Neste trabalho de tese é apresentado um estudo sistemático do crescimento e das propriedades magnéticas e estruturais dos compostos semicondutores magnéticos diluídos (SMD) Zn1-xMnxIn2Se4. Amostras monocristalinas com concentrações de Mn $0leqslant xleqslant 1.00$ foram obtidas pela técnica de transporte químico em fase vapor (CVT). Através de técnicas de difração de raios-X (padrões de pó e diagramas de Laue), estudou-se a evolução da estrutura desde a fase romboédrica do MnIn2Se4, até a fase tetraédrica do ZnIn2Se4. Observou-se a existência de uma fase pura romboédrica para $xgeqslant 0.87$, uma fase pura tetragonal para$xleqslant 0.25$ e uma mistura destas duas fases para x entre 0.67 e 0.35. Espectros de ressonância paramagnética eletrônica (EPR) foram obtidos em temperaturas T desde 4 K até 300 K, para diversas concentrações de Mn. Todos os espectros têm o pico de absorção principal centrado em g=2. A largura pico a pico da linha de ressonância, $deltaHpp$, decresce com o aumento da concentração de manganês e com o aumento da temperatura. Este comportamento está intimamente relacionado com a interação de troca entre os íons magnéticos. A partir de análises da dependência de $deltaHpp$ com a temperatura se extraíram estimativas para a temperatura de Curie-Weiss, $ heta$ e para a temperatura de congelamento Tf das fases de vidro de spin. Medidas da susceptibilidade magnética em modo DC foram realizadas para temperaturas $2Kleqslant Tleqslant 300K$. Dos dados de alta temperatura (T>100 K), resultaram estimativas para o parâmetro $ heta$, bem como para a concentração x de Mn. Os valores de $ heta$ obtidos destas medidas concordam bem com os obtidos de EPR e foram utilizados para estimar as constantes de troca antiferromagnéticas nos dois extremos de concentração da série. Nas amostras com alta concentração de Mn, $xgeqslant 0.67$, irreversibilidades características de uma transição para uma fase de vidro de spin foram observadas em temperaturas abaixo de 4K. Para melhor caracterizar estas transições, medidas da susceptibilidade AC e DC para diversas freqüências e campos magnéticos foram realizadas em temperaturas entre 2 e 20 K. A susceptibilidade DC em baixos campos mostra picos agudos em Tf ~ 2.5, 3.0 e 3.5 K, respectivamente para as amostras com x=0,67; 0,87 e 1,00. Nas três amostras, observa-se irreversibilidade entre medidas com esfriamento na ausência de campo (ZFC) e em presença de campo (FC). Evidência de um verdadeiro fenômeno de transição de fase é fornecida pelo crescimento da susceptibilidade não linear, $chiNL$, próximo de Tf.. Análises de escala estáticas de $chiNL$ e análises de escala dinâmicas desenvolvidas a partir de dados da susceptibilidade AC resultaram em expoentes críticos consistentes com os obtidos em outros vidros de spin com interação de curto alcance. / This work reports a systematic study on the crystal growth and magnetic and structural properties of the diluted magnetic semiconductors (SMD) Zn1-xMnxIn2Se4. Crystals with Mn concentrations $0leqslant xleqslant 1.00$ were grown by chemical vapour transport (CVT). Through x-ray powder diffraction patterns and Laue diagrams of single crystal we studied the transformation from the layered rhombohedral structure of MnIn2Se4 to the tetragonal structure of ZnIn2Se4. We observe the presence of a purely rhombohedral phase for $xgeqslant 0.87$, a purely tetragonal phase for $xleqslant 0.25$ and a two-phase mixture for x between 0.67 and 0.35. Electron Paramagnetic Resonance (EPR) spectroscopy has been performed in the temperature range 4.2 K to 300 K for various concentrations of Mn. All the spectra had the main absorption peak centered at g=2. The resonance peak-to-peak line width, ÄHpp, decreases with increasing concentration of manganese and with increasing temperature. This behavior is intimately related with the exchange interaction between magnetic ions. The analysis of the temperature dependence of ÄHpp resulted in estimates for the Curie-Weiss temperature $ heta$ and for the freezing temperature Tf of the spin glass phase. The DC magnetic susceptibility has been investigated in the range between $2Kleqslant Tleqslant 300K$. The high temperature data (T>100 K) gave estimates for the parameter $ heta$, as well as for the concentration x of Mn. The values of $ heta$ obtained in this way are in good agreement with those obtained from EPR and were used to estimate the antiferromagnetic exchange constants for the two extremes of concentration of the system. For the samples with high concentration of Mn, $xgeqslant 0.67$, irreversibilities characteristic of a spin glass transition have been observed at temperatures below 4K. To better characterize this transition, AC and DC susceptibility measurements for different frequencies and magnetic fields were made in the temperature range between 2 and 20 K. The low field AC susceptibility displayed sharp peaks at Tf ~2.5, 3.0 and 3.5 K for samples with x= 0.67, 0.87 and 1.00, respectively. Irreversibility between zero field cooled (ZFC) and field cooled (FC) DC measurements was observed in the three samples. Evidence of a true phase transition phenomenon is given by the steep increase of the nonlinear susceptibility $chiNL$ when approaching Tf from above. Static scaling analysis of $chiNL$ and dynamic scaling analysis of the AC susceptibility data have been carried out, which yielded critical exponents consistent with those obtained in other spin-glasses with short-range interactions.

