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Vc colocou: ferromagnetism, reação de combustão, BMP, spintrônica, combustion reaction spintonics on 2015-10-16T11:10:10Z (GMT) / Submitted by Cláudia Bueno (claudiamoura18@gmail.com) on 2015-10-16T16:35:28Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertação - Hermínia Veridiana dos Santos Pessoni e Silva - 2014.pdf: 14354082 bytes, checksum: d89e39503d04982864d158786be9e59b (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2015-10-19T14:33:30Z (GMT) No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertação - Hermínia Veridiana dos Santos Pessoni e Silva - 2014.pdf: 14354082 bytes, checksum: d89e39503d04982864d158786be9e59b (MD5) / Made available in DSpace on 2015-10-19T14:33:30Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertação - Hermínia Veridiana dos Santos Pessoni e Silva - 2014.pdf: 14354082 bytes, checksum: d89e39503d04982864d158786be9e59b (MD5) Previous issue date: 2014-05-13 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This work has as a general proposition the production of a DMS based on ZnO semiconductor and Co transition metal, as well as the study of the origin of ferromagnetism in these materials using the models of magnetic interaction in the literature. The combustion reaction was chosen as a synthesis route for the production of samples Zn1 - x Co x O with molar concentrations of x= 0,00, 0,005, 0,01, 0,03, 0,05, 0,07 and 0,09. The samples were obtained in the form of powders and chemical, structural and morphological characterization performed by EDS, DRX e MET measures. These characterizations indicated good crystallinity with the formation of the hexagonal crystalline phase wurtizita characteristic of ZnO without adding spurious phase for all molar concentrations. Powders presented nanometric dimension (~ 18 nm) with spherical particle shape and molars contractions of very similar elements of stoichiometric proposals. The optical characterizations via FTIR e UV-Vis confirmed the presence of the bond Zn - O and the replacement of Zn for Co in tetrahedral sites in ZnO crystal structure in the samples with x 0,00. By UV-Vis spectroscopy was also possible to obtain the optical bandgap values of the samples and confirm the semiconducting property, as well as calculate the dielectric constant of these materials. Magnetic characterizations performed by VSM measures (room temperature and high temperature) and Magnetometry at low temperatures indicated wide variety of magnetic systems in the studied temperature range. The x = 0,05 sample showed ferromagnetism across the temperature range of 5< Y (K) < 594. This same sample showed a value of 2,66 effective moment magnetic dipole and 2,52 for the saturation magnetization at low temperatures, that are significant results compared to several papers published. Through adjustments to the theoretical models, the ferromagnetism at low temperatures was attributed to the BMP magnetic interaction model. The room temperature ferromagnetismo was attibuted to the RKKY interaction model due to the high density of defects and free charge carriers in the samples. / Este trabalho teve como proposta geral a produção de um DMS com base no semicondutor ZnO e no metal de transição Co, bem como o estudo da origem do ferromagnetismo neste material utilizando de modelos de interação magnética existentes na literatura. A reação de combustão foi escolhida como rota de síntese para a produção das amostras de Zn1 -xCox O com concentrações molares de x= 0,00, 0, 005, 0,01, 0,03, 0,05, 0,07 e 0,09. As amostras foram obtidas na forma de pós e as caracterizações químicas, estruturais e morfológicas realizadas por meio de medidas de EDS, DRX e MET. Estas caracterizações indicaram boa cristalinidade com formação da fase cristalina hexagonal wurtizita característica do ZnO sem adição de fase espúria para todas as concentrações molares. Os pós apresentaram dimensão nanométrica (~ 18 nm) com formato esférico das partículas e contrações molares dos elementos muito próximas das propostas estequiometricamente. As caracterizações ópticas via FTIR e UV-Vis confirmaram a presença da ligação e a substituição do Zn por Co nos sítios tetraédricos da estrutura cristalina do ZnO nas amostras com x diferente 0,00. Pela espectroscopia UV-Vis também foi possível obter os valores de bandgap óptico das amostras e confirmar a propriedade de semicondutividade, bem como, calcular a constante dielétrica destes materiais. Já as caracterizações magnéticas realizadas via medidas de VSM (temperatura ambiente e a altas temperaturas) e Magnetometria à baixas temperaturas indicaram grande variedade de ordenamentos magnéticos em toda a faixa de temperatura estudada sendo ferromagnetismo identificado na faixa de temperatura de 5 < T (K) < 594 para a amostra x = 5. Essa amostra também apresentou valor de 2,66 para o momento de dipolo magnético efetivo e 2,52 para a magnetização de saturação à baixas temperaturas que são resultados expressivos em comparação à diversos trabalhos publicados. O ferromagnetismo à baixas temperaturas foi atribuído, por meio de ajustes com os modelos teóricos, ao modelo de interação magnética BMP, já o ferromagnetismo à temperatura ambiente ao modelo de interação RKKY devido à alta densidade de defeitos e portadores de cargas livres nas amostras.

