Le développement des dispositifs issus de l'électronique de spin nécessite de nouveaux matériaux qui permettent d'injecter de manière efficace le courant polarisé en spin dans des semiconducteurs. Parmi de nombreux matériaux utilisés comme injecteurs de spin, les semiconducteurs ferromagnétiques dilués (DMS), obtenus en dopant des semiconducteurs avec des impuretés magnétiques tels que Mn ou Co, sont considérés comme des candidats potentiels pour l'injection de spin. Ces matériaux dopés deviennent ferromagnétiques tout en conservant leurs propriétés semiconductrices. Par conséquent, ils présentent une similarité d'impédance électrique par rapport aux substrats semiconducteurs, ce qui rend efficace l'injection de courant polarisé en spin dans ces derniers. Dans ce contexte, l'objectif principal de cette thèse consiste à étudier la cinétique de croissance des semiconducteurs ferromagnétiques dilués GeMn. Nous cherchons à déterminer les paramètres clés de la croissance des couches de GeMn, à savoir la température du substrat, et la concentration en Mn. Pour la fabrication de dispositifs électroniques fonctionnels, le challenge crucial est d'obtenir des DMS ayant une température de Curie (TC) bien supérieure à la température ambiante. Nous nous sommes donc concentrés sur la cinétique de formation de la phase nanocolonnaire GeMn possédant une TC au-delà de 400 K. / The development of active spintronic devices requires new materials, which enable to efficiently inject spin-polarized currents into non-magnetic semiconductors. Among numerous materials that can be used as spin injectors, diluted magnetic semiconductors (DMS), obtained by doping standard semiconductors with magnetic impurities, such as Mn or Co, have emerged as potential candidates for spin injection. The materials become ferromagnetic while conserving their semiconducting properties. They exhibit therefore natural impedance match to host semiconductors and are expected to efficiently inject spin-polarized currents into semiconductors. In this context, the main objectives of this thesis work consist in studying the growth kinetics of GeMn-based diluted magnetic semiconductors. We aim at determining the main growth parameters, such as the substrate temperature and the Mn concentration, that govern the growth process of GeMn layers. Since for device applications it is crucial to obtain DMS exhibiting a Curie temperature (TC) well above room temperature, we have focused our attention to the kinetic formation of the GeMn nanocolumn phase, which exhibits a Curie temperature higher than 400 K.

O presente trabalho é um estudo sobre a importância da informação dos orbitais atômicos no cálculo de propriedades optoeletrônicas de heteroestruturas semicondutoras de baixa dimensionalidade. O trabalho é dividido em duas partes: na primeira parte, é estudada a simetria de reversão temporal no hamiltoniano k . p, analisando a preservação da informação de spin presente nos orbitais atômicos. O hamiltoniano obtido é inserido na equação de massa efetiva expandida para superredes. São calculadas estruturas de bandas de alguns poços quânticos de semicondutores III-V e grupo-IV. Compara-se o novo método com os tradicionais, e então são analisadas algumas grandezas que apresentam alteração significativa entre os métodos usados; A segunda parte é composta por um estudo detalhado do potencial de troca-correlação em semicondutores dopados. A matriz que descreve este potencial é escrita usando a distribuição de portadores presentes nos orbitais atômicos da rede cristalina, e os coeficientes desta matriz foram calculados usando quatro modelos para a correção de muitos corpos, baseadas nas aproximações LDA (Local density approximation) e LSDA (Local spin density approximation), com o objetivo de comparar as diversas parametrizações. Usando o método k . p tradicional, expandido para superredes, foram simulados sistemas δ-doped e hMni-δ-doped de Si, através de um cálculo autoconsistente baseado na equação de Poisson. A magnetização dos portadores é descrita por um modelo de campo médio. Foram analisados os perfis de potencial, estruturas de bandas, polarização de portadores e espectros de fotoluminescência para determinar as diferenças entre as aproximações utilizadas. / This work is a study about the atomic orbitals information importance in the calculation of optoelectronics properties of low dimensionality semiconductors. The work is divided in two parts. In the first one, a study of the time reversal symmetry of the k . p Hamiltonian is realized analyzing the preservation of the spin information present in the atomic orbitals. The obtained Hamiltonian is applied in the effective mass equation expanded to superlattices. Some calculations of quantum wells band structures are made using III-V and group-IV semiconductors, comparing the new method with the conventional ones to obtain an analysis of the difference of some physics properties. The second part is a detailed study of the exchangecorrelation potential in doped semiconductors. The matrix coefficients are calculated using the charge distribution of the crystalline lattice atomic orbitals, applied in some LDA (Local density approximation) and LSDA (Local spin density approximation) parameterizations to compare them. Using the conventional k . p method expanded to superlattices, Si δ-doped and hMni-δ-doped systems were calculated through a self consistent calculation based on Poissons equation. The carriers magnetization is described by an average field model. The potential profiles, band structures, carrier polarization and photoluminescence spectra were analyzed to obtain the difference between the approaches.

