Neste trabalho analisamos o comportamento dinâmico de um modelo clássico e de um modelo quântico de spins na presença de um campo magnético externo. Para estudar a dinâmica de um sistema de spins clássico utilizamos um modelo de Ising bidimensional com interações entre spins primeiros vizinhos na direção vertical diferente daquelas entre spins primeiros vizinhos na horizontal. Através do formalismo da equação mestra, e considerando o processo estocático de Glauber dentro da aproximação de pares dinâmica, determinamos os diagramas de fases estacionários para o modelo na presença de campos magnéticos estáticos e oscilantes no tempo. Dependendo dos valores da razão entre os acoplamentos na horizontal e na vertical, da frequência e da amplitude do campo oscilante, obtemos diagramas de fases onde estão presentes os ordenamentos ferromagnético, paramagnético e antiferromagnético. Além disso, a transição entre as fases pode ser contínua ou descontínua dependendo dos valores dos parâmetros. O modelo também pode apresentar um comportamento tricrítico. O modelo de Ising em um campo transverso unidimensional à temperatura nula foi o modelo escolhido para estudarmos a resposta de sistemas quânticos de spins sujeitos a campos magnéticos que oscilam periodicamente no tempo. Usamos a aproximação de campo médio e simulações de Monte Carlo para determinar a linha de transição contínua entre as fases ferromagnética e paramagnética presentes no diagrama de fases dinâmico do modelo. / In this work we have analized the dynamical behavior of a classical and of a quantum spin model subject external magnetic fields. For a better understanding of the dynamics of a classical spin system we have chosen a two-dimensional Ising model with interactions between first neighbors in the horizontal direction different from that of the vertical direction. By using the master-equation formalism and taking the stochastic Glauber process, within the dynamical pair approximation, we have determined the stationary phase diagrams of the model for static and oscillating magnetic fields. Depending on the values of the ratio between the horizontal and vertical couplings, the frequency and the amplitude of the time dependent field, we have obtained phase diagrams where the ferromagnetic, paramagnetic and antiferromagnetic phases are present. Besides, the transition between these phases can be continuous or discontinuous depending on the values of the parameters. The model may display also a tricritical behavior. We have also chosen the transverse Ising model in one dimension at zero temperature to study the response of the quantum spin systems subject to time dependent external fields. We have used the mean-field approximation and the Monte Carlo simulations to determine the continuous transition line between the ferromagnetic and paramagnetic phases.

Made available in DSpace on 2015-04-22T22:07:25Z (GMT). No. of bitstreams: 1 DISSERTACAO-JOSE DILSON.pdf: 2263133 bytes, checksum: bbf5adbba042abf3bd96506b874e8845 (MD5) Previous issue date: 2009-04-02 / Fundação de Amparo à Pesquisa do Estado do Amazonas / The scattering matrix technique is used to calculate the transmissivity of polarized spins through semiconductors heterostructures of asymmetrical double barrier. The movement of electrons is described in the effective mass approach of the Dresselhaus-Rashba.models.The transmissivity and polarization are calculated as a function of electron energy with kk = 0.5 × 106 cm−1, kk = 1 × 106 cm−1, kk = 1.5 × 106 cm−1 e kk = 2 × 106 cm−1 varying the angle φ InAs/GaSb/InAs/GaSb/InAs system. Fixing the parallel moment kk and varying φ = 0◦, 15◦, 30◦, 60◦, 75◦, and 90◦ we observed that the positions of the resonant picks vary faintly with the energy and the transmission curves change more strongly in the areas out of the resonance with the polarization reaching values among 10% − 82% in the resonant levels.For the directions φ = 45◦ and 135◦ the spin mixing produces an efficiency of polarization of 100% and the effects of the Dresselhaus and Rashba spin-orbit interactions are shown quite favorable to the engineered for fabricating of spin filters and spintronics devices. / A técnica da matriz de espalhamento é usada para calcular a transmissividade de spins polarizados, através de heteroestruturas semicondutoras de dupla barreira assimétrica. O movimento de elétrons de condução são descritos na aproximação da massa efetiva dos modelos de Dresselhaus-Rashba. A transmissividade e a polarização são calculadas como função da energia do elétron para kk = 0, 5×106 cm−1, kk = 1×106 cm−1, kk = 1, 5×106 cm−1 e kk = 2×106 cm−1, com vários valores de φ, para um sistema InAs/GaSb/InAs/GaSb/InAs. Fixando o momento paralelo kk e variando φ = 0◦, 15◦, 30◦, 60◦, 75◦, e 90◦ observamos que as posições dos picos ressonantes variam fracamente com a energia e as curvas de transmissão mudam mais fortemente nas regiões fora da ressonância com a polarização atingindo valores entre 10% −→ 82% nos níveis ressonantes. Para as direções φ = 45◦ e 135◦ o spin mixing produz uma eficiência de polarização de 100% e os efeitos das interações spin-órbita de Dresselhaus e Rashba mostram-se bastante favoráveis à engenharia na fabricação de filtrode spin e dispositivos spintrônicos.

