Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films.

In this report, the operation of a normally-off vertical gallium nitride (GaN) metal-oxide field effect transistor with a threshold voltage of 5 V is demonstrated. A crucial step during device fabrication is the formation of the highly n-doped source layer. The authors infer that the use of molecular beam epitaxy (MBE) is highly beneficial for suppressing diffusion of the magnesium (Mg) p-type dopants from the body layer grown by metal-organic vapor phase epitaxy into the source cap. Repassivation of the previously activated Mg acceptors by a hydrogen out-diffusion treatment is suppressed in the ultrahigh vacuum growth environment. Structural and electrical data indicate that the defect density of the GaN substrate is currently limiting device performance much more compared to other effects like varying surface morphology resulting from fluctuations in III/N stoichiometry during the MBE growth.

A relação entre os grupos de tranças de superfícies e os grupos de homotopia das esferas é atualmente um tópico de bastante interesse. Nos últimos anos tem sido feitos avanços consideráveis no estudo desta relação no caso dos grupos de tranças de Artin com n cordas, denotado por Bn, da esfera e do plano projetivo. Nessa tese analisamos com detalhes as interações entre a teoria de tranças e a teoria de homotopia, e mostramos novos resultados que estabelecem conexões entre os grupos de homotopia da 2-esfera S2 e os grupos de tranças sobre qualquer superfície. No andamento deste trabalho, descobrimos uma conexão surpreendente dos grupos de tranças com os grupos cristalográficos e de Bieberbach: para n maior ou igual que 3, o grupo quociente Bn/[Pn, Pn] é um grupo cristalográfico que contém grupos de Bieberbach como subgrupos, onde Pn é o subgrupo de tranças puras de Bn. Com isto obtivemos uma formulação de um Teorema de Auslander e Kuranishi para 2-grupos finitos e exibimos variedades Riemannianas compactas planas que admitem difeomorfismo de Anosov e cujo grupo de holonomia é Z2k . Além disso, durante esta tese, detectamos e, quando possível, corrigimos algumas imprecisões em dois importantes artigos nessa área de estudo, escritos por J. Berrick, F. R. Cohen, Y. L. Wong e J. Wu (Jour. Amer. Math. Soc. - 2006) assim como por J. Y. Li e J.Wu (Proc. London Math. Soc. - 2009). / The relation between surface braid groups and homotopy groups of spheres is currently a subject of great interest. Considerable progress has been made in recent years in the study of these relations in the case of the n-string Artin braid groups, denoted by Bn, the sphere and the projective plane. In this thesis we analyse in detail the interactions between braid theory and homotopy theory, and we present new results that establish connections between the homotopy groups of the 2-sphere S2 and the braid groups of any surface. During the course of this work, we discovered an unexpected connection of braid groups with crystallographic and Bieberbach groups: for n greater or equal than 3, the quotient group Bn/[Pn, Pn] is a crystallographic group that contains Bieberbach groups as subgroups, where Pn is the pure braid subgroup of Bn. This enables us to obtain a formulation of a theorem of Auslander and Kuranishi for finite 2-groups, and to exhibit Riemannian compact flat manifolds that admit Anosov diffeomorphisms and whose holonomy group is Z2k. In addition, during the thesis, we have detected, and where possible, corrected some inaccuracies in two important papers in the area of study, by J. Berrick, F. R. Cohen, Y. L. Wong and J. Wu (Jour. Amer. Math. Soc. - 2006), and by J. Y. Li and J. Wu (Proc. London Math. Soc. - 2009).

