Die magnetische Kühlung ist eine etablierte Technologie im Bereich der Tieftemperaturphysik. Allerdings bieten die Skalierbarkeit des magnetokalorischen Effektes und die Möglichkeit zur kompakten Bauweise auch ein breites Anwendungsspektrum für den Einsatz bei Raumtemperatur. Besonders hervorzuheben ist die Möglichkeit zur Anpassung der magnetostrukturellen Umwandlungstemperatur in La(Fe, Si)13-basierten Materialien an die Arbeitstemperatur einer Kühleinheit. Die Herstellung von Ausgangsmaterial über das Schmelzspinnen, ist von hoher technologischer Relevanz, da im Vergleich zu konventionell gegossenem Massivmaterial die anschließende Glühdauer drastisch reduziert werden kann [1]. In der vorliegenden Arbeit wird zunächst auf die optimalen Glühbedingungen in rasch-erstarrtem Bandmaterial für die Bildung der relevanten magnetokalorischen Phase eingegangen. Durch Variation der Glühtemperatur wird der Einfluss von Sekundärphasen auf den magnetokalorischen Effekt bewertet. Darüber hinaus können bei optimaler Wahl der Legierungszusammensetzung ein großer magnetokalorischer Effekt und der gewünschte Arbeitstemperaturbereich eingestellt werden. Besonderes Augenmerk wird auf die Verknüpfung des Substitutionseffektes (hier: Si für Fe) und der Aufweitung des Gitters durch Hydrierung mit dem resultierenden magnetokalorischen Effekt gelegt. Ein weiterer Punkt, sind die Untersuchungen zur Langzeitstabilität der Eigenschaften von hydriertem Band- und Massivmaterial. Grundlegende und umfassende Untersuchungen zur Substitution von Eisen durch Mangan und zum daraus folgenden Einfluss auf Phasenbildung, Umwandlungstemperatur sowie auf den magnetokalorischen Effekt, insbesondere nach der Hydrierung, werden ebenfalls dargestellt. Die Ergebnisse der vorliegenden Arbeit erlauben damit die Bewertung verschiedener Strategien zur Optimierung der magnetokalorischen Eigenschaften von La(Fe, Si)13.

Demands for new, energy-efficient appliances have greatly increased in response to our growing need for a more environmentally friendly society. Magnetic refrigeration is a technique that utilizes the magnetocaloric effect, with possible energy savings of up to 30% compared to commercial gas compression refrigerators. A material appropriate for commercial magnetocaloric devices should be both cheap and non-toxic; it should also exhibit a first-order magnetic transitions close to room temperature. The magnetic properties of Fe2P-related materials can be relevant in this context, since their magnetic properties can be finely tuned through the substitution of Fe by Mn and P by Si, As, Ge or B to meet the general requirements for a magnetocaloric device. An in-depth study has therefore here been made of the structural and magnetic properties of the (Fe,Mn)2(P,Si)-system. The phase diagram of the FeMnP1-xSix-system has been carefully re-examined. It is found to contain two single-phase regions: an orthorhombic Co2P-type structure (x < 0.15) and a hexagonal Fe2P-type structure (0.24 ≤ x < 0.50). Selected compounds within the Fe2P-type region of the phase diagram have been shown to exhibit potential for use in magnetic refrigeration applications. Neutron powder diffraction has here been used to determine the magnetic structures of selected crystalline compositions within the FeMnP1-xSix-system to gain a better understanding of its magnetic properties. The Fe2P-type region is mainly ferromagnetic, but an incommensurate antiferromagnetic structure has also been identified close to the Co2P/Fe2P-type phase border for x ≈ 0.25. The so-called ''virgin effect'' in the Fe2P-type region of the FeMn(P,Si) phase diagram is found to be accompanied by an irreversible structural phase transition induced by magnetostriction. This new phase is found to be preserved during successive cooling-heating cycles. Furthermore, the magnetic properties of the substituted Fe2P-type structure changes significantly for metal:non-metal ratios away from 2:1. Such deviations could well explain the apparently conflicting structure-property relationships described in earlier literature for the FeMnP1-xSix-system.