Apresentaremos uma série de estudos realizados sobre o tema anisotropias magnéticas induzidas (Kind) em 1igas amorfas. Foram usados vários tipos de tratamentos térmico, com o objetivo de obtermos uma visão amp1a do assunto. A análise teórica dos dados de Kind e de \"after-effect\" magnético (MAE) foi feita com um modelo baseado em sistemas de dois níveis (TLS). A partir da análise dos dados experimentais obtém-se um largo espectro de energias de ativação. Estas energias estão relacionadas aos tempos de re1axação, através da equação de Arrhenius: = 0 exp (E/kT), onde o pré-fator 0 é da ordem do inverso da frequência de Debye. Construiu-se um forno para tratamentos térmicos em ligas amorfas, que opera em um eletroimã de 6 kOe e desenvolveu-se os programas para análise de dados experimentais. Essas facilidades, associadas ao traçador de curvas de histerese, permitiram os seguintes estudos de anisotropias induzidas em ligas amorfas ferromagnéticas: 1. Estudo da cinética de indução de anisotropia por tratamentos térmicos na faixa de temperatura de 190 a 250 °C, para fitas amorfas \"as cast\" de composição C070.4Fe4.6 Si15B10. Com o modelo TLS pode-se obter o espectro de energias de ativação, que se apresentou na faixa de energia de 1.50 a 1.85eV, com 0 = 1.6 x 10-13s. 2. Realizou-se também o estudo da indução de anisotropia na presença de um campo magnético de 5 kOe, para amostras pré-tratadas a 400 °C por 10 minutos. Estas são de composição Co77-X Mnx Si14 B9, com X = 2 e 6 e foram submetidas a tratamentos isotérmicos a temperaturas entre 240 e 325 °C. Neste caso pode-se verificar que para as duas composiQ6es 0 pré-fator é maior ( 10-8S). Quanto ao espectro de energias de ativação, em ambos os casos, apresenta-se na faixa de energias de 1.10 a 1.55 eV. 3.Outro experimento, neste caso com amostras de composição Co67Fe4Mo1Si12B16 consistiu na aplicação, à temperatura ambiente, de tensão mecânica ( 800 MPa) na fita, enquanto se monitorava a energia de anisotropia magnética. Pudemos então observar uma variação continua dessa propriedade e posteriormente uma recuperação completa da condição inicial, com a remoção da tensão aplicada, mostrando um processo de caráter anelástico. Procuramos estudar os efeitos de tensões mecânicas sobre as propriedades magnéticas de amostras com composição Co67Fe4Mo1Si12B16. Duas linhas de trabalho foram adotadas: 4) Numa olhou-se para a isotropia induzida em amostras pré-tratadas, quando submetidas a tratamentos térmicos na faixa de temperatura de 200 a 400 °C, sob tensão mecânica de 500 MPa e posteriormente na ausência de tensão. A partir desses ensaios foi possível se separar uma componente plástica e outra anelástica na anisotropia induzida. 5) Noutra, obteve-se o comportamento do MAE, para amostras com e sem tensão mecânica aplicada, no intervalo de 300 a 500 K. Os resultados obtidos demonstram a não existência de efeitos plásticos ou anelásticos nas energias de ativação dos processos envolvidos no \"after-effect\". Por outro lado verificamos uma grande alteração na intensidade do MAE, devido à tensão. / In this thesis we will present a series of studies related to induced magnetic anisotropies (Kind) in amorphous alloys. In order to get a more general view of this theme, we used several different kinds of annealings. The theoretical analysis of the data of Kind and magnetic after-effect (MAE) was performed using a model based on two-level systems (TLS). From the analysis of the experimental data we get a large actiyation energy spectrum. These energies are related to the relaxation times, through the Arrheniu \'s expression: = 0 exp(E/kT), where the pre-factor 0 is of the order of the inverse of the Debye frequency . We constructed a furnace for thermal annealing of the amorphous alloys, which operates within an electromagnet producing 6 kOe. We also wrote, the computer programs for the analysis of the experimental data. These facilities, together with the hysteresis loop tracer permitted the followings studies of the induced anisotropies in amorphous alloys: 1) A study of the kinetics of the induced anisotropy by annealing in the temperature range from 190 to 250 °C, in as cast amorphous ribbons of composition Co70.4Fe4.6Si15B10. Using the TLS