Made available in DSpace on 2017-07-21T20:42:39Z (GMT). No. of bitstreams: 1 Ederson Pauletti.pdf: 5739200 bytes, checksum: 362611be295a3342251b72d168512c61 (MD5) Previous issue date: 2013-03-22 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work the ductile crack growth resistance curves (J-R curves) was evaluated for the steels used in the oil and gas pipelines from the X60 and X70 API 5L grades using the unloading compliance technique. The non standardized SE (T) specimens were used due to the same has low constraint, it is feature of pipelines. The plastic area for the obtaining the J integral was performed by measuring from the crack mouth open displacement, (CMOD). The central emphasis this research was evaluated the delamination phenomenon influence from the fracture toughness and along the J-R curve. As also the microtexture in close to delamination and ductile fracture regions, by using the Electron Backscatter Diffraction (EBSD) technique. The results have shown that value average of 355kJ/m2 was similar for both the shallow and deep crack specimens. The J-R curve values average from the shallow crack specimens have displayed most steep J-R curves, namely, it obtained higher energy values along the ductile crack propagation. Specimens that showed delaminations, during fracture process, obtained higher energy values along the ductile crack propagation. The occurrence of delaminations was more common in deep crack specimens. The microtexture results have shown a random grain sizes distribution. The components with higher intensity in the delamination region were α-fiber textures: {113} <110>, {112} <110>, {223} <110> the same cause anisotropy of mechanical properties and can be influenced out-of-plane constraint during the test. Moreover, there was the occurrence of the {100} <110> recrystallization texture component, which have deleterious effect in occurrence of delamination in microalloyed steels. Whereas in the ductile fracture region occurred the {111} <231>, {111} <132> and {111} <123> γ-fiber, which it can be influenced the ductile crack propagation process. / Neste trabalho foram investigadas as curvas de resistência ao crescimento de trinca dúctil para aços utilizados em tubulações de petróleo e gás, das classes API 5L X60 e X70, utilizando-se da técnica de variação da flexibilidade elástica. Espécimes SE(T) não padronizados foram utilizados, pois o mesmo possui baixa restrição ao fluxo plástico, característica de tubulações. A área plástica para obtenção da integral J foi obtida através da medida de deslocamento de abertura de trinca (crack mouth open displacement, CMOD). A ênfase central desta pesquisa foi avaliar a influência do fenômeno da delaminação na tenacidade à fratura (JIC) e na curva J-R em geral. Como também a obtenção da microtextura em regiões próximas à delaminação e fratura dúctil, que ocorrem durante o processo de fratura no ensaio de tenacidade à fratura, pelo uso da técnica de difração de elétrons retroespalhados (EBSD). Os resultados mostraram que o valor médio de tenacidade à fratura, de 355 kJ/m2 foi equivalente para espécimes com tamanho de trinca rasa e profunda. A média dos valores de curva J-R para espécimes com tamanho de trinca rasa mostrou curvas J-R mais íngremes, ou seja, obtiveram maiores valores energia ao longo da propagação dúctil de trinca. Os espécimes que apresentaram delaminações, durante o processo de fratura, obtiveram maior valor de energia ao longo do crescimento de trinca dúctil. A ocorrência das delaminações foi mais comum em espécimes com tamanho de trinca profunda. Os resultados de microtextura mostraram uma distribuição aleatória de tamanhos de grão. As componentes com maior intensidade na região próxima à delaminação foram as texturas de fibras α: {113}<110>, {112}<110>, {223}<110> que causam anisotropia das propriedades mecânicas e podem ter influenciado no estado triaxial de tensões durante o ensaio. Além disso, houve ocorrência da componente de textura de recristalização {100}<110>, a qual tem efeito deletério na ocorrência de delaminações em aços microligados. As componentes de fibra mais intensas na região próxima à fratura dúctil foram as texturas de fibra γ: {111}<231>, {111}<132> e {111}<123>, as quais podem influenciar no processo de propagação de trinca dúctil.

