In this work were synthesized and studied four new coordination compounds of copper(II) with phenylseleninato ligand, three of them polymeric and one molecular. The reaction of phenylseleninic acid with copper(II) acetate led to microcrystalline light blue powder of polymeric compound catena-poli-[Cu(μ2-O2SePh)2], 1. That powder was used to obtain crystalls of 1 and three others: catena-poli-[Cu(μ2-O2SePh)2(NH3)], 2, obtained by the dissolution of 1 in dmf and aqueous ammonia, catena-poli-[Cu(μ2-O2SePh)2(NC5H5)]·NC5H5, 3, obtained by the dissolution of 1 in pyridine and aqueous ammonia, and poli-[Cu(O2SePh)2(NH3)2(H2O)2], 4, obtained by the dissolution of 1 in water and aqueous ammonia. Compound 1 is a two-dimensional coordination polymer with layers formed by the phenylseleninato double bridges between copper(II) units. The copper(II) centers in the same layer form a (4,4) net topology. Magnetic susceptibility measurements of the crystalline sample of 1 was carried out on a SQUID magnometer and strong ferromagnetic coupling was verified between 10 and 6.3 K. Below 6.3 K was verified a weak antiferromagnetic interactions between partially ordered ferromagnetic layers. All the new compounds were characterized by X-ray diffraction, infrared spectroscopy and elemental analysis. Compounds 2 and 3 have supramolecular nature due to hydrogen bonds in those structures. Only compound 3 is a one-dimensional coordination polymer. / Neste trabalho foram sintetizados e estudados quatro novos compostos de coordenação de cobre(II) com o ligante fenilseleninato, três deles poliméricos e um molecular. A reação do ácido fenilselenínico com acetato de cobre(II) originou um pó microcristalino e azul-claro, referente ao composto polimérico catena-poli-[Cu(μ2-O2SePh)2], 1. Esse foi o composto de partida para a obtenção de cristais de 1, além da obtenção de outros três compostos cristalinos: catena-poli-[Cu(μ2-O2SePh)2(NH3)], 2, obtido pela dissolução de 1 em dmf e amônia aquosa; catena-poli-[Cu(μ2-O2SePh)2(NC5H5)]·NC5H5, 3, obtido pela dissolução de 1 em piridina e amônia aquosa; e [Cu(O2SePh)2(NH3)2(H2O)2], 4, obtido a pela dissolução de 1 em água e amônia aquosa. O composto 1 é um polímero bidimensional em camadas formadas pelo centro metálico e o ligante, no qual os centros de cobre(II) são conectados por pontes duplas de fenilseleninato. Esses centros distribuem-se nas camadas com uma topologia de rede (4,4). Estudos de propriedades magnéticas com um magnetômetro do tipo SQUID evidenciaram fortes interações ferromagnéticas entre 10 e 6,3 K, quando ocorre uma transição antiferromagnética, com queda brusca dos valores do inverso da susceptibilidade magnética, provavelmente por interações antiferromagnéticas entre os planos de Cu(II) paralelos. Todos os novos compostos foram caracterizados por difração de raios-X, espectroscopia de infravermelho e análise elementar. A instabilidade e decomposição, por perda de solvente, das estruturas 2 e 4 impediram estudos de magnetismo, porém a elucidação de suas estruturas cristalinas evidenciaram a existência de interessante caráter supramolecular devido a presença do ligante NH3, em 2, e NH3 e H2O, em 3. As ligações de hidrogênio nesses compostos mostraram-se importantes na manutenção de suas estruturas no estado sólido, assim como na formação de camadas com topologia de rede (4,4). Somente o composto 3 mostrou-se como um polímero de coordenação unidimensional.

Abstract The subjects of this thesis are the dynamics of liquid crystals in external electric and magnetic fields as well as the magnetic properties of small molecules, both studied by liquid crystal nuclear magnetic resonance (LC NMR) spectroscopy. Director dynamics of a liquid crystal 5CB in external magnetic and electric fields was studied by deuterium NMR and spectral simulations. A new theory was developed to explain the peculiar oscillations observed in the experimental spectra collected during fast director rotation. A spectral simulation program based on this new theory was developed and the outcome of the simulations was compared with the experimental results to verify the tenability of the theory. In the studies on the properties of small solute molecules, LC NMR was utilised to obtain information about anisotropic nuclear magnetic interaction tensors. The nuclear magnetic shielding tensor was studied in methyl halides, the spin-spin coupling tensor in methyl mercury halides and the quadrupolar coupling tensor in deuterated benzenes. The effects of small-amplitude molecular motions and solvent interactions on the obtained parameters were considered in each case. Finally, the experimental results were compared to the corresponding computational NMR parameters calculated in parallel with the experimental work.

