Influência das propriedades estruturais e magnetostrictivas na dinâmica da magnetização de multicamadas magnéticas amorfas

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Representações espectrais de sistemas complexos: aplicações à síntese de superfícies brownianas fracionárias anisotrópicas, filtragem de sinais e identificação de correlações

In this thesis, we study the application of spectral representations to the solution of problems in seismic exploration, the synthesis of fractal surfaces and the identification of correlations between one-dimensional signals. We apply a new approach, called Wavelet Coherency, to the study of stratigraphic correlation in well log signals, as an attempt to identify layers from the same geological formation, showing that the representation in wavelet space, with introduction of scale domain, can facilitate the process of comparing patterns in geophysical signals. We have introduced a new model for the generation of anisotropic fractional brownian surfaces based on curvelet transform, a new multiscale tool which can be seen as a generalization of the wavelet transform to include the direction component in multidimensional spaces. We have tested our model with a modified version of the Directional Average Method (DAM) to evaluate the anisotropy of fractional brownian surfaces. We also used the directional behavior of the curvelets to attack an important problem in seismic exploration: the atenuation of the ground roll, present in seismograms as a result of surface Rayleigh waves. The techniques employed are effective, leading to sparse representation of the signals, and, consequently, to good resolutions

Efeito Kerr magneto-óptico; Ressonância ferromagnética; Acoplamento bilinear; Acoplamento biquadrático

In this work, we investigated the magnetic properties of a monocrystalline Fe thin film and of Fe(80 Å)/Cr(t)/Fe(80 Å) tri-layers, with the nonmagnetic metallic Cr spacer layer thickness varying between 9 Å < t < 40 Å. The samples were deposited by the DC Sputtering on Magnesium Oxide (MgO) substrates, with (100) crystal orientation. For this investigation, experimental magneto-optical Kerr effect (MOKE) magnetometry and ferromagnetic resonance (FMR) techniques were employeed. In this case, these techniques allowed us to study the static and dynamical magnetization properties of our tri-layers. The experimental results were interpreted based on the phenomenological model that takes into account the relevant energy terms to the magnetic free energy to describe the system behavior. In the case of the monocrystalline Fe film, we performed an analytical discussion on the magnetization curves and developed a numerical simulation based on the Stoner-Wohlfarth model, that enables the numerical adjustment of the experimental magnetization curves and obtainment of the anisotropy field values. On the other hand, for the tri-layers, we analyzed the existence of bilinear and biquadratic couplings between the magnetizations of adjacent ferromagnetic layers from measurements of magnetization curves. With the FMR fields and line width angular dependencies, information on the anisotropy in three layers was obtained and the effects of different magnetic relaxation mechanisms were evidenced. It was also possible to observe the dependence of the epitaxy of the multilayers with growth and sputtering parameters. Additionally it was developed the technique of AC magnetic susceptibility in order to obtain further information during the investigation of magnetic thin films

Evolução top-hat hidrodinâmica em campos de velocidades peculiares primordiais

We investigate the cosmology of the vacuum energy decaying into cold dark matter according to thermodynamics description of Alcaniz & Lima. We apply this model to analyze the evolution of primordial density perturbations in the matter that gave rise to the first generation of structures bounded by gravity in the Universe, called Population III Objects. The analysis of the dynamics of those systems will involve the calculation of a differential equation system governing the evolution of perturbations to the case of two coupled fluids (dark matter and baryonic matter), modeled with a Top-Hat profile based in the perturbation of the hydrodynamics equations, an efficient analytical tool to study the properties of dark energy models such as the behavior of the linear growth factor and the linear growth index, physical quantities closely related to the fields of peculiar velocities at any time, for different models of dark energy. The properties and the dynamics of current Universe are analyzed through the exact analytical form of the linear growth factor of density fluctuations, taking into account the influence of several physical cooling mechanisms acting on the density fluctuations of the baryonic component of matter during the evolution of the clouds of matter, studied from the primordial hydrogen recombination. This study is naturally extended to more general models of dark energy with constant equation of state parameter in a flat Universe

Propriedades magnéticas quase-estáticas de filmes ferromagnéticos amorfos de FeCuNbSiB

