Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work, we study the magnetic properties of CoFe2O4 nanoparticles dispersed in silica (SiO2) matrix. Samples were produced by the sol-gel method and synthesized at different temperatures. The average distance between the particles and thus the nature of inter-particle interactions were controlled for using different concentrations of iron and cobalt salts and the starting material tetraethylorthosilicate (TEOS). The samples were characterized by thermogravimetry (TG), X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetization as a function of magnetic field and temperature. Thermogravimetric analysis showed that the weight loss increases with increasing concentration of salts in the starting solution. The results of X-ray diffraction combined with the transmission electron microscope images confirmed the presence of CoFe2O4 nanoparticles dispersed into the SiO2 matrix. Magnetization measurements showed a typical behavior generally showed by a magnetic nanoparticles system, that is, the observation of a peak in the curve FC-ZFC with a thermal hysteresis in the low temperature region. In particular, we observed a dependence of the coercive field with temperature of CoFe2O4 nanoparticles that did not follow the predictions of the model Bean-Livingston. We believe that this fact is linked more to the effect of size distribution than the possible effects of interaction between the nanoparticles. In this regard, a model considering a particle size distribution in the coercivity of nanoparticles was used. The model was able to fit the experimental data to wide temperature range. / Neste trabalho, nós estudamos as propriedades magnéticas de nanopartículas de CoFe2O4 dispersas em uma matriz de sílica (SiO2). As amostras foram produzidas através do método sol-gel para diferentes temperaturas de síntese. A distância média entre as partículas e, deste modo, a natureza das interações inter-partículas foram controladas utilizando diferentes concentrações dos sais de ferro e cobalto e do reagente de partida tetraetilortossilicato (TEOS). As amostras foram caracterizadas através de medidas de termogravimetria (TG), fluorescência de raios X (FRX), difração de raios X (DRX), microscopia eletrônica de transmissão (MET) e magnetização em função do campo magnético e da temperatura. As análises termogravimétricas mostraram que a perda de massa aumenta com o aumento da concentração dos sais de partida na solução. Os resultados de difração de raios X combinados com as imagens de microscopia eletrônica de transmissão confirmaram a presença das nanopartículas de CoFe2O4 dispersas dentro da matriz de SiO2. As medidas de magnetização mostraram um comportamento típico de um sistema de nanopartículas magnéticas, ou seja, o aparecimento de um pico na curva de ZFC-FC com uma histerese térmica na região de baixas temperaturas. Em particular, nós observamos uma dependência do campo coercivo com a temperatura das nanopartículas de CoFe2O4 que não seguiu as previsões do modelo de Bean Livingston. Nós acreditamos que esse fato está ligado muito mais ao efeito da distribuição de tamanhos do que aos possíveis efeitos de interação entre as nanopartículas. Neste sentido, um modelo que considera a distribuição de tamanhos de partículas na coercividade das nanopartículas foi utilizado. O modelo foi capaz de ajustar os dados experimentais para amplo intervalo de temperatura.

In this work is described a study of the magnetic anisotropy of a magnesioferrite nanoparticle system (MgFe2O4) obtained by precipitation in solid solution of magnesiowüstite [Mg, Fe]O. The anisotropy field was obtained from system’s resonance spectra as measured in the X band. Such spectra can be considered as composed by two spectral lines: A broad line assigned to blocked particles at the measuring temperature and a thin line, which can be attributed to unlocked (superparamagnetic) particles. The anisotropy field of four samples treated at 700 °C during different annealing times were obtained from the spectra measured with their easy magnetization axes oriented at 0 ° and 45 ° with the applied field. The use of a theoretical model to adjust the obtained anisotropy field data enabled the obtaining of the model’s parameter values and their posterior comparison with those reported by other authors. Beyond the dependence of the anisotropy field with the measurement temperature and the sample’s treatment time, it was observed the presence of low field resonance lines that may be related to transitions usually considered as forbidden, which can be enabled by the degenerescence breaking caused by dipolar magnetic interactions between particles. Such phenomenon, which is considerably rare to observe in magnetic nanoparticles systems, was clearly observed in the studied system not only for the half-field resonance line (2Q), but also for lines appearing at one third of field (3Q) and at one quarter of field (4Q). This feature shows that, at least in the investigated experimental system, the dipolar interactions between nearby particles cannot always be neglected as has been proposed by several other authors. / O presente trabalho aborda o estudo da anisotropia magnética do sistema de nanopartículas de magnesioferrita (MgFe2O4) precipitadas em solução sólida de magnesiowüstita [Mg;Fe]O a partir da análise de seus espectros de ressonância magnética medidos na banda X. Tais espectros podem ser aproximadamente decompostos em duas linhas, sendo uma atribuída às partículas bloqueadas na temperatura de medição (linha larga) e a outra às partículas desbloqueadas (linha estreita). Os campos de anisotropia de quatro amostras tratadas a 700 oC por diferentes tempos de tratamento foram obtidos a partir dos espectros medidos com seus eixos de magnetização fácil orientados a 0° e a 45° em relação ao campo aplicado. O emprego de um modelo teórico de anisotropia magnética para ajustar os dados obtidos possibilitou a obtenção dos valores dos parâmetros considerados no mesmo e a comparação destes valores com os que foram reportados por outros autores. Além da dependência do campo de anisotropia com a temperatura de medição e com o tempo de tratamento térmico das amostras, observou-se a presença de linhas de ressonância na região de baixos campos que podem estar relacionadas a transições normalmente consideradas como proibidas que ser viabilizadas com a quebra da degenerescência causada por interações magnéticas dipolares entre partículas. Tal fenômeno, que é consideravelmente raro de se observar em sistemas de nanopartículas magnéticas, foi claramente observado no sistema estudado não somente para a linha de ressonância a meio campo (2Q), mas também para as linhas a um terço (3Q) e um quarto (4Q) de campo. Isso evidencia que, ao menos no sistema experimental investigado, as possíveis interações dipolares entre partículas próximas não podem ser sempre desprezadas a priori como tem sido proposto por vários outros autores.

In this work we studied the dependence of two organic precursors, sucrose and glycerin, in the process to obtain CoFe2O4 nanoparticles by co-precipitation method. The samples were studied by different techniques of structural and magnetic characterization. X-ray diffraction Measurements associated with the Rietveld refinement analysis confirm that all samples obtained with and without the addition of organic precursors have a single phase identified in a cubic crystalline system belonging to the space group . The average crystallite size calculated by Scherrer's equation using the full width at half maximum (FWHM) for the samples synthesized without and with concentration of 0.020 mol.L-1 of glycerin and sucrose showed values approximately equal to 14(2), 8(1) and 5(1) nm, respectively. The Williamson-Hall curves show that both organic precursors used in this work influence on the homogeneity of the samples. Transmission electron microscopy analysis indicates that the inclusion of sucrose during the process of synthesis of these materials reduces the average size of the crystallite, it induces a narrowing in the size distribution and change in the particles shape. The X-ray absorption measurements performed in the Co and Fe K-edge of samples synthesized with sucrose and glycerin show a low variation in the density of unoccupied state in the p sublevel however reveal an increase in the interatomic distances between the absorber and the scattering atoms. Magnetization measurements as a function of temperature show a decrease in the blocking temperature (TB) even as a superparamagnetic behavior at room temperature for sample with 0.020 mol.L-1 of sucrose. / Neste trabalho foi realizado um estudo sobre a influência do uso de dois precursores orgânicos, sacarose e glicerina, no processo de obtenção de nanopartículas de ferrita de cobalto (CoFe2O4) pelo método de co-precipitação. As amostras foram estudadas através de diferentes técnicas de caracterização estrutural e magnética. As medidas de difração de raios X associadas às análises de refinamento Rietveld confirmaram que todas as amostras obtidas com e sem a adição de precursores orgânicos têm única fase identificadas em um sistema cristalino cúbico pertencente ao grupo de espaço . O tamanho médio de cristalito estimado pela equação de Scherrer, usando a largura a meia altura dos picos de difração, mostra que para as amostras sintetizadas sem e com concentração de 0,020 mol.L-1 de glicerina e de sacarose revelaram valores aproximadamente iguais a 14(2), 8(1) e 5(1) nm, respectivamente. As análises das curvas de Williamson-Hall mostram que ambos os precursores orgânicos utilizados neste trabalho influenciam na homogeneidade das amostras. As análises de microscopia eletrônica de transmissão indicam que a incorporação de sacarose durante o processo de síntese desses materiais reduz o tamanho médio do cristalito, assim como, induz um estreitamento na distribuição de tamanho, além de alterar a morfologia das amostras. Medidas de absorção de raios X realizadas nas bordas K do Co e Fe nas amostras sintetizadas com sacarose e glicerina apresentam uma baixa variação da densidade de estado desocupados no subnível p, no entanto revela um aumento na distância interatômica entre o átomo absorvedor e o espalhador. Medidas de magnetização como uma função da temperatura mostram uma diminuição na temperatura de bloqueio na amostra obtida com adição de sacarose, assim como, um comportamento superparamagnético à temperatura ambiente na amostra obtida com 0,020 mol.L-1 de sacarose.