L'importance des effets relativistes dans la chimie a été reconnu depuis les années 1980. Par exemple, sans la relativité (a) l'or aurait la même couleur que l'argent (b) le mercure ne serait pas liquide à la température ambiante et (c) nos voitures ne démarrent pas avec une batterie de plomb. Pour une description théorique de la structure et la réactivité des éléments lourds, la relativité est un ingrédient essentiel. Le hamiltonien pour les calculs moléculaires relativistes à 4 composantes est construit en remplaçant la partie mono-électronique de l'hamiltonien électronique non-relativiste par le hamiltonien de Dirac. La partie bi-électronique est approchée par le terme de r Coulomb comme dans le cas non relativiste, ce qui donnel'hamiltonien de Dirac-Coulomb (DC). Pour réduire le coût de calcul, on peut utiliser des hamiltoniens relativistes à 2 composantes. Parmi eux, l'hamiltonien exact à 2 composantes (X2C) est le plus précise. La corrélation électronique est, cependant, une contribution très importante pour obtenir une description théorique à la fois qualitative et quantitative des spectroscopies moléculaires, réactions, etc. Dans cette thèse, nous avons étudié l'interaction entre la relativité et de la corrélation. à la fois par des développements méthodologiques et par des applications moléculaires. Dans la première partie de la thèse, nous avons calculé les constantes spectroscopiques dimères des gaz rares lourds. La liaison faible de ces dimères ne peut être décrit que par l'inclusion de la corrélation électronique. Les dimères des gaz rares les plus lourds, le radon et l'eka-radon, nécessite de plus un traitement adéquat de la relativité. Nos calculs sont basés sur l'hamiltonien X2Cmmf, à la fois avec des méthodes de corrélation basés sur une fonction d'onde et séparation de porte (srDFT). La deuxième partie de cette thèse concerne la simulation de la spectroscopie des rayons X, où l'on sonde la région du cœur d'une molécule, ou la relativité joue un rôle très important. Nous avons étudié la spectroscopie L-edge de la série isoélectronique: UO22 +, UNO+, et UN2, où le couplage spin-orbite joue un rôle majeur. Au niveau des méthodes, nous avons considéré MP2 à couches ouvertes et la théorie de la fonctionnelle de la densité dépendante de temps (TDDFT). Dans un autre étude, nous avons simulé la spectroscopie K-edge de la série H2X (X = O, S, Se, Te) et XH3 (X = N, P, As) ainsi que les molécules N2 et N2O2. Pour ces systèmes, l'interaction spin-orbite est moins important. Par conséquent, nous avons utilisé un hamiltonien DC sans spin (SF). Certains des systèmes pris en compte dans ce travail sont de caractère multi-référentielles ; nous avons utilisé la methode Coupled Cluster Multi-référentielle de type State Universal et adapté au groupe unitaire (UGA-SUMRCC) comme une méthode de corrélation. Dans la troisième et partie principale de la thèse, l'attention est de nouveau sur la relativité et de la corrélation, mais pour le calcul des propriétés électriques et magnétiques moléculaires. Nous avons développé et mis en œuvre un module pour le calcul des valeurs moyennes au niveau relativiste à 4-composantes coupled cluster monoréferentiel. Les propriétés qui sondent la densité électronique près de noyaux (lourds), telles que la résonance paramagnétique électronique (RPE), les paramètres des gradients de champ électrique et la non-conservation de la parité (NCP) des molécules chirales ,sont parfaitement adaptés pour l'application de cette méthode. Pour l'instant, nous avons étudié que la NCP. Ce module dans le logiciel DIRAC pour les calculs moléculaires relativistes fournit un cadre propice pour la mise en œuvre de méthodes de CC relativistes employant la symétrie de groupes doubles et de permutation de manière très efficace. En perspective, nous ciblons la mise en œuvre de la réponse linéaire CC pour le calcul des énergies d'excitation et propriétés moléculaires de second ordre tels que les paramètres de RMN. / The importance of relativistic effects in chemistry has been recognized since the 1980s. Without relativity (a) gold would have the same colour as silver (b) mercury would not be liquid at room temperature (c) our cars would not start (lead-battery). For a theoretical description of the structure and reactivity of heavy-elements, relativity is considered as an essential ingredient. The Hamiltonian for the 4-component relativistic molecular calculations is constructed by replacing the one-electronic part of the non-relativistic molecular Hamiltonian by the Dirac Hamiltonian. The two-electronic part of the Hamiltonian is approximated by the Coulombic repulsion term as in the non-relativistic case. The resulting Hamiltonian is called the Dirac-Coulomb (DC) Hamiltonian. For chemical applications there exist a class of relativistic Hamiltonians, where one-electronic part of the DC Hamiltonian is transformed to a 2-component one. Among them the eXcact 2-component (X2C) Hamiltonian is the most