Neste trabalho de tese é apresentado um estudo sistemático do crescimento e das propriedades magnéticas e estruturais dos compostos semicondutores magnéticos diluídos (SMD) Zn1-xMnxIn2Se4. Amostras monocristalinas com concentrações de Mn $0leqslant xleqslant 1.00$ foram obtidas pela técnica de transporte químico em fase vapor (CVT). Através de técnicas de difração de raios-X (padrões de pó e diagramas de Laue), estudou-se a evolução da estrutura desde a fase romboédrica do MnIn2Se4, até a fase tetraédrica do ZnIn2Se4. Observou-se a existência de uma fase pura romboédrica para $xgeqslant 0.87$, uma fase pura tetragonal para$xleqslant 0.25$ e uma mistura destas duas fases para x entre 0.67 e 0.35. Espectros de ressonância paramagnética eletrônica (EPR) foram obtidos em temperaturas T desde 4 K até 300 K, para diversas concentrações de Mn. Todos os espectros têm o pico de absorção principal centrado em g=2. A largura pico a pico da linha de ressonância, $deltaHpp$, decresce com o aumento da concentração de manganês e com o aumento da temperatura. Este comportamento está intimamente relacionado com a interação de troca entre os íons magnéticos. A partir de análises da dependência de $deltaHpp$ com a temperatura se extraíram estimativas para a temperatura de Curie-Weiss, $ heta$ e para a temperatura de congelamento Tf das fases de vidro de spin. Medidas da susceptibilidade magnética em modo DC foram realizadas para temperaturas $2Kleqslant Tleqslant 300K$. Dos dados de alta temperatura (T>100 K), resultaram estimativas para o parâmetro $ heta$, bem como para a concentração x de Mn. Os valores de $ heta$ obtidos destas medidas concordam bem com os obtidos de EPR e foram utilizados para estimar as constantes de troca antiferromagnéticas nos dois extremos de concentração da série. Nas amostras com alta concentração de Mn, $xgeqslant 0.67$, irreversibilidades características de uma transição para uma fase de vidro de spin foram observadas em temperaturas abaixo de 4K. Para melhor caracterizar estas transições, medidas da susceptibilidade AC e DC para diversas freqüências e campos magnéticos foram realizadas em temperaturas entre 2 e 20 K. A susceptibilidade DC em baixos campos mostra picos agudos em Tf ~ 2.5, 3.0 e 3.5 K, respectivamente para as amostras com x=0,67; 0,87 e 1,00. Nas três amostras, observa-se irreversibilidade entre medidas com esfriamento na ausência de campo (ZFC) e em presença de campo (FC). Evidência de um verdadeiro fenômeno de transição de fase é fornecida pelo crescimento da susceptibilidade não linear, $chiNL$, próximo de Tf.. Análises de escala estáticas de $chiNL$ e análises de escala dinâmicas desenvolvidas a partir de dados da susceptibilidade AC resultaram em expoentes críticos consistentes com os obtidos em outros vidros de spin com interação de curto alcance. / This work reports a systematic study on the crystal growth and magnetic and structural properties of the diluted magnetic semiconductors (SMD) Zn1-xMnxIn2Se4. Crystals with Mn concentrations $0leqslant xleqslant 1.00$ were grown by chemical vapour transport (CVT). Through x-ray powder diffraction patterns and Laue diagrams of single crystal we studied the transformation from the layered rhombohedral structure of MnIn2Se4 to the tetragonal structure of ZnIn2Se4. We observe the presence of a purely rhombohedral phase for $xgeqslant 0.87$, a purely tetragonal phase for $xleqslant 0.25$ and a two-phase mixture for x between 0.67 and 0.35. Electron Paramagnetic Resonance (EPR) spectroscopy has been performed in the temperature range 4.2 K to 300 K for various concentrations of Mn. All the spectra had the main absorption peak centered at g=2. The resonance peak-to-peak line width, ÄHpp, decreases with increasing concentration of manganese and with increasing temperature. This behavior is intimately related with the exchange interaction between magnetic ions. The analysis of the temperature dependence of ÄHpp resulted in estimates for the Curie-Weiss temperature $ heta$ and for the freezing temperature Tf of the spin glass phase. The DC magnetic susceptibility has been investigated in the range between $2Kleqslant Tleqslant 300K$. The high temperature data (T>100 K) gave estimates for the parameter $ heta$, as well as for the concentration x of Mn. The values of $ heta$ obtained in this way are in good agreement with those obtained from EPR and were used to estimate the antiferromagnetic exchange constants for the two extremes of concentration of the system. For the samples with high concentration of Mn, $xgeqslant 0.67$, irreversibilities characteristic of a spin glass transition have been observed at temperatures below 4K. To better characterize this transition, AC and DC susceptibility measurements for different frequencies and magnetic fields were made in the temperature range between 2 and 20 K. The low field AC susceptibility displayed sharp peaks at Tf ~2.5, 3.0 and 3.5 K for samples with x= 0.67, 0.87 and 1.00, respectively. Irreversibility between zero field cooled (ZFC) and field cooled (FC) DC measurements was observed in the three samples. Evidence of a true phase transition phenomenon is given by the steep increase of the nonlinear susceptibility $chiNL$ when approaching Tf from above. Static scaling analysis of $chiNL$ and dynamic scaling analysis of the AC susceptibility data have been carried out, which yielded critical exponents consistent with those obtained in other spin-glasses with short-range interactions.