Nesta tese discutimos sistematicamente os efeitos magnetocalórico e barocalórico em sistemas físicos com transição de fase magnética de primeira e de segunda ordem em sistemas físicos cujo magnetismo tem carater local. Para essa finalidade, utilizamos um modelo de momentos magnéticos locais interagentes, incluindo um termo de acoplamento magnetoelástico. Nossos cálculos mostram que a relação entre a interação de troca e o acoplamento magnetoelástico é responsável pela ordem da transição de fase e pelo aparecimento da histerese térmica e magnética. Os resultados mostram que as grandezas magnetocalóricas exibem grandes valores quando o sistema sofre uma transição de fase de primeira ordem. Além disso, quando existe uma histerese visível as grandezas magnetocalóricas dependem do processo de aquecimento e resfriamento do sistema. Ainda de acordo com nossos resultados, existe um efeito barocalórico normal (sistema aquece sob aumento de pressão) quando a pressão aplicada aumenta a temperatura de ordenamento magnético sem alterar a ordem da transição de fase magnética. O efeito barocalórico inverso (sistema resfria sob aumento de pressão) ocorre quando a pressão aplicada diminui a temperatura crítica sem mudar a ordem da transição de fase. Nossos cálculos mostram, ainda, que um efeito barocalórico anômalo (mudança de sinal nas grandezas barocalóricas) ocorre em casos especiais onde a pressão aplicada muda a natureza da transição de fase magnética do primeira para segunda ordem e vice-versa. / In this thesis we systematically discuss the magnetocaloric and barocaloric effects in physical systems, whose magnetism has a localized character and undergo both second and first order phase transitions. For this purpose, we use a model of interacting localized magnetic moments, including the magnetoelastic coupling. Our calculations show that the ratio between the exchange and magnetoelastic parameters is responsible for the appearance of the first order transition and consequently the thermal and magnetic hysteresis. Our calculations also show that the magnetocaloric quantities exhibit large values when the system undergoes a first order transition. Furthermore, when the hysteresis is not negligible, the magnetocaloric quantities depend on the heating and cooling processes. As far as the barocaloric effect is concerned, our calculations show that the normal barocaloric effect occurs (the system heat up with increasing applied pressure) when the applied pressure increases the magnetic ordering temperature without changing the order of the magnetic phase transition. On the other hand, the inverse barocaloric effect (the system cools down with increasing applied pressure) occurs when the applied pressure reduces the magnetic ordering temperature without changing the order of the magnetic phase transition. Finally, our calculations show that an anomalous barocaloric effect (i.e., change of sign in the barocaloric quantities) occurs in special cases when the applied pressure change the nature of the magnetic phase transition from first to second order and vice-versa.

Energy conversion devices using solid-state magnetocaloric materials have the potential to reduce energy consumption and mitigate environmental pollutants. To overcome the limited magnetic entropy change of magnetocaloric materials, magnetic refrigeration devices typically use the active magnetic regenerator (AMR) cycle. AMR devices have demonstrated promising performance, however costs must be reduced for broad market penetration. Although the magnet cost is of greatest importance for commercialization, literature has decoupled magnet design from AMR optimization. And while multilayered regenerators can improve performance without increasing cost, a number of questions remain unanswered as a result of the prohibitive parameter space. This dissertation explores methods of improving AMR performance and decreasing cost both at the subsystem level, namely the magnetocaloric regenerator, fluid flow system and magnetic field source, and the device level by coupling the regenerator and magnet design problems in a cost optimization framework. To improve AMR performance, multilayered regenerators with second-order magnetocaloric materials are experimentally and numerically investigated, yielding insight on how individual layers behave and interact over a wide range of regenerator compositions and