Orientadores: Sergio Gama, Nilson Antunes de Oliveira / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-08T01:08:08Z (GMT). No. of bitstreams: 1 Campos_Arianade_D.pdf: 9214751 bytes, checksum: 227e7e0b1cc697ea73f810066346ff6d (MD5) Previous issue date: 2006 / Resumo: Neste trabalho descreveremos a obtenção dos compostos da família MnA s1-xAx (A= Te,P, Sb) e Mn1-xFe xAs para várias concentrações. Dividimos este trabalho em duas etapas, a primeira via obtenção em forno de alta pressão e a segunda via obtenção em forno tubular em tubos de quartzos. A primeira etapa, ainda se dividiu em obtenção indireta e direta dos materiais. Na obtenção indireta dos materiais, focamos nosso trabalho nos compostos de MnAs e MnSb para a produção da série MnAs1-xSbx. Na obtenção direta, partimos dos elementos para sintetizar os materiais, utilizando o mesmo método adotado na obtenção indireta. Na segunda etapa do trabalho, obtemos os compostos diretamente em tubos de quartzo. As amostras produzidas foram caracterizadas por difração de raios-X, microscopia óptica, microscopia eletrônica de varredura utilizando a técnica de WDS e, finalmente, análises magnéticas para a obtenção do efeito magnetocalórico de cada material, e assim a avaliação destes materiais como possíveis candidatos a materiais refrigerantes. Após o cálculo do efeito magnetocalórico, utilizamos um modelo fenomenológico que considera a dependência da temperatura crítica da fase magnética na mudança de volume, o modelo utilizado parte das descrições propostas por Bean e Rodbell que correlaciona fortes interações magnetoelásticos com a transição de fase de primeira ordem / Abstract: In this work we describe the obtaining processes of the MnAs1-x Ax (A= Te, P, Sb) and Mn1-xFexAs series for several concentrations. We divided this work in two stages: in the first one the samples were obtained using a high pressure furnace and in the second one using a resistive furnaces with the samples sealed in quartz tubes. The first stage, can be split in direct and indirect obtaining of the materials. In the indirect obtaining of the materials, our work was focused on the MnAs and MnSb compounds for the production of the series MnAs1-xSbx. In the direct obtaining, we synthesized the materials directly from the elements, using the same method adopted in the indirect obtaining. In the second stage of the work, we obtained the samples directly from the elements in quartz tubes. The produced samples were characterized by ray-X diffraction, optical microscopy, electron microscopy using the WDS technique and finally magnetic analysis for the calculation of the magnetocaloric effect of each material and, in this way evaluate these materials as possible candidates to refrigerant materials. After the calculation of the magnetocaloric effect, we used a phenomenological model that considers the dependence of the critical temperature of the magnetic phase in the volume change, the model used part of the descriptions proposed by Bean and Rodbell [1] that correlates strong magnetoelastic interaction with the first order phase transition / Doutorado / Física da Matéria Condensada / Doutor em Ciências

A macroscale Internal State Variable (ISV) constitutive model coupling magnetism effects with thermal, elastic, and damage effects is developed. Previous models for magnetic and mechanical fields included constitutive equations describing their effects on the material system studied independently. Some models explain the mechanisms behind mechanical deformations caused by magnetization changes that are presented in the literature. They mainly focus on the nanoscale level. Other models, describe the behavior of one specific magnet that is mostly a permanent magnet. However permanent magnets are made of rare-earth elements that are subjected to a high supply risk. In attempt to find an alternative to permanent magnets, a mathematical model that captures the physical behavior of magnets is needed, to help develop a tool to create a new permanent magnet. The ISV constitutive model herein describes the macroscale mechanical deformation caused by magnetic fields on ferromagnetic materials, Iron (Fe), Cobalt (Co) and Nickel (Ni) precisely. The ISV model internally coheres the kinematic, thermodynamic, and kinetic relationships of deformation using the evolving histories of internal variables. For the kinematics, a multiplicative decomposition of deformation gradient is employed including a magnetization term, and the Jacobian that represents the conservation of mass and conservation of momentum. The First and Second Law of Thermodynamics are used to constrain the appropriate constitutive relations through the Clausius-Duhem inequality. The kinetic framework employs a stress-strain relationship with a flow rule that couples the thermal, mechanical, and damage terms. To determine the ISVs needed to mimic the behavior of magnetic materials, we conducted various magnetic experiments on three different specimens made of Iron, Nickel and Cobalt. Experiments captured the mechanical deformation of a rod sample when subjected to a magnetic field using the Michelson Interferometer. To study the magnetic hysteresis of Iron, Nickel, and Cobalt, previous literature data were used. It was shown that the magnetization equation modeled the hysteresis of Iron, Nickel, and Cobalt. The magnetostrictive strain equation shows good agreement for Nickel and Cobalt, but further investigation should be done for Iron.

We examine the relaxed minimization problem for ferromagnetic bodies using the nonlocal exchange energy model. We show that the model possesses a wide range of phenomena including hysteresis, hysteresis subloops, Barkhausen effect, and demagnetization. The results are in three parts. First, we examine analytically the problem of a unit sphere of ferromagnetic material. We show that when the exchange energy is zero we duplicate De Simone's model which has a wide range of measure-valued minimizers. As the exchange energy grows our model stabilizes at the saturated solutions of the Stoner-Wohlfarth model. Here, the measure-valued minimizers are eliminated. Next, we examine numerically the problem of a body composed of several unit spheres of ferromagnetic material. We show that a constrained problem that focuses on the resultant field energy produces results similar to the unconstrained problem with considerable savings in time and resources. Finally, we examine numerically the constrained problem on a moderately large body. It is shown that the constrained problem contains all the hysteresis phenomena mentioned above. / Ph. D.

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