L’objectif fondamental de cette étude est de montrer la faisabilité de l’élaboration des couches épaisses de zircone non dopée, en contrôlant la microstructure et l’état mécanique, par MOCVD et par Sol-Gel. Dans un premier temps, nous avons essayé d’optimiser les conditions de dépôt de MOCVD, en faisant varier ou en jouant sur les différents paramètres du procédé, conduisant à l’obtention des couches de ZrO2 micrométriques et denses. La stabilité de la phase quadratique de la zircone est conditionnée par la pression partielle en oxygène, la température du substrat ainsi que l’épaisseur du dépôt. La texture cristallographique de type {100} est obtenue pour les dépôts réalisés à une température de substrat T ≤ 850°C et pour de faibles pressions totales. Concernant l’état mécanique des couches de zircone, l’augmentation de l’épaisseur de la couche peut relaxer les contraintes résiduelles de tension au sein du dépôt. Ce phénomène s’accentue au-delà d’une épaisseur critique suite à la création des espacements entre les colonnes de croissance de la couche. Parallèllement, nous avons montré que la qualité des dépôts Sol-Gel est maitrisée par le choix du substrat, l’utilisation de « sols » vieillis, la multiplication du nombre de couches « spin-coating », le mode de dépôt ainsi que la température de recuit. Certaines propriétés caractéristiques du dépôt telles que la cristallisation, la composition de phase et l’adhérence sont aisément contrôlées respectivement par l’âge du sol, la température de recuit et le coefficient de dilatation thermique associé au substrat utilisé. La microstructure (changement de phases, taille des cristallites, texture cristallographique) et les contraintes internes (thermiques et intrinsèques) ont été caractérisées. Le Sol-Gel présente l’avantage de proposer des couches de zircone très peu contraintes par rapport aux films obtenus par le procédé MOCVD. Quel que soit le procédé de dépôt, MOCVD et/ou Sol-Gel, l’élaboration des films de ZrO2 orientés demeure fonction de la température du traitement. La tentative d’élaborer des multicouches de zircone par un couplage MOCVD/Sol-Gel montre la possibilité de sélectionner des paramètres de dépôt propices à la fabrication d’un film présentant un état microstructural et mécanique contrôlé et voulu. / The fundamental purpose of this study is to demonstrate the feasibility to obtain an undoped zirconia thick film, by controlling the microstructure and mechanical state, using MOCVD and Sol-Gel technique. Firstly, we try to optimize the MOCVD deposition conditions, by varying the different process parameters, leading to the production of ZrO2 micrometric and dense films. The stability of the tetragonal zirconia phase depends on the oxygen partial pressure, the substrate temperature and the film thickness. The crystallographic texture of {100} type is obtained for the deposits obtained under a substrate temperature T ≤ 850°C and a low total pressure. Concerning mechanical state of the zirconia films, the thickness increasing can relax the tensile residual stress within the deposit. This phenomenon accents beyond a critical thickness due to the creation of columns spaces during film growth. In the second part, we show that the quality of the Sol-Gel deposition is controlled by substrate origin, use of aged sol, increase of “spin-coating” layers number, deposition mode and annealing temperature. Some deposit characteristics such as crystallization, phase composition and film adhesion are easily controlled by sol aging, annealing temperature and thermal expansion coefficient associated to the used substrate, respectively. The microstructure (phase change, crystalline size, crystallographic texture) and the internal stresses (thermal and residual) were characterized. The Sol-Gel technique has the advantage of providing zirconia films with low stress level compared to the films obtained by MOCVD. Regardless of the deposition process, MOCVD and / or Sol-Gel, the development of ZrO2 oriented films is in function of the treatment temperature. The attempt to get multilayer zirconia by coupling MOCVD/Sol-Gel methods shows the possibility to choose the deposition parameters in order to produce films with controlled and wanted microstructure and mechanical state.