Au cours de cette thèse, nous avons développé la réaction d'hydroamination d'arylacétylènes en présence de quelques amines secondaires aliphatiques. Dans un premier temps, nous avons pu montrer que cette réaction peut être catalysée par différents sels de cuivres. L'utilisation catalytique de CuCN a permis la formation régio- et stéréosélective d'énamines issues d'addition d'orientation de type anti-Markovnikov d'isomèrie (E). Les conditions développées n'ont pas permis la purification des énamines observées. Pour cette raison une réduction en présence de l'agent réducteur NaBH3CN a été effectuée afin de pouvoir isoler les amines correspondantes. L'utilisation catalytique de CuCl a permis à son tour, dans certaines conditions, la synthèse régio-et stéréosélective de 1,3-diènes (1E,3E)-1,4-disubstitués. La nature des électroniques des substituants des noyaux aromatiques des alcynes employés a joué un rôle majeur en ce qui concerne la chimiosélectivité de la réaction. Dans un second temps, nous nous sommes concentrés sur l'amélioration des conditions développées et la recherche d'autres moyens plus efficaces, moins coûteux et plus verts, nous avons été en mesure de montrer que la réaction étudiée peut également s'effectuer uniquement en présence d'éthylène glycol employé en tant que solvant et promoteur de la réaction. Cette méthode permet l'accès direct aux énamines issues de l'addition d'orientation de type anti-Markovnikov d'isomérie (E) avec d'excellents rendements isolés sans qu'il soit nécessaire de purifier les énamines obtenues. / In this thesis, we developed the hydroamiantion arylacétylènes reaction in the presence of some aliphatic secondary amines. At first, we could show that this reaction can be catalyzed by various copper salts. The catalytic use of CuCN allowed the regional training and sétéréosélective enamines derived from anti-Markovnikov addition type orientation isomerism (E). Developed conditions have not allowed the purification of the observed enamines. For this reason a reduction in the presence of the reducing agent NaBH3CN was performed in order to isolate the corresponding amines. The catalytic use of CuCl enabled in turn, under certain conditions, the regio-and stereoselective synthesis of 1,3-dienes (1E, 3E) -1,4-disubstituted oxanilides. The electronic nature of substituents of the aromatic rings alkynes employed played a major role as regards the chemoselectivity of the reaction.Secondly, we concentrate developed to improve conditions and find other more efficient ways, cheaper and greener, we were able to show that the test reaction can also be carried out only in the presence ethylene glycol used as solvent and the reaction promoter. This method allows direct access to enamines from the addition anti-Markovnikov orientation type of isomerism (E) with excellent isolated yields without the need to purify the resulting enamines.

A l’échelle micro-nanométrique, la fiabilité et la durée de vie des microsystèmes (MEMS),généralement réalisés en silicium, sont fortement affectées par les effets de frottement, d’adhesion,d’usure... L’objectif de ce travail est d’étudier les mécanismes de frottement et d’usure sur deswafers de silicium. Le comportement micro-nanotribologique de monocouches auto-assemblées(SAMs) d’alkyltrichlorosilane, déposées sur des wafers de silicium de différentes orientationscristallographiques – i.e, Si(100), Si(111) et Si(110), a été étudié à l’aide d’un nanotribomètre. Lesparamètres modifiés au cours de l’étude sont les suivants : la longueur de la chaîne, les paramètrestribologiques (charge normale, vitesse de glissement, distance de glissement, taux d’humidité relativeet température du substrat) et les propriétés de surface du silicium (orientation cristallographique,topographie). Les résultats expérimentaux montrent que le comportement nanotribologique desmonocouches greffées sur des substrat polis est influencé par l’homogénéité du film et la fractiond’aire qu’il couvre. Ces deux paramètres étant contrôlés par le temps d’immersion et l’orientationcristallographique du substrat. La topographie du silicium a également été modifiée de manièreà créer des motifs périodiques (microstructure par DRIE). Le comportement tribologique de cessurfaces revêtues d’OTS est contrôlé par les variations des propriétés physico-chimiques dessurfaces et la fragilité de la microstructure. Pour s’affranchir des problèmes de fragilité, des motifsstructurés à l’échelle nanométrique sont réalisés par nano-impression. / At micro and nanoscale, fiability and durability of micromechanical devices (MEMS), usuallymanufactured of silicon, are strongly affected by the friction effects, adhesion, wear... The aim ofthis work is to study the mechanisms of friction and wear of silicon wafers. Micro/nanotribologicalstudy of self-assembled monolayers (SAMs) derived from n-alkyltrichlorosilanes deposited on siliconwafers displaying various crystallographic orientations – i.e, Si (100), Si (111) and Si (110) – hasbeen conducted using a nanotribometer (ball-on-disc). The parameters that have been varied are: the alkyl chain length, the tribological parameters (normal load, sliding velocity, sliding distance,relative humidity level and substrat’s temperature) and surface characteristics of the silicon substrates(crystallographic orientation, roughness). On smooth silicon substrats, experimental results show thatthe tribological behaviour of SAMs is control by the film’s homogeneity and the surface coverageof the monolayer in connection with the time immersion and the crystallographic orientation of thesubstrate. The topography of silicon was also modified by changing the microstructure by DRIE inorder to create periodic patterns. The tribological behavior of OTS SAM grafted on microstructuredsurfaces was controlled by the changes in physico-chemical properties and the fragility of thepatterns. To overcome the problems of fragility of these surfaces, patterns at the nanoscale areachieved by nanoimprint.