Neste trabalho, nossa principal motivação foi o entendimento da dinâmica de pacotes de onda em isolantes topológicos 2D. Como excitações de carga se movem nesses materiais? De que maneira essas trajetórias dependem das condições iniciais, e de que forma as condições de contorno influenciam nessa dinâmica? Essas foram algumas das perguntas que guiaram nosso trabalho. Através de simulações computacionais, estudamos o movimento de pacotes de onda gaussianos em poços quânticos de HgTe/CdTe. O comportamento de isolante topológico para essa heteroestrutura foi prevista teoricamente no importante trabalho de Bernevig et al. (Science, vol. 314, no. 5806, 2006) e confirmada experimentalmente por König et al. (Science, vol. 318, no. 5851, 2007). Estudando-se a evolução temporal desse sistema, foi possível observar trajetórias que dependem de forma evidente, não apenas da orientação de spin, mas também da orientação de um pseudo-spin proveniente do modelo BHZ. Em sistemas com condições de contorno periódicas em ambas as dimensões e sem a aplicação de campos externos, foram observadas trajetórias com formato de espiral, acompanhadas por um \"side-jump\" dependente da direção do spin e do pseudo-spin. Em especial, para o caso em que o pseudo-spin está inicialmente orientado na direção-z, as trajetórias espiraladas foram subtituidas por um padrão do tipo \"zitterbewegung\" dependente de um potencial de \"bias\". Para sistemas confinados com bordas impenetráveis, observou-se a formação de estados de borda helicais característicos de isolantes topológicos. / In this work, our main motivation was the understanding about the dynamics of wave packets in 2D topological insulators. How charge excitations move throughout theses materials? In what way their trajectories depend on the initial conditions, and how boundary conditions change this dynamics? These were some of the questions that have guided us in our work. Using numerical simulations, we have studied the movement of gaussian wave packets in HgTe/CdTe quantum wells. The topological insulator behavior for this heterostructure was theoretically predicted on the important work conducted in 2006 by Bernevig et al. (Science, vol. 314, n. 5806, 2006), and experimentally confirmed by König et al. (Science, vol. 318, no. 5851, 2007) a year later. Studing the time evolution of this system, was possible to observe trajectories that depend evidently, not only from the spin projection, but also from the pseudospin orientation coming from the BHZ model. From simulations with periodic boundary conditions in both of the two dimensions, and without the application of any external fields, we observed spiral trajectories accompanied by a spin and pseudospin dependent side-jump. Especially, for the case in which the pseudospin was iniatially oriented in \"z\" direction, the spiral trajectories were replaced by a pattern of the type \"zitterbewegung\" dependent of a bias potential. For the confined systems with barriers of hardwall type, was observed the formation of helical edge states, that is the fingerprint of topological insulators.