A relação entre os grupos de tranças de superfícies e os grupos de homotopia das esferas é atualmente um tópico de bastante interesse. Nos últimos anos tem sido feitos avanços consideráveis no estudo desta relação no caso dos grupos de tranças de Artin com n cordas, denotado por Bn, da esfera e do plano projetivo. Nessa tese analisamos com detalhes as interações entre a teoria de tranças e a teoria de homotopia, e mostramos novos resultados que estabelecem conexões entre os grupos de homotopia da 2-esfera S2 e os grupos de tranças sobre qualquer superfície. No andamento deste trabalho, descobrimos uma conexão surpreendente dos grupos de tranças com os grupos cristalográficos e de Bieberbach: para n maior ou igual que 3, o grupo quociente Bn/[Pn, Pn] é um grupo cristalográfico que contém grupos de Bieberbach como subgrupos, onde Pn é o subgrupo de tranças puras de Bn. Com isto obtivemos uma formulação de um Teorema de Auslander e Kuranishi para 2-grupos finitos e exibimos variedades Riemannianas compactas planas que admitem difeomorfismo de Anosov e cujo grupo de holonomia é Z2k . Além disso, durante esta tese, detectamos e, quando possível, corrigimos algumas imprecisões em dois importantes artigos nessa área de estudo, escritos por J. Berrick, F. R. Cohen, Y. L. Wong e J. Wu (Jour. Amer. Math. Soc. - 2006) assim como por J. Y. Li e J.Wu (Proc. London Math. Soc. - 2009). / The relation between surface braid groups and homotopy groups of spheres is currently a subject of great interest. Considerable progress has been made in recent years in the study of these relations in the case of the n-string Artin braid groups, denoted by Bn, the sphere and the projective plane. In this thesis we analyse in detail the interactions between braid theory and homotopy theory, and we present new results that establish connections between the homotopy groups of the 2-sphere S2 and the braid groups of any surface. During the course of this work, we discovered an unexpected connection of braid groups with crystallographic and Bieberbach groups: for n greater or equal than 3, the quotient group Bn/[Pn, Pn] is a crystallographic group that contains Bieberbach groups as subgroups, where Pn is the pure braid subgroup of Bn. This enables us to obtain a formulation of a theorem of Auslander and Kuranishi for finite 2-groups, and to exhibit Riemannian compact flat manifolds that admit Anosov diffeomorphisms and whose holonomy group is Z2k. In addition, during the thesis, we have detected, and where possible, corrected some inaccuracies in two important papers in the area of study, by J. Berrick, F. R. Cohen, Y. L. Wong and J. Wu (Jour. Amer. Math. Soc. - 2006), and by J. Y. Li and J. Wu (Proc. London Math. Soc. - 2009).

Abstract This research is designed to investigate the occurrence of interdiffusion-induced mesopores, Brownian motion, cleaving and healing and defect clusters in three binary composites, i.e. Ni1-xO/Co1-xO, Ni1-xO/ZrO2 and Co1-xO/ZrO2 of the Ni1-xO-Co1-xO-ZrO2 system. Firstly, the (NimCo1-m)1-£_O/Ni-doped Co3-dO4 composites prepared by reactive sintering Ni1-xO and Co1-xO powders (1:2 molar ratio, denoted as N1C2) at 1000oC with or without further annealing at 720oC in air were studied by X-ray diffraction and electron microscopy to clarify the formation mechanism of mesoporous spinel precipitates. Submicron-sized inter- and intragranular pores, due to incomplete sintering and grain boundary detachment, prevails in (Ni0.33Co0.67)1-£_O protoxide with rock salt structure; whereas nanosize pores due to Kirkendall effect were restricted to the spinel precipitates having Ni component