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / In the last decades, several applications of niobium nitride thin films has been proposed or effectively implemented. In the cubic δ − NbN phase, the bulk material presents a Tc for the superconducting transition near 17 K, what is far larger than those found in other normal (BCS) superconductors and useful in, for example, Josephson tunnel junctions. More recently, other phases have been also focus of interest, like the hexagonal δ-NbN phase. The hardness and resistance to chemical corrosion make this material well fitted for mechanically improved surface. Thin film preparation or deposition of niobium nitrides by physical methods (PVD) is not a trivial task. Stoichiometry, crystal structure and morphology of the resulting films are strongly affected by the deposition conditions, and even a qualitative model for the growth mechanisms of niobioum nitride is still lacking. In this work we have studied the effect of some parameters on the structural and morphologic properties of NbN thin films. The samples have been produced by reactive magnetron sputtering for different nitrogen partial pressures, substrate temperatures, bias voltages and deposition times. The crystallographic structure, preferred orientations, grain sizes and surface roughness were stablished by XR diffraction and, for some samples, atomic force microscopy. The results have shown that without bias voltage cubic NbN thin films are obtained, with or without substrate heating, when the partial pressure of N2 in the reactive atmosphere is between 13 and 25 %. Films produced with 17 % N2 are preferentially oriented in the <200> direction and this texture is enhanced by substrate heating. The analysis of the results in two samples with different thickness clearly indicates that for cubib NbN, the growth is remarkable different in the <111> and <200> directions. A possible mechanism to explain this difference is presented. The main effect of the voltage bias was to induce a hexagonal δ - NbN structure even for voltages as low as -10 V. These films present larger densities values than those found in the films with cubic phase, being the highest density achieved with -70V bias. All samples deposited with bias present a compressive stress and small grain size. The connections between stress, grain size and density are presented and discussed. In summary, we have identified a group of key parameters that makes possible the deposition of NbN thin films by reactive magnetron sputtering, either for superconductivity or tribological applications. / Nas últimas décadas, têm sido propostas e implementadas muitas aplicações para filmes finos de nitreto de nióbio. Na fase cúbica δ − NbN, o material na sua forma bulk apresenta Tc de transição supercondutora próxima a 17 K, o qual é de longe muito maior do que os valores encontrados para outros supercondutores normais (BCS). E, proveitoso, por exemplo, para junções túnel Josephson. Mais recentemente, outras fases também têm sido foco de interesse, como a hexagonal δ0 − NbN. A dureza e resistência à corrosão química fazem deste material bem equipado para melhoramento mecânico de superfícies. A preparação de filmes finos de nitretos de nióbio por PVD não é uma tarefa trivial. Estequiometria, estrutura cristalina e morfologia dos filmes resultantes são fortemente afetadas pelas condições de deposição. E, mesmo um modelo qualitativo para os mecanismos de crescimento do nitreto de nióbio ainda está faltando. Neste trabalho estudamos o efeito de alguns parâmetros sobre as propriedades estruturais e morfológicas de filmes finos de NbN. As amostras foram produzidas por magnetron sputtering reativo com diferentes pressões parciais de nitrogênio, temperaturas do substrato, voltagem bias e tempos de deposição. Os resultados mostraram que sem bias aplicado são obtidos filmes finos de NbN na fase cúbica, com ou sem aquecimento do substrato, quando a pressão parcial de N2 na atmosfera reativa está entre 13 e 25%. Filmes produzidos com 17% de N2 estão preferencialmente orientados na direção (200) e sua textura é aumentada por aquecimento do substrato. A análise dos resultados em duas amostras com diferentes espessuras indicou claramente que, para NbN cúbico, o crescimento é notavelmente diferente nas direções (111) e (200). Um possível mecanismo capaz de explicar esta diferença é proposto neste trabalho. O efeito significativo da aplicação do bias foi induzir a estrutura hexagonal δ − NbN mesmo para voltagens pequenas como -10 V. Estes filmes apresentam valores de densidade maiores do que para os filmes com fase cúbica, sendo a maior densidade alcançada para -70 V de bias. Todas amostras depositadas com aplicação de bias apresentaram estresse compressivo e tamanho de grãos pequeno. As conexões entre estresse, tamanho de grão e densidade são apresentados e discutidos. Em resumo, identificamos um grupo de parâmetros chave que tornam possível a deposição de filmes finos de NbN por magnetron sputtering reativo, seja para supercondutividade seja para aplicações tribológicas.