accurate one. Electron correlation, however, is a very important contribution to achieve a both qualitative and quantitative correct description of molecular spectroscopies, reactions etc. It is, therefore, essential to study the interplay between relativity and correlation. In this thesis, we have studied this interplay both in terms methodological developments and molecular applications. In the first part of the thesis we have studied the spectroscopic constants of the heavy rare gas dimers. The weak bonding of those dimers can only be described by the inclusion of electron correlation. The heavier analogues in the rare gas series i.e, Radon and eka-Radon, in addition require adequate treatment of relativity. Our calculations are based on the eXact 2-Component molecular-mean field (X2Cmmf) Hamiltonian both with wave function methods and range-separated DFT methods. The second part of this thesis simulates X-ray spectroscopy, where one probes the core region of a molecule. In the core region relativity plays a very significant role. Removal and excitation of electrons from that region involve various processes, which are beyond a mean-field description. We have studied L-edge spectroscopy of the isoelectronic series: UO22+, UNO+, and UN2, where spin-orbit coupling plays a major role. For the theory we have considered single reference open-shell MP2 and Time Dependent Density functional Theory (TDDFT). In another work, we have studied K-edge spectroscopy of the H2X (X= O, S, Se, Te) and XH3 (X= N, P, As) series as well as N2, N2O2 molecules. For this study spin-orbit coupling is less important, therefore, we have treated them with the Spin-Free (SF) DC Hamiltonian. Some of the systems considered in this work are Multi-Reference in nature; we have used Unitary Group Adapted (UGA) State Universal Multi-reference Coupled Cluster (UGA-SUMRCC) theory as a correlation method. In the third and major part of the thesis, the thrust is again on relativity and correlation, but for the calculation of molecular electric and magnetic properties. We have developed and implemented a module for the calculation of expectation values at the 4-component Relativistic Single Reference Coupled Cluster level. Properties that probe the electron density near (heavy) nuclei, such as Electron Paramagnetic Resonance (EPR) parameters, electric field gradients and parity non-conservation (PNC) in chiral molecules are ideally suited for the application of this method. However, we have only studied PNC so far. This module in the DIRAC software for relativstic molecular calculations provides a convenient framework for the implementation of relativistic CC methods employing double group and permutation symmetry very efficiently. In the near future we therefore target the implementation of Linear Response CC for the calculation of excitation energies and second-order molecular properties such as NMR parameters.

>Magister Scientiae - MSc / In the current study, we give a report when oxalic acid was used as an electrolyte to synthesize an AAO template with hexagonal pore array. Optimum parameters were observed as 0.4 M of oxalic acid, anodizing voltage of 45 V, temperature of approximately 8 °C and the period of 120 minutes. Atomic force microscope (AFM) and High resolution scanning electron microscope (HRSEM) showed that template has an average pore diameter of 103 nm. Co and MnOx (x = 1,2) nanostructures were selectively deposited in the pores of the template using a novel atomic layer deposition (ALD) technique. The diameter sizes and the array of the nanostructures and the template were corresponding. Energy dispersive xrays (EDX) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of Co and MnOx (x =1,2) on the samples while x-ray diffraction (XRD) provided an indication of their orientations. Magnetic force microscopy as main characterization tool showed the existence of multi-domains on both Co and MnOx (x =1,2) nanostructures. / South Africa

Cobalt Ferrite has important, size-dependent magnetic properties. Consequently, an overview of particle size is important. Co-precipitation in air was the fabrication method used because it is comparatively simple and safe. The effects of three different reaction times including 1, 2, 3 hour(s) on particle size were compared. Also, the effectiveness of three different capping agents (Oleic Acid, Polyvinylpyrollidone (PVP), and Trisodium Citrate) in reducing aggregation and correspondingly particle size were examined. Using Welch’s analysis of variance (ANOVA) and the relevant post hoc tests, there was no significant difference (p=0.05) between reaction times of 1 hour and 2 hours, but there was a significant difference between reaction times of 2 hours and 3 hours. Potentially, because of increased coarsening for the 3 hour reaction time. PVP and Oleic Acid were shown to be effective in reducing aggregation; however, Citrate was not effective. Possibly, the synthesis procedure was inadequate.