model we obtained the activation energy spectrum. It presents two peaks in the energy range from 1.50 to 1.85eV and a pre-factor 0= 1.6x10-13 s. 2) Using a 5 KOe magnetic field we studied the effects of a Field annealing treatment in samples pre-annealed at 400 °C for 10 minutes. The isothermal annealings were made in Co70-XMnXSi14B9 with x = 2 and 6, in the temperature range from 240 to 325 °C. In this case we observed for these two compositions a larger pre-factor ( 10-8s) than before. The activation-energy spectra, for the both composition, are found in the energy range from 1.10 to 1.55eV. 3) Another experiment was done using samples of Co67Fe4Mo1Si12B16. We applied a tensile stress (800MPa) to the ribbon and measured the magnetic anisotropy energy. We observed a continuous variation of this energy and, after removal of the stress, the sample recuperated its initial condition, showing a process characteristically anelastic. We studied the effects of mechanic stress on the magnetic properties of samples of composition Co67Fe4Mo1Si12B16. We worked in two directions: 4) We studied the induced anisotropy in pre-annealed samples, submitted to annealing in the range from 200 to 400 °C, under a tensile stress of 500MPa and without applied stress. From these resul ts we can separate a plastic and an anelastic component in the induced anisotropy. 5) In other, we studied the behavior of the MAE, for samples with and with and without applied tensile stress, in the range from 300 to 500 K. The results obtained show neither plastic nor anelastic effects on the activation energies of the processes involved in the MAE. On the other h~nd we can see a strong alteration in the intensity of the MAE, due the stress.

Le but de la thèse est de modéliser la génération d’ondes élastiques ultrasonores émises par EMATs dans une pièce ferromagnétique, modélisation appliquée au domaine du contrôle non destructif (CND). Les traducteurs EMATs combinent deux physiques différentes : électromagnétisme et élastodynamique. L’enjeu est d’intégrer dans la plateforme de simulation CIVA, dédiée notamment aux CND par courant de Foucault et par ultrasons, les éléments de modélisation inhérents à la problématique multi-physique posée. Ces éléments sont multiples et concernent premièrement la modélisation des forces électromagnétiques créées par un EMAT : la force de Lorentz, existant dans tous milieux conducteurs, et les forces d’aimantation et de magnétostriction, spécifiques aux milieux ferromagnétiques. Deuxièmement, la modélisation proposée prend aussi en compte le phénomène de création d’harmoniques, phénomène dû aux différentes forces et traduisant le fait que les fréquences de l’onde ultrasonore émise peuvent être des harmoniques des fréquences du signal d’excitation de l’EMAT. Le modèle de forces et de création d’harmoniques permet de modéliser des situations de contrôle non prises en compte par les modèles de la littérature, notamment lorsque les champs statiques créés par l’EMAT sont faibles ou lorsque le courant d’excitation possède une forte intensité. Enfin, les forces électromagnétiques sont transformées en contraintes surfaciques équivalentes pour correspondre au formalisme des données d’entrée des modèles de rayonnement des ondes élastiques implémentés dans CIVA. L’outil informatique développé permet donc de traiter toute configuration et condition d’utilisation des EMATs, constituant un outil d’optimisation de leur conception. Les caractéristiques électromagnétiques de différents matériaux, données par l’expérience, sont utilisées pour mener diverses études paramétriques. / The present study aims at modelling the ultrasonic wave generation by EMATs into a ferromagnetic part in the context of non-destructive testing (NDT). Wave radiation by EMATs combines two different kinds of physical phenomena: electromagnetism and elastodynamics. The issue is to implement into the NDT simulation platform CIVA, dedicated in particular to eddy current and ultrasonic NDT, the models developed in the present work. These developments are of various nature and firstly concern the modelling of the electromagnetic forces created by an EMAT: the Lorentz force, existing in any conductive media, and the magnetization