Deux systèmes de spins uniques sont étudiés dans cette thèse : d’abord un unique atome de Mn dans une boîte quantique chargée positivement, puis un unique atome de Cr dans une boîte quantique neutre. Cette étude a été entièrement réalisée par des moyens optiques, via la photoluminescence d’une boîte unique. Ces deux systèmes sont fortement couplés aux phonons et aux contraintes. Ce couplage permet de ne plus se limiter à une détection et une manipulation du spin optique, mais ouvre également la possibilité de le faire en modulant le champ de contrainte.Par des expériences de photoluminescence résonante, nous montrons l’existence de niveaux optique en Λ dans les boîtes quantiques chargées positivement et dopées d’un unique atome de Mn. Ces niveaux en Λ sont utilisés pour étudier la dynamique du spin hybride trou-Mn. Cette étude révèle l’existence de flip-flops des spins du trou et du Mn à l’échelle de la nanoseconde, dus à l’interaction d’échange trou-Mn et à l’interaction avec les phonons acoustiques. Un couplage par l’anisotropie des contraintes dans le plan de niveaux électron-Mn dégénérés et séparé par un spin flip de deux unités est aussi démontré, et dynamique cohérente induite par les contraintes est étudiée en détail.Grâce à des expériences magnéto-optiques, nous avons pu détecter la photoluminescence d’un atome de Cr unique dans une boîte quantique, et extraire une anisotropie magnétique du spin du Cr de quelques meV. De nombreux paramètres des boîtes étudiées peuvent être extrait de ces expériences, et nous montrons notamment que le couplage trou-Cr est antiferromagnétique. Nous démontrons que le spin du Cr peut être préparé par pompage optique résonant. Ces expériences ont permis de montrer que, sous excitation, le spin du Cr fait des flip-flops avec celui du trou, causé par l’interaction d’échange trou-Cr et le couplage aux phonons acoustiques. Ces flip-flops causent une relaxation rapide du spin du Cr sous excitation, mesurée comme de l’ordre de quelques dizaines de nanosecondes. Dans le noir, nous avons trouvé que le spin du Cr relaxe en quelques microsecondes. Finalement, nous avons aussi démontré la possibilité de contrôler le spin du Cr par l’effet Stark optique. / Two different single spin systems are studied in this thesis: single Mn spin in positively charged quantum dots, and single Cr atom in a neutral quantum dots. Both are probed optically, using the photoluminescence of a single dot. Both systems are strongly coupled to phonons and strains. This coupling opens new ways to probe and control the spins of the magnetic atoms, not only optically, but also by modulating the strain state at their position.Using resonant photoluminescence, we demonstrate the existence of optical Λ-level systems in Mn-doped positively charged quantum dots, and use them to study the dynamics of the hole-Mn hybrid spin. It reveals hole-Mn spin flip-flops in the nanosecond range due to the interplay of the interaction with acoustic phonons and the hole-Mn exchange interaction. We also show that degenerated electron-Mn states separated by spin flips of two units are coupled through the in-plane strain anisotropy, and study this strain induced coherent dynamics.Using magneto-optic experiments, we are able to probe the spin of a single Cr atom and extract a magnetic anisotropy D0 in the meV range. Several parameters of the QD are extracted from these experiments, and, especially, we show that the h-Cr coupling is anti-ferromagnetic. We demonstrate that the Cr spin can be prepared using resonant optical pumping. These experiments evidence a Cr relaxation time under excitation in the 10 nanoseconds range. This relaxation is driven by hole-Cr spin flip-flops caused by the interplay of the interaction with acoustic phonons and the hole-Cr exchange interaction. A relaxation in the dark in the microseconds range was also measured. Possibility of control of the Cr spin was also demonstrated using the optical Stark effect.