Graphene is a single layer of carbon atoms arranged in honeycomb lattice. Over a long period of time it was treated as a hypothetical material to understand the properties of other allotropes of carbon, such as graphite, carbon nanotube etc. Half decade back, a single layer of graphene was finally isolated and since then the field has observed a flurry of activities. Low energy excitations in graphene are massless Dirac Fermions and quantum electrodynamic effects can be observed at room temperature in graphene, which makes it very popular among the condensed matter community. In addition graphene also shows many interesting mesoscopic effects, which is the focus of the present work. We study the electronic, magnetic and structural properties of the graphene nanostructures. The entire thesis based on the results and findings obtained from the present investigation is organized as follows. Chapter 1: provides a general introduction to the properties of graphene and graphene based nanostructures. Chapter2:describes the theoretical tools used in this thesis to investigate the properties of graphene nanoribbons. The first two chapters are meant to give the reader an overview about the field of graphene and a few of the computational techniques commonly used to investigate the properties of graphene. The following chapters are the new findings reported in this thesis. Chapter3:shows how zigzag graphene nanoribbons respond in a non-linear fashion when edges are subjected to some external potential such as magnetic field. Such response originates from the edge states present in zigzag ribbons and thus not observed in armchair nanoribbons. In the limit of ribbon width W→∞, an edge magnetic field produces a moment of ~ 1/3 per edge atom even for an infinitesimally small field, which is clearly a signature of non-linear response. Response of a finite width nanoribbon is size dependent and also depends on ln(V), the applied field. This is akin to Weber-Fechner law of audio visual perceptions. It is interesting to note that nature does provide a “quantum realization” of this in the form of biological sensing organs like the ear and eye. The magnetic response is found to scale inversely with the ribbon width. Chapter4:deals with the magnetic properties of the zigzag graphene nanoribbon. This is also a special property of the geometry of the zigzag edges and not observed in armchair nanoribbons. Our investigation reveals that the electron-electron repulsion (Hubbard U) energy creates a delta function like edge magnetic field in zigzag graphene nanoribbons. Starting from this, magnetic properties of zigzag graphene nanoribbons can be qualitatively and quantitatively explained from the non-linear response of zigzag nanoribbons. Zigzag graphene nanoribbons can exist in two possible ‘magnetic states’: antiferro (AF) where the two opposite edges have antiparallel magnetic moment and ferro (FM) where moment is parallel in the two opposite edges. First we describe the properties of undoped zigzag nanoribbons. They have AF ground state. Continuum theory can explain the size dependent bandgap and magnetic moment of the ground state. We present the first explicit derivation of the gap. Then we show that hole doping can change the ground state to FM, which is metallic. Thus the system has the property of magnetoresistance, which can be exploited by doping zigzag graphene nanoribbons externally with some gate voltage or internally by some electron acceptor element, such as boron. The critical doping for transition depends inversely with the ribbon width. We have found that the ‘phase transition’ on hole doping is a common phenomena for zigzag terminated nanostructures, such as hexagonal nanodots. Chapter5:discusses the effects of random edge shapes and random potential (Anderson disorder) on the magnetic properties of zigzag graphene nanostructures. Defects and disorders in the form of edge shape randomness and random potentials arising from substrate are very common in graphene. Our study reveals that edge state magnetism is very robust to shape randomness of the terminating edges of nanostructures; as long as there are three to four repeat units of a zigzag edge, the edge state magnetism is preserved. We also discover some “high energy” edges (ones where the edge atoms have only one nearest neighbor) can have very large moments compared to even the zigzag edges. Edge magnetism is also found to be robust to relatively small Anderson disorders, because a slowly varying small potential does not scatter the edge states. Chapter6:reveals how edge functionalization by O atom and OHgroup changes the properties of the zigzag graphene nanoribbons. Functionalization by various different molecules is a very popular method of tuning the properties of graphene. We have shown that it is possible to tune the properties of zigzag graphene nanoribbons by edge functionalization. Further, we have found that structures with clustered functionalization leads to “spatially” varying electronic structure, which can lead to interesting possibilities for electronic devices. Chapter7:describes structural stability, electronic and magnetic properties of graphene nanoribbons in presence of topological defects such as Stone-Wales defects. Our study reveals that the sign of stress induced by a SW defect in a graphene nanoribbon depends on the orientation of the SW defect with respect to the ribbon edge and the relaxation of the structure to relieve this stress determines its stability. Local warping or wrinkles arise in graphene nanoribbon when the stress is compressive, while the structure remains planar otherwise. The specific consequences to armchair and zigzag graphene nanoribbon can be understood from the anisotropy of the stress induced by a SW defect embedded in bulk graphene. We also have found localized electronic states near the SW defect sites in a nanoribbon. However, warping results in delocalization of electrons in the defect states. We have observed that, in zigzag graphene nanoribbons magnetic ordering weakens due to the presence of SW defects at the edges and the ground state is driven towards that of a nonmagnetic metal.