As novas descobertas petrolíferas, em especial as localizadas no pré sal tem despertado uma grande preocupação na área técnica em consequência das dificuldades na sua exploração e extração. As maiores preocupações no segmento de exploração de petróleo e gás natural em águas profundas estão relacionadas, às propriedades mecânicas e a resistência à corrosão dos diversos componentes. Assim, para evitar o processo de corrosão na parte interna de uma tubulação ou componente do setor petrolífero, uma alternativa é usar um revestimento com elevada resistência à corrosão. Neste trabalho utilizou-se um aço AISI 8630 modificado, que foi revestido por soldagem GTAW (Gas Tungsten Arc Welding) ou TIG (Tungsten Inert Gas), empregando uma liga especial de níquel 625 (Inconel 625). No entanto, existe a possibilidade de formação de fases frágeis no material de base (MB) - AISI 8630, no material de adição (MA) - liga de níquel 625 ou ainda na interface entre os materiais dissimilares. Para atenuar a formação da fase martensítica oriunda do resfriamento da soldagem foram realizados diferentes ciclos térmicos de alívio de tensão (AT), mas outras fases frágeis podem surgir nas diversas juntas. Os tratamento térmicos de ATs foram realizados nas temperaturas de 620, 640, 660 e 680 C. A presença e a influência de fases frágeis foram analisadas na junta soldada, por meio de ensaios mecânicos, tais como, tração, impacto e dureza. Investigou-se a existência de fases frágeis por caracterização microestrutural por meio de microscopia ótica e microscopia eletrônica de varredura com EDS. Observou-se que nos ensaios de tração o material de base manteve sua tensão de escoamento dentro de valores estabelecidos por normas do setor, como a NACE MR0175. No entanto, no ensaio de impacto ocorreu uma perda significativa da tenacidade na temperatura de 680 C, enquanto temperaturas menores de alívio de tensão os valores míninos de impacto foram acima de 73 J. Os valores de durezas encontrados próximos da zona de ligação indicaram que houve um endurecimento, em consequência da difusão dos elementos de liga do MA para o aço baixa liga. Observações realizadas com auxílio de microscopia ótica indicaram zonas frágeis denominadas de Zonas Parcialmente Diluídas (ZPD) que reduziram a resistência ao impacto. Análise por EDS mostrou variações dos elementos de liga ao longo da interface da junta. / New oil fields discovered, especially those one located in pre-salt basin, has called attention to great concern in technical area as results of difficulties in exploration and extraction. The difficulties focused in the area of oil and natural gas exploration in ultra deep waters are related to mechanical properties and corrosion resistance of various components. Coatings with high corrosion resistance are used to prevent corrosion process internally of the pipes or components used. In this work was used an AISI 8630 modified that was cladded with nickel alloy 625 (inconel 625) through GTAW (Gas Tungsten Inert Gas) or TIG process (Tungsten Inert Gas). However there is a possibility of generate embrittlement phases in base material (MB) - AISI 8630, addition material (MA) alloy nickel 625 or in intermediate phase between dissimilar materials. Several Post Weld heat treatment (PWHT) cycles were performed to minimize the martensitic phase formation by welding cooling. It was performed PWHT cycle at 620, 640, 660 e 680 C temperature. The presence and influence of embrittle phases were analyzed in welded joint by mechanical tests such as tensile, charpy and hardness. It was investigated embrittle phases by microstructural characterization using optic microscope and scanning electron microscope with EDS. It was observed in tensile test the base material maintained its yield strength within ranges established by industry standards such as NACE MR0175. However, it was observed in the impact test the significant lost of the tenacity in 680ºC, while in lower PWHT temperatures the values of charpy test were over than 73 J. The values founded near the area fusion in hardness test indicated the XVI hardening as consequence of elements diffusion of the MA alloy to low alloy steel. It was observed by optic microscope fragile areas denominated by Partially Diluted Area (ZPD) that reduces the resistance of impact. EDS analysis showed variation of alloy elements along the joint.

