Propagação de danos para o modelo Ising em uma rede quadrada totalmente frustrada

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Caracterização magnética e estrutural de filmes depositados por gaiola catódica

Iron nitrite films, with hundred of nanometers thick, were deposited using the Cathodic cage plasma nitriding method, with a N2/H2 plasma, over a common glass substract. The structure, surface morphology and magnetic properties were investigated using X-ray diffractometry (XRD), atomic force microscopy (AFM) and vibrating sample magnetometer (VSM). XRD shows the formation of γ FeN phase and a combination of ζFe2N + ɛFe3N phases. The film s saturation magnetization and coercivity depends on morphology, composition, grain size and treatment temperature. Temperature raising from 250 ºC to 350 ºC were followed by an increase in saturation magnetization and film s surface coercivity on the parallel direction in relative proportion. This fact can be attributed to the grain sizes and to the different phases formed, since iron rich fases, like the ɛFe3N phase, emerges more frequently on more elevated treatment s temperature. Using this new and reasonably low cost method, it was possible to deposit films with both good adhesion and good magnetic properties, with wide application in magnetic devices

Propriedades físicas de bicamadas magnéticas

The magnetic order of bylayers composed by a ferromagnetic film (F) coupled with an antiferromagnetic film (AF) is studied. Piles of coupled monolayers describe the films and the interfilm coupling is described by an exchange interaction between the magnetic moments at the interface. The F has a cubic anisotropy while the AF has a uniaxial anisotropy. We analyze the effects of an external do magnetic field applied parallel to the interface. We consider the intralayer coupling is strong enough to keep parallel all moments of the monolayer an then they are described by one vector proportional to the magnetization of the layer. The interlayer coupling is represented by an exchange interaction between these vectors. The magnetic energy of the system is the sum of the exchange. Anisotropy and Zeeman energies and the equilibrium configuration is one that gives the absolute minimum of the total energy. The magnetization of the system is calculated and the influence of the external do field combined with the interfilm coupling and the unidirectional anisotropy is studied. Special attention is given to the region near of the transition fields. The torque equation is used to study dynamical behavior of these systems. We consider small oscillations around the equilibrium position and we negleet nonlinear terms to obtain the natural frequencies of the system. The dependence of the frequencies with the external do field and their behavior in the phase transition region is analized

Renderizações não fotorealísticas para estilização de imagens e vídeos usando areia colorida

Non-Photorealisitc Rendering (NPR) is a class of techniques that aims to reproduce artistic techniques, trying to express feelings and moods on the rendered scenes, giving an aspect of that they had been made "by hand". Another way of defining NPR is that it is the processing of scenes, images or videos into artwork, generating scenes, images or videos that can have the visual appeal of pieces of art, expressing the visual and emotional characteristics of artistic styles. This dissertation presents a new method of NPR for stylization of images and videos, based on a typical artistic expression of the Northeast region of Brazil, that uses colored sand to compose landscape images on the inner surface of glass bottles. This method is comprised by one technique for generating 2D procedural textures of sand, and two techniques that mimic effects created by the artists using their tools. It also presents a method for generating 21 2D animations in sandbox from the stylized video. The temporal coherence within these stylized videos can be enforced on individual objects with the aid of a video segmentation algorithm. The present techniques in this work were used on stylization of synthetic and real videos, something close to impossible to be produced by artist in real life

Aplicação de resíduo industrial para isolamento térmico: uma proposta para utilização do poliuretano de mamona com agregado de resíduo plástico termofixo

A aplicação de isolantes térmicos em sistemas construtivos promove vantagens com vistas ao conforto térmico de ambientes. E com isso, o aumento da produtividade em locais de trabalho, a sensação de bem-estar e a diminuição dos custos com climatização. A demanda por conforto ambiental, no âmbito da isolação térmica, somada ao advento de novas leis que regulam os requisitos mínimos de conforto, as exigências dos consumidores pela adoção de métodos de produção mais “limpos”, a fiscalização quanto à destinação de resíduos industriais, além da inserção de produtos no mercado com apelos ambientais, incentivaram o desenvolvimento da presente pesquisa. O presente trabalho trata da aplicação do poliuretano, visando comparar o desempenho térmico do derivado de origem vegetal (óleo de mamona) com adição de resíduo plástico termofixo em diferentes proporções (5%, 10%, 15% e 20%), com o poliuretano petrolífero, a lã de vidro e a lã de rocha através da análise de suas propriedades térmicas (condutividade térmica – k, difusividade térmica – e capacidade calorífica – Cp). . Após a realização dos ensaios, os compósitos estudados foram moídos e reutilizados como carga para novos compósitos. Com base nos resultados dos ensaios de propriedades térmicas, constatou-se que o material desenvolvido conduz menos calor que o poliuretano de petróleo, a lã de vidro e a lã de rocha, além de oferecer alta inércia térmica, bom desempenho térmico e baixo custo. Assim como foi comprovada a possibilidade de fabricação de novos compósitos para fins de isolamento, reutilizando os compósitos testados