operating parameters. An efficient AMR modeling approach is presented where individual layers are treated as cascaded AMR elements, and simulations are in excellent agreement with experiments. Insights from the computationally efficient model are used to inform device modifications, and a no-load temperature span of 40 K is measured in close proximity to the simulated optimum; one of the highest in literature. To simultaneously decrease AMR costs, a permanent magnet optimization framework is explored that is conducive to nonlinear objectives and constraints. This is used to investigate the optimal design of permanent magnet structures with reduced rare-earth permanent magnet materials. The regenerator and magnet design problems are then coupled in a permanent magnet topology optimization to minimize the combined capital and operating costs of an AMR. The optimal magnetic field waveform and the optimal means of producing this waveform are simultaneously obtained. The lifetime ownership costs of the optimized AMR device are shown to be in the realm of existing entry-level cooling devices. The presented cost optimization framework is of interest to both scientists and engineers, and demonstrates the importance of fast AMR models in identifying system designs, regenerator compositions and operating regimes that increase AMR performance and decrease cost. / Graduate

La réfrigération magnétique, basée sur l'effet magnétocalorique (EMC), est une alternative intéressante aux méthodes de réfrigération traditionnelles, basées sur des cycles de compression/détente, car elle présente des rendements énergétiques nettement plus élevés et permet d'éviter l'utilisation de gaz nocifs contribuant à l'effet de serre et problématiques pour l'environnement. Cette technologie s'appuie sur l’EMC géant de certains matériaux magnétiques autour de la température ambiante. Cet effet permet d'augmenter ou de diminuer la température du matériau lors de son aimantation ou désaimantation adiabatique autour de sa température de transition magnétique.La majeure partie des travaux de thèse se focalise sur la famille des matériaux de type La(Fe,Si)13 dans lesquels un effet magnétocalorique géant a été mis en évidence et pour lesquels la faisabilité industrielle semble la plus favorable. Dans un premier temps, les propriétés structurales et magnétiques de ces alliages sont explorées et optimisées, en remplaçant aussi bien la terre rare que le métal de transition par d'autres éléments. Les méthodes d’élaboration, des traitements thermiques, ainsi que le contrôle de la stœchiométrie sont guidées par les caractérisations structurales, microstructurales, physiques (thermiques et magnétiques).D’autre part, l'effet de l'insertion d'éléments interstitiels légers est également étudié et une grande partie du travail porte sur la détermination des conditions de stabilité de ces interstitiels dans les matériaux. Grâce à l'extension des distances Fe-Fe, la température de Curie de la phase magnétocalorique peut être augmentée jusqu'à des plages proches de latempérature ambiante. L'influence d’une faible concentration en carbone sur les propriétés magnétiques des échantillons est examinée avant hydrogénation et la teneur en carbone est optimisée.Afin d'étudier la diffusion des éléments interstitiels, la cinétique de sorption d'hydrogène est étudiée par la méthode de Sieverts ainsi que par diffraction neutronique. La diffraction neutronique in situ et à haute résolution permet une localisation des atomes interstitiels et donne accès au schéma d’insertion. Cette étude permet de préciser l’effet de l’insertion d’interstitiels légers et des substitutions d’éléments de terre rare sur la structure des alliages métalliques complexes de type La-Fe-Si. Nous montrons que la dépression ou l’accélération de la cinétique d'hydrogénation peut être liée à la variation hétérogène particulière de la maille et des liaisons dans la structure de type NaZn13. Un mécanisme pour le chemin de diffusion est suggéré.Le mécanisme d'insertion d'atomes légers est non seulement fortement lié à l'espace disponible, mais aussi associés à la facilité du chemin de diffusion dans le réseau. Nous démontrons avec des résultats expérimentaux qu'une addition modérée de carbone dans la phase La(Fe,Si)13 avant l'hydrogénation peut effectivement ralentir la cinétique d'insertion de l'hydrogène. Dans les phases La-Ce-Fe-Si, une