As novas descobertas petrolíferas, em especial as localizadas no pré sal tem despertado uma grande preocupação na área técnica em consequência das dificuldades na sua exploração e extração. As maiores preocupações no segmento de exploração de petróleo e gás natural em águas profundas estão relacionadas, às propriedades mecânicas e a resistência à corrosão dos diversos componentes. Assim, para evitar o processo de corrosão na parte interna de uma tubulação ou componente do setor petrolífero, uma alternativa é usar um revestimento com elevada resistência à corrosão. Neste trabalho utilizou-se um aço AISI 8630 modificado, que foi revestido por soldagem GTAW (Gas Tungsten Arc Welding) ou TIG (Tungsten Inert Gas), empregando uma liga especial de níquel 625 (Inconel 625). No entanto, existe a possibilidade de formação de fases frágeis no material de base (MB) - AISI 8630, no material de adição (MA) - liga de níquel 625 ou ainda na interface entre os materiais dissimilares. Para atenuar a formação da fase martensítica oriunda do resfriamento da soldagem foram realizados diferentes ciclos térmicos de alívio de tensão (AT), mas outras fases frágeis podem surgir nas diversas juntas. Os tratamento térmicos de ATs foram realizados nas temperaturas de 620, 640, 660 e 680 C. A presença e a influência de fases frágeis foram analisadas na junta soldada, por meio de ensaios mecânicos, tais como, tração, impacto e dureza. Investigou-se a existência de fases frágeis por caracterização microestrutural por meio de microscopia ótica e microscopia eletrônica de varredura com EDS. Observou-se que nos ensaios de tração o material de base manteve sua tensão de escoamento dentro de valores estabelecidos por normas do setor, como a NACE MR0175. No entanto, no ensaio de impacto ocorreu uma perda significativa da tenacidade na temperatura de 680 C, enquanto temperaturas menores de alívio de tensão os valores míninos de impacto foram acima de 73 J. Os valores de durezas encontrados próximos da zona de ligação indicaram que houve um endurecimento, em consequência da difusão dos elementos de liga do MA para o aço baixa liga. Observações realizadas com auxílio de microscopia ótica indicaram zonas frágeis denominadas de Zonas Parcialmente Diluídas (ZPD) que reduziram a resistência ao impacto. Análise por EDS mostrou variações dos elementos de liga ao longo da interface da junta. / New oil fields discovered, especially those one located in pre-salt basin, has called attention to great concern in technical area as results of difficulties in exploration and extraction. The difficulties focused in the area of oil and natural gas exploration in ultra deep waters are related to mechanical properties and corrosion resistance of various components. Coatings with high corrosion resistance are used to prevent corrosion process internally of the pipes or components used. In this work was used an AISI 8630 modified that was cladded with nickel alloy 625 (inconel 625) through GTAW (Gas Tungsten Inert Gas) or TIG process (Tungsten Inert Gas). However there is a possibility of generate embrittlement phases in base material (MB) - AISI 8630, addition material (MA) alloy nickel 625 or in intermediate phase between dissimilar materials. Several Post Weld heat treatment (PWHT) cycles were performed to minimize the martensitic phase formation by welding cooling. It was performed PWHT cycle at 620, 640, 660 e 680 C temperature. The presence and influence of embrittle phases were analyzed in welded joint by mechanical tests such as tensile, charpy and hardness. It was investigated embrittle phases by microstructural characterization using optic microscope and scanning electron microscope with EDS. It was observed in tensile test the base material maintained its yield strength within ranges established by industry standards such as NACE MR0175. However, it was observed in the impact test the significant lost of the tenacity in 680ºC, while in lower PWHT temperatures the values of charpy test were over than 73 J. The values founded near the area fusion in hardness test indicated the XVI hardening as consequence of elements diffusion of the MA alloy to low alloy steel. It was observed by optic microscope fragile areas denominated by Partially Diluted Area (ZPD) that reduces the resistance of impact. EDS analysis showed variation of alloy elements along the joint.