and the magnetostriction forces, which add up when the medium is further ferromagnetic. Secondly, the proposed models take into account the generation of harmonics due to the three forces. Ultrasonic frequencies of the generated ultrasonic waves can be harmonics of the frequencies of the excitation electrical signal. The model of force and harmonic generation allows us to deal with configurations not treated in the literature, namely, when the static field created by the EMAT is low or when the excitation current intensity is high. Finally, the three electromagnetic forces are transformed into equivalent surface stresses readily usable as entries of an existing model of ultrasonic wave radiation already implemented in CIVA. Thus, the combination of all these models and their translation as a numerical tool running within the CIVA platform can be used for optimizing the design of EMAT. Experimentally measured electromagnetic characteristics of different materials are used to perform various parametric studies.

Cette thèse est consacrée à l’étude de la structure et des propriétés magnétiques de nanofils ferromagnétiques de diamètre entre 2 et 7 nm, formés par auto-assemblage dans des couches minces de CeO2 ou SrTiO3 déposées sur substrat SrTiO3(001).Grâce à la Spectroscopie d’Absorption des rayons X (XAS) ainsi que la Magnétométrie à l’échantillon Vibrant (VSM) nous avons étudié des nanofils de Co, Ni et CoNi. Cette étude montre qu’à température ambiante et à pression atmosphérique les nanofils de Co peuvent exister dans les phases hcp et/ou fcc alors que l’ajout de Ni stabilise la structure fcc.Nous présentons aussi la stabilité de la structure et des propriétés magnétiques en fonction de la température et nous décrivons en particulier les effets d’une procédure de recuit oxydant, suivi d’un recuit sous vide comme un possible outil de modulation de la structure et des propriétés des nanofils.Les études XAS mettent en évidence l’existence de dichroïsme linéaire pour des nanofils de Co et CoNi avec des diamètres extrêmement faibles (moins de 2nm). Nous en discutons les possibles origines de ce dichroïsme : effets d’étirement et de taille finie.In fine, une tentative de croissance auto-organisée de couches minces de CeO2 ou SrxBayTiO3 dopées au fer est présentée. Dans la matrice CeO2 la microscopie électronique à transmission ne détecte pas de nanofils et le XAS montre une grande variété d’oxydes de fer différents, malgré des conditions de croissance très proches. La formation de nanofils en matrice SrxBayTiO3 est suggérée par les propriétés magnétiques (hystérèse et anisotropie) et la composition des nanofils déterminée par le XAS met en évidence les phases α-Fe et FeO. / This PhD thesis is dedicated to the structural and magnetic studies of self-assembled ferromagnetic nanowires with diameters of 2-7 nm embedded in CeO2 (or SrTiO3) thin films grown on SrTiO3 (001) substrates.By means of X-rays Absorption Spectroscopy (XAS) and Vibrating Sample Magnetometry (VSM) Co, Ni and CoNi nanowires are studied. This study shows that at ambient temperature and pressure cobalt nanowires may contain Co-hcp or Co-fcc phase, while adding Ni stabilizes the fcc.This study also discusses structural and magnetic stability as a function of temperature under different thermal treatments. We describe in particular the effects of oxidative treatment followed by high vacuum annealing as a tool to tune the structure and magnetic properties of cobalt nanowires. XAS evidences linear dichroism in Ni and CoNi embedded nanowires of extremely small diameter (less than 2nm). We discuss possible origins of these phenomena considering the effects of strain and finite size of the objects.Finally, an essay of growing CeO2 or SrxBayTiO3 thin films doped by Fe is presented. It shows that within the CeO2 matrix no nanowires are evidenced by TEM, while XAS shows a large variety of different iron oxides even if the growth conditions are not drastically modified from one sample to another. As it is suggested by the magnetic measurements (hysteresis and anisotropy), formation of nanowires in SrxBayTiO3 matrix is very likely and the nanowires composition determined by XAS evidences a mixture of αFe and FeO phases.