Au laboratoire, une famille de tyrosine kinases propres aux bactéries et ne présentant aucune ressemblance structurale avec les protéine-kinases d’origine eucaryote a été identifiée. Ces enzymes, appelées BY-kinases, sont notamment impliquées dans la biosynthèse des polysaccharides extracellulaires, mais leurs rôles précis ainsi que leurs mécanismes catalytiques sont encore peu compris.Dans la première partie de ce travail, nous avons caractérisé le rôle physiologique de la phosphorylation sur la tyrosine de la protéine Ugd, une UDP-glucose déshydrogénase, par les BY-kinases Wzc et Etk d’E. coli. Nous avons démontré que la phosphorylation d’Ugd sur un site commun à Wzc et Etk augmente son activité. Nous avons également établi que la phosphorylation d’Ugd par Wzc participe à la régulation de la quantité d’acide colanique produit, tandis que la phosphorylation d’Ugd par Etk influence la résistance de la bactérie à la polymyxine.Nous avons également effectué une analyse structure-fonction du domaine cytoplasmique de deux BY-kinases, CapA1/CapB2 de S. aureus et Wzc d’E. coli. Nous avons montré que ces deux protéines s’associent en octamère, grâce au motif EX2RX2R et qu’elle s’autophosphoryle selon un mécanisme intermoléculaire. Nous avons, de plus, identifié le mécanisme d’activation de ces protéines et révélé l’importance d’un domaine particulier dans l’autophosphorylation de Wzc et la biosynthèse de l’acide colanique.La caractérisation structurale et fonctionnelle des BY-kinases représente une approche prometteuse et originale en vue de l’élaboration de molécules inhibant spécifiquement leur activité et pouvant affecter le pouvoir virulent des bactéries pathogènes. / A new class of bacterial enzymes, named BY-kinases, has been shown to catalyze protein-tyrosine phosphorylation. These enzymes share no structural and functional similarities with their eukaryotic counterparts. Evidence of their involvement in extracellular polysaccharide biosynthesis has been provided, but their accurate functions and their catalytic mechanism remain largely unknown.First, we characterized the physiological role of tyrosine phosphorylation of Ugd, a UDP-glucose dehydrogenase, by the BY-kinases Wzc and Etk of E. coli. We demonstrated that Ugd phosphorylation by Wzc or Etk occurs on the same site and increases its activity. We also established that Wzc-mediated phosphorylation of Ugd participates in the regulation of colanic acid production whereas Ugd phosphorylation by Etk influences resistance to polymyxin.In addition, we performed a structure-function analysis of the cytoplasmic domain of two BY-kinases, namely CapA1/CapB2 from S. aureus and Wzc from E. coli. We showed that these two proteins associate in a ring-shaped octamer in which the motif EX2RX2R plays a crucial role. In addition, we showed that BY-kinases autophosphorylate using an intermolecular mechanism. We also identified the activation mechanism of BY-kinases and we revealed the role of a particular domain, found specifically in BY-kinases from proteobacteria, in Wzc autophosphorylation and colanic acid biosynthesis.Structural and functional characterization of BY-kinases represents an original and promising approach in order to develop new molecules inhibiting specifically these enzymes and to affect the virulence of bacterial pathogens.

Cette tbese comprend une étude sur les composés AB2O6 qui ont attiré un large intérêt ces dernieres années grâce à leurs propriétés de magnétisme de basse dimensionalité. Pour mieux étudier les propriétes magnétiques et les parametres qui les influencent oous oous sommes appuyé sur une bonne connaissance de la stmcture cristalline. Tous les écbantillons ont été étudié par diffraction de rayons X (pression ambiante ou sous hautes pressions) et neutroDique (basse et/ou baute température). Nous avons également effectué d'autres caractérisations, telles que mesures de susceptibilité, de cbaleur spécifique et spectroscopie Mossbauer. Nous avons produit la série MnNb2_6 V 80 6 afio de faire varier les distances intercbaines magnétiques et donc la nature des interactions d'échanges. Ces composés sont intéressants pour préseoter une faible anisotropie et un couplage des moments magnétiques dans les chaines en zig-zag le long de l'axe c, ce qui leur confere un caractere unidimensionnel et un couplage antiferromagnétique entre les chames. Nous avons fait une comparaison avec le modele théorique de la chaine d'Heisenberg classique et obtenu des constantes d'échange intra et intercbatnes, J et J', et sa dépendance par rapport à la teneur 8. Ensuite, nous avons fait une étude des séries MnxAt-xNb206 avec A = Fe, Co et Ni. Notre but a été de vérifier comment la nature et la différence de rayons atomiques peuvent influer sur le comportement structural et magnétique de ces composés. Pour ceux qui n'ont pas présenté de mise en ordre magnétique à 1,5 K nous avons fait une étude des corrélations à courtes distances en utilisant le modele de Bertaut La mise en reuvre de synthese HP et HT nous a permis de stabiliser une phase Mn V 20 6 en symétrie orthorhombique et de mettre en jour ses propriétés physiques. Nous avons ensuite comparé avec le MnV 20 6 monoclinique et également avec le MnNb20 6 orthorhombique. Les comportements magnétiques des composés isostructuraux sont tres similaires mais dans le cas de MnNb20 6, les moments magnétiques forment des chaí'nes du type + - + - et dans la pbase Mo V 20 6 nous observons des cbalnes du type + + - -, stmct.ure inédite dans ces composés. / Esta tese compreende um estudo sobre os compostos AB2O6 que chamaram a atenção nos últimos anos graças às suas propriedades de magnetismo de baixa dimensionalidacle. Para melhor entender as propriedades magnéticas e os parâmetros que as influenciam, nos apoiamos no conhecimento da estrutura cristalina. Todas as amostras foram estudadas por difração de raios X (pressão ambiente e sob altas pressões) e de nêutrons (em baixa e em alta temperatura). Também fizemos outras caractetizações como medidas ele susceptibilidade magnética, calorespecífico e espectroscopia Mossbauer. A sé ti e MnNb2_6 V 60 6 foi produzida a fim de variar as distâncias entre as cadeias magnéticas e, com isso, a natureza das interações de troca. Estes compostos são interessantes por apresentarem baixa anisotropia, um acoplamento dos momentos magnéticos nas cadeias em zig-zag ao longo elo eixo c, conferindo-lhe o caráter