Cette thèse porte sur l'étude des ondes de spin dans des puits quantiques CdMnTe dopés n, par rotation Kerr résolue en temps (TRKR) et par mélange à quatre ondes (FWM). Nous avons étudié trois échantillons de haute mobilité et de caractéristiques différentes.La technique TRKR donne accès uniquement aux excitations de vecteur d'onde nul, dans notre cas l'onde spin-flip en q=0. Nous avons étudié l'anticroisement qui apparait entre l'onde spin-flip et l'excitation spin-flip des ions manganèse. Nous avons étudié la variation du gap, et donc de l'énergie de couplage, entre ces modes en fonction de la puissance d'excitation et du champ magnétique. En particulier nous avons étendu les mesures des modes mixtes à plus basse concentration en Mn (jusqu'à 0.07%) et contrairement à ce qui était attendu, nous avons trouvé que le régime de couplage fort persiste à cette concentration.Nous nous sommes ensuite intéressés à la détermination de la polarisation en spin ζ du gaz d'électrons bidimensionnel, qui peut être déduite de l'énergie de couplage entre les modes mixtes. Nous avons trouvé que la polarisation mesurée par cette méthode excède la polarisation théorique calculée en prenant en compte le renforcement de la susceptibilité par les effets à N corps. Nous avons également mesuré les temps de relaxation des électrons confinés dans le puits quantique, et nous avons montré l'influence de l'échauffement de l'échantillon par le laser sur le temps de relaxation de spin des électrons.Dans la deuxième partie de cette thèse, nous avons étudié par FWM l'amortissement et la dispersion des ondes de spin de vecteur d'onde non nul pour l'un de nos échantillons. Nous avons démontré qu'on peut effectivement générer les ondes de spin en excitation femtoseconde, et les détecter en FWM. Nous avons trouvé que leur dispersion est plus faible que celle observée dans les expériences de Raman. Cette faible dispersion pourrait être imputable à la forte densité d'excitation utilisée dans les expériences de FWM (typiquement trois à quatre ordres de grandeur supérieurs à celle du Raman), et/ou au fait que deux ondes de vecteur d'ondes q et –q, ayant des dispersions différentes, sont sondées simultanément en FWM. / This thesis focuses on the study of spin waves in n-doped CdMnTe quantum wells using respectively time-resolved Kerr rotation (TRKR) and four-wave mixing (FWM) techniques. We studied three high mobility samples with different characteristics.The TRKR technique gives access only to zero wave vector excitations, in our case the spin- flip wave q = 0 . We studied the anticrossing that appears between the spin -flip wave and the manganese spin -flip excitation. We studied the gap variation energy between these modes as function on the power excitation and the magnetic field. In particular, we have extended the measurements of mixed modes at lower Mn concentration (up 0.07 %) and contrary to what were expected; we found that the strong coupling regime persists at this concentration.We are then interested in determining the two dimensional electron gas spin polarization ζ, which can be deduced from the energy coupling between the mixed modes. We found that the measured polarization exceeds the theoretical polarization calculated taking into account the increased susceptibility by many-body effects. We also measured the electron spin relaxation time and we have shown that it is influenced by thermal effects inherent to optical pump-probe experiments on this time.In the second part of this thesis, we studied by FWM the damping and the dispersion of the non-zero wave vector spin waves for one of our samples. We have demonstrated that we can actually generate spin waves in femtosecond excitation and deted them by FWM. We found that the dispersion is lower than that observed in the Raman experiments. This low dispersion may be due to the strong excitation density used in the FWM experiments (typically three to four orders of magnitude higher than the Raman ones) and / or the fact that two waves of wave vector q and - q, having different dispersions are simultaneously probed in FWM .