Spin-polarized two-dimensional electron states (2DESs) at surfaces and interfaces of magnetically active materials attract immense interest because of the idea of exploiting fermion spins rather than charge in next generation electronics. Applying angle-resolved photoelectron spectroscopy, we show that the silicon surface of GdRh2Si2 bears two distinct 2DESs, one being a Shockley surface state, and the other a Dirac surface resonance. Both are subject to strong exchange interaction with the ordered 4f-moments lying underneath the Si-Rh-Si trilayer. The spin degeneracy of the Shockley state breaks down below ~90 K, and the splitting of the resulting subbands saturates upon cooling at values as high as ~185 meV. The spin splitting of the Dirac state becomes clearly visible around ~60 K, reaching a maximum of ~70 meV. An abrupt increase of surface magnetization at around the same temperature suggests that the Dirac state contributes significantly to the magnetic properties at the Si surface. We also show the possibility to tune the properties of 2DESs by depositing alkali metal atoms. The unique temperature-dependent ferromagnetic properties of the Si-terminated surface in GdRh2Si2 could be exploited when combined with functional adlayers deposited on top for which novel phenomena related to magnetism can be anticipated.

Des structures prismées sont fréquemment observées dans les coulées de lave comme la Chaussée des Géants (Irlande). Plusieurs théories existent pour expliquer ces formations, dont la plus répandue est celle de la contraction thermique. Or cette théorie permet difficilement de comprendre certaines observations de terrain comme la séparation fréquente des coulées en plusieurs niveaux. Afin de mieux comprendre la structuration au sein des coulées basaltiques, nous avons procédé à une étude pluridisciplinaire basée sur les propriétés magnétiques, les variations structurales et géochimiques de deux coulées prismées du Massif Central (La Palisse, Ardèche et Saint-Arcons-d’Allier, Haute-Loire). Notre approche permet de montrer que les fabriques cristallographiques et magnétiques sont gouvernées par l’écoulement de la lave. L’orientation du plagioclase contrôle la distribution des titanomagnétites à l’origine des fabriques magnétiques. Notre étude montre également que l’utilisation de l’ASM est un outil fiable pour déterminer l’orientation de l’écoulement à condition d’être contrôlée par des mesures de fabriques cristallographiques. Les mesures de la quantité d’eau et les analyses isotopiques (H et O) montrent que l’effet de l’altération météorique est faible et que l’eau contenue dans la roche est essentiellement de l'eau de constitution. De plus, à l’échelle du prisme, des variations de deuxième ordre sont observées comme celle des paramètres d’hystérésis qui indique des tailles de grains de titanomagnétites plus importantes vers le centre. Ces variations au sein du prisme semblent difficilement compatibles avec une structuration des coulées par la simple contraction thermique. / Columnar jointing is frequently observed in lava flows, as in the Giant Causeway (Ireland). The most common theory explaining the formation of prisms is by the thermal contraction. However, this theory hardly explains some field observations such as the frequent existence of three parts within the lavas flows, from the base to the top. To complete our understanding of the structuring lava flows, we carried out a multidisciplinary study based on the magnetic properties, structural and geochemical characterization of two basaltic flows from the French Massif Central (La Palisse, Ardèche and Saint-Arcons-d'Allier, Haute-Loire). Our approach shows that crystallographic and magnetic fabrics are governed by the flow. The distribution of titanomagnetite grains carrying the magnetic fabrics is mainly controlled by the plagioclase orientation. Our study also shows that the use of the AMS to determine the flow direction is a reliable tool, provided punctual control by measurements of crystallographic fabrics are performed. Measurements of the water content and isotopic analyses (H and O) show a limited weathering effect in the studied areas: rock water is mostly primary water in equilibrium with the magma. Moreover, second order changes are noted across the prism section such as hysteresis parameters associated to grain size variation of titanomagnetite (larger grains in the center). The variations of magnetic properties across the prism section suggest a gradient of the crystallization rate from the center to the edge of the prism, which seems difficult to reconcile with the structuring of the flow by thermal contraction only.