insertion de carbone peut aider à retenir les atomes d'hydrogène lors de la désorption, par conséquent, offre une possibilité d'avoir une meilleure stabilité des matériaux hydrogénés pour des applications à long terme. La stabilité des matériaux hydrogénés est mesurée par DSC et une amélioration de la stabilité thermique du matériau est réalisée par un dopage au carbone.Un volet exploratoire est consacré aux alliages Fe-Cr-Ni et Fe-Cr-Mn qui pourraient potentiellement avoir un effet magnétocalorique exploitable. Les transitions magnétiques et structurales de ces alliages de compositions différentes sont étudiées et leur potentiel d'application magnétocalorique est discuté. / The magnetocaloric effect (MCE) is characterized by a magnetic entropy change and an adiabatic temperature change. The NaZn13-type La(Fe,Si)13 system has attracted wide interest because of its first-order ferromagnetic phase transition with a large magnetocaloric effect. The transition temperature can be flexibly adjusted through substitution or interstitial insertion. Particularly, hydrogen interstitials can adapt the temperature range to room-temperature applications. Precise adjustment can be achieved by full hydrogen absorption then partial desorption. However, fully hydrogenated alloys are unstable upon heating. It is important to have a better understanding of its hydrogen stability to optimize its application potential.In the first part, the structural, magnetic, and magnetocaloric properties of La(Fe,Si)13 phases are studied. In particular, we have investigated the effect of substitution of Ce on the La site and Mn on the Fe sites. The partial substitution of Ce results in the decrease of TC with decreasing lattice constant. At the same time, Ce substitution for La results in a reduced volume of the octahedral interstitial site due to steric effect. The interstitial insertion is impeded by Ce partial substitution.Secondly, the effects of interstitial atoms such as hydrogen and carbon are examined. These elements are able to enter the interstitial voids in the La(Fe,Si)13 phase, expanding the lattice. Through the extension of Fe-Fe distances, the Curie temperature of the magnetocaloric phase can be raised up to room temperature range. The influence of small concentration of carbon on the magnetic properties of samples is examined prior to hydrogenation and carbon content is optimized. In order to investigate the interstitial dynamics, the hydrogen sorption kinetics is studied by the means of Sieverts’ volumetric method and neutron diffraction. Particular attention has been given to the adjustment of the structure in the course of hydrogen/deuterium interstitial absorption and desorption.Steady-state and in-situ neutron diffractions provide precise information of the interstitial atom location of the sequential filling of the accommodating sites. The structural investigation allows specifying the deformations undergone in the complex metallic alloys La-Fe-Si when subjected to light interstitial insertion or rare earth substitution at the cation site. We show that the depression or enhancement of the hydrogenation kinetics may be related to the particular inhomogeneous cell variation of bonding in the structure. A mechanism for the diffusion path is suggested.The mechanism is light atom insertion into the interstitial sites is not only strongly related to the available space for accommodation, but also associated with the facility of the diffusion path in the lattice. We demonstrate with experimental results that a modest addition of carbon in the La-Fe-Si phase prior to hydrogenation can effectively slow down the hydrogen insertion kinetics. In Ce-substituted La-Ce-Fe-Si phases, carbon insertion can help retain hydrogen atoms during desorption, therefore, offering a prospect to have improved stability of hydrogenated materials for long-term applications. The hydrogen stability of the material is examined by means of thermal desorption in DSC and an enhancement of the thermal stability of the material is achieved with carbon-doping.Lastly, in the search of new rare-earth-free materials for magnetocaloric applications, we have explored the capacity of alloys of types FeCrNi and FeCrMn. The magnetic and structural transitions of these alloys of different compositions are studied and their potential for magnetocaloric application is examined in this thesis.