Modelos algébricos para prever o comprimento de uma chama turbulenta têm sido foco de estudo de diversos grupos de pesquisa por suas aplicações na área de engenharia. O método experimental para obter o modelo varia desde visualizações simples, até técnicas fotográficas, este último com parâmetros fotográficos variando entre os autores. Técnicas fotográficas são usadas para estimar a altura de levantamento da base da chama, (Lift-Off) e o comprimento médio visível de chama (Visible Flame Length, VFL). Duas técnicas comuns que podem ser encontradas na literatura: por imagens de chama com baixo tempo de exposição e longo tempo de exposição, são comparados com um terceiro que se baseia na intensidade luminosa e na frequência de imagens de chama que ocupam um pixel. O melhor método foi utilizado para caracterizar o comportamento das chamas turbulentas de gás natural para diferentes regimes de velocidade do escoamento. Modelos algébricos que preveem o comprimento de chama, altura de levantamento e a velocidade crítica de extinção de chama são avaliados com os novos resultados experimentais. Logo após, os coeficientes numéricos dos melhores modelos algébricos são reajustados Finalmente, foram obtidos mapas de estabilidade relacionados à altura de levantamento e à velocidade crítica de extinção de chama para cada diâmetro em função da diluição com CO2 e do número adimensional de Reynolds. A terceira parte deste trabalho está focada no estudo da distribuição de radiação térmica. Em particular, foram consideradas três distâncias radiais medidas em comprimentos de chama (0,5 Lf, 1 Lf, 2 Lf) visando obter a distribuição do fluxo radiante experimental ao longo de um eixo vertical adjacente às chamas. Finalmente, os dados experimentais foram utilizados como dados de entrada em uma análise inversa com o objetivo de calcular os fatores de ponderação do modelo das múltiplas fontes ponderadas (por suas siglas em inglês WMPS). Nesta última parte, são apresentados frações radiantes e distribuições de fluxo de calor radiante de chamas de gás natural diluídas para diversas diluições com dióxido de carbono e diâmetros do queimador. / Predicting models for turbulent diffusion flame lengths have several applications driven the attention of many research groups. Since several studies use photographs to measure the flame length, with photographic parameters varying among authors, in other cases simple visualizations were used. It is important to explore possible discrepancies among measurement technics that could affect the results. Optical visualizations of turbulent diffusion flames are used to estimate the visible average flame length (VFL) and the lift-off. The study presents a study of three different methods to measure the VFL using optical techniques. The effect on the image of the main optic parameters such as focus, exposure time and ISO sensibility are analyzed. The VFL obtained with images in low exposure time and long exposure time are compared with a third optical method that is based on the luminous intensity and the frequency of flame images occupying a pixel. One method was used to characterize the behavior of turbulent diffusion flames of natural gas for a range of flames in function of the flow velocity. Universal non-dimensional models that describe the VFL, lift-off and the blow-out stability limit of gaseous jet diffusion flames in the still air have been compared with new experimental data. The numerical coefficients of the best models are adjusted. Finally, maps of stability related to lift-off and blow-out were obtained for each diameter in function of the dilution with CO2 and flow exit velocity expressed in non-dimensional Reynolds number The third part of this work focuses on the estimation of the thermal distribution of radiative flux from turbulent diffusion flames in laboratory-scale. The experimental measurements were gotten from the previous stability study. In particular, was considered three radial distances measured in flame lengths (0,5 Lf, 1 Lf, 2 Lf) aiming at obtaining the experimental radiant flux along a vertical axis adjacent to the flames. Finally, the experimental data was used as input data in an inverse analysis with the purpose of computing weight coefficients of the weighted multi-point source (WMPS) model. Then, experimental data that include: radiant fractions and radiative heat flux are presents for several flames with different dilutions with carbon dioxide and burner´s diameters.

