This work studied sputter deposited conventional spin valves (SV) and related structures. In SV layered structures, two ferromagnetic layers are separated by a non-magnetic spacer. Under an external magnetic field, the relative orientation of the magnetization changes in the ferromagnets, exhibiting the giant magnetoresistive effect. The controlled switching of ferromagnets in convention SV is facilitated by the exchange bias (EB) effect, which is achieved by depositing an antiferromagnetic layer next to one of the ferromagnetic layers in a magnetic field. Two highly related investigations were performed in this work. In the first part the exchange bias effect in the Ni80Fe20/Fe50Mn50/Co trilayer structure was studied. Samples were deposited in a low field condition that permitted EB to be established in NiFe/FeMn but not in FeMn/Co bilayer structures. Temperature-dependent magnetic measurements were performed on the trilayer sample, as well as the corresponding NiFe/FeMn and FeMn/Co bilayer samples. Recent literature on similar system showed that an AF spiral could be formed in the trilayer, which was probed by relative EB directions of the NiFe and Co layers. In this work, no exchange bias was found to propagate from the NiFe/FeMn system into the FeMn/Co system, showing that the AF spiral was induced by the specific magnetic treatment and was not the cause of EB effect. Besides, exchange bias field and coercivity of the samples indicated the influence of the EB system in the presence of an adjacent EB system. Explanations of the effect were made with some existing EB models. In the second part of the work, conventional SV of target structure Nb/NiFe/Cu/Co/FeMn/Nb was studied in a 'built-up samples' strategy. A batch of these built-up samples, which corresponded to the different stages of the deposition of the target top conventional SV structure, were prepared by terminating the sputtering process after a certain number of layers were deposited. These samples were thoroughly characterized by structural, magnetic and electrical measurements. In terms of structural characterization by x-ray techniques, more reliable information concerning the morphology and microstructure of the layers was obtained by probing the built-up samples, instead of relying solely on the information of the full SV structure. For the electric and magnetic measurements, a number of unexpected observations were made in the built-up samples, although the final performance of the full SV structure was of comparable quality to the literature. These results showed the ability of the 'built-up samples' strategy in critical characterization and optimization of magnetic multilayered structures.

Plastic strains during investment casting of single crystal Ni-based superalloys arise from differences in thermal contraction between the metal and ceramic mould-and-core. If deformation is above a critical limit, subsequent solution heat treatment of the alloy causes recrystallisation. Crack nucleation and propagation is preferred at the recrystallisation grain boundaries, and this significantly reduces the creep and fatigue properties of the alloy. The as-cast microstructure (Chapter 3) is characteristic of high temperature deformation (~1050�(), where dislocations primarily form loops and networks at the y/y' interface. This validates the process modelling performed by our collaborators. The dislocation density was found to be higher in the interdendritic areas and the dendrite cores were virtually dislocation-free, indicating that deformation occurs at temperatures close to they' solvus, which is 1250-1310�C for CMSX 4, where dislocations accumulate in the interdendritic areas that are the first to precipitate they'. Critical strains for recrystallisation were determined as a function of temperature (Chapter 3). Recrystallisation was found to be sensitive to the surface finish, where alloys with the cast surface were more prone to recrystallisation, tolerating plastic strains of 1-2% at temperatures ~1000�C. On examining the cast surface of CMSX 4, two sources of nucleation for recrystallisation were identified (Chapter 4). Firstly, micro-grains of y', 2-30 ?m deep, forming high angle boundaries with the bulk single crystal were found within the surface eutectic; these grow larger during the heat treatment and maintain high angle misorientations with the matrix. Secondly, in regions where surface eutectic is absent, the metal adheres to the mould and forms intense local deformation, 5- 20 ?m deep, during subsequent detachment. During the heat treatment local surface recrystallisation occurs, where small grains develop in orientations similar to the deformed matrix and subsequently twin to form high angle boundaries. Experimental trials show that in the presence of deformation in the bulk the nuclei in the casting surface can cause recrystallisation. By removing the cast surface with etching, recrystallisation can be completely mitigated . Recrystallisation studies on alloys varying systematically in composition (Chapter 5) show that high . Co alloys (with up to 8 wt%) are more prone to recrystallisation. Co lowers they' solvus temperature and the stacking fault energy of y. Ru, Mo and W appear to have no direct effect on recrystallisation. The nucleating grains for recrystallisation form in orientations similar to the deformed matrix, and subsequently twin to form high angle boundaries and proliferate within the deformed microstructure. They' phase and topologically close packed phases hinder grain boundary migration.