O efeito Hall quântico surge em gases de elétrons bidimensionais (2DEG) na presença de altos campos magnéticos B. O campo magnético quantiza o movimento planar dos elétrons em órbitas ciclotrônicas caracterizadas pelos níveis de Landau. Neste regime a resistividade transversal (ou Hall) ρxy em função de B exibe platôs em submúltiplos inteiros de e2/h, i.e., ρxy = ν-1 e2/h, sendo ν o fator de preenchimento dos níveis de Landau. Por sua vez, a resistividade longitudinal ρxx apresenta picos nas transições entre platôs de ρxy. Em primeira instância, ρxx é uma medida indireta da densidade de estados no nível de Fermi g(εF), e os picos dos mesmos indicam cruzamentos do nível de Fermi εF com niveis de Landau. Assim, o diagrama de densidade de elétrons n2D e B dos picos de ρxx ~ g(εF) fornece um mapa topológico da estrutura eletrônica do sistema. Em sistemas de duas subbandas, ρxx(n2D, B) exibe estruturas em forma de anel devido a cruzamentos de níveis de Landau de subbandas distintas [experimentos do grupo do Prof. Jiang (UCLA)]. Estes cruzamentos podem ainda levar a instabilidades ferromagnéticas. Investigamos estas instabilidades usando a teoria do funcional da densidade (DFT) para o cálculo da estrutura eletrônica, e o modelo de Ando (formalismo de Kubo) para o cálculo de ρxx e ρxy. Para temperaturas mais altas (340 mK) obtemos as estruturas em forma de anel em ρxx. Para temperaturas mais baixas (70 mK), observamos uma quebra dos anéis devido a transições de fase ferromagnéticas. Variando-se o ângulo θ de B com relação ao 2DEG observa-se o encolhimento do anel. Nossos resultados mostram que o ângulo de colapso total do anel depende de uma competição entre o termo de troca da interação de Coulomb (princípio de Pauli) e cruzamentos evitados devido ao ângulo θ finito. As transições de fase exibem ainda o fenômeno de histerese. Na região de instabilidade ferromagnética obtemos diferentes soluções variando B de forma crescente ou decrescente. Estas soluções possuem energias total diferentes, de forma que representam estados fundamental e excitado de muitos corpos. Esta observação, juntamente com resultados anteriores do grupo [Freire & Egues (2007)], representam as primeiras realizações teóricas da previsão da possibilidade de estados excitados como mínimos locais do funcional de energia do estado fundamental [Perdew & Levy (1985)]. O modelo aqui proposto fornece excelente acordo com os experimentos considerados. Adicionalmente, a observação sistemática e experimentalmente verificada dos estados excitados valida as previsões de Perdew & Levy. Aplicamos ainda estas mesmas ideias no cálculo da estrutura eletrônica e condutância de fios quânticos na presença de campos magnéticos, mostrando que cruzamentos de modos transversais também exibem instabilidades ferromagnéticas observadas em experimentos recentes [Dissertação de Mestrado de Filipe Sammarco, IFSC/USP], fortalecendo a validade do modelo apresentado nesta tese. / The quantum Hall effect arises in two dimensional electron gases (2DEG) under high magnetic fields B. The magnetic field quantizes the planar motion of the electrons into cyclotron orbits given by the Landau levels. In this regime the transversal (Hall) resistivity ρxy shows plateaus as a function of B at integer sub-multiples of e2/h, i.e., ρxy = ν-1 e2/h, where n is the filling factor of the Landau levels. The longitudinal resistivity ρxx shows peaks at the transition between the plateaus of ρxy. In principle, ρxx is an indirect measure of the density of states at the Fermi level g(εF), so that the peaks indicate when the Fermi level εF crosses a Landau level. Therefore, a density-B-field diagram n2D-B of the ρxx ~ g(εF) peaks shows a topological map of the electronic structure of the system. In two-subband systems, ρxx( n2D, B) shows ringlike structures due to crossings of spin-split Landau levels from distinct subbands [experiments from the group of Prof. Jiang (UCLA)] that could lead to ferromagnetic instabilities. We study these instabilities using the density functional theory (DFT) to calculate the electronic structure, and Ando\'s model (Kubo formalism) for ρxx and ρxy. At higher temperatures (340 mK) we also obtain the ringlike structures in ρxx. At lower temperatures (70 mK) we see broken rings due to quantum Hall ferromagnetic phase transitions. Tilting B by theta with respect to the 2DEG normal we find that the ring structure shrinks. Our results show that the angle of full collapse depends on a competition between the exchange term from the Coulomb interaction (Pauli principle) and the anticrossing of Landau levels due to the finite angle theta. Additionally, at the instabilities we observe hysteresis. Sweeping the B field up or down near these regions we obtain two different solutions with distinct total energies, corresponding to the ground state and an excited state of the many-body system. This result, together with previous results of our group [Freire & Egues (2007)], are the first realizations of the theoretical prediction of the possibility of excited states as local minima of the ground state energy functional [Perdew & Levy (1985)]. The model proposed here shows an excellent agreement with the experiments. Additionally, the systematic and experimentally verified observation of excited states corroborates the predictions of Perdew & Levy. Similar ideas as presented here when applied to the electronic structure and conductance of quantum wires with an in-plane magnetic field show ferromagnetic instabilities at crossings of the wire transverse modes [Master Thesis of Filipe Sammarco, IFSC/USP], also with excellent experimental agreement. This strengthen the range of validity of the model proposed in this Thesis.

