Resumo: O estudo dos regimes dinâmicos de vórtices em supercondutores do tipo II tem atraído grande interesse do ponto de vista teórico e experimental, por um lado pela riqueza de comportamentos que é apresentado e, por outro, pela compreensão dos mecanismos de pinning, o qual é utilizado para aumentar e manter a densidade de corrente crítica, permitindo seu uso em aplicações práticas. Usando simulações de dinâmica molecular, analisamos os efeitos de pinning artificiais na dinâmica de vórtices em filmes finos supercondutores do tipo II. Além da distribuição aleatória de centros de pinning, dois tipos de arranjos periódicos foram considerados: hexagonal e kagomé. Para o arranjo kagomé analisamos a dinâmica para a força externa aplicada nas direções transversal e longitudinal em relação à simetria da rede. Em cada caso são observados comportamentos dinâmicos distintos da rede de vórtices, uma vez que existem simetrias diferentes para cada direção. Diferentemente dos resultados obtidos para a distribuição aleatória e hexagonal de pinnings, para o arranjo de kagomé, a rede de vórtices apresenta uma dinâmica e um segundo pico na resistência diferencial ainda não descritos na literatura. Quando a força é aplicada na direção longitudinal, são observados regimes dinâmicos complexos da rede de vórtices. Para esse caso, além dos regimes ordenados típicos de redes periódicas de pinnings, os vórtices se movem em canais desordenados quando a corrente de transporte é aumentada, lembrando o regime smectic visto em simulações com pinning aleatório. Subsequentemente, para altas correntes, os vórtices atingem um regime dinâmico onde eles novamente se movem em canais bem definidos, revelando uma ordem transversal. Para o arranjo hexagonal, os vórtices sempre se movem em linhas retas. Apresentamos também um estudo da densidade de corrente crítica para diferentes valores de densidade de vórtices. / Abstract: The study of current-driven vortex lattices in type II superconductors has attracted a great interest from both experimental and theoretical point of view, on the one side for the riches of behaviors that is showed and, otherwise, for the comprehension of pinning mechanisms, which has been used to increase and to maintain the critical current density, to allow their use in practical applications. Using molecular dynamics simulations, we analyze the effects of artificial pinning on the vortex dynamics of type II superconductors thin films. Yonder random distribution of pinning centers, two types of periodic pinning arrays are considered: the hexagonal and kagomé. For the kagomé array, we analyze the dynamics for the applied external force in the transversal and longitudinal directions to the lattice symmetry. For each case are observed different dynamical behaviors of vortex lattice, once there are different symmetries for each direction. Differently than the results obtained for the random and hexagonal distribution of pinnings, for the kagomé array, the vortex lattice show a dynamic and a second peak in the differential resistance still not reported in the literature. When the force is applied in the longitudinal direction, the dynamical behavior of vortices is much more complex. For this case, yonder the ordered regimes, typically observed in systems with periodic pinning, the vortices moving in disordered channels when the transport current is increased, resembling the smetic dynamical regime that was found in simulations for random pinning distributions. Subsequently, for high currents, the vortices reach dynamical regime where they move in well defined channels again, revealing transversal ordering. For the hexagonal array, the vortices always move in straight trajectories. We also show a study of critical current density for different values of vortex density. For all densities, the hexagonal pinning network shows higher critical currents. / Orientador: André Luiz Malvezzi / Coorientador: Pablo Antonio Venegas Urenda / Banca: Ladir Candido da Silva / Banca: Paulo Noronha Lisboa Filho / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Mestre