Today, one of the topics that attracts interest of the scientific community is the understanding of magnetic properties of magnetic systems with reduced dimensions, in particular, ferromagnetic thin films. In this case, the comprehension and control of these properties, as well as the development of routes to obtain them, are crucial issues in many aspects of current and future technologies for storage and transmission of information in the electro-electronic industry. There are several materials that exhibit soft magnetic properties, and we highlight the amorphous alloys and that ones obtained by partial crystallization, so-called nanocrystalline materials. The production of these alloys as magnetic ribbons is very common in scientific and technological area, but there are just a few works related to the production of these alloys as thin films. In this work, we studied the quasi-static magnetic properties of ferromagnetic thin films based on FeCuNbSiB in a wide range of thicknesses, from 20 to 500 nm, produced by sputtering. In particular, after the structural characterization performed via X-ray diffraction, the magnetic properties of the sets of samples were investigated using experimental magnetization curve, obtained using a vibrating sample magnetometer, as well as through theoretical curves obtained by theoretical modeling and numerical computation. The modeling process is based on the Stoner Wohlfarth model applied to three dimensions, and adds some energy terms, using as reference experimental results of magnetization. In this case, from the comparison between theoretical and experimental results and the analysis of the constant anisotropy behavior as a function of film thickness, we aim to obtain further information on the magnetization process of the samples, to identify routes for the production of thin films and develop a theoretical to films to use it, in the future, in the obtainment of the theoretical curves of some magnetic measurements, such as magnetoimpedance and magnetoresistance

Criação da matéria em cosmologias com velocidade da luz variável e eletrônico não-linear

In this work we obtain the cosmological solutions and investigate the thermodynamics of matter creation in two diferent contexts. In the first we propose a cosmological model with a time varying speed of light c. We consider two diferent time dependence of c for a at Friedmann-Robertson- Walker (FRW) universe. We write the energy conservation law arising from Einstein equations and study how particles are created as c decreases with cosmic epoch. The variation of c is coupled to a cosmological Λ term and both singular and non-singular solutions are possible. We calculate the "adiabatic" particle creation rate and the total number of particles as a function of time and find the constrains imposed by the second law of thermodynamics upon the models. In the second scenario, we study the nonlinearity of the electrodynamics as a source of matter creation in the cosmological models with at FRW geometry. We write the energy conservation law arising from Einstein field equations with cosmological term Λ, solve the field equations and study how particles are created as the magnetic field B changes with cosmic epoch. We obtain solutions for the adiabatic particle creation rate, the total number of particles and the scale factor as a function of time in three cases: Λ = 0, Λ = constant and Λ α H2 (cosmological term proportional to the Hubble parameter). In all cases, the second law of thermodynamics demands that the universe is not contracting (H ≥ 0). The first two solutions are non-singular and exhibit in ationary periods. The third case studied allows an always in ationary universe for a suficiently large cosmological term

Um estudo sobre a interação estrela-planeta a partir da relação massa-período planetário com a atividade estelar

n this work, we analyze the behavior of the chromospheric and coronal activities as a function of the mass and the orbital period of extrasolar planets which were detected by transit technique. So we look for possible effects of the planet on the star s chromosphere and corona. For this study we selected a sample of 48 stars with chromospheric activity indicator and 23 with coronal activity indicator. Our work is based on the work from Pont et al. (2011) in order to study stars with planets which were obtained by transit technique. Furthermore, we studied the relationship between planetary mass and orbital period with the chromospheric and coronal activity in order to better understand which influences the planets cause in the outer layers of stellar atmosphere. In our analysis we can observe that the mass of the planets exerts no influence in the stellar activity. However, we observed that the stellar coronal and chromospheric activities decrease with the increase of the orbital period of the planet

Um estudo sobre a interação estrela-planeta a partirda relação massa-período planetário com a atividade estelar

In this work, we analyze the behavior of the chromospheric and coronal activities as a function of the mass and the orbital period of extrasolar planets which were detected by transit technique. So we look for possible effects of the planet on the star s chromosphere and corona. For this study we selected a sample of 48 stars with chromospheric activity indicator and 23 with coronal activity indicator. Our work is based on the work from Pont et al. (2011) in order to study stars with planets which were obtained by transit technique. Furthermore, we studied the relationship between planetary mass and orbital period with the chromospheric and coronal activity in order to better understand which influences the planets cause in the outer layers of stellar atmosphere. In our analysis we can observe that the mass of the planets exerts no influence in the stellar activity. However, we observed that the stellar coronal and chromospheric activities decrease with the increase of the orbital period of the planet

Estudos de modelos Estocásticos: processo de contato, séries aleatórias e processos difusivos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Influência da filogenia em redes de interações ecológicas

In this thesis we deal with a class of composed networks that are formed by two tree networks, TP and TA, whose end points touches each other through a bipartite network BPA. We explore this network using a functional approach. We are interested in what extend the topology, or the structure, of TX (X = A or P) determines the links of BPA. This composed structure is an useful model in evolutionary biology, where TP and TA are the phylogenetic trees of plants and animals that interact in an ecological community. We use in this thesis two cases of mutualist interactions: frugivory and pollinator networks. We analyse how the phylogeny of TX determines or is correlated with BPA using a Monte Carlo approach. We use the phylogenetic distance among elements that interact with a given species to construct an index κ that quantifies the influence of TX over BPA. The algorithm is based in the assumption that interaction matrices that follows a phylogeny of TX have a total phylogenetic distance smaller than the average distance of an ensemble of Monte Carlo realizations generated by an adequate shuffling data. We find that the phylogeny of animals species has an effect on the ecological matrix that is more marked than plant phylogeny