No description available.

Disorder in materials often brings in new and exotic physical properties along with it. It is thus very important to study different kinds of disorder and their implications on various material properties. In this thesis, we study selected transition metal oxide families of compounds, each being associated with a specific kind of disorder and investigate effects of that disorder on their dielectric and magnetic properties. In Chapter1, we have given brief introductions on the origin of magnetic and dielectric properties in materials and have also discussed various mechanism which give rise to multiferroism in materials which exhibit both spontaneous magnetic ordering and spontaneous electric ordering in the same phase of the material. In Chapter2,we describe the various methodologies adopted in this thesis. In Chapter3,we mainly study the effect of cationic size-disorder in deciding un-usually robust dielectric properties of Ln2CuTiO6(Ln=Y,Dy,Ho,Er,Yb) family of compounds. We discover that these materials, in addition to possessing large dielectric constant values, also exhibit exceptional stabilities of their dielectric constants with respect to large changes in temperature and frequency. We further find that this class of materials are non-ferroelectrics though its hares the same non-centrosymmetric space group, P63cm,with the well-known multiferroic YMnO3 that undergoes a ferroelectric transition at 940K. Using first principles calculations, we establish that exceptional dielectric properties result from a combination of two separate effects. Extensive size disorders at the Cu/Ti B-site suppress the expected ferroelectric transition, leading to relatively large values of the dielectric constant for every compound investigated in this series. Additionally, it is shown that the majority contribution to the dielectric constant arises from intermediate-frequency polar vibrational modes, making it relatively stable against temperature variations. In Chapter4, we study the effect of cation anti-site disorder on the magnetic, electric and dielectric properties of the solid solution series of (x)Fe2O3-(1-x)FeTiO3 for several values of x. For intermediate values of x, these solid solution members are found to be strong ferrimagnets. Anti-site cation disorder, between Fe and Ti, however strongly reduces the magnetic moment values. By tuning the degree of cation anti-site disorder, we attain multi-functionality in these samples. We have performed detailed characterizations of valence states of Fe and Ti across the solid solution series using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. Using x-ray magnetic circular dichroism, we validate the microscopic model of magnetism and suggest a microscopic picture of the anti-site cation disorder for these samples. In Chapter5,we study the effects of controlled chemical disorder in SrTiO3 lattice, by performing site-specific doping of Mn in SrTiO3. We find that site specific Mn doping has decisive influence on their dielectric properties with qualitatively and quantitatively different behaviors between these doped samples .Using electron paramagnetic resonance experiments, we establish the site specific doping of Mn in SrTiO3 lattice. We find that while Mn doping at Ti site continues to remain paraelectric, Mn doping at the Sr site becomes a relaxor ferroelectric. We find samples having Mn substituted at both Sr and Ti sites simultaneously to be also relaxor ferroelectrics. Combining experiments with first principles calculations, we understand the origin for the high temperature dielectric properties of various Mn doped SrTiO3 samples. We show that Mn ions doped at the Sr sites off-centers and gives a significant dipolar contribution to their dielectric constants. While demonstrating the superior dielectric properties of Mn doped SrTiO3 ceramics ,we also elucidate their magnetic properties in details. In Chapter6,we study the effect of cation anti-site disorder on the magnetic and dielectric properties of undoped and Lu doped La2NiMnO6 samples. Using detailed spectroscopic characterizations of these samples with x-ray absorption experiments, coupled with d.c.magnetization and a.c.susceptibility measurements, we demonstrate that while the doped samples are ferromagnetic, the undoped samples