Cette thèse est consacrée à l'amélioration de la cohérence dans le régime d'effet Hall quantique entier (EHQE) à facteur de remplissage ν=2, obtenu en appliquant un fort champ magnétique perpendiculairement au plan d'un gaz bidimensionnel d'électrons formé à l'interface d'une hétérostructure semiconductrice d'AlGaAs/GaAs. On obtient alors des conducteurs unidimensionnels chiraux (états de bord) permettant de réaliser l'équivalent électronique de l'interféromètre de Mach-Zehnder (IMZ), pour étudier la cohérence dans ce régime. L'observation inattendue d'une structure périodique en forme de lobes dans la visibilité des interférences en fonction de la tension appliquée en entrée suggère un rôle non négligeable des interactions.Dans un première partie nous expliquons l'émergence des états de bord dans le régime d'EHQE. Nous faisons ensuite l'état de l'art des connaissances concernant leur cohérence, puis nous présentons l'IMZ électronique du point de vue expérimental.Ensuite, nous détaillons les résultats expérimentaux, d'abord concernant la visibilité à tension finie: nos mesures confirment une prédiction théorique concernant un transition de phase quantique en fonction de la dilution de l'état de bord qui interfère ; nous ne voyons pas d'effet flagrant de la relaxation en énergie. Enfin, de précédents travaux(1) ayant identifié clairement l'état de bord voisin de celui qui interfère comme l'environnement limitant la cohérence du système, nous avons réalisé un nouveau type d'échantillon afin de diminuer le couplage à cet environnement de manière contrôlée. Nous avons ainsi augmenté la cohérence de moitié en accord quantitatif avec la théorie issue de précédents travaux(1).(1)P. Roulleau, F. Portier, P. Roche, A. Cavanna, G. Faini, U. Gennser, and D. Mailly. Noise Dephasing in Edge States of the Integer Quantum Hall Regime. Physical Review Letters, 101(18):186803–4, October 2008 / This PhD thesis is devoted to the engineering of quantum coherence in the integer quantum Hall effect regime (IQHE) at filling factor ν=2, obtained by applying a strong perpendicular magnetic field to a bidimensional electron gas formed at the interface of a GaAlAs/GaAs semiconducting heterostructure. Then unidimensional chiral conductors called edge states appear which can be used as electron beams to build the equivalent in condensed matter of a Mach-Zehnder interferometer (MZI) so as to study coherence in this regime. The unexpected periodic lobe structure of the visibility as function of the bias voltage suggests that interactions play an important role.In the first part, we explain how edge states emerge in the IQHE regime. We picture the state of the art on the edge states coherence. Then we present the MZI from the experimental point of view.Next we show our results, first concerning the visibility at finite bias: our measurements confirm a prediction about a quantum phase transition as function of the interfering edge state dilution. We don't see any significant manifestation of energy relaxation in the visibility. Finally, having identified the adjacent edge state as the noisy environment limitating coherence thanks to previous works, we have designed a new kind of sample to decrease the coupling of the system to this environment in a controlled manner. We thus decreased dephasing by half, in quantitative agreement with the theory developped previously in our group.