Desenvolvimento de um sistema a jato de plasma obtido em cátodo oco para deposição de filmes finos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Excitações em cristais fotônicos unidimensionais

In this work, we present a theoretical study of the propagation of electromagnetic waves in multilayer structures called Photonic Crystals. For this purpose, we investigate the phonon-polariton band gaps in periodic and quasi-periodic (Fibonacci-type) multilayers made up of both positive and negative refractive index materials in the terahertz (THz) region. The behavior of the polaritonic band gaps as a function of the multilayer period is investigated systematically. We use a theoretical model based on the formalism of transfer matrix in order to simplify the algebra involved in obtaining the dispersion relation of phonon-polaritons (bulk and surface modes). We also present a quantitative analysis of the results, pointing out the distribution of the allowed polaritonic bandwidths for high Fibonacci generations, which gives good insight about their localization and power laws. We calculate the emittance spectrum of the electromagnetic radiation, in THZ frequency, normally and obliquely incident (s and p polarized modes) on a one-dimensional multilayer structure composed of positive and negative refractive index materials organized periodically and quasi-periodically. We model the negative refractive index material by a effective medium whose electric permittivity is characterized by a phonon-polariton frequency dependent dielectric function, while for the magnetic permeability we have a Drude like frequency-dependent function. Similarity to the one-dimensional photonic crystal, this layered effective medium, called polaritonic Crystals, allow us the control of the electromagnetic propagation, generating regions named polaritonic bandgap. The emittance spectra are determined by means of a well known theoretical model based on Kirchoff s second law, together with a transfer matrix formalism. Our results shows that the omnidirectional band gaps will appear in the THz regime, in a well defined interval, that are independent of polarization in periodic case as well as in quasiperiodic case

Modelo de Ashkin-Teller anisotrópico: uma abordagem por grupo de renormalização

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Análises estatísticas em redes complexas: propriedades topológicas, críticas e dinâmicas

In this thesis, we address two issues of broad conceptual and practical relevance in the study of complex networks. The first is associated with the topological characterization of networks while the second relates to dynamical processes that occur on top of them. Regarding the first line of study, we initially designed a model for networks growth where preferential attachment includes: (i) connectivity and (ii) homophily (links between sites with similar characteristics are more likely). From this, we observe that the competition between these two aspects leads to a heterogeneous pattern of connections with the topological properties of the network showing quite interesting results. In particular, we emphasize that there is a region where the characteristics of sites play an important role not only for the rate at which they get links, but also for the number of connections which occur between sites with similar and dissimilar characteristics. Finally, we investigate the spread of epidemics on the network topology developed, whereas its dissemination follows the rules of the contact process. Using Monte Carlo simulations, we show that the competition between states (infected/healthy) sites, induces a transition between an active phase (presence of sick) and an inactive (no sick). In this context, we estimate the critical point of the transition phase through the cumulant Binder and ratio between moments of the order parameter. Then, using finite size scaling analysis, we determine the critical exponents associated with this transition

Em busca de um novo indicador Espectroscópico do período de rotação das Estrelas do tipo solar

The study of sunspots consistently contributed to a better understanding of magnetic phenomena of the Sun, as its activity. It was found with the dynamics of sunspots that the Sun has a rotation period of twenty-seven days around your axis. With the help of Project Sun-As-A-Star that solar spectra obtained for more than thirty years we observed oscillations of both the depth of the spectral line and its equivalent width, and analysis of the return information about the characteristics of solar magnetism. It also aims to find patterns of solar magnetic activity cycle and the average period of rotation of the Sun will indicate the spectral lines that are sensitive to magnetic activity and which are not. Sensitive lines how Ti II 5381.0 Å stands as the best indicator of the solar rotation period and also shows different periods of rotation cycles of minimum and maximum magnetic activity. It is the first time we observe clearly distinct rotation periods in the different cycles. The analysis also shows that Ca II 8542.1 Å and HI 6562.0 Å indicate the cycle of magnetic activity of eleven years. Some spectral lines no indicated connection with solar activity, this result can help us search for programs planets using spectroscopic models. Data analysis was performed using the Lomb-Scargle method that makes the time series analysis for unequally spaced data. Observe different rotation periods in the cycles of magnetic activity accounts for a discussion has been debated for many decades. We verified that spectroscopy can also specify the period of stellar rotation, thus being able to generalize the method to other stars