Histerese térmica de sistemas magnéticos nanoestruturados

We report a theoretical investigation of thermal hysteresis in magnetic nanoelements. Thermal hysteresis originates in the existence of meta-stable states in temperature intervals which may be tuned by small values of the external magnetic field, and are controlled by the systems geometric dimensions as well as the composition. Two systems have been investigated. The first system is a trilayer consisting of one antiferromagnetic MnF2 film, exchange coupled with two Fe lms. At low temperatures the ferromagnetic layers are oriented in opposite directions. By heating in the presence of an external magnetic field, the Zeeman energy induces a gradual orientation of the ferromagnets with the external field and the nucleation of spin- op-like states in the antiferromagnetic layer, leading eventually, in temperatures close to the Neel temperature, to full alignment of the ferromagnetic films and the formation of frustrated exchange bonds in the center of the antiferromagnetic layer. By cooling down to low temperatures, the system follows a different sequence of states, due to the anisotropy barriers of both materials. The width of the thermal hysteresis loop depends on the thicknesses of the FM and AFM layers as well as on the strength of the external field. The second system consists in Fe and Permalloy ferromagnetic nanoelements exchange coupled to a NiO uncompensated substrate. In this case the thermal hysteresis originates in the modifications of the intrinsic magnetic

Nucleação de vortices e paredes de domínio em nanoestruturas magnéticas

We report a theoretical investigation of the magnetic phases and hysteresis of exchange biased ferromagnetic (F) nanoelements for three di erent systems: exchange biased nanoparticles, exchange biased narrow ferromagnetic stripes and exchange biased thin ferromagnetic lms. In all cases the focus is on the new e ects produced by suitable patterns of the exchange energy coupling the ferromagnetic nanoelement with a large anisotropy antiferromagnetic (AF) substrate. We investigate the hysteresis of iron and permalloy nanoparticles with a square basis, with lateral dimensions between 45 nm and 120 nm and thickness between 12 nm and 21 nm. Interface bias is aimed at producing large domains in thin lms. Our results show that, contrary to intuition, the interface exchange coupling may generate vortex states along the hysteresis loop. Also, the threshold value of the interface eld strength for vortex nucleation is smaller for iron nanoelements. We investigate the nucleation and depinning of an array of domain walls pinned at interface defects of a vicinal stripe/AF bilayer. The interface exchange eld displays a periodic pattern corresponding to the topology of the AF vicinal substrate. The vicinal AF substrate consists of a sequence of terraces, each with spins from one AF subalattice, alternating one another. As a result the interface eld of neighboring terraces point in opposite direction, leading to the nucleation of a sequence of domain walls in the ferromagnetic stripe. We investigated iron an permalloy micrometric stripes, with width ranging from 100 nm and 300 nm and thickness of 5 nm. We focused in domain wall sequences with same chirality and alternate chirality. We have found that for 100nm terraces the same chiraility sequence is more stable, requiring a larger value of the external eld for depinning. The third system consists of an iron lm with a thickness of 5 nm, exchange coupled to an AF substrate with a periodic distribution of islands where the AF spins have the opposite direction of the spins in the background. This corresponds to a two-sublattice noncompensated AF plane (such as the surface of a (100) FeF2 lm), with monolayer-height islands containing spins of one sublattice on a surface containing spins of the opposite sublattice. The interface eld acting in the ferromagnetic spins over the islands points in the opposite direction of that in the spins over the background. This a model system for the investigation of interface roughness e ects. We have studied the coercicivity an exchange bias hysteresis shift as a function of the distance between the islands and the degree of interface roughness. We have found a relevant reduction of coercivity for nearly compensated interfaces. Also the e ective hysteresis shift is not proportional to the liquid moment of the AF plane. We also developed an analytical model which reproduces qualitatively the results of numerical simulations

A utilização da modelagem física no estudo do desenvolvimento, geometria e cinemática de falhas distensionais

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Pirometamorfismo nos arenitos da Formação Açu, Bacia Potiguar, NE do Brasil

Pyrometamorphism results from conditions of high temperatures and very low pressures provoked by the intrusion of hypabyssal basic bodies into sedimentary or metassedimentary hosting rocks. The onshore portion of the Potiguar Basin in NE Brazil offers examples of this type of metamorphism nearby the contacts of Paleogene to Neogene plugs, sills and dikes of diabases and basalts crosscutting sandstones, siltstones and shales of the Açu Formation (Albian-Cenomanian). The thermal effects over these rocks are reflected on textures and minerals assemblages that characterize the sanidinite facies of metamorphism, often with partial melting of the feldspathic and mica-rich matrix. The liquid formed is potassic and peraluminous, with variably colored rhyolitic glass (colorless, yellow, brown) comprising microcrystals of tridymite, sanidine and clinoenstatite, besides residual detrital clasts of quartz and rarely zircon, staurolite and garnet. Lenses of shale intercalated within the sandstones display crystallites of Fe-cordierite (sekaninaite), mullite, sanidine, armalcolite (Fe-Ti oxide) and brown spinel. The rocks formed due to the thermal effect of the intrusions are called buchites for which two types are herein described: a light one derived from feldspathic sandstone and siltstone protoliths; and a dark one derived from black shale protoliths. Textures indicating partial melting and minerals such as sanidine, mullite, tridymite and armalcolite strongly demonstrate that during the intrusion of the basic bodies the temperature reached 1,000-1,150°C, and was followed by quenching. Cooling of the interstitial melts has as consequences the closure of pores and decrease of the permeability of the protolith, which varies from about 17-11% in the unaffected rocks to zero in the thermally modified types. Although observed only at contacts and over small distances, the number of basic intrusions hosted within the Potiguar Basin in both onshore and offshore portions leaves opened the possibility of important implications of the thermal effects over the hydrocarbon exploration in this area as well in other Cretaceous and Paleozoic basins in Brazil