progressively expelled upon annealing. A rapid net vacancy flux and a tensile misfit stress perpendicular to the protoxide/spinel interface caused the formation of elongated and aligned {100}-faceted mesopores in the spinel precipitates with a relatively low equilibrium vacancy concentration. Aligned mesopores in diffusion zone of nonstoichiometric metal oxides have potential applications on thermal barrier bond coating and mass-transport limited heterogeneous catalysis. Also, this thesis deals with the reorientation and shape change of low-crystal-symmetry (non-cubic) ZrO2 within the high-crystal-symmetry grains of Co1-xO/Ni1-xO cubic rock salt-type structure. ZrO2/Co1-xO composites 1:99 and ZrO2/Ni1-xO composites 1:9 in molar ratio were sintered and then annealed at 1650oC for 24 and 100 h in air to induce reorientation of the embedded particles. Transmission electron microscopic observations in both systems indicated that the submicron tetragonal/monoclinic (t/m) ZrO2 particles fell into three topotaxial relationships with respect to the host Co1-xO/Ni1-xO grain: (1) parallel topotaxy, (2) ¡§eutectic¡¨ topotaxy i.e. [100]Z//[111]C,N, [010]Z//[0 1]C,N and (3) ¡§occasional¡¨ topotaxy [100]Z//[111]C,N, [01 ]Z//[0 1]C,N. The parallel topotaxy has a beneficial low energy for the family of {100}Z/C,N and {111}Z/C,N interfaces. The change from the occasional topotaxy to an energetically more favorable eutectic topotaxy was likely achieved by a rotation of the ZrO2 particles over a specific (100)Z/(111)C,N interface. Brownian-type rotation is probable for the embedded t-ZrO2 particles in terms of anchorage release at the interphase interface with the Co1-xO/Ni1-xO host. Detachment or bypassing of rock salt type grain boundaries could also cause orientation as well as shape changes of intergranular ZrO2 particles. Zirconia-polymorphism-induced cleaving and spontaneous healing by precipitation was studied in Co1-xO polycrystals containing a dispersion of ZrO2 particles. Conventional, analytical, and high-resolution transmission electron microscopy indicated that the Co1-xO matrix cleaves parallel to {100} and {110} planes and heals itself by co-precipitation of parallel-topotaxial ZrO2/Co3-£_O4 particles upon cooling. Due to size effect and matrix constraint, nanometer-size ZrO2 precipitates at cleavages were able to retain tetragonality upon further cooling to room temperature. Paracrystalline array of defect cluster was shown to form in Zr-doped Ni1-xO and Co1-xO polycrystals while prepared by sintering at relative high temperature, i.e., 1650oC to increase the defect concentration. Paracrystalline array of defect clusters in Co3-£_O4 spinel structure also occurred when doped with Zr4+ at high temperature or cooled below 900oC to activate oxy-precipitation of Co3-dO4 at dislocations. transmission electron microscopic observations indicated the spinel precipitate and its paracrystal predominantly formed at the ZrO2/Co1-xO interface and the cleavages/dislocations of the Co1-xO host. Defect chemistry consideration suggests the paracrystal is due to the assembly of charge- and volume-compensating defects of the 4:1 type with four octahedral vacant sites surrounding one Co3+-filled tetrahedral interstitial site. The spacing of paracrystalline distribution is 3.3, 2.9 and 4.9 times the lattice parameter for Zr-doped Ni1-xO, Zr-doped Co1-xO and Zr-doped Co3-dO4. This spacing between defect clusters is about 0.98 times that of the previously studied undoped Co3-dO4. There is much larger (3.4 times difference) paracrystalline spacing for Zr-doped Co3-£_O4 than its parent phase of Zr-doped Co1-xO.