Submitted by Geyciane Santos (geyciane_thamires@hotmail.com) on 2015-08-06T15:41:08Z No. of bitstreams: 1 Dissertação - Efraim Fernandes Marques.PDF: 1576792 bytes, checksum: 9f5d857f0e1015473fb55bdbd1206731 (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2015-08-07T13:48:53Z (GMT) No. of bitstreams: 1 Dissertação - Efraim Fernandes Marques.PDF: 1576792 bytes, checksum: 9f5d857f0e1015473fb55bdbd1206731 (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2015-08-07T13:50:56Z (GMT) No. of bitstreams: 1 Dissertação - Efraim Fernandes Marques.PDF: 1576792 bytes, checksum: 9f5d857f0e1015473fb55bdbd1206731 (MD5) / Made available in DSpace on 2015-08-07T13:50:56Z (GMT). No. of bitstreams: 1 Dissertação - Efraim Fernandes Marques.PDF: 1576792 bytes, checksum: 9f5d857f0e1015473fb55bdbd1206731 (MD5) Previous issue date: 2012-10-02 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this dissertation, the scattering matrix technique is used to calculate the transmissivity of polarized spins through semiconductors heterostructures of symmetrical double barrier. The movement of electrons is described in the effective mass approach of the complete Dresselhaus models. The transmissivity and polarization are calculated as a function of electron energy with kjj = 0; 5x106cm􀀀1, kjj = 1x106cm􀀀1 and kjj = 2x106cm􀀀1 varying the angle GaSb=GaxAl1􀀀xSb=GaSb=GaxAl1􀀀xSb=GaSb system. Fixing the parallel moment kjj and varying = 30o, 45o, 60o and 90o, we observed that the positions of the resonant picks vary faintly with the energy and the transmission curves change more strongly in the areas out of the resonance, with the polarization reaching values among 54%􀀀!82% in the resonant levels. For the direction = 45o, the spin mixing produces a spin polarization efficiency of nearly 100 %. Applying an external voltage, the peak transmissivity and spin polarization undergo a shift towards regions to lower energies. / Neste trabalho, a técnica da matriz de espalhamento é usada para calcular a transmissividade de spins polarizados através de heteroestruturas semicondutoras de dupla barreira simétrica. O movimento de elétrons de condução são descritos na aproximação da massa efetiva do modelo completo de Dresselhaus. A transmissividade e a polarização são calculadas como função da energia do elétron para kjj = 0; 5x106cm􀀀1, kjj = 1x106cm􀀀1 e kjj = 2x106cm􀀀1 com vários valores de , para um sistema GaSb=GaxAl1􀀀xSb=GaSb=GaxAl1􀀀xSb=GaSb. Fixando o momento paralelo kjj e variando = 30o, 45o, 60o e 90o, observamos que as posições dos picos ressonantes variam fracamente com a energia e as curvas de transmissão mudam mais fortemente nas regiões fora da ressonância, com a polarização atingindo valores entre 54%􀀀!82% nos níveis ressonantes. Para a direção = 45o, o spin-mixing produz uma eficiência de polarização de quase 100%. Ao aplicarmos uma tensão externa, os picos de transmissividade e spin polarização sofrem um desclocamento para regiões de energias mais baixas.