Over the last decades, several works have been devoted to the investigation of a new class of hybrid spin systems with nodal localized Ising spins and interstitial delocalized electrons. In a diamond-like geometry, this class of models depicts magnetization plateaus and an enhanced magnetocaloric effect. In square lattices geometry, the quantum correlations may lead to ferromagnetic or antiferromagnetic ordering depending on the filling fraction of the delocalized orbitals. In the present work, we study the magnetic and thermodynamic properties of a hybrid spin ladder consisting of Ising spins placed in distinct chains, interacting with each other through a ferromagnetic exchange coupling φ. The interaction between nearest neighbor’s localized spins on the same chain is mediated by a pair of interstitial electrons that may hop between chains with hopping amplitude t obeying Pauli’s Exclusion Principal. The exchange interaction between each localized spins and an interstitial electron is J. The interplay of hopping amplitude and exchange interaction may give rise to a rich phase diagram for the ground state of the system. Such a model may be exactly solvable by using decoration-iteration transformation, transfer matrix method and exact diagonalization. Our results show that quantum correlations betweeen delocalized spins induce an antiferromagnetic coupling between chains that competes with the ferromagnetic exchange coupling. A resulting kinecally-driven frustration is predominant in the regime of low temperatures and large hopping amplitudes. We provide the full ground-state phase diagram as well as the frustration diagram on distinct coupling regimes. The effect of an external magnetic field are also investigated. Further, we determine how the hopping amplitude and an external magnetic field affect different thermodynamics properties of the system, such as total magnetization, specific heat and magnetocaloric rate. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nas últimas décadas, vários trabalhos têm sido dedicados à investigação de uma nova classe de sistemas de spins híbridos, formados por spins de Ising nodais localizados e elétrons intersticiais delocalizados. Na geometria do tipo diamante, esta classe de modelos apresenta platô de magnetização e um efeito magnetocalórico pronunciado. Em uma geometria de rede quadrada, as correlações quânticas podem levar a um ordenamento ferromanético ou antiferremagnético dependendo da fração de preenchimento dos orbitais delocalizados. No presente trabalho, nós estudamos as propriedades magnéticas e termodinâmicas de escada de spins híbridos, consistindo em spins de Ising localizados em cadeias distintas, que interagem por meio de acoplamento ferromagnético de troca φ. A interação entre os primeiros vizinhos de spins localizados numa mesma cadeia é mediada por um par de elétrons intersticiais que podem saltar entre as cadeias com amplitude de hopping t, obedecendo ao Princípio de Exclusão de Pauli. A interação de troca entre os spins localizados e os spins intersticiais é J. A competição entre as interações de troca e a amplitude de hopping pode dar origem a um rico diagrama de fases para o estado fundamental do sistema. Este modelo pode ser exatamente solúvel usando a transformação de iteração-decoração, método da matriz de transferência e diagonalização exata. Nossos resultados mostram que as correlações quânticas entre os spins delocalizados induzem um acoplamento antiferromagnético entre as cadeias que compete com o acoplamento ferro-magnético de troca. Uma frustração induzida cineticamente é predominante no regime de baixas temperaturas e grandes amplitudes de hopping. Nós fornecemos um diagrama de fases completo para o estado fundamental do sistema, bem como o diagrama de frustração para diferentes regimes dos acoplamentos. Os efeitos de um campo magnético externo também são investigados. Além disso, nós determinamos como a amplitude de hopping e um campo magnético externo afetam diferentes propriedades termodinâmicas do sistema, tais como a magnetização total, o calor específico e a taxa magnetocalórica.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho foram estudadas as propriedades estruturais e termomagnéticas dos pseudobinários Ho1-yGdyAl2, através de abordagens experimentais e teóricas. A parte experimental envolveu a preparação de cinco amostras, com as concentrações y = 0, 0,25, 0,5, 0,75 e 1, assim como medidas de magnetização, calor especifico e da variação adiabática da temperatura. Na parte teórica usamos um hamiltoniano modelo que leva em consideração a interação dos íons com o campo magnético aplicado, com o campo elétrico cristalino e a troca entre os íons magnéticos. A entropia da rede foi considerada na aproximação de Debye e a entropia eletrônica na aproximação do gás de elétrons livres. A influência das reorientações de spin, espontâneas e induzidas pelo campo magnético, na magnetização e no calor especifico foram investigadas sistematicamente tanto a partir de dados experimentais quanto teoricamente. Também obtemos resultados teóricos para a variação de entropia e variação adiabática da temperatura alterando a intensidade ou a direção do campo magnético. / In this work the structural and thermomagnetic properties of the pseudobinaries Ho1-yGdyAl2 have been studied by experimental and theoretical approaches. The experimental part included the preparation of five samples, with concentrations y = 0, 0,25, 0,5, 0,75 and 1, as well as magnetization, specific heat and adiabatic temperature change measurements. In the theoretical part we used a model Hamiltonian which includes the interaction of the ions with the applied magnetic field, the crystalline electrical field and exchange interactions with others magnetic ions. The lattice entropy has been considered within Debyes approach and the electronic entropy as that of a free electron gas. The spin reorientation influence, spontaneous and magnetic field induced ones, in magnetization and specific heat has been systematically investigated either from experimental data as well as theoretically. We have also obtained theoretical results for the isothermal entropy change and the adiabatic temperature change by modifying either the magnetic field strength or its direction.