Nous avons étudié les propriétés dynamique d'un spin individuel dans une boite quantique de semiconducteur II-VI (spin d'un atome de Mn ou electron résident). Une boîte quantique comportant un atome de manganese présente six raies qui permettent de sonder optiquement l'état de spin du Manganese. Des expériences pompe-sonde réalisées sur boîte unique ont permit de montrer que le spin du Mn peut être orienté optiquement en quelques dizaines de ns, que le temps de vie $T_1$ de ce spin est supérieur à la $mu$s, et que le pompage optique en champ nul est controlé par une anisotropie magnétique induite par les contraintes. Par ailleurs, dans le but d'identifier les mechanismes du pompage optique, nous avons mis en évidence des processus de relaxation de spin au sein du système exciton-manganese, durant la durée de vie de ce dernier. Enfin, nous avons mis en evidence un effet Stark optique sur chacune des raies d'une boîte quantique magnétique. Concernant la dynamique d'un électron dans une boîte quantique II-VI, nous avons mis en évidence le pompage du spin de l'électron résident ainsi que des noyaux. / We have studied the dynamic properties of a single spin (Mn impurity or resident electron) in a II-VI semiconductor quantum dot. A quantum dot doped with a single Mn atom presents six lines which allow to probe optically the spin-state of the Mn atom. Pump-probe experiments at a single dot level were carried out to demonstrate that the Mn spin could be oriented in a few tens of ns, and that the spin-distribution prepared by such means was perfectly conserved over a few $mu$s. The optical pumping of the Mn spin at zero magnetic field is controlled by a strain-induced magnetic anisotropy. Furthermore, seeking for a microscopic mechanism controlling the optical pumping of the Mn atom, we have evidenced spin relaxation channels within the exciton-Mn complex. At last, we have evidenced an optical Stark effect on any of the lines of a Mn-doped quantum dot. Concerning the dynamics of an electron in a II-VI quantum dot, we have evidenced optical pumping of the resident electron, and dynamic nuclear spin polarization.

A dependência com a temperatura dos tempos de relaxação nuclear e da forma da linha de ressonância dos núcleos F0, nos antiferromagnetos diluidos Fex Zn1-x F2 e Mnx Zn1-x F2, foram estudados na faixa TN&#8804;T&#8804;300K. Os tempos de relaxação spin, rede (1/T1) dos núcleos F0, os quais não estão acoplados aos spins Fe (ou Mn) através da interação hiperfina transferida, foram medidos e calculados teoricamente em função da concentração x. Os resultados teóricos apresentam uma boa concordância com os dados experimentais para concentrações na faixa 0.1&#8804;x&#8804;0.8. A dependência com a temperatura de 1/T1, para TN<T&#8804;300K foi entendido qualitativamente em ambos sistemas. Os resultados obtidos para 1/T1, em temperaturas próximas de TN foram utilizados para estudar os \"Efeitos de Campo Aleatório\" no comportamento crítico do Mn.65Zn.35 F2 com o campo aplicado paralelamente e perperdicularmente ao eixo fácil (C). Encontrou-se neste caso, que a temperatura de transição TN decresce substancialmente com o aumento da intensidade do campo somente quando H0 || C. Os resultados experimentais obtidos estão de acordo com as teorias do Efeito de Campos Aleatórios em antiferromagnetos anisotrópicos diluídos. A divergência crítica do segundo momento da linha de ressonância não homogeneamente alargada do F0 foi estudada acima de TN. Os resultados experimentais concordam com os cálculos de Heller para o alargamento não homogêneo causado por Efeitos de Campos Aleatórios. Observou-se que a forma da linha se altera na região crítica. Longe de TN ela é Gaussiana e para t&#8804;10-2 ela mostra uma tendência a se tornar Lorentziana. Abaixo de TN a sua meia largura aumenta, seguindo qualitativamente o aumento da magnetização das sub-redes. / The temperature dependence of the nuclear relaxation rates and line shapes of the F0 resonance in the diluted antiferromagnet Fex Zn1-x F2 and Mnx Zn1-x F2 are studied over a large temperature range TN&#8804;T&#8804;300K. The high (room) temperature spinlattice relaxation rates (1/ T1) of the F0 nuclei, which are not transfer hiperfine coupled to the Fe (or Mn) spins, have been measured and calculated as a function of the concentration x. Good agreement with experiment is found for the theoretical results, which have been obtained in the range 0.1&#8804;x&#8804;0.8. The temperature dependence of TN<T&#8804;300K was qualitatively understood \"Random Field Effects\" on the critical behavior of Mn.65Zn.35 F2, for fields applied parallel and perpendicular to the easy (C) axis. It was found that the transition temperature TN depressed substantially with field only for H0 || C. The experimental results are in general accord with the theory for Random Field Effects in desordered, anisotropic antiferromagnets. The critical divergence of the in homogeneously broadened F0 NMR was studied in Fe.6Zn.4F2 above TN. The experimental results agree with Heller\'s calculation of the NMR line broadening by Random Field Effects. With H0 || C the line shape changes from Gaussian towards Lorentzian for t&#8804;10-2 and below TN its line width increase qualitatively following increase in the sublattice magnetization.