L’effet Hall quantique (EHQ) apparaissant dans des gaz bidimentionnels d’électrons places à basse température et sous fort champ magnétique a révolutionné la métrologie des résistances depuis sa découverte en 1980 par Klaus von Klitzing. Cet effet apporte une représentation de l’ohm uniquement basé sur la constante de Planck et la charge de l’électron. En 2004, le graphène, un arrangement purement bi-dimensionnel d’atomes de carbone en nid d’abeille, dans lequel les porteurs de charge se comportent comme des fermions de Dirac, a permis de mettre à jour une nouvel effet Hall quantique. Du point de vue de la métrologie des résistances l’EHQ dans le graphène est très prometteur car plus robuste que celui apparaissant dans les hétérostructures semi-conductrices. Ceci pourrait mener à la création d’un étalon de résistance plus pratique, fonctionnant à plus haute température et plus faible champ magnétique ce qui serait un avantage notable pour une dissémination accrue d’un étalon de résistance précis vers les acteurs industriels. Dans ce manuscrit une étude complète de l’impact des défauts linéaires, omniprésent dans le graphène crû par dépôt chimique en phase vapeur sur métal, dans le régime d’effet Hall quantique est menée. Nous avons montré que ces défauts linéaires mènent à des processus de dissipation non-conventionnels qui viennent altérer la quantification de la résistance de Hall. Cette étude pointe vers l’utilisation de monocristaux pour les prochaines investigations du graphène CVD pour une application en métrologie des résistances. La deuxième partie de ce manuscrit est dédiée à l’étude du graphène crû par dépôt chimique en phase vapeur sur carbure de silicium. Nous avons comparé précisément la résistance de Hall d’un échantillon de graphène entre 10 et 19 T à la température de 1.4 K avec celle donnée par un étalon de résistance en GaAs/AlGaAs avec une incertitude relative de ( -2 ± 4 ) × 10⁻¹⁰. Pour la première fois un étalon de résistance en graphène a pu fonctionner dans les mêmes conditions de température et de champs magnétique que celui fabriqué en GaAs/AlGaAs et de plus sur un intervalle de champ magnétique plus de dix fois plus grand. Nous avons également étudié les processus de dissipation apparaissant dans cet échantillon de graphène. Cette étude montre que la longueur de localisation des porteurs de charge sature à une valeur proche de l’extension de la fonction d’onde et ce sur une grande plage de champs magnétique, ce qui soulève des questions intéressantes concernant le désordre présent dans ce type de graphène. Finalement dans un second échantillon provenant de la même technique de fabrication nous avons comparé précisément la résistance de Hall de l’échantillon de graphène avec celle d’un étalon de résistance en GaAs/AlGaAs. Il apparait que la résistance de Hall dans l’échantillon de graphène est quantifié avec une précision métrologique pour des champs magnétiques allant jusqu’à 3.5 T, des températures atteignant 9 K et reste dans un état non dissipatif jusqu’à des courants de 500 µA. Ceci ouvre une voie directe à la réalisation d’étalons quantiques de résistance réalisés en graphène. / The quantum Hall effect (QHE) observed in two dimensional electron gas placed at low temperature and under a strong perpendicular magnetic field, has revolutionized the resistance metrology since its discovery in 1980 by Klaus von Klitzing. It provides a representation of the ohm based on the Planck constant and the electron charge only. In 2004, graphene, a purely two dimensional arrangement of carbon atoms in an honeycomb lattice, where the charge carriers behave as Dirac fermions, has revealed a new flavor of the QHE. From the metrological point of view the QHE in graphene is very promising since it is much more robust than the effect appearing in conventional semiconductors and it could lead to a more convenient resistance standard operating at higher temperature and lower magnetic induction which is an advantage for a broader dissemination of a precise standard towards industrial end-users. In this manuscript, a complete study about the impact in the QHE regime of line defects such as wrinkles or grain boundaries, ubiquitous in graphene grown by chemical vapor deposition on metal is treated. We show that these line defects lead to a non conventional dissipation mechanism that jeopardize the quantum Hall effect accuracy pointing to the use of wrinkle-free monocrystals for further metrological studies. The second part of my manuscript is focused on monolayer graphene grown by chemical vapor deposition on silicon carbide. We precisely compared the Hall resistance of the graphene sample from 10 T to 19 T at the temperature of 1.4 K with a GaAs/AlGaAs resistance standard with a relative uncertainty of ( -2 ± 4 ) × 10⁻¹⁰. For the first time a graphene-based standard was able to operate in the same temperature and magnetic field conditions as semiconductor-based one, furthermore, on a magnetic range more than ten times larger. We thus made a careful study of the dissipation mechanisms taking place in this sample and measured precisely the magnitude of the localization length in the QHE regime that saturate interestingly at the extension of the charge carrier wavefunction itself, opening interesting questions about the close link between Hall quantization and localization physics in graphene grown on SiC. Finally in a second sample grown using the same technique we precisely compared the Hall resistance of the graphene sample and a GaAs/AlGaAs resistance standard that turned out to be in agreement at the metrological level for magnetic fields as low as 3.5 T current as high as 500 µA and temperature as high 9 K. This paves the way for the realization of easy to use quantum Hall resistance standards made out of graphene.