Nesta tese discutimos sistematicamente os efeitos magnetocalórico e barocalórico em sistemas físicos com transição de fase magnética de primeira e de segunda ordem em sistemas físicos cujo magnetismo tem carater local. Para essa finalidade, utilizamos um modelo de momentos magnéticos locais interagentes, incluindo um termo de acoplamento magnetoelástico. Nossos cálculos mostram que a relação entre a interação de troca e o acoplamento magnetoelástico é responsável pela ordem da transição de fase e pelo aparecimento da histerese térmica e magnética. Os resultados mostram que as grandezas magnetocalóricas exibem grandes valores quando o sistema sofre uma transição de fase de primeira ordem. Além disso, quando existe uma histerese visível as grandezas magnetocalóricas dependem do processo de aquecimento e resfriamento do sistema. Ainda de acordo com nossos resultados, existe um efeito barocalórico normal (sistema aquece sob aumento de pressão) quando a pressão aplicada aumenta a temperatura de ordenamento magnético sem alterar a ordem da transição de fase magnética. O efeito barocalórico inverso (sistema resfria sob aumento de pressão) ocorre quando a pressão aplicada diminui a temperatura crítica sem mudar a ordem da transição de fase. Nossos cálculos mostram, ainda, que um efeito barocalórico anômalo (mudança de sinal nas grandezas barocalóricas) ocorre em casos especiais onde a pressão aplicada muda a natureza da transição de fase magnética do primeira para segunda ordem e vice-versa. / In this thesis we systematically discuss the magnetocaloric and barocaloric effects in physical systems, whose magnetism has a localized character and undergo both second and first order phase transitions. For this purpose, we use a model of interacting localized magnetic moments, including the magnetoelastic coupling. Our calculations show that the ratio between the exchange and magnetoelastic parameters is responsible for the appearance of the first order transition and consequently the thermal and magnetic hysteresis. Our calculations also show that the magnetocaloric quantities exhibit large values when the system undergoes a first order transition. Furthermore, when the hysteresis is not negligible, the magnetocaloric quantities depend on the heating and cooling processes. As far as the barocaloric effect is concerned, our calculations show that the normal barocaloric effect occurs (the system heat up with increasing applied pressure) when the applied pressure increases the magnetic ordering temperature without changing the order of the magnetic phase transition. On the other hand, the inverse barocaloric effect (the system cools down with increasing applied pressure) occurs when the applied pressure reduces the magnetic ordering temperature without changing the order of the magnetic phase transition. Finally, our calculations show that an anomalous barocaloric effect (i.e., change of sign in the barocaloric quantities) occurs in special cases when the applied pressure change the nature of the magnetic phase transition from first to second order and vice-versa.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Nesta dissertação, foram investigadas as propriedades magnéticas e magnetocalóricas nos compostos intermetálicos de terras-raras Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) usando abordagens teórica e experimental. Do ponto de vista teórico, a série Gd1-xDyxAl2 foi descrita através de um modelo para o hamiltoniano magnético, incluindo o efeito Zeeman, interação de troca e a anisotropia de campo elétrico cristalino. As entropias da rede e eletrônica foram consideradas nas aproximações de Debye e de gás de elétrons livres, respectivamente. A parte experimental inclui a preparação do material, sua caracterização e medidas das quantidades magnéticas e magnetocalóricas. Os resultados experimentais e os cálculos teóricos da variação adiabática da temperatura (ΔTad) e da variação isotérmica da entropia (ΔS T), sob variações de campo magnético ao longo da direção de fácil magnetização, estão de bom acordo. O efeito da aplicação do campo magnético ao longo de uma direção de difícil magnetização foi estudado e as componentes da magnetização em função da temperatura foram investigadas. Também foi observado que a temperatura de reorientação de spin, TR, diminui quando a intensidade do campo magnético aumenta. Além disso, as concentrações molares ótimas de um material híbrido formado pelos compostos Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) foram simuladas usando um método numérico de matriz proposto por Smaili e Chahine. O compósito apresenta um bom intervalo de temperatura para um refrigerador