Pipeline steels that carry oil and natural gas in severe environments suffer from two important modes of failure: stress corrosion cracking (SCC) and hydrogen-induced cracking (HIC). The SCC has been studied extensively in the literature; however, HIC phenomenon in pipeline steels is less investigated by researchers. Nevertheless, HIC is recognized as the most important damage mode in sour environment. Hydrogen atoms produced due to surface corrosion of the steel diffuse into it through microstructural defects. When a critical amount of hydrogen is accumulated in such defects, HIC cracks initiate and propagate. The main objectives of this thesis are to find the HIC crack nucleation and propagation sites, evaluate a role of texture and grain boundary character distribution in crack growth and finally establish the effect of different microstructural parameters contributing to the HIC related failure in pipeline steel. In this thesis, HIC standard test and electrochemical hydrogen-charging experiments were used to induce HIC cracks in pipeline steels. HIC cracks at the cross section of tested samples were observed using scanning electron microscope (SEM). The SEM observations clearly indicate that the investigated X60 and X70 steels are susceptible to HIC while the X60SS steel showed a higher resistance to HIC. This experiment also proved that the X70 steel has higher susceptibility to HIC than the other investigated steel. Energy dispersive spectroscopic (EDS) analyses indicated that two types of inclusion namely manganese sulfide and carbonitiride precipitates serve as crack nucleation sites. HIC cracks were observed to propagate at the center of cross section where the segregation of some elements such as carbon and manganese occurred. Moreover, two other experiments were carried out in order to evaluate the capability of pipeline steels for hydrogen-trapping. The first test, hydrogen-permeation experiment, showed that all pipeline steel specimens, such as X70, X60 and X60SS steels, contain both reversible and irreversible hydrogen traps. However, the density of traps at the center of cross section was higher than other regions in all tested specimens. The hydrogen-discharging experiments also showed that all specimens keep a considerable amount of hydrogen inside their traps. The hydrogen traps, based on their binding energy with the metal matrix, are categorized as reversible (weak) and irreversible (strong) traps and the roles of each type of traps are explained. Electron backscatter diffraction (EBSD) measurements were done along the HIC crack in X70 steel after standard HIC test. The results showed that the {100} texture was strong while the {111} texture was weak. Some special texture components, such as the {110}, {332} and {112}, were observed after the HIC crack-stoppage. EBSD results also documented that fine grain colonies were prone to intergranular HIC crack propagation and IPF and PF, calculated in both sides of HIC cracks, showed the preferences of ND||<100> orientation. Both susceptible X60 and non-susceptible X60SS steel to HIC were compared based on the EBSD results. It was observed that the high amount of recrystallization fraction with no stored energy is one of the main reasons for a higher HIC resistance of X60SS steel to HIC. Moreover, Kernel Average Misorientation (KAM) data showed that the deformation is more concentrated in the as-received and HIC tested X60 specimens. The effect of hydrogen-charging during tensile/fatigue loading of X60SS steel was studied and it was observed that some HIC cracks at the cross section of X60SS steel were appeared after hydrogen-charging at stresses below the yield stress. Experiments were carried out to understand the effect of cold-rolling and annealing on HIC susceptibility in pipeline steels. The results documented that the {100} dominant texture is more pronounced in 50% and 90% cold-rolled and annealed specimens. The effect of different factors such as KAM degree and recrystallized fraction affecting HIC susceptibility on cold-rolled and annealed specimens was investigated. The obtained results showed that the cold-rolling and annealing process may not be considered as an effective method to increase HIC resistance in pipeline steels.