This thesis deals with the study of transport properties of integer and fractional QH edge states and it is based on the work I performed during my Ph.D. studies. The focus of this thesis is on Luttinger liquids far from equilibrium and their relaxation dynamics. Since Boltzmann, a fundamental aspect of statistical mechanics has been the understanding of the emergence of an equilibrium state. Interactions play a crucial role in the thermalization process that drives a system through states described by the Gibbs equilibrium ensemble. Therefore, it seems counterintuitive that a strongly interacting system, such as the Luttinger liquid, should not present any relaxation dynamics. This peculiar fact is due to the integrability of the Luttinger model, i.e. the existence of an infinite number of conserved quantities that precludes the equilibration process. However, in the past few years it has become clear that integrable systems can present some kind of relaxation, even though not towards the Gibbs equilibrium ensemble. Remarkably, the necessity of correctly taking into account some particular non-equilibrium configurations, also revealed the necessity of modifying bosonization, a technique widely used to study strongly interacting systems in one dimension. In this work we focus on three different cases: • Relaxation of high energy electrons injected in a ν = 1/3 chiral Luttinger liquid and in a standard Luttinger liquid. • Heating and the emergence of effective temperatures in a Quantum Hall system at fractional filling fraction ν = 2/3 partitioned by a Quantum Point Contact. • Effect of relaxation on shot-noise measurement of the quasi-particle charge in a ν = 2 QH state.

This work focuses on improving the thrust-to-power ratio of Hall effect thrusters using in-channel electrodes to reduce ion-wall neutralization and focus the ion beam. A higher thrust-to-power ratio would give Hall thrusters increased thrust with the limited power available on spacecraft. A T-220HT Hall thruster is modified in this work to include a pair of ring electrodes within inside the discharge channel. The electrodes are biased above anode potential to repel ions from the walls and toward the channel centerline. Theoretical analysis of ion loss factors indicate that ion-wall neutralizations remove almost 13% of the total ions produced. Reduced wall losses could significantly improve the thruster performance without increased discharge power or propellant consumption. The thruster performance, plume ion characteristics, and internal plasma contours are experimentally measured. The plume and internal plasma measurements are important to determine the cause of the performance changes. The thruster is tested in three conditions: no electrode bias, low bias (10 V), and high bias (30 V). The performance measurements show the electrodes do indeed improve the thrust and thrust-to-power ratio, the latter only at the low bias level. Adding bias increases the ion density and decreases the plume angle compared to the no bias case. The plume measurements indicate that the performance improvements at low bias are due to increased ion number density as opposed to increased ion energy. The increased ion density is attributed to reduced wall losses, not increased ionization. The in-channel measurements support this due to little change in the acceleration potential or the electron temperature. At the high bias level, a drop in thrust-to-power ratio is seen, even though a larger increase in thrust is observed. This is due to increased power draw by the electrodes. Plume measurements reveal the increased thrust is due to ion acceleration. The internal measurements show increased acceleration potential and electron energy which can lead to increased ionization. At the high bias condition, the electrodes become the dominant positive terminal in the thruster circuit. This causes the increased ion acceleration and the creation of domed potential contours that conform to the near-wall cusp-magnetic fields. The domed contours produce focused electric fields, which cause the decreased wall losses and plume angle.