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After nearly 15 years of research effort, High Temperature Superconductors (HTS) are finding a wide range of practical applications. A clear understanding of the factors controlling the current carrying capacity of these materials is a prerequisite to their successful technological development. The critical current density (Jc) in HTS is directly dependent on the structure and pinning of the Flux Line Lattice (FLL) in these materials. This thesis presents an investigation of the Jc anisotropy in HTS. The use of thin films grown on off c-axis (vicinal) substrates allowed the effect of current directions outside the cuprate planes to be studied. With this experimental geometry Berghuis, et al. (Phys. Rev. Lett. 79,12, pg. 2332) observed a striking flux channelling effect in vicinal YBa2Cu3O7-δ (YBCO) films. By confirming, and extending, this observation, it is demonstrated that this is an intrinsic effect. The results obtained, appear to fit well with the predictions of a field angle dependent cross-over from a three dimensional rectilinear FLL to a kinked lattice of strings and pancakes. The pinning force density for movement of strings inside the cuprate planes is considerably less than that on vortex pancake elements. When the FLL is entirely string-like this reduced pinning leads to the observed channelling minima. It is observed that anti-phase boundaries enhance the Jc in vicinal YBCO films by strongly pinning vortex strings. The effect on the FLL structure cross-over of increasing anisotropy has been elucidated using de-oxygenated vicinal YBCO films. Intriguingly, the counter intuitive prediction that the range of applied field angle for which the kinked lattice is fully developed reduces with increasing anisotropy, appears to be confirmed. Although vortex channelling cannot be observed in c-axis YBCO films, the pinning force density for vortex string channelling has been extracted by observing string dragging. By studying the effect of rotating the applied field at a constant angle to the cuprate planes, it is possible to observe the cross-over into the string pancake regime in c-axis films. In the 3D region, the observed behaviour is well explained by the anisotropic Ginzburg-Landau model. Measurements were also made on thin films of the much more anisotropic Bi 2Sr2CaCu2O8+x material, grown on vicinal substrates. The absence of any flux channelling effect and clear adherence to the expected Kes-Law behaviour in the observed Jc characteristics does not provide evidence for the existence of the predicted 'crossing lattice' in Bi 2Sr2CaCu2O8+x.

Superconductivity and ferromagnetism are two antagonistic physical phenomena which their coexistence in a uniform material can be resolved only under extraordinary conditions. The reason for that is the phonon-mediated attraction energy between electrons which results in the formation of the so-called Cooper pairs, is usually smaller that the exchange (Zeeman) interaction between electrons which tend to align the electron spins. However, non-zero total momentum Cooper pairs can be accomplished even in the presence of an exchange field as surprisingly! predicted first by Fulde and Ferrel [1] and independently by Larkin and Ovchinikov [2] nearly 50 years ago. This coexistence has already been observed experimentally in both bulk samples [3, 4] and in thin films [5-7] which result from a different type of electron-pairing mechanism which electrons with spin pointing in the same direction team up to form Cooper pairs with one unit of spin, resulting in the so-called triplet superconductivity. Apart from this so-called ferromagnetsuperconductors which both superconducting and ferromagnetism order parameters are present in a uniform material, hybrid systems [8] are made form materials with different or even mutually exclusive properties. Therefore the overall property can be strongly affected by the interaction between constituent materials. The present work, concerns such a hybrid system where Nb, a superconducting metal having transition temperature below 9.5K, is placed in contact with a ferromagnetic metal, Gd with bulk Curie temperature of around 290 K in a form of a nanocomposite. The mutual immiscibility of these two elements gives us the opportunity to take advantage of both the superconduction and ferromagnetism properties of the constituents and further study the transport and magnetic behavior of the system and their effects on each other specially on the critical current of the superconductor which is expected to be modified by the proximity of the ferromagnetic metal.

