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No. of bitstreams: 1 SilasSarmentoPedrosa_TESE.pdf: 13307553 bytes, checksum: 616db20e12e34747afe3ca24c303b31a (MD5) Previous issue date: 2017-09-15 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico (CNPq) / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / H? presentemente grande interesse de pesquisa em aglomerados de nanopart?culas superparamagn?ticas, devido em parte ? alta demanda para aplica??es biom?dicas, e ao mesmo tempo ao grande interesse, do ponto de vista fundamental, em novas fases magn?ticas. A suscetibilidade magn?tica inicial e o campo de fuga, s?o fatores essenciais para otimiza??o de sistemas para aplica??es biom?dicas. H?, ao mesmo tempo, grande interesse em confirmar a exist?ncia de ferromagnetismo dipolar, em sistemas onde a energia de troca n?o ? fator dominante. Desenvolvemos um estudo te?rico do impacto da intera??o dipolar sobre as fases magn?ticas de nanopart?culas superparamagn?ticas, confinadas em aglomerados esf?ricos e elipsoidais. Consideramos nanopart?culas de Fe3O4 com tamanhos no intervalo de 9 nm a 12 nm, arranjadas com densidade uniforme em aglomerados de tamanho da ordem de centenas de nan?metros. Mostramos que as fases magn?ticas, e a suscetibilidade inicial, s?o controladas pela intera??o dipolar, e que a topologia do arranjo de nanopart?culas, o tamanho das nanopart?culas e a densidade de empacotamento s?o fatores que controlam as propriedades magn?ticas. Mostramos que a intera??o dipolar pode estabilizar fases magn?ticas cl?ssicas, conhecidas apenas para sistemas com alto conte?do de energia de troca e de anisotropia. Al?m disso, as fases magn?ticas em reman?ncia t?m uma caracter?stica peculiar: a m?dia t?rmica do momento de cada nanopart?cula pode se aproximar do valor de satura??o, mantendo o aglomerado superparamagn?tico. Aglomerados elipsoidais de alta excentricidade s?o os sistemas de escolha para aplica??es biom?dicas porque podem exibir expressivo aumento de suscetibilidade magn?tica, mantendo um campo de fuga de baixa intensidade em reman?ncia. O modelo te?rico reproduz satisfatoriamente resultados experimentais de aglomerados esf?ricos de Fe3O4, e de sistemas de part?culas de Fe e Co de baixa dimensionalidade. / Superparamagnetic nanoparticles clusters are currently driving considerable research attention. The interest stems from chances of designing systems with promising potential for technological applications, and from the fundamental viewpoint, tailoring new magnetic phases. The initial magnetic susceptibility and the stray field, at remanence, are key features for the optimization of magnetic systems for biomedical applications. Also, the existence of dipolar ferromagnetism, in the absence of exchange energy, has been one of the focus of magnetism for decades. We report a theoretical discussion of the impact of the dipolar interactions on the magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles, with size ranging from 9 nm to 12 nm, arranged with uniform density in hundreds nanometer size volumes. We show that the magnetic phases, and the initial susceptibility, are controlled by the dipolar interaction. Also, the topological nanoparticle arrangement, the nanoparticle size, and the packing density, are key features. We show that the dipolar interaction alone may stabilize classical magnetic phases, well known for systems with large content of exchange and anisotropy energies. In addition, we have found that at remanence the nanoparticles clusters magnetic phase have a unique property. The dipolar energy leads to thermal stabilization of the individual nanoparticles moments. Large nanoparticles densities may allow nearly full thermal value of the nanoparticles magnetic moments. Despite this, the nanoparticles cluster is superparamagnetic, with a rather small stray field at remanence, as required for biomedical safety. Nanoparticle clustering in large eccentricity ellipsoidal volumes are promising systems for both low field and large field biomedical applications. For low field applications, there is a large increase in the initial susceptibility, with enhancement in the efficacy of vector targeting and also for hyperthermia absorption rate. For high field applications, the enhancement of the stray is much stronger than that for spherical clusters. Our theoretical model reproduces typical properties of Fe3O4 nanoparticles spherical clusters, as well as intriguing results for Fe and Co quasi-one-dimensional systems.