magnético de 60 até 170 K. / In this work, the magnetic and magnetocaloric properties were investigated in the rare earth intermetallic compounds Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) using both theoretical and experimental approaches. From theoretical point of view, the Gd1-xDyxAl2 series was described through a magnetic model Hamiltonian including the Zeeman effect, exchange interaction and the crystalline electrical field anisotropy. The crystalline and electronic entropies were considered in Debye and free electrons gas approximations, respectively. The experimental work included the material preparation, characterization and measurements of magnetic and magnetocaloric quantities. Good agreement between experimental data and calculations were obtained for the adiabatic temperature change (ΔTad) and the isothermal entropy change (ΔS T), upon applied magnetic field changes along the easy magnetic direction. The effect of magnetic field applied along non easy magnetic direction was studied and the temperature dependence of the magnetization components was full investigated. Also, we observed that the spin reorientation temperature, TR, decreases when the magnetic field intensity increases. Besides, the optimal molar concentrations of the hybrid material formed by compounds Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) were simulated using numerical matrix method proposed by Smaïli and Chahine. The obtained composite presents a good temperature interval for magnetic refrigeration from 60 to 170 K.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Nesta dissertação, foram investigadas as propriedades magnéticas e magnetocalóricas nos compostos intermetálicos de terras-raras Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) usando abordagens teórica e experimental. Do ponto de vista teórico, a série Gd1-xDyxAl2 foi descrita através de um modelo para o hamiltoniano magnético, incluindo o efeito Zeeman, interação de troca e a anisotropia de campo elétrico cristalino. As entropias da rede e eletrônica foram consideradas nas aproximações de Debye e de gás de elétrons livres, respectivamente. A parte experimental inclui a preparação do material, sua caracterização e medidas das quantidades magnéticas e magnetocalóricas. Os resultados experimentais e os cálculos teóricos da variação adiabática da temperatura (ΔTad) e da variação isotérmica da entropia (ΔS T), sob variações de campo magnético ao longo da direção de fácil magnetização, estão de bom acordo. O efeito da aplicação do campo magnético ao longo de uma direção de difícil magnetização foi estudado e as componentes da magnetização em função da temperatura foram investigadas. Também foi observado que a temperatura de reorientação de spin, TR, diminui quando a intensidade do campo magnético aumenta. Além disso, as concentrações molares ótimas de um material híbrido formado pelos compostos Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) foram simuladas usando um método numérico de matriz proposto por Smaili e Chahine. O compósito apresenta um bom intervalo de temperatura para um refrigerador magnético de 60 até 170 K. / In this work, the magnetic and magnetocaloric properties were investigated in the rare earth intermetallic compounds Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) using both theoretical and experimental approaches. From theoretical point of view, the Gd1-xDyxAl2 series was described through a magnetic model Hamiltonian including the Zeeman effect, exchange interaction and the crystalline electrical field anisotropy. The crystalline and electronic entropies were considered in Debye and free electrons gas approximations, respectively. The experimental work included the material preparation, characterization and measurements of magnetic and magnetocaloric quantities. Good agreement between experimental data and calculations were obtained for the adiabatic temperature change (ΔTad) and the isothermal entropy change (ΔS T), upon applied magnetic field changes along the easy magnetic direction. The effect of magnetic field applied along non easy magnetic direction was studied and the temperature dependence of the magnetization components was full investigated. Also, we observed that the spin reorientation temperature, TR, decreases when the magnetic field intensity increases. Besides, the optimal molar concentrations of the hybrid material formed by compounds Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) were simulated using numerical matrix method proposed by Smaïli and Chahine. The obtained composite presents a good temperature interval for magnetic refrigeration from 60 to 170 K.