the fractal parameters were calculated with the semivariogram method. The puddle delineation program was applied in this study to spatially delineate soil surface and to accurately determine MDS and NCA. It has been found that fractal parameters can better characterize surface microtopography. More importantly, fractal and anisotropic analyses can help to better understand the overland flow generation process.

It is well known that the formation and propagation of microcracks within concrete is anisotropic in nature, and has a degrading effect on its mechanical performance. In this thesis an anisotropic damage mechanics model is formulated for concrete which can predict the behavior of the material subjected to monotonic loading, fatigue loading, and freeze-thaw cycles. The constitutive model is formulated using the general framework of the internal variable theory of thermodynamics. Kinetic relations are used to describe the directionality of damage accumulation and the associated softening of mechanical properties. The rate independent model is then extended to cover fatigue loading cycles and freeze-thaw cycles. Two simple softening functions are used to predict the mechanical properties of concrete as the number of cyclic loads as well as freeze-thaw cycles increases. The model is compared with experimental data for fatigue and freeze-thaw performance of plain concrete. / DOT-MPC grant / Department of Civil Engineering, North Dakota State University