Le couplage d’échange entre une couche ferromagnétique (F) et une couche antiferromagnétique (AF) permet de piéger l’aimantation de la couche ferromagnétique. Ce phénomène est largement utilisé dans des systèmes magnétiques complexes, telles que les vannes de spins, ou les mémoires MRAM, où il permet de constituer des couches de références, normalement insensibles aux cycles d’écriture des couches de stockage. On remarque aux petites dimensions, lorsque la taille des cellules diminue en dessous de la centaine de nm, des renversements partiels ou complets des électrodes de référence, dus à un basculement du réseau de spins dans l’AF. L’objectif de cette thèse est de comprendre ces phénomènes de renversement, de les quantifier en fonction de la dimension latérale des dispositifs, et de présenter des solutions viables afin d’accroître la stabilité des systèmes de stockage. Ce travail essentiellement expérimental, comprenant dépôts, lithogravure et caractérisations, se déroulera pour la majeure partie au sein du laboratoire SPINTEC (UMR8191). L’étudiant sera cependant amené à collaborer avec plusieurs entités du pôle grenoblois, notamment pour les mesures magnéto-optiques, les analyses cristallographiques, ainsi que pour une partie de simulation atomistique ; il devra aussi s’intéresser à l’intégration industrielle de ses études en rendant compte de ses résultats, en les discutant, afin que Crocus Technology en bénéficie directement. La thèse, se déroulant sur trois ans, explorera les points suivants : i) Etude de la stabilité thermique en fonction de la taille des motifs (0-15mois) L’étudiant déposera par pulvérisation cathodique des bicouches F/AF (AF=FeMn, PtMn ou IrMn) qui seront gravées sur la plate forme de technologie amont (PTA) localisée sur le site du CEA/Grenoble. Il caractérisera par des mesures d’effet kerr ou de magnétotransport les propriétés magnétiques des bicouches, notamment les distributions de TB, de champ d’échange, en fonction de la taille des motifs. Il participera aux analyses cristallographiques en collaboration avec le Service général des rayons X et le laboratoires d'Etude des Matériaux par Microscopie Avancée (LEMMA) (du CEA/Grenoble/INAC/SP2M). Ces analyses qui donneront des renseignements sur les tailles de grains et leur distribution seront utilisées pour comprendre les mesures magnétiques dans un premier temps, et seront un point de départ pour optimiser via des recuits, ou l’ajout d’éléments d’addition, la stabilité des systèmes aux dimensions réduites, typiquement <100nm. ii) Etude du couplage inter-grain dans l’AF (15-22mois) L’étudiant réalisera des mesures de trainage magnétique et déterminera les volumes de nucléation dans l’AF et les comparera aux données cristallographiques. Il essaiera de déterminer l’importance de ce couplage dans la stabilité des points mémoire en jouant sur son intensité (recuits, éléments d’addition…), ceci participant de l’optimisation de l’anisotropie d’échange aux petites dimensions présentée dans la partie précédente. iii) Volet de simulations atomistiques (22-30mois) L’étudiant collaborera avec le laboratoire LSIM et notamment F. Lançon afin de simuler, grâce à un code de calcul développé localement, l’impact de la cristallographie (taille de grains, couplage inter-grains, désordre interfacial et rugosité) sur les propriétés de l’anisotropie d’échange dans les systèmes F/AF de taille réduite. Ces simulations permettront de comprendre les mesures expérimentales réalisées en parallèle et d’ouvrir de nouvelles voies exploratoires pour optimiser les valeurs de champ d’échange en vue de leur intégration dans les dispositifs. / Exchange coupling between a ferromagnetic (F) and an antiferromagnetic (AF) layer is responsible of a higher coercivity and of a shift in the hysteresis loop. This phenomenon is widely used in magnetic systems like spin-valves and MRAM to set the reference layer, that remains fixed during the writing processes of the storage layer. It has been noticed that, for systems with reduced lateral size , the magnetization of the reference layer can (completely or partially) reverse because of spin switches in the AF layer. The aim of this thesis project is the understanding of these reversal phenomena, the quantification as a function of lateral dimension and the proposal of feasable solutions in order to increase the stability of the storage layer. The thesis will be maily experimental, including deposition, lithography and characterization processes. The main part of the thesis will be spent at SPINTEC (UMR8191) laboratories. The student will also collaborate with other research groups in Grenoble, in particular for magneto-optical measurements, crystallographic analysis, and atomic simulations. He will also manage the industrial integration of his studies, by sharing and discussing his results with Crocus Technology. The thesis, during a period of three years, will cover the