Researchers have recently become interested in two-dimensional materials such as graphene, hexagonal boron nitride (h-BN), Transition Metal Dichalcogenides (TMDs), etc. Their 2D hexagonal structures result in unique properties, which make these materials attractive for scientists and engineers. In this work, we investigated the electronic properties of graphene, h-BN, and MoS2 based on density functional theory (DFT). We first studied the electronic properties of monolayers of different materials. We found a zero bandgap and observed massless Dirac Hamiltonian in graphene. For h-BN, a large bandgap at K-point was observed. Also, we observed the bandgap opening in MoS2 and a strong splitting of its bands. Then, we extended these studies to graphene and h-BN bilayers. For graphene bilayer, we observed a gapless material and massive Dirac fermions. For h-BN bilayer, an indirect bandgap was observed, smaller in comparison with its monolayer. The main focus of this study was the investigation of graphene/h-BN heterostructures for different stacking configurations. The suitability of h-BN as a substrate for graphene is due to its small lattice constant mismatch with graphene and its high insulating gap (~ 5 eV). Another important aspect to be observed in graphene/h-BN heterostructures is the gap opening brought by the h-BN layer proximity to the initially gapless graphene layer. We found the effect of bandgap opening in graphene/h- BN and determined the most stable configuration which is the AB[CB]. This work supports the findings of many researchers who demonstrate that graphene/h-BN heterostructures are very useful as building blocks for nanodevices with desirable electronic properties.

Studies of influence deposition parameters on heterostructures by SIMS

Cette thèse porte sur l'exploration de différentes phases topologiques présentes dans des hétérostructures à base de mercure, cadmium et tellure (HgCdTe). Ces systèmes sont de parfaits cas d'études des états topologiques dans la matière condensée. En effet, leur structure de bande peut aisément être modifiée d'inversée à non-inversée par le biais de paramètres internes ou externes.Lorsqu'un système présente une structure de bande inversée, il a une topologie non triviale. Il est impossible de modifier cet ordre topologique sans fermer son gap, ce qui inévitablement entraîne l'apparition de particules sans masse dans son volume. Un système présentant une structure de bande inversée et un gap d'énergie finie dans lequel se trouve le niveau de Fermi, est appelé isolant topologique. Ce nouveau type de matériau est isolant dans son volume, mais abrite des états métalliques sans gap sur ses bords. Ces derniers ont une relation de dispersion linéaire et sont protégés des effets liés au désordre et de la rétrodiffusion par des impuretés non magnétiques. Ces états particuliers apparaissent à l'interface de matériaux présentant des ordres topologiques différents. Ainsi, un isolant topologique 2D se caractérise par des canaux 1D de conductance polarisés en spin à ses bords, alors qu'un isolant topologique 3D accueille des fermions de Dirac 2D, polarisés en spin, aux surfaces.L'existence de fermions sans masse 2D et 3D a déjà été démontrée expérimentalement. Cependant, la transition de phase topologique durant laquelle apparaissent les particules sans masse n'a que très peu été explorée. Il est possible de modifier la structure de bande de HgCdTe d'inversée à non inversée par le biais de la composition chimique, la pression, la température ou le confinement quantique. Ces paramètres permettent ainsi de sonder le système au voisinage de différentes transitions de phase topologiques. Dans ce travail, l'utilisation de la température comme paramètre d'ajustement continu du gap permet d'étudier au point de transition de phase l'apparition de fermions semi-relativistes de Dirac (2D) et de Kane (3D) ainsi que leurs propriétés.Les systèmes étudiés au cours de ces travaux de recherche sont des cristaux massifs de Hg1-xCdxTe et des puits quantiques HgTe/CdTe présentant des structures de bandes inversées et non inversées, ainsi que des couches minces de HgTe contraintes pouvant être considérées comme des isolants topologiques 3D ayant un confinement quantique résiduel. Tous ces systèmes possèdent des propriétés topologiques. L'interprétation des résultats s'appuie sur les prédictions théoriques basées sur le modèle de Kane. En annexe, une vue d'ensemble des puits quantiques composites InAs/GaSb, structures également identifiées comme isolants topologiques, est présentée, comportant les résultats préliminaires obtenus sur ces dernières.Toutes les structures ont été étudiées par magnétospectroscopie en transmission dans les domaines de fréquence terahertz et infra-rouge moyen à l'aide d'un dispositif expérimental spécifiquement conçu pour permettre des mesures sur une large plage de températures. / This thesis presents an investigation of different topological phases in mercury-cadmium-telluride (HgCdTe or MCT) based heterostructures. These solid state systems are indeed a perfect playground to study topological states, as their band structure can be easily varied from inverted to non-inverted, by changing internal or external parameters.If a system has an inverted band ordering, its electronic structure has a non-trivial topology. One cannot change its topological order without closing the band gap, which is inevitably accompanied with the appearance of massless particles in the bulk. A system, that has an inverted band structure and a finite gap in which the Fermi level is positioned, is called a topological insulator. These novel materials are insulators in the bulk, but host gapless metallic states with linear dispersion relation at boundaries, protected against disorder and backscattering on non-magnetic impurities. These states arise at the interfaces between materials characterized by a different topological order. A 2D topological insulator is thus characterized by a set of 1D spin-polarized channels of conductance at the edges, while a 3D topological insulator supports spin-polarized 2D Dirac fermions on its surfaces.The 2D and 3D massless fermions have already been demonstrated experimentally in HgCdTe-based heterostructures. However, the topological phase transitions during which the massless particles appear remain barely explored. The HgCdTe band structure can be tuned from inverted to non-inverted using chemical composition, pressure, temperature, or quantum confinement. These parameters therefore allow to probe the system in the vicinity of different topological phase transitions. In this thesis, the use of temperature as continuous band gap tuning parameter allows to study the appearance and the parameters of semi-relativistic 2D Dirac and 3D Kane fermions emerging at the points of phase transitions.The systems investigated were Hg$_{1-x}$Cd$_x$Te bulk systems and HgTe/CdTe quantum wells characterized by an inverted and regular band order, and strained HgTe films which can be considered as 3D topological insulators with a residual quantum confinement. All these systems exhibit topological properties, and the experimental results are interpreted according to theoretical predictions based on the Kane model. This thesis is complemented by an overview and the preliminary results obtained on a different compound -- a InAs/GaSb broken-gap quantum well, which was also identified as a topological insulator. The structures were studied by means of terahertz and mid-infrared magneto-transmission spectroscopy in a specifically designed experimental system, in which temperature could be tuned in a broad range.