Orientador: Flávio César Guimarães Gandra / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-30T17:33:36Z (GMT). No. of bitstreams: 1 Monteiro_JoseCarlosBotelho_D.pdf: 10216375 bytes, checksum: 06d25402d8c5828939f2e7fa0710efbe (MD5) Previous issue date: 2016 / Resumo: Esta tese tem como principal objetivo desenvolver, apresentar e justificar a utilização de uma metodologia experimental que permita avaliar o efeito magnetocalórico (EMC), em qualquer tipo de material, de modo que as medidas reflitam a resposta real que a amostra fornece ao ser submetida a ciclos de magnetização similares àqueles que ocorrem em sistemas de refrigeração magnética. Para tal, construímos sistemas de medidas que utilizam dispositivos Peltier como sensores de fluxo de calor, capazes de realizar medidas diretas da quantidade de calor que a amostra absorve ou libera em situações aonde há variação de temperatura, campo magnético ou do ângulo entre direção do cristal e o campo aplicado. Na primeira parte do trabalho, foram realizadas medidas no sistema com dispositivos Peltier desenvolvido para uso no equipamento comercial PPMS - Physical Property Measurement System (Sistema de medidas de propriedades físicas) da Quantum Design. Utilizamos os métodos indiretos de medida do EMC mais comuns na literatura (medidas via curvas de magnetização e calor específico) para comparação com as medidas diretas de fluxo de calor através de varredura de campo obtidas pelo nosso sistema. Esta análise foi feita inicialmente em duas amostras com transições magnéticas de primeira e segunda ordem, consideradas como amostras padrão na área do EMC: Gadolínio e a liga Gd5Ge2Si2. Discutimos as diferenças encontradas e definimos aquele que acreditamos ser o protocolo de medidas mais correto para a avaliação do EMC para fins práticos. A partir desta conclusão, analisamos três outras amostras que apresentam comportamentos não usuais e alto potencial magnetocalórico e discutimos as diferenças. Perdas do EMC por histerese foram obtidas experimentalmente. Na segunda parte, com o auxílio de um calorímetro com o elemento Peltier capaz de realizar um giro de até 80° sob campo constante de até 1,9 T, realizamos o estudo do efeito magnetocalórico anisotrópico (EMC-ani) em amostras monocristalinas da família RAl2, obtidas pelo processo de Czochralski. Primeiramente medidas de calor específico e do EMC convencional foram realizadas nos monocristais, através do protocolo definido como ideal na primeira parte do trabalho, utilizando o sistema Peltier do PPMS. A partir desses dados, fomos capazes de obter o EMC-ani, de modo indireto, pela subtração das curvas obtidas. Por fim utilizamos o sistema Peltier de giro para realizar medidas diretas do EMC-ani em monocristais de DyAl2. Os resultados das medidas indiretas e diretas foram comparados com cálculos obtidos através de um modelo autoconsistente / Abstract: This thesis aimed to develop, present and justify the use of a methodology that allows one to evaluate the magnetocaloric effect (MCE), for any kind of material, such that the results reflects the real behavior of the sample submitted to magnetization cycles similar to those of magnetic refrigeration systems. To do so, we built measurement systems that uses Peltier devices as heat flux sensors to determine the heat absorbed or released by the sample in situations where the temperature, magnetic field, or angle between a given crystal direction and field changes. In the first part of the work, we report measurements using a Peltier device system developed for use with the Quantum Design PPMS (Physical Property Measurement System). We evaluated the indirect MCE measurements by using the most common techniques found in literature (through magnetization or specific heat curves) and compared to the direct heat flux measurements obtained through field sweep scans with our system. This analysis was initially made with two samples that present a first and a second order magnetic transition, considered as standard samples at MCE research area: Gadolinium and the Gd5Ge2Si2 alloy. We discussed the differences found and defined the measurement protocol that we believe to be correct to the practical evaluation of the MCE. From this conclusion, we analyzed three other samples that present uncommon behavior and high magnetocaloric potential and discussed their differences. MCE hysteresis losses were experimentally obtained. In the second part, with the aid of a calorimeter built with Peltier devices capable of perform an 80° rotation under constant magnetic field up to 1,9 T, we made the study of the