unidimensional, e ainda um acoplamento antiferromagnético mais fraco entre as cadeias. É feita uma comparação com o modelo teórico de cadeia de Heisenberg clássica e são obtidas as constantes ele troca intra e intercadeias, J e J', e sua dependência em relação à ó. Em seguida, foi feito um estudo das séries MnxAt-xNb20 6 com A = Fe, Co e Ni, com o objetivo de verificar como a natureza e a diferença dos raios atômicos podem influenciar o comportamento estrutural e magnético destes compostos. Para aqueles que não apresentaram ordem magnética à l ,5 K, foi feito um estudo das conelações ele curta distância utilizando o modelo de Bertaut. Graças a síntese AP e AT conseguimos estabilizar o MnV 20 6 em simetria ortorrômbica e determinar suas propriedades físicas. Fizemos então uma comparação com o polimorfo monoclínico Mn V 206 e também com o composto ortonômbico MnNb20 6 . Os comportamentos magnéticos macroscópicos dos compostos isoestruturais são muito similares, porém no caso ela fase MnNb20 6, os momentos magnéticos formam cadeias elo tipo +- + - e na fase Mn V 20 6, as cadeias são do tipo + + - -, estrutura inédita, até então não observada nestes compostos. / This thesis comprises a study of tbe compounds of AB20 6 type that have attracted wide interest in recent years due to tbeir low-dimensional magnetism properties. To better understand tbeir magnetic properties and tbe parameters lhat influence it, we have relied on a good knowledge of lhe crystal stmcture. Ali samples were studied by X-ray diffraction (ambient pressure or under hlgh pressure) and neutron diffraction (low and/or high temperature). We also performed other cbaracterizations, such as magnetic susceptibility, specific heat and Mbssbauer spectroscopy measurements. We have produced tbe MnNb2_6 V 60 6 serie in order to vary the distances between magnetic interchains and thus the nature of lhe excbange interactions. Tbese compounds are interesting due to tbe presence of Jow anisotropy and a coupling of magnetic moments in zigzag cbains along the c-axis, whlch gives lhem a one-dimensional character, and ao antiferromagnetic coupling between cbains. We made a comparison wilh the theoretical modei of the classical Heisenberg chain and obtained the exchange constants intra- and inter-cbain, J and J', and its dependency on the content b. Then we made a study o f MnxA1_.,Nb20 6 series with A = Fe, Co and Ni. Our aim was to check how the nature and tbe difference in atomic radii can influence the stmctural and magnetic properties of tbese compounds. For those whlcb sbowed no magnetic ordering at 1.5 K we made a study of correlations at short distances using Bertaut's model. Thanks to tbe use of HP and HT synthesis we managed to stabilize MnV20 6 in orthorbombic symetry and determine its physical properties. We then performed a comparison witb the monoclinic MnV20G and with tbe 011horbombic MnNb206 compound as well. Macroscopic magnetic behaviour o f isostructural compounds are ve1y similar but in the case of MnNb20 6, the magnetic moments form + - + - type chains while the Mn V 20 6 presents cbains of + + - - type, a structure never observed before in tbis family o f compounds.

Bien que l’amorçage de fissure joue un rôle important en fatigue, ses mécanismes ne sont pas encore pleinement compris. Des critères d'amorçage basés sur des mécanismes physiques de déformation plastique ont été proposés mais ne sont pas faciles à utiliser et à valider, car ils nécessitent des variables locales à l'échelle du grain. L'étude présente vise à établir un critère d'amorçage en fatigue oligocyclique, utilisable sous chargement d'amplitude variable.Le comportement mécanique de l'acier inoxydable AISI 316L étudié a été caractérisé en fatigue oligocyclique. Il a été modélisé par un schéma autocohérent utilisant une loi de plasticité cristalline basée sur l'évolution des densités de dislocations. L'endommagement de surface a été suivi pendant un essai de fatigue à l'aide d'un dispositif de microscopie optique in situ. Les fissures présentes après 2000 cycles ont été analysées et leurs caractéristiques cristallographiques calculées.Comme les grains de surface montrent une déformation plus importante à cause d'un moindre confinement par les grains voisins, il est nécessaire de définir une loi de localisation spécifique aux grains de surface. La forme proposée fait intervenir une variable d'accommodation intergranulaire, sur le modèle de la loi de localisation de Cailletaud-Pilvin. Elle a été identifiée à partir de simulations par éléments finis. L'état des contraintes et des déformations dans les grains de surface a alors été simulé. Des indicateurs d'amorçage potentiels ont ensuite été comparés sur une même base expérimentale. Deux indicateurs pertinents de l'endommagement en fatigue ont pu être obtenus. / Although crack initiation is proved to play an important role in fatigue, its mechanisms have not been fully understood. Some crack initiation criteria based on physical mechanisms of plastic deformation have been defined. However, these criteria are not easy to use and valid, as they need local variables at the grain scale. The present study aims at establishing a crack initiation criterion in low-cycle fatigue, which should be usable under variable amplitude loading conditions.Tension-compression fatigue tests were first carried out to characterize the mechanical behavior of the stainless steel AISI 316L studied. The mechanical behavior was simulated using a self-consistent model using a crystalline plastic law based on dislocation densities. The evolution of surface damage was observed during a fatigue test using an in situ optical microscopic device. Cracks were analyzed after 2000 cycles and their crystallographic characteristics calculated. As surface grains exhibit larger strain because they are less constraint by neighbor grains, a specific numerical frame is necessary to determine stress state in surface grains. A localization law specific to surface grains under cyclic loading was identified from finite element simulations. The proposed form needs an intergranular accommodation variable, on the pattern of the localization law of Cailletaud-Pilvin. Stress-strain state in surface grains was simulated. Potential indicators for crack initiation were then compared on a same experimental data base. Indicators based on the equivalent plastic strain were found to be suitable indicators of fatigue damage.