Die elektrische Leitfähigkeit und der Hall-Effekt der halbleitenden Silicide Beta-FeSi2 und MnSi1,73 werden im Temperaturbereich zwischen 4,2 und 300 K untersucht. In ergänzenden Untersuchungen werden strukturelle und optische Eigenschaften dieser Materialien bestimmt. Die Ergebnisse der Messungen an MnSi1,73 - Schichten werden im Rahmen der Boltzmann-Gleichung in Relaxationszeitnäherung interpretiert. Die Temperatur- abhängigkeit der elektrischen Leitfähigkeit und der Hall-Beweglichkeit der Mangansilicid-Schichten kann unter Einbeziehung der Ladungsträgerstreuung an Korngrenzen und akustischen Phononen erklärt werden. In einer kritischen Diskussion werden die Grenzen des verwendeten Transportmodells aufgezeigt. Den Schwerpunkt der Untersuchungen an Beta-FeSi2 bildet die Analyse des Hall-Koeffizienten in Abhängigkeit von der Temperatur und dem Magnetfeld. Mit einem neuen dynamischen Meßverfahren werden umfassende Ergebnisse für den Hall-Koeffizienten in dünnen Schichten und Einkristallen erhalten, die eine herkömmliche Interpretation des Hall-Effekts in Beta-FeSi2 in Frage stellen. Unter Einbeziehung eines wesentlichen Einflusses des anomalen Hall-Effekts in die Interpretation, können die Eigenschaften des Hall-Effekts in Beta-FeSi2 verstanden werden.

The concept of Berry phase, since its proposition in 1984, has found numerous applications and appears in almost every branch of physics today. In this work, we study several physical effects in ferromagnetic metal materials which are manifestations of the Berry phase. We first show that when a domain wall in a ferromagnetic nanowire is undergoing precessional motion, it pumps an electromotive force which follows a universal Josephson-type relation. We discover that the integral of the electromotive force over one pumping cycle is a quantized topological invariant equal to integer multiples of h/e, which does not depend on the domain wall geometry nor its detailed dynamic evolution. In particular, when a domain wall in a nanowire is driven by a constant magnetic field, we predict that the generated electromotive force is proportional to the applied field with a simple coefficient consisting of only fundamental constants. Our theoretical prediction has been successfully confirmed by experiments. Similar effect known as spin pumping occurs in magnetic multilayer heterostructures, where a precessing free magnetic layer pumps a spin current into its adjacent normal metal layers. Based on this effect, we propose two magnetic nanodevices that can be useful in future spintronics applications: the magnetic Josephson junction and the magneto-dynamic battery. The magnetic Josephson junction has a drastic increase in resistance when the applied current exceeds a critical value determined by the magnetic anisotropy. The magneto-dynamic battery acts as a conventional charge battery in a circuit with well-defined electromotive force and internal resistance. We investigate the condition under which the power output and efficiency of the battery can be optimized. Finally we study the side jump contribution in the anomalous Hall effect of a uniformly magnetized ferromagnetic metal. The side jump contribution, although arises from disorder scattering, was believed to be independent of both the scattering strength and the disorder density. Nevertheless, we find that it has a sensitive dependence on the spin structure of the disorder potential. We therefore propose a classification scheme of disorder scattering according to their spin structures. When two or more classes of disorders are present, the value of side jump is no longer fixed but depends on the relative disorder strength between classes. Due to this competition, the side jump contribution could flow from one class dominated limit to another class dominated limit when certain system control parameter changes. Our result indicates that the magnon scattering plays a role distinct from the normal impurity scattering and the phonon scattering in the anomalous Hall effect, because they belong to different scattering classes. / text