Anisotropic Magnetocaloric Effect (MCE-ani) in monocrystalline samples of the RAl2 family grown by the Czochralski method. First, we made specific heat and conventional MCE measurements with the ideal protocol that was defined in the first part of the work, using the PPMS Peltier system. From these data, we were able to calculate indirectly the MCE-ani by subtracting the acquired curves. Finally, we used the Peltier rotation system to perform direct measurements of the MCE-ani in DyAl2 single crystals. The results of the indirect and direct measurements were compared with calculations achieved using a self-consistent process / Doutorado / Física / Doutor em Ciências / 1060137/2011 / CAPES

<p style="text-align: justify;">Kroz istoriju, otkrivanje novih materijala i njihovog dizajna dovodilo je do tehnolo&scaron;kih revolucija. U pro&scaron;lom veku, novi materijali naprednih svojstava uveli su elektronske uređaje u svakodnevni život čoveka. Industrija mikročipova predstavlja ogroman deo svetskog trži&scaron;ta, i traži neprestan razvoj da bi pružila bolje proizvode potro&scaron;ačima. Početkom ovog veka, nova grupa materijala, pod nazivom multiferoici, privukla je pažnju naučno-istraživačkog dru&scaron;tva u svetu. Ovi materijali poseduju jedinstvenu karakteristiku da istovremeno ispoljavaju vi&scaron;e od jedne feroične osobine (feroelektričnost, feromagnetizam, feroelastičnost), a &scaron;to je jo&scaron; važnije, mogu da ostvare interakciju među njima. Naime, magnetizacija multiferoika se može postići primenom spolja&scaron;njeg električnog polja, a takođe se mogu i polarisati primenom spolja&scaron;njeg magnetnog polja. Ovo vrlo interesantno svojstvo otvara potencijlanu primenu u oblasti hibridne računarske memorije, senzora, aktuatora, i dr. Sredinom pro&scaron;log veka, jednofazni multiferoici su prvi privukli pažnju, ali poseldnjih godina, kompozitni multiferoici su pokazali bolje rezultate u pogledu funkcionalnih karakteristika. Trend minijaturizacije je takođe prisutan u ovoj oblasti, stoga su multiferoični tanki filmovi vrlo atraktivni u istraživačkih krugovima ne samo zbog niske potro&scaron;nje električne energije, malog utro&scaron;ka meterijala i malih dimenzija, već i zbog dobre magnetoelektrične interakcije.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Glavni cilj ove doktorske disertacije je bio određivanje optimalnog procesa sinteze/depozicije, i vr&scaron;enje strukturne i funkcionalne karakterizacije multiferoičnih vi&scaron;eslojnih tankih filmova, sačinjenih od naizmenično deponovanih feroelektričnih BaTiO₃ i fero/ferimagnetnih NiFe₂O₄ slojeva (uglavnom na silicijumskim supstratima sa slojem platine). Različite strukture slojeva dizajnirane su u cilju određivanja optimalne, koja bi dala najvi&scaron;e vrednosti magnetoelektričnog efekta.&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; U prvom koraku, sintetisani su stabilni solovi/rastvori prekursora, veličine čestica od nekoliko nanometara, reolo&scaron;kih karakteristika pogodnih za depoziciju tehnikama iz tečne faze. Vi&scaron;eslojni filmovi su dobijeni &quot;spin&quot; procesom nano&scaron;enja, pri čemu je termički tretman svakog sloja na 500 &deg;C bio neophodan radi potpunog otparavanja zaostalog rastvarača. Filmovi bez pukotina, ukupne debljine ispod 1 &mu;m, uniformne debljine sloja (60 nm sloj BaTiO₃ i 40 nm sloj NiFe₂O₄) i ravne povr&scaron;ine mogu biti dobijeni sinterovanjem u temperaturnom opsegu od 750 do 900 &deg;C. Strukturna karakterizacija je potvrdila sistem bez prisustva sekundarnih faza, sačinjen od perovskitnog BaTiO₃ i spinelnog NiFe₂O₄. Dielektrična merenja su bila u saglasnosti sa mikrostrukturnom analizom, i vrednostima dielektrične konstante tipične za nanostrukturni sistem, niske vrednosti dielektričnih gubitaka i male provodljivosti. Uticaj međuslojne polarizacije, koja nalikuje Debajeovoj relaksaciji, izražena kroz povećanje dielektrične konstante uspod 100 kHz, bio je jači u nižoj frekventnoj regiji na povi&scaron;enim temperaturama usled termičke aktivacije nosilaca naelektrisanja u feritnoj fazi. Samo čist BaTiO₃ film je pokazao slab feroelektrični histerezis nepotpune saturacije, malo vi&scaron;e polarizacije filma sinterovanog na 900 &deg;C usled ogrubljivanja strukture. Meuđuslojni efekti su takođe primećeni kod feroelektričnih merenja na sobnoj temperaturi, sa izraženijim prisustvom kada se primeni jače električno polje. Na osnovu dielektričnih i feroelektričnih merenja, zaključeno je da film sa debljim titanatnim i tanjim feritnim slojevima ima najverovatnije najbolji dizajn slojeva. Magnetne histerezisne petlje su snimljene na sobnoj temperaturi za čiste NiFe₂O₄ filmove i vi&scaron;eslojne filmove. Analizom vi&scaron;eslojnih filmova različitog dizajna slojeva, pretpostavljeno je da zatezanje nastalo mehaničkom interakcijom između titanatnih i feritnih slojeva jeste prisutno, i da raste sa povećanjem broja kontaktnih povr&scaron;ina, stoga film sa tanjim titanatnim i feritnim slojevima verovatno predstavlja najbolji izbor sa aspekta megnetnih osobina.