O uso de análise quantitativa em radiologia médica tem sido de grande valia na detecção de alterações não acessíveis à análise visual simples, dita qualitativa, seja por serem muito sutis, seja por não estarem presentes nas técnicas de imagem de ressonância magnética convencional. Porém, certos tipos de quantificação exigem a aquisição softwares e de plataformas computacionais de alto custo, além de mão de obra especializada com conhecimento técnico em computação para operar em ambientes não intuitivos. Neste cenário o objetivo deste trabalho foi a implementação de um software para análise de transferência de magnetização em imagens de ressonância magnética nuclear que funcionasse na plataforma IBM-PC e em sistemas operacionais livres como GNU/Linux. Com este intuito foi elaborado um algoritmo para leitura de imagens codificadas no padrão DICOM 3.0, um algoritmo para a construção dos mapas de Razão de Transferência de Magnetização do volume adquirido e um visualizador com interface amigável para a segmentação e análise dos resultados. Ao final, software possibilitou a abertura da imagem DICOM. Também construiu de maneira eficiente, os mapas de diferença de porcentagem entre as imagens sem e com o pulso de transferência de magnetização (MTR), possibilitando, inclusive, correções de artefatos de movimentos, quando pouco intensos. Permitiu o delineamento de regiões de interesse irregular, com boa visibilidade dos resultados. Como controle padrão, os resultados foram comparados com o conjunto de ferramentas da Universidade McGill (Brain Imaging Center, McGuill University, Montreal, Quebec, Canadá), amplamente testado em artigos publicados. / The use of quantitative analysis in medical radiology has been of great value in the detection of not accessible alterations in the simple visual analysis, said qualitative, for being very subtle, or for not being present in conventional magnetic resonance image techniques. However, certain types of quantification demand the acquisition of high cost softwares and computational platforms, beyond specialized workmanship, with technical knowledge in computation, to operate in non intuitive environments. In this scenery the objective of this work was the implementation of a software for analysis of transference of magneti zation in nuclear magnetic resonance images that works in IBM-PC platform and free operational systems as GNU/Linux. So, an algorithm for reading of standard DICOM 3.0 codified images was elaborated, an algorithm for the construction of Magnetization Transfer Ratio maps of acquired volume, and a visualizer with friendly interface for segmentation and analysis of the results. Finally the software made the opening of DICOM image possible. It also generated in efficient way the maps of percentage difference among the images without and with the pulse of magnetization transfer (MT), also making devices of movement corrections possible, when they are not very intense. It allowed the delineation of regions of irregular interest, with good visibility of the results. As standard control, the results were compared with the set of tools of the McGill University (Brain Imaging Center, McGuill University, Montreal, Quebec, Canada), widely tested in published articles. The elaborated program took care of the considered objectives.

L’objectif de cette est de développer une source harmonique permettant de couvrir la gamme d’énergie 50 -70 eV du spectre électromagnétique par la génération d’harmoniques d’ordre élevé dans un gaz rare. Cette source appelée "tabletop" est obtenue à partir d’un système laser femtoseconde basé sur la technique d’amplification à dérive de fréquence. Une telle source permet de réaliser des expériences de dynamique des charges et de l’aimantation aux seuils M des métaux de transition. L’intérêt de sonder la dynamique de l’aimantation dans le domaine des rayons X est la sélectivité chimique sur les éléments. Grâce à la sélectivité chimique, on peut sonder la dynamique des spins des espèces chimiques qui sont responsables du magnétisme. Par exemple, dans un système contenant plusieurs éléments, on peut accéder aux propriétés magnétiques de chacun de ses composants en ajustant l’énergie des photons au seuil d’absorption de l’élément choisi. / The goal of this thesis is to develop an harmonic source to cover the energy range of 50-70 eV of the electromagnetic spectrum by high harmonic generation in a rare gas. This source called "tabletop" is obtained from a femtosecond laser system based on the chirped pulse amplification technique. Such source allows probing the magnetization dynamics at the M-edges of transition metals. The interest to probe the magnetization dynamics with X-rays is the chemical selectivity on the elements. This, one can probe the spin dynamics of the chemical element which are responsible for magnetism. For example, in a system containing several elements, one can reach the magnetic properties of each one of its components by adjusting the photon energy to the absorption edge of the element.