following subjects: i) Study of the termal stability as a function of lateral size (0-15 months). The student will deposit F/AF bilayers being AF FeMn, PtMn or IrMn) by magnetron sputtening. These layers will be etched at Pthe TA cleanroom facility, in CEA-Grenoble. TB and exchange field distributions will be characterized by Kerr effect and magnetotrasport measurements as a function of lateral size. He will collaborate to the crystallographic analysis with the X-ray general service and the 'laboratoires d'Etude des Matériaux par Microscopie Avancée' (LEMMA) (CEA/Grenoble/INAC/SP2M). These analysis, that will give informations about grain size and distribution, will help the understanding of the magnetic measurements and will be a starting point for an optimization, through annealing steps or additional elements, of the stability of systems with reduced lateral size (typically below 100nm). ii) Study of AF inter-grain coupling (15-22 months) The student will perform magnetic training measurements. He will determine the nucleation volumes in the AF and compare them with the crystallographic results. He will manage to establish the importance of this coupling in the stability of magnetic memories and to vary its intensity (annealing, additional elements). This study will contribute to optimize the exchange anisotropy at reduced dimensions presented in the previous point. iii) Atomic simulations (22-30 months) The student will collaborate with LSIM laboratory, in particular with F.Lançon. He will simulate the impact of crystallography (grain size, inter-grain coupling, interfacial disorder, rugosity) on exchange anisotropy properties in F/AF systems with reduced lateral size. Simulations will be performed with a code developped in the lab. These simulations will help in understanding the experimental measures performed previously, and will give new suggestions in the optimization process of the exchange field for technological integration.

This work aims to study the niobium influence in the crystallographic texture and the corrosion resistance of ferritic stainless steels on aerated and de-aerated environments. For this objective the ferritic stainless steels P409, P410, P430A, P430E (Nb stabilized) were used; and austenitic stainless steel, P304, and carbon steel, A36, were used as comparative materials. These materials had a mechanical characterization (hardness and tension test), and the steels P430A and P430E had a crystallographic characterization too. That crystallographic characterization (by EBSD) was analyzed through to inverse pole figure (IPF) and crystal orientation distribution function (CODF). The samples were submitted to anodic potentiodynamic polarization test in solutions: 3.56% NaCl, and 1N H2SO4 on aerated environment, and 3.56% NaCl at de-aerated environment. The samples were examined by SEM after the polarization tests. The analysis of the results clearly showed that the crystallographic texture influence the corrosion resistance. The niobium in the stainless steel reduces the presence of preferential orientation, therefore, the influence of the texture in the corrosion resistance, but helps to increase the corrosion resistance by the formation of niobium carbbonites. Finally it was observed that for polarization tests in aerated environments and de-aerated have a very small variation in behavior that depends on the steel, but this variation is not statistically significant. / O objetivo deste trabalho é estudar a influência do nióbio na textura cristalográfica e resistência à corrosão de aços inoxidáveis ferríticos em ambientes aerados e desaerados. Usou-se para o estudo os aços inoxidáveis ferríticos P409, P410, P430A, P430E (estabilizado ao Nb) e como objetos de comparação usaram-se os aços inoxidáveis austenítico P304 e o aço ao carbono A36. Neles realizou-se uma caracterização mecânica (dureza, ensaio de tração), e para os aços P430A e P430E foi feita uma caracterização cristalográfica (via EBSD) por meio de figura de polos inversa (IPF) e da função de distribuição de orientação cristalina (FDOC). As amostras foram submetidas a ensaios de polarização potenciodinâmica anódica para soluções de 3,56% NaCl e 1N de H2SO4 em ambiente aerado e solução de 3,56% NaCl em ambiente desaerado. As amostras foram examinadas por microscopia MEV após os ensaios de polarização. A análise dos resultados mostrou claramente que a textura cristalográfica influência a resistência à corrosão. O nióbio diminui a presença de orientações preferenciais, e, por consequência, a influência da textura na resistência à corrosão, no entanto aumenta a resistência à corrosão nos aços inoxidáveis pela formação de carbonetos de nióbio. Por último observou-se que para ensaios de polarização em ambientes aerados e desaerados existe uma ligeira variação no comportamento que depende de cada aço, mas que não é estatisticamente significativa. / Mestre em Engenharia Mecânica