</p> / <p>Through history, discovery of new materials and material design have led to technological revolutions. In the last century, new materials with advanced properties have introduced electronic devices in our everyday lives. Microchip industry represents one huge part of world market, and needs constant development to provide better products to consumers. In the beginning of this century, a novel group of materials, called multiferroics, have attracted close attention of research society around the world. These materials have a unique property to express more than one ferroic property simultaneously (feroelectricity, ferromagneticity, ferroelasticity), and more important, to achieve coupling between them. Namely, magnetization of multieferroic may be changed with application of external electric field, and they can be polarized with application of the external magnetic field. This is a very interesting property that opens the potential applications in fields of hybrid computer memory, sensors, actuators, etc. In the middle of last century, single phase multiferroics were the first to trigger interest in this special property, but in recent years, composite multiferroics have shown more promising results in terms of functional properties. The trend of miniaturization is also present in this field, so multiferroic thin films are very attractive for research not only because of low power and material consumption or small size, but also because of strong magnetoelectric coupling.<br />The main goal of this thesis was to determine optimal synthesis/deposition process, and perform structural and functional characterization of multiferroic multilayer thin films, composed of ferroelectric BaTiO3 and ferro/ferrimagnetic NiFe2O4 layers in alternating order (mostly on platinum coated silicon substrates). Different layer structures were designed in order to find optimal one which could show the strongest magnetoelectric effect.<br />In the first step, stable precursor sols/solutions were synthesized, with particle size of a few nanometes, and rheological properties suitable for solution deposition. The multilayered thin films were obtained by spin coating and thermal treatment of each layer on 500 &deg;C was necessary in order to completely evaporate traces of residual solvents. Crack free films with overall thickness below 1 &mu;m, uniform single layer thickness (60 nm of BaTiO3 layer and 40 nm of NiFe2O4) and flat surface can be obtained by sintering in temperature range from 750 to 900 &deg;C. Structural characterization confirmed that secondary phase free system with microstructure on nanometer scale was obtained, composed of perovskite BaTiO3 and spinel NiFe2O4. Dielectric measurements were in agreement with microstructural characterization, showing the values of dielectric constant typical for nanostructured system, low values of dielectric losses and low conductivity. The influence of interfacial polarization, resembling Debye behavior, expressed as a rise of dielectric constant below 100 kHz, was stronger in lower frequency range on higher temperatures due to thermal activation of mobile charge carriers in ferrite phase. Only the pure BaTiO3 films showed weak unsaturated ferroelectric hysteresis loops, with slightly higher polarization of films sintered on 900 &deg;C due to coarsening of the structure. The interface effects were also detected in ferroelectric measurements on room temperatures, showing increased presence when higher field is applied. Regarding dielectric and ferroelectric characterization, it was concluded that the multilayered films with thick titanate and thin ferrite layers may probably have the most promising layer design. Magnetic hysteresis loops were recorded on room temperatures for the pure NiFe2O4 and multilayers films. By analysis of different layer design of multilayers, it was assumed that mechanical straining between the ferrite and titanate layers may be present, and increases with the number of contact surfaces, thus the films with thinner titanate and ferrite layers may probably have the best layer design from aspect of magnetic properties.</p>