Cette tbese comprend une étude sur les composés AB2O6 qui ont attiré un large intérêt ces dernieres années grâce à leurs propriétés de magnétisme de basse dimensionalité. Pour mieux étudier les propriétes magnétiques et les parametres qui les influencent oous oous sommes appuyé sur une bonne connaissance de la stmcture cristalline. Tous les écbantillons ont été étudié par diffraction de rayons X (pression ambiante ou sous hautes pressions) et neutroDique (basse et/ou baute température). Nous avons également effectué d'autres caractérisations, telles que mesures de susceptibilité, de cbaleur spécifique et spectroscopie Mossbauer. Nous avons produit la série MnNb2_6 V 80 6 afio de faire varier les distances intercbaines magnétiques et donc la nature des interactions d'échanges. Ces composés sont intéressants pour préseoter une faible anisotropie et un couplage des moments magnétiques dans les chaines en zig-zag le long de l'axe c, ce qui leur confere un caractere unidimensionnel et un couplage antiferromagnétique entre les chames. Nous avons fait une comparaison avec le modele théorique de la chaine d'Heisenberg classique et obtenu des constantes d'échange intra et intercbatnes, J et J', et sa dépendance par rapport à la teneur 8. Ensuite, nous avons fait une étude des séries MnxAt-xNb206 avec A = Fe, Co et Ni. Notre but a été de vérifier comment la nature et la différence de rayons atomiques peuvent influer sur le comportement structural et magnétique de ces composés. Pour ceux qui n'ont pas présenté de mise en ordre magnétique à 1,5 K nous avons fait une étude des corrélations à courtes distances en utilisant le modele de Bertaut La mise en reuvre de synthese HP et HT nous a permis de stabiliser une phase Mn V 20 6 en symétrie orthorhombique et de mettre en jour ses propriétés physiques. Nous avons ensuite comparé avec le MnV 20 6 monoclinique et également avec le MnNb20 6 orthorhombique. Les comportements magnétiques des composés isostructuraux sont tres similaires mais dans le cas de MnNb20 6, les moments magnétiques forment des chaí'nes du type + - + - et dans la pbase Mo V 20 6 nous observons des cbalnes du type + + - -, stmct.ure inédite dans ces composés. / Esta tese compreende um estudo sobre os compostos AB2O6 que chamaram a atenção nos últimos anos graças às suas propriedades de magnetismo de baixa dimensionalidacle. Para melhor entender as propriedades magnéticas e os parâmetros que as influenciam, nos apoiamos no conhecimento da estrutura cristalina. Todas as amostras foram estudadas por difração de raios X (pressão ambiente e sob altas pressões) e de nêutrons (em baixa e em alta temperatura). Também fizemos outras caractetizações como medidas ele susceptibilidade magnética, calorespecífico e espectroscopia Mossbauer. A sé ti e MnNb2_6 V 60 6 foi produzida a fim de variar as distâncias entre as cadeias magnéticas e, com isso, a natureza das interações de troca. Estes compostos são interessantes por apresentarem baixa anisotropia, um acoplamento dos momentos magnéticos nas cadeias em zig-zag ao longo elo eixo c, conferindo-lhe o caráter unidimensional, e ainda um acoplamento antiferromagnético mais fraco entre as cadeias. É feita uma comparação com o modelo teórico de cadeia de Heisenberg clássica e são obtidas as constantes ele troca intra e intercadeias, J e J', e sua dependência em relação à ó. Em seguida, foi feito um estudo das séries MnxAt-xNb20 6 com A = Fe, Co e Ni, com o objetivo de verificar como a natureza e a diferença dos raios atômicos podem influenciar o comportamento estrutural e magnético destes compostos. Para aqueles que não apresentaram ordem magnética à l ,5 K, foi feito um estudo das conelações ele curta distância utilizando o modelo de Bertaut. Graças a síntese AP e AT conseguimos estabilizar o MnV 20 6 em simetria ortorrômbica e determinar suas propriedades físicas. Fizemos então uma comparação com o polimorfo monoclínico Mn V 206 e também com o composto ortonômbico MnNb20 6 . Os comportamentos magnéticos macroscópicos dos compostos isoestruturais são muito similares, porém no caso ela fase MnNb20 6, os momentos magnéticos formam cadeias elo tipo +- + - e na fase Mn V 20 6, as cadeias são do tipo + + - -, estrutura inédita, até então não observada nestes compostos. / This thesis comprises a study of tbe compounds of AB20 6 type that have attracted wide interest in recent years due to tbeir low-dimensional magnetism properties. To better understand tbeir magnetic properties and tbe parameters lhat influence it, we have relied on a good knowledge of lhe crystal stmcture. Ali samples were studied by X-ray diffraction (ambient pressure or under hlgh pressure) and neutron diffraction (low and/or high temperature). We also performed other cbaracterizations, such as magnetic susceptibility, specific heat and Mbssbauer spectroscopy measurements. We have produced tbe MnNb2_6 V 60 6 serie in order to vary the distances between magnetic interchains and thus the nature of lhe excbange interactions. Tbese compounds are interesting due to tbe presence of Jow anisotropy and a coupling of magnetic moments in zigzag cbains along the c-axis, whlch gives lhem a one-dimensional character, and ao antiferromagnetic coupling between cbains. We made a comparison wilh the theoretical modei of the classical Heisenberg chain and obtained the exchange constants intra- and inter-cbain, J and J', and its dependency on the content b. Then we made a study o f MnxA1_.,Nb20 6 series with A = Fe, Co and Ni. Our aim was to check how the nature and tbe difference in atomic radii can influence the stmctural and magnetic properties of tbese compounds. For those whlcb sbowed no magnetic ordering at 1.5 K we made a study of correlations at short distances using Bertaut's model. Thanks to tbe use of HP and HT synthesis we managed to stabilize MnV20 6 in orthorbombic symetry and determine its physical properties. We then performed a comparison witb the monoclinic MnV20G and with tbe 011horbombic MnNb206 compound as well. Macroscopic magnetic behaviour o f isostructural compounds are ve1y similar but in the case of MnNb20 6, the magnetic moments form + - + - type chains while the Mn V 20 6 presents cbains of + + - - type, a structure never observed before in tbis family o f compounds.