exhibit re-entrant spin-glass magnetism. We also show that the dielectric properties of undoped La2NiMnO6,crys-tallizing in monoclinic and rhombohedral phases are distinctly different and we study their dielectric relaxations in details. We also demonstrate multiferroism in Lu doped La2NiMnO6 samples. In Appendix A, we study the electronic origin of ferroelectric polarization in the spin spiral compound ,MnWO4. Using x-ray absorption spectroscopy on synthesized MnWO4 samples, coupled with configuration interaction calculations, we establish quantitatively a significant population of Mn 3d states beyond the expected half filling and provide a critical insight into the significant presence of spin-orbit coupling and consequent finite polarization in this system. In Appendix B, we study charge-transfer doping in few-layer grapheme covered with electron acceptor (Tetracyanoethylene) and donor (Tetrathiafulvalene) molecules using x-ray photoelectron spectroscopy. We give quantitative estimates of the extent of doping in these samples and thereby elucidate the origin of unusual shifts of their Raman G bands in contrast to electrochemically doping schemes. In conclusion, we investigate, in this thesis, properties of different classes of compounds in presence of distinctly different kinds of disorder and establish the critical role of disorder in each case in tuning their desirable physical properties.

Válvulas de Spin têm sido utilizadas na fabricação de sensores magnéticos e memórias de acesso randômico, sendo muito importantes do ponto de vista tecnológico. Neste trabalho, foram exploradas as análises das curvas de reversão de primeira ordem da magnetorresistência (MR-FORC), bem como ajustes das curvas de histereses da magnetização e magnetorresistência, para estudar o fenômeno de exchange-bias, anisotropia magnética e propriedades magnetorresistivas. As válvulas de spin estudadas foram do tipo NiFe/Cu/NiFe/IrMn, tendo camadas semente e de cobertura de Ta, preparadas por sputtering. Um modelo fenomenológico de parede de domínios no material antiferromagnético (AFM) foi utilizado, levando em conta as anisotropias magnéticas e interações entre as camadas. Também foram consideradas certas dispersões da anisotropia dos grãos ferromagnéticos (FM) e antiferromagnéticos (com distribuições Gaussianas) em torno dos respectivos eixos de anisotropia uniaxiais. Para o ajuste da magnetização para algumas amostras, foi necessário utilizar uma rotação no plano de um ângulo nos eixos de anisotropia uniaxiais do FM e AFM, em relação à direção do campo magnético aplicado durante a deposição dos filmes. Bons ajustes das curvas de histereses das magnetizações foram obtidos nas direções medidas do campo magnético aplicado. Um método baseado em medidas de variações angulares da magnetorresistência em campos constantes foi proposto para extrair este ângulo para cada amostra. Foram obtidas razoáveis concordâncias entre estes ângulos e os correspondentes extraídos dos ajustes das curvas de magnetização. Através da análise dos diagramas da MR-FORC e de simulações indicados dos resultados dos ajustes das histereses da magnetização, foi encontrada uma relação direta entre os campos de interação (e suas incertezas) com os campos de exchange-bias (HEB) dos grãos da distribuição (extraídos das simulações, usando a largura da distribuição obtida do ajuste). Resumindo, esta análise mostrou que esta técnica permite extrair informações comparativas sobre a dispersão dos eixos de anisotropia dos grãos FM e AFM em torno do eixo de anisotropia uniaxial, o que pode ser importante na caracterização dos sensores magnetorresistivos. Além disso, análise dos diagramas MR-FORC indicaram início da presença de descontinuidade na camada de NiFe presa em 27, com um aumento acentuado (acima do previsto) para a amostra com 25. Este aumento acima do previsto corrobora com nossa hipótese. As simulações das curvas de histerese da magnetorresistência não foram muito bons, indicando que melhorias devem ser introduzidas no modelo utilizado para a simulação da histerese da magnetorresistência, obtidos a partir dos ângulos das camadas ferromagnéticas livre e presa. A questão referente a presença em algumas das amostras de um desalinhamento entre os eixos fácéis do FM e do AFM ainda é uma questão em aberta, mas neste