Over the last decades, several works have been devoted to the investigation of a new class of hybrid spin systems with nodal localized Ising spins and interstitial delocalized electrons. In a diamond-like geometry, this class of models depicts magnetization plateaus and an enhanced magnetocaloric effect. In square lattices geometry, the quantum correlations may lead to ferromagnetic or antiferromagnetic ordering depending on the filling fraction of the delocalized orbitals. In the present work, we study the magnetic and thermodynamic properties of a hybrid spin ladder consisting of Ising spins placed in distinct chains, interacting with each other through a ferromagnetic exchange coupling φ. The interaction between nearest neighbor’s localized spins on the same chain is mediated by a pair of interstitial electrons that may hop between chains with hopping amplitude t obeying Pauli’s Exclusion Principal. The exchange interaction between each localized spins and an interstitial electron is J. The interplay of hopping amplitude and exchange interaction may give rise to a rich phase diagram for the ground state of the system. Such a model may be exactly solvable by using decoration-iteration transformation, transfer matrix method and exact diagonalization. Our results show that quantum correlations betweeen delocalized spins induce an antiferromagnetic coupling between chains that competes with the ferromagnetic exchange coupling. A resulting kinecally-driven frustration is predominant in the regime of low temperatures and large hopping amplitudes. We provide the full ground-state phase diagram as well as the frustration diagram on distinct coupling regimes. The effect of an external magnetic field are also investigated. Further, we determine how the hopping amplitude and an external magnetic field affect different thermodynamics properties of the system, such as total magnetization, specific heat and magnetocaloric rate. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nas últimas décadas, vários trabalhos têm sido dedicados à investigação de uma nova classe de sistemas de spins híbridos, formados por spins de Ising nodais localizados e elétrons intersticiais delocalizados. Na geometria do tipo diamante, esta classe de modelos apresenta platô de magnetização e um efeito magnetocalórico pronunciado. Em uma geometria de rede quadrada, as correlações quânticas podem levar a um ordenamento ferromanético ou antiferremagnético dependendo da fração de preenchimento dos orbitais delocalizados. No presente trabalho, nós estudamos as propriedades magnéticas e termodinâmicas de escada de spins híbridos, consistindo em spins de Ising localizados em cadeias distintas, que interagem por meio de acoplamento ferromagnético de troca φ. A interação entre os primeiros vizinhos de spins localizados numa mesma cadeia é mediada por um par de elétrons intersticiais que podem saltar entre as cadeias com amplitude de hopping t, obedecendo ao Princípio de Exclusão de Pauli. A interação de troca entre os spins localizados e os spins intersticiais é J. A competição entre as interações de troca e a amplitude de hopping pode dar origem a um rico diagrama de fases para o estado fundamental do sistema. Este modelo pode ser exatamente solúvel usando a transformação de iteração-decoração, método da matriz de transferência e diagonalização exata. Nossos resultados mostram que as correlações quânticas entre os spins delocalizados induzem um acoplamento antiferromagnético entre as cadeias que compete com o acoplamento ferro-magnético de troca. Uma frustração induzida cineticamente é predominante no regime de baixas temperaturas e grandes amplitudes de hopping. Nós fornecemos um diagrama de fases completo para o estado fundamental do sistema, bem como o diagrama de frustração para diferentes regimes dos acoplamentos. Os efeitos de um campo magnético externo também são investigados. Além disso, nós determinamos como a amplitude de hopping e um campo magnético externo afetam diferentes propriedades termodinâmicas do sistema, tais como a magnetização total, o calor específico e a taxa magnetocalórica.

Entanglement is one of the most technologically important quantum phenomena and its con-trolled creation brings us a step closer to the realisation of a quantum computer. Hybrid electron and nuclear spin systems which combine long nuclear decoherence times with the high polarisation and rapid processing times of electron spins are considered reliable candidates for the representation of the fundamental building block of a quantum computer, the qubit. In the literature electron spins quite often play the role of a mediator which can access, manipulate and couple states with long coherence times, beneficial for storing quantum information. Despite the fact that an electron spin can be a useful resource for nuclear spin systems, its permanent presence can be a source of decoherence. The use of transient photoexcited electron spins provide an additional advantage and once the operations which involve the electron spin are completed, the electron spin can decay and not interfere further with the evolution of the system. In this thesis we report magnetic resonance experiments and density functional theory calculations for the demonstration of nuclear - nuclear entanglement using photoexcited triplet states. We study homonuclear and heteronuclear fullerene derivatives and we identify in each case the relevant parameters that can lead to high fidelity entangling operations. The hyperfine interaction in a homonuclear system is the key parameter which determines the degree of entanglement between the nucelar spins according to a recent theoretical proposal. We measure and calculate the hyperfine interaction in homonuclear systems with ¹³C nuclear spins in order to prove the feasibility of this scheme. Further experiments on a fullerene system with two nuclear spins a ³¹P and a ¹H show that entangling operations of high fidelity which involve the demonstration of CNOT gates, are possible within the lifetime of the triplet state.