Recent theoretical studies have suggested that the observed suppression of superconductivity in superconductor/ferromagnet (S/F) heterostructures could be modulated by controlling the ferromagnetic exchange interaction in the superconductor. The exchange interaction in the superconductor is the sum of the exchange interaction from the ferromagnetic regions, which has a phase and magnitude that depends upon the direction that the ferromagnet is magnetised and the distance. As the exchange interaction has a phase it is possible that the contribution from two regions will cancel out. The exchange interaction, which can be viewed as an imbalance in the spin populations, suppresses superconductivity so any reduction in the exchange interaction will increase the superconducting transition temperature (Tc) of the heterostructure. Thus by changing the magnetisation of the ferromagnetic regions it is possible to control the exchange interaction in the superconductor and so the superconducting properties of the heterostructure. I have measured the superconducting properties of niobium/cobalt bilayers as a function of the applied magnetic field. I have observed that one component of the superconducting properties is controlled by the net magnetisation rather than the magnitude of the applied field. I have been able to show that this component of the observed change in the superconducting properties is not simply due to stray magnetic field but that the superconductivity is being actively controlled by means of the exchange interaction. This is the first experimental evidence that the superconducting properties of a S/F heterostructure can be controlled in this way, which opens up the possibilities for the construction of future devices.

YBa2Cu3O7-d (YBCO) was one of the first high temperature superconductors discovered, and its superconducting properties are strongly dependent on oxygen stoichiometry. A large amount of work has been done on the variation of stoichiometry and its effect on the superconducting properties of the material. However, in spite of all the work done, the results published in the literature are very scattered. This thesis presents a thermogravimetric study of oxygen diffusion in YBCO under isothermal and non-isothermal conditions and tries to reconcile the data available based on the results obtained and taking into account the factors that may have affected the data presented by other groups, such as the effects of the microstructure and the different diffusion coefficients measured with the techniques used. An Arrhenius expression for the chemical diffusion of oxygen has been calculated from the analysis of isothermal oxygenation data, and it has been corroborated by a study of the nonisothermal experiments carried out. This work includes the development of a macroscopic model for oxygen diffusion in YBCO based on the diffusion coefficient calculated from experimental data. The model is used to simulate for the first time oxygenations under both isothermal and non-isothermal conditions. The study of non-isothermal oxygenations has led directly to the design of novel cooling procedures that can be introduced at the end of the processing stage of YBCO samples, producing highly oxygenated specimens in shorter times than for conventional isothermal and ramped oxygenation procedures. The final section of this dissertation presents a study of the Direct Current Zoning effect. The generation of a mobile hot zone in a polycrystalline YBCO bar when passing a current across it is directly linked to the diffusion of oxygen ions in the material. A mechanism for the motion of the zone along the sample has been suggested. A computer model has been developed to reproduce this process taking into account the motion of ions due to chemical diffusion and the potential difference established. The results from this model have corroborated the mechanism suggested and give for the first time the opportunity to study this phenomenon in more detail.

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Cílem této práce je studium systému UCo1−xRuxGe nacházejícího se na rozhraní magnetické nestability. Podrobná studie magnetických a transportních vlastností po- lykrystal· s r·zným obsahem ruthenia odhalila komplexní změnu těchto parametr· v rámci celé série. UCoGe je slabý ferromagnet s TC ∼ 3 K a malým magnetickým momentem 0.035 µB/f.u. Pozorovali jsme strmý nár·st těchto hodnot až po hodnotu koncentrace ruthenia xmax ≈ 0.1(TC,max = 9 K a µ0 = 0.11 µB). Podstata tohoto ná- r·stu byla studována na mono krystalu o složení UCo0.88Ru0.12Ge získaném metodou plovoucí zóny. Difrakce polarizovaných neutron· provedená na tomto krystalu se slo- žením UCo0.88Ru0.12Ge vysvětlila posílení magnetismu změnou vzájemné orientace magnetických moment· na iontu uranu a kobaltu. Antiparalelní uspořádání, které bylo dříve publikováno pro UCoGe je změněno na paralelní uspořádání v dopova- ném UCo0.88Ru0.12Ge kdy dojde k reorientaci momentu na kobaltu. Silná anizotropie UCo0.88Ru0.12Ge se promítá do teplotní závislosti odporu a teplotní roztažnosti, které se dramaticky liší pro všechny tři osy. Tento prvotní nár·st TC a magnetického mo- mentu je následován poklesem směřujícím ke koncentraci xcr ≈ 0.31 kde magnetické uspořádání mizí. Podrobná studie kritických exponent· teplotních závislostí elek- trického odporu,...