trabalho foi encontrado que este ângulo é igual a 2. / Spin Valves have been employed as magnetic sensors and used in random access memories, showing they are very important in terms of technological point of view. In this work, analyses of the magnetoresistance first order reversal curves (MR-FORC) have been used, as well as fittings of the magnetization and magnetoresistance hysteresis, to study the exchange-bias phenomena, magnetic anisotropies and magnetoresistance in spin valves. Sputtering has been used to the deposition of NiFe/Cu/NiFe/IrMn, and Ta has been deposited as seed and buffer layers. A domain wall model (in the antiferromagnetic layer) taking into account the magnetic anisotropies and the interactions between the layers has been employed to fit the magnetization hysteresis. Some textures have been also introduced to take into account the ferromagnetic (FM) and antiferromagnetic (AFM) grains dispersion (with Gaussian distributions) centered around the respective uniaxial anisotropy axes. However, to obtain good fits for some samples, it has been necessary to include an in-plane rotation of an angle of the both FM and AFM easy axes in relation to the field direction applied during the growing of the films. Good fits of the magnetization hysteresis have been obtained for all measured directions of the applied field. A new method based on the angular variation of the magnetoresistance to constant fields has been proposed to extract directly these angles. Reasonable agreements have been obtained between these angles and the corresponding ones extracted from the fits of the magnetization loops. Through the analyses of the MR-FORC and from the simulations indicated by the parameters (obtained from the fittings of magnetization loops), a direct relation between the interaction fields (and its uncertainties) and the exchange-bias fields of the grains of the distribution (extracted from the simulations, using the width of the distribution obtained from the magnetization fittings) has been identified. In summary, this analysis has showed that this technique allows to extract comparative information about the dispersion of the anisotropy axes of the FM and AFM grains around the uniaxial axis, which can be very import to the characterization of spin-valve based sensors. Besides, MR-FORC analyses have also indicated the presence of a threshold of discontinuity of the pinned NiFe layer at 27, showing a huge increase (above of the expected) to the sample at 25, and this unexpected increasing has corroborated with our hypothese. Simulations of the magnetoresistance loops have not been good, indicating that improvements should be included in the model employed to simulate these curves, obtained from the pinned and free angles of the NiFe layers. Concerning the case of the presence of misalignments of FM and AFM for some samples, it is still an open question, but in this work, we have found that this angle () is equal to 2.

Titanium is one of the earth-abundant elements, and its oxides including titanium dioxide (TiO2) and strontium titanium oxide (SrTiO3) are widely used in technologies of electronics, energy conversion, catalysis, sensing, and so on. Generally, the Ti ions in these compounds have a valence of 4+ with the outer shell electron configuration of 3d0. In this thesis, we explore interface and titanite containing Ti3+ ions with 3d1 itinerate electrons, which we believe open new doors towards some new titanite-based technologies. In the first part of this thesis (Chapter 3), we will discuss the nanoscale chemical and valence evolution at a metal/oxide interface: Ti/SrTiO3. In many devices, metal-oxide interfaces are ubiquitous and play important roles in the performance of a wide range of electronic and optoelectronic devices. This motivated us to examine the microscopic structure of the interfaces between strontium titanium oxide and metals. In this work, one unit cell of cubic perovskite Ti2O3 was observed at the Ti/SrTiO3 interface, and oxygen diffusion depth of ~3.2 nm was observed in the sample fabricated at room temperature. Meanwhile, oxygen vacancy domains in the SrTiO3 substrates was observed and characterized by low angle annular dark field (LAADF) imaging and electron energy loss spectra (EELS). In the main part of this thesis, we will focus