Notre étude à caractère pluridisciplinaire comprend l'élaboration de composés intermétalliques ainsi que la caractérisation de leurs propriétés tant structurales que magnétiques. Nos travaux ont porté sur des borures RT12B6 où R est un élément de terre rare ou l'yttrium et T un métal de transition 3d ainsi que des phases de Laves (Hf,Ta)Fe2. Pour appréhender les propriétés physiques de ces composés, nous avons mis en œuvre diverses variables externes (température, champ magnétique, pression) mais aussi internes telle que la pression chimique liée à la substitution d'un élément par un autre. Nous apportons une contribution à l'étude des propriétés magnétiques des composés RCo12B6. Les propriétés magnétiques de ces composés sont caractérisées à la fois par une température d'ordre qui varie peu avec l'élément de terre rare R et un moment magnétique de Co remarquablement faible. Nous montrons que les interactions d'échange R-Co sont de plus d'un ordre de grandeur plus faibles que les interactions Co-Co existant dans ces composés. La substitution du fer au cobalt dans les composés RCo12B6 est possible et donne lieu à une localisation préférentielle. Grâce à la spectroscopie Mössbauer et à la diffraction neutronique, nous avons démontré l'extrême sensibilité de l'orientation des moments magnétiques à la substitution Fe/Co. Le composé LaFe12B6 présente des propriétés magnétiques remarquables avec un état fondamental antiferromagnétique (AFM) et une transition vers un état ferromagnétique (FM) qui peut être induite par le champ appliqué ou par la température. A basse température la transition métamagnétique AFM-FM est accompagnée d'une hystérésis très large et est caractérisée par des sauts spectaculaires comme l'illustre nos mesures magnétiques, de magnétostriction ou de transport. La transition métamagnétique s'avère également fort sensible à la pression appliquée. Le composé intermétallique LaFe12B6 est caractérisé par une forte expansion thermique linéaire, un large effet magnétovolumique et présente à la fois des effets magnétocaloriques inverse et normal. L'effet de la substitution du cobalt ou du manganèse au fer ou du cérium au lanthane sur les propriétés structurales et magnétiques a été étudié de façon détaillée. La substitution Co/Fe ou Mn/Fe entraine dans les deux cas une forte augmentation du champ critique de la transition métamagnétique. Inversement la substitution Ce/La, quant à elle, réduit fortement le champ de transition. L'étude de l'alliage amorphe LaFe12B6, préparé par hypertrempe, montre des propriétés magnétiques radicalement différentes puisque la phase amorphe devient alors ferromagnétique avec une haute température de Curie. Enfin nous avons étudié les propriétés magnétiques intrinsèques du système intermétallique Hf1-xTaxFe2 pour lequel la solution solide est complète. L'analyse de l'ensemble des mesures a mis en lumière des comportements originaux du magnétisme du fer et ceci tant dans l'état ordonné que dans l'état paramagnétique. Le caractère inhabituel du magnétisme de ces composés est attribué au comportement d'électrons itinérants, lequel est à l'origine de la transition métamagnétique entre l'état AFM et l'état FM. / Our multidisciplinary study includes the synthesis of intermetallic compounds and the characterization of their structural and magnetic properties. Our work has focused on RT12B6 borides where R is a rare earth element or yttrium and T a 3d transition metal as well as (Hf, Ta)Fe2 Laves phases. In order to understand the physical properties of these compounds, we have implemented various external variables (temperature, magnetic field, pressure) as well as internal variables such as the chemical pressure due to the substitution of one element with another. Through this experimental work, we investigated the magnetic properties of RCo12B6 compounds. The magnetic properties of these compounds present both an ordering temperature which is quasi independent of the rare earth element R and a remarkably small magnetic moment of Co. We show that the R-Co exchange interactions are more than an order of magnitude smaller that the Co-Co occurring in these compounds. We demonstrated that the iron for cobalt substitution in RCo12B6 compounds gives rise to a preferential substitution scheme. Combining Mössbauer spectroscopy and neutron diffraction, we have found that the magnetic ordering direction is extremely sensitive to Fe/Co substitution. LaFe12B6 compound presents remarkable magnetic properties with an antiferromagnetic (AFM) ground state but it can be transformed into a ferromagnetic (FM) state by the applied magnetic field or by the temperature. At low temperature, the field-induced AFM-FM metamagnetic transition has a large hysteresis and exhibits ultra sharp jumps as shown in our magnetic, magnetostriction and transport measurements. The metamagnetic transition is also very sensitive to the applied pressure. LaFe12B6 intermetallic compound shows a large linear thermal expansion, a huge volume magnetostriction and both normal and inverse magnetocaloric effects. The effect of cobalt or manganese for iron substitution or cerium for lanthanum substitution on the structural and magnetic properties was deeply investigated. Co/Fe or Mn/Fe substitution in both cases leads to a strong increase of the critical field of the metamagnetic transition. However Ce/La substitution reduces strongly the transition field. The investigation of LaFe12B6 amorphous alloy, prepared by melt spinning, shows radically different magnetic properties since the amorphous phase becomes ferromagnetic with a high Curie temperature. Finally we studied the intrinsic magnetic properties of the Hf1-xTaxFe2 system for which the solid solution is complete. The analysis of all the measurements highlighted original behaviours of the iron magnetism and this both in the ordered state and in the paramagnetic state. These remarkable properties are attributed to the itinerant character of the Fe 3d band magnetism, which gives rise to the metamagnetic transition between the AFM and FM states.