Cette thèse est consacrée à l'étude par résonance magnétique nucléaire (RMN) des propriétés magnétiques du composé Cu2(C5H12N2)2Cl4 (CuHpCl). Ce système de spins quantiques 1/2 a longtemps été considéré comme une réalisation expérimentale d'un système d'échelles de spins. Son diagramme de phase en fonction du champ magnétique est très riche, avec en particulier un état fondamental liquide de spin en champ nul et deux transition de phase quantique aux champ Hc1=7.5 Tesla et Hc2 = 13 Tesla. Le but premier de cette étude est une clarification de certaines zones de ce diagramme de phases, en particulier la phase basse température comprise entre Hc1 et Hc2 qui présente un ordre magnétique tridimensionnel. A travers l'étude d'un composé spécifiquement deutéré, nous avons montré la nature commensurable de cette phase ordonnée et étudié la nature de la transition en fonction de H et de T. Mais l'apport principal de cette étude, est la mise en évidence de l'apparition à basse température d'une aimantation transverse alternée due à des interactions anti-symétriques, similaire à celle observée dans d'autres composés liquide de spins tels que NENP ou SrCu2(BO3)2. Cette aimantation apparaît pour des champs inférieurs à Hc1 et persiste pour H> Hc2. CuHpCl semble être le seul composé à présenter la coexistence de cette aimantation alternée et d'une phase magnétique 3D. Cette aimantation alternée est également observée dans les mesures d'aimantations par couple menées sur ce composé. Enfin, dans le but de comprendre les chemins d'échanges dans CuHpCl, nous avons entrepris un étude préliminaire (RMN et mesures d'aimantation) de composés dérivés (méthylés) de CuHpCl.

L'échange multiple décrit de manière effective les propriétés magnétiques d'un système de fermions quasiment localisés. Nous considérons ce modèle pour des spins 1/2 sur le réseau triangulaire avec des processus à 2, 3, 4, 5 et 6 particules. Cet hamiltonien est aujourd'hui reconnu comme étant un bon candidat pour décrire le magnétisme nucléaire des films d'3He solide adsorbés sur du graphite. Nous l'étudions par diagonalisations exactes sur des échantillons jusqu'à 36 sites. Dans un large domaine de paramètres, l'analyse des effets de taille finie sur les énergies propres, la structure des spectres et les fonctions de corrélation amènent à conclure que les fluctuations quantiques détruisent l'ordre magnétique à température nulle. L'état fondamental est caractérisé par une longueur de corrélation finie, c'est un liquide de spins où les excitations magnétiques possèdent un gap. Nous calculons la chaleur spécifique et trouvons une entropie importante à basse température, en accord avec les mesures expérimentales. La réponse à un champ extérieur révèle un plateau d'aimantation à M/M(sat)=1/2. Nous discutons les conditions d'apparition d'un tel plateau et les comparons avec les situations, mieux comprises, de la dimension un ainsi que celle de spins classiques. Pour le plus grand système étudié (36 sites), le fondamental possède une quasi-dégénérescence quatre. Cette dégénérescence ne s'explique pas par une brisure spontanée de symétrie mais elle conduit à une interprétation topologique. Nous suggérons une image de ``resonating valence-bond'' (RVB) à courte portée pour la fonction d'onde du fondamental et discutons du lien éventuel avec la phase de HALDANE et les états de type ``valence-bond solid''. Le modèle d'échange multiple sur le réseau triangulaire semble un des tous premiers exemples d'hamiltonien bidimensionnel de spins 1/2 dont le fondamental ne brise aucune de symétrie à température nulle.