Dans cette thèse nous allons présenter nos résultats sur les différentes solutions pour le gaz d'électrons dans une bicouche de graphène sous fort champ magnétique en fonction de la différence de potentiel entre les deux couches et du facteur de remplissage entier de v = -3 à v = 3. Nos résultats seront comparés aux résultats expérimentaux pour expliquer, entre autres, l'apparition de sous plateau dans la conductivité pour les états de N = O. Nos états fondamentaux furent calculés à partir de l'approximation Hartree-Fock pour prendre en compte l'interaction coulombienne. Nous avons obtenu différentes solutions que l'on peut classifier comme les phases cohérentes : inter-couche simple, inter-couche double, inter-spin simple, inter-spin double et orbitale. Le reste des solutions que nous avons trouvées sont des phases sans cohérence. Ces phases peuvent être décrites comme des ferro-aimants de pseudo-spin de Hall. Pour chacune de ces phases, nous avons calculé les modes collectifs et l'absorption électromagnétique à l'aide de la théorie "Generalized Random-Phase Approximation" (GRPA) et le gap d'excitation du système. Nous avons porté une attention particulière aux modes collectifs dans la phase orbitale qui montrent une instabilité et à montrer que cette instabilité peut être décrite par une interaction de type Dzyaloshinskii-Moriya (DM) dans un hamiltonien effectif de spin pour décrire les modes collectifs. Nous avons aussi calculé des effets magnéto-électriques en calculant le changement de polarisation de spin qui se produit en appliquant un champ électrique dans le plan des couches aux facteurs de remplissage v = ±1, ±2.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Granular magnetic systems can be composed by magnetic particles or clusters with size of some nanometers. These magnetic nanoparticles present different magnetic order phases, as superparamagnetic, and they can be embedded in both, metallic or insulating matrix. These systems present several phenomena such as the giant magnetoresistance (GMR), extraordinary Hall effect (EHE), tunnel magnetoresistance (TMR) and Coulomb blockade. That phenomena use to disappear when a small termal fluctuation is high enough to reverse the magnetization of the clusters leading the lost the magnetic information in a very short time range. When it occurs, the particles are in superparamagnetic phase. In order to maintain the magnetic information at high values of temperature, or even to suppress the superparamagnetic limit, many works have tried to use an antiferromagnetic matrix, to induce the increase of the energy barrier among the two easy directions of magnetization by the exchange coupling in the grain(FM)/matrix(AFM) interfaces, with the purpose of stabilizing the particles magnetization. In this work, we have produced Co granular samples embedded in SiO2/CoO insulating/antifferomagnetic matrix through a sequential deposition by magnetron sputtering, in order to study the exchange bias of Co grains laterally surrounded by CoO.Were perfomed measurements of transmission eletronic microscopy (TEM), x-ray diffraction (XRD), measures low fields thermomagnetics (ZFC-FC), to obtain answers of the structural and magnétic charater of the samples. The Exchange Bias were investigated by extraordinary Hall effect, meauresments in differents temperature values in a cooling field (FC) of 5kOe. These non conventional measurements, are carried out with field cooling and the applied external field to perfomed the loop hysteresis both with perpendicular direction to the substrate. The effective anistropy, changes the magnitude up to three ordes of magnitude for different thicknesses of CoO in the limit T!0. On the other hand, the HEB linearily decreases, and goes to zero at a certain temperature. This temperature, is approximately, equal to that where the split of ZFC-FC curves occurs. This split is associated with the CoO blocking temperature, i.e. the ordering temperature of CoO. These results are reported from the finite size effect present in thin films. For the specific case of CoO, these effects lead to a nonzero net magnetization coming from uncompensed moments present on the surface that, in some cases, are responsible for the magnetic