Theoretical studies of point defect interactions and structural stability of compounds have been performed using density functional theory. The defect-related properties, such as activation energy of diffusion, electronic and magnetic structure of selected materials have been studied. The major part of the present work is devoted to a very important material for semiconductor industry, GaAs. The formation energies of intrinsic point defects and the solution energies of 3d transitions in GaAs have been calculated from first principles. Based on the calculated energies, we analysed the site preference of defects in the crystal. The tendency of defects to form clusters has been investigated for the intrinsic defects as well as for impurities in GaAs. The magnetic moment of 3d impurities has been calculated as a function of the chemical environment. The possibility of increasing the Curie temperature in (Ga,Mn)As by co-doping it with Cr impurities has been examined on the basis of calculated total energy difference between the disordered local moment and the ferromagnetically ordered spin configurations. We found that, in order to reach the highest critical temperature, GaAs should be separately doped with either Cr or Mn impurities. Also, we have shown that diffusion barrier of interstitial Mn depends on the charge state of this impurity in (Ga, Mn)As. The formation of defect complexes between interstitial and substitutional Mn atoms, and their influence on the value of diffusion barrier for interstitial Mn, has been studied. The pair interactions energies between interstitial oxygen atoms in hcp Zr, Hf and Ti have been calculated using first principles. Based on the calculated energies, the oxygen ordering structures in IVB transition metal solid solutions have been explained. A prediction of nitrogen ordering in Hf-N solid solution has been made. The thermodynamic description of intermetallic compounds in the Zr-Sn binary system has been obtained. The conclusion has been made that Zr substitution on the Sn sites takes place in the Zr4Sn phase, which accounts for the unusual stoichiometry of this Cr3Si structure type compound. The influence of pressure on the phase stability in the Fe-Si system has been investigated. We have found instability of the hcp Fe0.9Si0.1 random alloy with respect to the decomposition onto the Si-poor hcp Fe alloy and the B2 FeSi under high pressure. The tendency of this decomposition becomes stronger with increasing the applied pressure. / QC 20100624

Magnetic properties of diluted magnetic semiconductors and magnetic metallic multilayers are investigated by SQUID magnetometry. By doping GaAs with magnetic Mn2+ ions under well defined growth conditions, one obtains a diluted magnetic semiconductor, (Ga,Mn)As, in which the randomly-distributed magnetic ions act as acceptor centers. At high enough dopant and hole concentration a carrier-induced ferromagnetic state results between the magnetic ions. Due to peculiarities of the growth process sizable amounts of donor defects, such as Mn interstitials and As antisites, are also introduced into the GaAs host. The magnetic properties of (Ga,Mn)As are altered by the presence of such defects through the compensation effect of the holes. The Mn interstitials are thermally unstable above a certain threshold temperature and therefore their concentration can be controlled by post-growth annealing. The influence of the interfaces on the magnetic moment of FeNi/V and FeNi/Co superlattices has been studied. A decrease of the `FeNi' magnetic moment at the interfaces is observed for FeNi/V superlattices while in case of FeNi/Co an enhanced magnetic moment is obtained at the interfaces. Changes of the interlayer exchange coupling have been studied in a series of Fe/V(Fe) multilayers in which the V spacer was alloyed with small amounts of Fe. The dynamic magnetic properties of discontinuous metal-insulator multilayers of Ni81Fe19/Al2O3 have been investigated. By varying the thickness of the insulator the system exhibits a superferromagnetic, a 3d spin-glass-like and a superparamagnetic behavior.