Les réseaux de nanoaimants à frustration géométrique font l'objet d'investigations depuis maintenant une décennie. Ils permettent de réaliser expérimentalement des modèles de spins théoriques qui n'ont parfois pas d'équivalent naturel. Ces réseaux présentent, entre autres, l'intérêt d'offrir un accès direct aux configurations locales de spin. Le travail présenté dans ce manuscrit vise à réaliser expérimentalement le modèle dit de "glace carrée" dans un réseau nanoaimants. Ce modèle hautement frustré présente un état fondamental massivement dégénéré, à l'origine de son entropie résiduelle à basse température. Dans ce travail, nous allons présenter deux approches permettant de réaliser le modèle de glace carrée.La première consiste à introduire dans le réseau carré conventionnel des nanoaimants supplémentaires. Sous certaines conditions, ceux-ci agissent en modifiant les couplages effectifs entre les aimants du réseau principal. Pour cela, les aimants additionnels doivent se comporter passivement vis à vis du réseau principal. Dans une étude théorique, nous montrerons que l'analyse du hamiltonien dans l'espace réciproque échoue à saisir les propriétés essentielles de ce nouveau modèle. C'est en calculant l'énergie de configurations aléatoires de spins que nous prouverons que notre système présente les caractéristiques recherchées. Nous porterons une attention particulière à l'effet de la portée et de la nature des interactions entre aimants. Grâce à des simulations micromagnétiques par différences finies, nous déterminerons les géométries pertinentes à employer pour une réalisation expérimentale. Grâce aux outils de microfabrication disponibles au laboratoire, nous avons pu fabriquer de tels réseaux. Les aimants sont constitués de permalloy mince, ce qui leur permet d'atteindre un régime superparamagnétique lors d'un recuit thermique. Nous avons observé que les aimants additionnels influencent comme prévu la physique du réseau carré. Un biais expérimental a cependant engendré une aimantation rémanente élevée dans certains réseaux. Cet effet a malheureusement masqué les corrélations caractéristiques attendues dans le modèle de glace carrée. Mais ces travaux ont permis de mettre en évidence un effet inattendu des aimants additionnels. Ceux-ci semblent stimuler les fluctuations thermiques dans les réseaux qui en sont pourvus.La seconde approche consiste à surélever les aimants orientés dans l'une des deux directions du réseau carré. Nous emploierons une méthodologie similaire à celle décrite précédemment pour l'étude de ce nouveau système. Théoriquement, le contrôle de la surélévation permet d'explorer trois modèles de spins différents, dont le modèle de glace carrée. Nous avons fabriqué des réseaux avec plusieurs surélévations, estimées au moyen de simulations micromagnétiques. Pour des raisons techniques, nous avons cette fois travaillé avec des nanoaimants de permalloy épais. Ils présentent la caractéristique d'être athermiques. Les fluctuations sont alors introduites grâce à un champ magnétique tournant et décroissant. Nous montrerons par des simulations que cette dynamique particulière stimule l'apparition de corrélations ferromagnétiques. La désaimantation réduit alors les surélévations nécessaires à la réalisation du modèle de glace carrée. Nous avons observé expérimentalement que l'effet de la surélévation est parfaitement cohérent avec nos prévisions. Les facteurs de structure que nous avons obtenus prouvent que nous avons réussi à réaliser le modèle de glace avec des nanoaimants. Cette approche nous a permis d'observer pour la première fois une phase de Coulomb dans l'espace direct. Ce travail ouvre des perspectives intéressantes pour l'étude des excitations existant dans cette phase, analogues à des monopoles magnétiques classiques. / Since a decade, a big interest has grown about geometrically frustrated nanomagnets arrays. They allow experimental realisation of theoretical spin models, that sometimes have no natural counterpart. In addition to their high flexibility, these networks provide a direct access to local spin configurations. The work presented in this manuscript aims to realise experimentally the so-called "square ice" model, using nanomagnets array. This highly frustrated model has already been theoretically studied. It is found to have a massively degenerated ground state, associated to a residual entropy at low temperature. In this thesis, we present two approaches that should achieve the square ice model.The first one consists to introduce additional magnets in the conventional square nanomagnets array. They can act through an effective coupling between the magnets of the initial network. The condition required is that additional magnets should behave passively against main magnets. We will show in a theoretical study that a reciprocal space analysis of the Hamiltonian fails to grasp the essential properties of the new model. By computing the energy of a number of random spins configurations, we will show that the square ice model can be achieved. A special attention will be paid to the influence of the range and the nature of magnets interactions. Using finite-difference micromagnetic simulations, we will determine the geometries adapted to an experimental realisation. We were able to make such networks using the microfabrication tools available in the laboratory. Our magnets are made of thin permalloy films. This allows the magnets to reach a superparamagnetic