on the structure and physical properties of Ti2O3, a titanite which has received less attention so far in the research community. Different from TiO2 and SrTiO3, Ti2O3 has a much narrower band-gap (~0.1 eV), and we will discuss some preliminary results of its physical properties and potential applications. In Chapter 4, we will discuss the photothermal application and mid-infrared photodetectors using Ti2O3 nanoparticles based on its ultra-narrow bandgap. Photo-thermal effect via a Ti2O3/membrane structure is further applied to seawater desalination. A high temperature of 70 °C was achieved when this Ti2O3/membrane double layer structure floating on top surface of water subjected to the white light illumination of 7 kW/cm2. Furthermore, room temperature mid-infrared (10 μm) photodetectors based on Ti2O3/graphene hybrid structure was fabricated and studied. The photoresponsivity of this hybrid device, operated from 4.5 to 10 μm, is above ~ 100 A/W, which, to our knowledge, is the highest value for the mid-infrared photodetectors operating in the photocurrent (PC) mode. In chapter 5, structure, optical, transport properties of Ti2O3 epitaxial thin films on sapphire fabricated by pulsed laser deposition (PLD) will be discussed. By tailoring growth conditions, two different: trigonal and orthorhombic, of Ti2O3 were stabilized on Al2O3 substrates. More interestingly, the orthorhombic Ti2O3 has never been reported, and, moreover, superconductivity (~8 K) and high temperature ferromagnetism (up to 700 K) was discovered in this new stabilized phase. More details of the physical properties of Ti2O3 will be discussed in the following chapters of this dissertation.

Tato práce je zaměřená na studium magnetických vlastností tří r·zných sloučenin na bázi uranu obsahujících 5f elektrony (U4Ru7Ge6, UAu2Si2 a UIrGe). V rámci této práce byly, za pomoci r·zných metod, připraveny vysoce kvalitní krystaly těchto sloučenin. Vlastnosti těchto systém· byly studovány r·znými objemovými meto- dami (magnetizace, měrné teplo, teplotní roztažnost, elektrický transport) a také neu- tronovým a rentgenovým rozptylem při širokém spektru vnějších podmínek (nízká teplota, vysoké magnetické pole, vysoký tlak). Kombinace těchto metod odhalila komplexní chování těchto systém· a pomohla sestrojit jejich magnetické fázové diagramy. První studovanou sloučeninou je feromagnet U4Ru7Ge6vykazující velmi nízkou mag- netokrystalovou anizotropii, která je neočekávaná pro sloučeninu na bázi uranu. To se promítá do izotropní závislosti téměř všech měřených fyzikálních vlastností. Bylo zjištěno, že osa snadné magnetizace se mění v uspořádaném stavu a tento jev je spojen s anomáliemi v teplotní roztažnosti ukazující na možnou rhomboedrickou distorzi. Ta vede k vytvoření dvou odlišných pozic uranu s odlišným magnetickým momentem. Tato před- pově¤ byla potvrzena teoretickými výpočty a pomocí polarizované neutronové difrakce. Rozdíl magnetických moment· na dvou odlišných uranových pozicích je zp·soben...

This thesis focuses on the study of magnetic properties of three 5f electron itinerant ferromagnets UCo0.990Ru0.010Al, UCoGa and URhGa and investigation of their phase diagrams. The single crystals of high-quality were prepared by Czochralski method for all three compounds. The physical properties at ambient pressure were studied by macroscopic methods (magnetization, electrical transport and heat capacity measurements) and also by magnetic force microscopy (MFM). The measurements were done under various external conditions (high pressure, low temperatures, high magnetic field). Through all these measurements and external conditions we investigated the interesting physical properties and the ferromagnetic phase diagrams. Effect of different conditions during the preparation and the thermal treatment on UCoGa was studied on two different single crystals. We show that annealing leads to improved quality of samples and the gallium evaporation from the melt during the growth leads to lower quality in parts of the ingot closer to melt. MFM images of UCoGa below the ordering temperature show domain branching and narrow magnetic domains wall made only of neighboring atoms with opposing moments. We have grown first ever single crystal of URhGa with ferromagnetic ordering temperature TC = 41 K. Anomalous maximum in...