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This dissertation is centered on investigation of metastable thermoelectric thin film materials and is split into 3 primary sections. Section 1 focuses on formation mechanisms of FeSbx compounds from layered precursors. It was found that a compositionally favorable and homogeneous nucleation environment allowed for the nucleation of a metastable phase, which surprisingly resembles the local coordination environment of the precursors, even in cases where they are compositionally unfavorable. Over the course of this work, the technique of normal-incidence thin film pair distribution function analysis is introduced, which allows for rapid acquisition and analysis of local structure data from intact thin films. Section 2 investigates changes in the stacking sequences of ([PbSe]1+δ)m(TiSe2)n nanolaminate materials, which consist of interleaved layers of each compound in the chemical formula, and how these changes effect the thermoelectric power factor. Homologous series of systematically varying m and n values are investigated and measured properties are correlated back to the designed nanoarchitecture of the laminate materials. It is found that the compounds are stabilized by electron exchange between constituents at the interfaces, and that ‘doping’ of the laminate structure by changing the relative amounts of each constituent is an effective means of optimizing their transport properties. It is also shown that interface density between constituents can be utilized to optimize performance. Section 3 moves from the case of PbSe layers, which maintain their structure, to SnSe layers that significantly distort as the layer size is changed. The distortions in SnSe are observed to occur from templating off TiSe2 layers. As the size of the SnSe layers increases, relatively fewer templated interfacial atoms exist and stabilization of interior atoms must also be considered. The coarse behaviors developed in ([PbSe]1+δ)m(TiSe2)n hold, but the structural distortions in SnSe likely change the band structure of this constituent and hence the composite material, complicating the analysis. In some cases, these changes allow for radically different behavior, best exemplified with high TiSe2 ratios in ([SnSe]1+δ)1(TiSe2)n displaying significant enhancement of the Seebeck coefficient at cryogenic temperatures over the low-n and PbSe-containing analogues. This dissertation includes previously published and unpublished coauthored material.