regime during a thermal annealing. We observed that additional magnets have the expected influence on square nanomagnets arrays. However, an experimental bias caused a high residual magnetisation in some networks. This effect has unfortunately hided the correlations expected in the square ice model. But this work shown an unexpected effect. Additional magnets appear to enhance thermal fluctuations in the networks.The second approach is to raise the magnets oriented in one of the two directions of the square lattice. For the study of this new system, a methodology similar to that described above will be used. Theoretically, the control of the elevation can explore three different spins models, including the square ice model. We made arrays for different elevations, estimated by micromagnetic calculations. For technical reasons, we worked with thick permalloy nanomagnets. Their thickness makes them insensitive to thermal fluctuations. Fluctuations are then introduced using a decreasing rotating magnetic field. Our simulations show that this particular dynamic stimulates emergence of ferromagnetic correlations. The field demagnetisation then reduce the elevations required for achieving the square ice model. We have experimentally observed that the elevation has an effect perfectly consistent with our previsions. Experimental structure factors show that we succeed to achieve the square ice model with nanomagnets. This approach allowed us to observe, for the first time, a Coulomb phase in the real space. This work opens interesting perspectives for studying the excitations of this phase. It has previously been shown that they are similar to classical magnetic monopoles.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-12-19T11:40:49Z No. of bitstreams: 1 ivodealmeidamarques.pdf: 899153 bytes, checksum: 26d551a7e59dfc04b2c846957605025f (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-12-19T13:07:30Z (GMT) No. of bitstreams: 1 ivodealmeidamarques.pdf: 899153 bytes, checksum: 26d551a7e59dfc04b2c846957605025f (MD5) / Made available in DSpace on 2016-12-19T13:07:30Z (GMT). No. of bitstreams: 1 ivodealmeidamarques.pdf: 899153 bytes, checksum: 26d551a7e59dfc04b2c846957605025f (MD5) Previous issue date: 2007-03-01 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nos últimos anos, o progresso da nanotecnologia permitiu a fabricação de amostras magnéticas na escala nanométrica, bem como a evolução de técnicas experimentais para medir suas propriedades. Verificou-se experimentalmente que o estado fundamental de um Nanodisco Magnético pode ser a configuração de Vórtice ou a configuração de domínio único, dependendo de parâmetros como raio e espessura do Nanodisco Magnético. Em escala nanométrica, o efeito de borda e a energia magnetostática tornam-se importantes, criando uma anisotropia de borda sobre os spins próximos a esta. Previsões teóricas indicam que o estado fundamental de um Nanodisco Magnético deve ser a configuração de domínio único. É importante enfatizar que tais previsões são baseadas em teorias que não levam em consideração a espessura da borda. No texto subseqüente analisamos, via simulações Monte Carlo, o efeito da espessura da borda sobre o estabelecimento do estado fundamental em Nanodiscos Magnéticos. Nossos resultados mostraram que a espessura da borda possui influência considerável sobre o estabelecimento do estado fundamental em tais sistemas. Analisamos também, via simulações Monte Carlo e Dinâmica de Spins, o efeito da inclusão de um buraco circular, no centro de um Nanodisco Magnético, sobre a configuração de Vórtice. Neste caso, nossos resultados mostraram que o buraco prende a configuração de Vórtice. Tal efeito pode ser usado para criar esquemas de chaveamento baseados no aprisionamento e soltura da configuração de Vórtice por buracos em Nanodiscos Magnéticos submetidos a campo externo. / In the last years, the progress of the nanotechnology allowed the production of magnetic samples with dimensions in the nanometric scale, as well as the evolution of experimental techniques to measure their properties. It was verified experimentally that the fundamental state of a Magnetic Nanodisk can be the Vortex configuration or the single domain configuration, depending on parameters as ray and thickness of Magnetic Nanodisk. In nanometric scale, the border effect and the magnetostatic energy become important, creating a border anisotropy on the close spins to the border. Theoretical forecasts indicate that the fundamental state of a Magnetic Nanodisk should be the single domain configuration. It is important to emphasize that such forecasts are based on theories that don't take into account the thickness of the border. In the subsequent text we analyzed, by Monte Carlo simulations, the effect of the thickness of the border on the establishment of the fundamental state in Magnetic Nanodisks. Our results showed that the thickness of the border possesses considerable influence on the establishment of the fundamental state in such systems. We also analyzed, by Monte Carlo simulations and Spin Dynamics, the effect of the inclusion of a circular hole, in the center of a Magnetic Nanodisk, about the Vortex configuration. In this case, our results showed that the hole pinning the Vortex configuration. Such effect might be used for switching schemes employing pinning and depinning of the Vortex configuration by holes in Magnetic Nanodisks submitted to external field.