coupling governed by a local disorder and frustration, a spin-glass-like behavior. The extraodinary Hall effect and giant magnetoresistance were study of thin Fe-rich amorphous films and Fe-rich/Cu multilayers. Were investigated and compared the extraordinary Hall effect in these two types of samples and discussed it in terms of thickness and sample structure. The thicker films exhibited a strong in-plane magnetic anisotropy, and by decreasing film thickness both saturated Hall resistivity and Hall sensitivity increase. Electrical conductance increases and Hall resistivity decreases when the films are sandwiched with Cu. / Sistemas granulares magnéticos podem ser formados por grãos ou aglomerados magnéticos cujo tamanho é de alguns nanômetros. Estes grãos magnéticos apresentam diferentes fases de ordenamento magnético, como o superparamagnetismo, e podem estar envolvidos tanto por matrizes metálicas como matrizes isolantes. Estes sistemas possuem uma riqueza de fenômenos, como a magnetorresitência gigante (GMR), Efeito Hall Extraordinário (EHE), magnetorresistência túnel (TMR), bloqueio de coulomb entre outros. Estes fenômenos muitas vezes desaparecem quando à energia térmica for suficiente para inverter a magnetização dos grãos, levando-os a perder informação magnética num intervalo de tempo muito curto. Quando isto ocorre dizemos que os grãos estão na fase superparamagnética. Para reter a informação magnética a temperatura ambiente, ou até mesmo suprimir o superparamagnetismo, tem-se tentado o uso de uma matriz antiferromagnética (AFM) onde os grãos ficam imersos, e o acoplamento de troca na interface grão (FM)/matriz (AFM) pode induzir um aumento na barreira de energia entre as duas direções de fácil magnetização e com isso estabilizar a magnetização dos grãos. Neste trabalho foram produzidas amostras granulares de Co imersos em matriz isolante/antiferromagnética de SiO2/CoO pela deposição alternada do metal e dos isolantes por magnetron sputtering , com a finalidade de estudar a evolução do exchange bias entre os grãos de Co que estão lateralmente envolvidos por CoO. Foram realizadas medidas de Microscopia Eletrônica de Transmissão (TEM), Difração de raios-X (XRD), medidas termomagnéticas a baixos campos (ZFC-FC), para obter respostas quanto ao caráter estrutural e magnético das amostras. O Exchange bias foi investigado por efeito Hall extraordinário, medidas à diferentes temperaturas sob um campo de resfriamento (FC) de 5kOe. Estas medidas, diferente do convencional, são realizadas com o campo de resfriamento e o campo externo aplicado para realizar a curva de magnetização ambas na direção perpendicular ao substrato. A anisotropia efetiva, varia, na sua magnitude, até três ordens de grandeza para diferentes espessuras de CoO para o limite T !0. Por outro lado, o HEB decresce linearmente, até ir à zero numa dada temperatura. Esta temperatura onde extingue-se o HEB é, aproxiamdamente, a mesma onde ocorre à separação das curvas ZFC-FC. Esta separação nas curvas está associado com a temperatura de bloqueio de CoO, ou seja, temperatura de ordenamento dos grãos de CoO responsáveis pelo acoplamento direto com os grãos de Co. Estes resultados reportados são oriúndos dos efeitos de tamanho de grão, presentes em filmes muitos finos. Para o caso específico do CoO, estes efeitos levam há presença de uma magnetização diferente de zero oriúndos dos momentos não compensados presentes na superfície que, para alguns casos, são responsáveis pelo acoplamento magnético governado por uma desordem local e frustação, um compotamento do tipo spin-glass-like . Efeito Hall extraordinário e magnetorresistência gigante foram estudados em filmes finos amorfos de FINEMET e multicamdas FINEMET/Cu. Foi investigado e comparado o efetio Hall nos dois tipos de amostras, e discutido em termos da espessura e estrutura da amostra. Para os filmes mais espessos foi observado uma forte anisotropia no plano do filme, por outro lado, quando a espessura diminui ambos, a resistividade Hall satura e a sensibilidade Hall aumenta considerável. A condutividade elétrica aumenta e a resistividade Hall diminui para as multicamadas FINEMET/Cu.