De nombreux phénomènes physiques ne peuvent être compris qu'en s'intéressant à la structure électronique. Cette dernière peut être interprétée en termes de propriétés électromagnétiques, chacune de ces propriétés révélant diverses informations sur le système étudié. Il est donc important d'avoir des outils efficaces afin de calculer de telles propriétés. C'est dans ce contexte que cette thèse a été écrite, notre principal objectif ayant été de développer une méthode générale donnant accès à une vaste gamme de propriétés électromagnétiques. Dans la première partie de cette thèse, nous décrivons le socle théorique au sein duquel nous travaillons, en particulier la théorie de la fonctionnelle de la densité de courant dépendante du temps (TDCDFT), qui est une approche qui permet de décrire la réponse du système à un champ magnétique. La seconde partie est consacrée à la méthode que nous avons mise au point pour calculer diverses propriétés magnétiques en préservant l'invariance de jauge. Nous démontrons en particulier qu'en utilisant une simple règle de somme, il est possible de placer les courants diamagnétique et paramagnétique sur un pied d'égalité, évitant par là même les écueils habituels intrinsèques au calcul de propriétés magnétiques, comme la dépendance en l'origine de la jauge du vecteur potentiel. Nous illustrons notre méthode en l'appliquant notamment au calcul de la magnétisabilité et du dichroïsme circulaire, qui est une propriété possédant d'importantes applications pratiques, notamment en biologie. Dans la dernière partie, plus exploratoire, nous tentons d'étendre notre formalisme aux systèmes périodiques. Nous y discutons plusieurs stratégies afin de calculer l'aimantation dans des systèmes décrits par des conditions aux limites périodiques. / Various phenomena of matter can only be understood by probing its electronic structure. The latter can be interpreted in terms of electromagnetic properties, each property revealing a different piece of information. Having a reliable method to calculate such properties is thus of great importance. This thesis is to be regarded in this context. Our main goal was to develop a general method that gives access to a wide variety of electromagnetic properties. In the first part of this thesis, we describe the theoretical background with which we work, and in particular time-dependent current-density-functional theory (TDCDFT), which is a density-functional approach that can describe the response due to a magnetic field. The second part is dedicated to the method we developed in order to calculate various magnetic properties in a gauge-invariant manner. In particular, we show that by using a simple sum rule, we can put the diamagnetic and paramagnetic currents on equal footing. We thus avoid the usual problems that arise when calculating magnetic properties, such as the dependence on the gauge origin of the vector potential. We illustrate our method by applying it to the calculation of magnetizabilities and circular dichroism, which has important applications, notably in biology. In the last part, which is more explorative, we aim at extending our formalism to periodic systems. We discuss several strategies to calculate magnetization in systems described with periodic boundary conditions.

This thesis is organized in to six chapters. The contents of each chapter are briefly summarized in the following sections. Chapter 1 introduces different Fe-based superconductors. Within a very short span of their discovery, they quickly expanded to include six different crystal structures. The crystal structure of these systems and their Tc’s are discussed in this chapter. In particular, the properties of Fe1+yTe are described with specific reference to isovalent doping and nonisovalent doping using different elements that affect the superconducting transition in this material. In chapter 2, the basic principles of growth and characterization techniques are explained. These are: 1. Modified Bridgman technique, 2. X-ray diffraction technique, 3. Electron Probe Micro Analyzer to determine chemical composition, 4. SQuID – For magnetization measurements, 5. Closed cycle refrigerator and He Cryostat – for resistivity measurements. The fabrication of high temperature furnace for Bridgman setup is also discussed. Crystal growth and characterization of parent Fe1+yTe and Sb doped Fe1+yTe system are discussed in chapter 3 and chapter 4 respectively. Details of crystal growth by modified Bridgman technique are described here. The characterization includes analysis of crystalline phase, structure and composition. Detailed structural information is extracted from Rietveld refinement of X-ray powder diffraction and the composition analyzed using EPMA. Temperature dependence of magnetization and transport behavior are also discussed. The ideal doping range of Sb in Fe1+yTe is defined in this chapter. In chapter 5, the effect of Sb doping of Fe1.05Te0.50Se0.50 is investigated. The crystal growth and characterization of the new superconductor, Fe1.05Te0.50Se0.48Sb0.02 are deliberated. The superconducting transition observed in transport measurements at zero field and the magnetic field dependence of transition temperatures are discussed. Estimation of upper critical Page|vi|Preface field using Ginzburg-Landau theory as well as the field dependence of magnetization of this crystal is explained in this chapter. The thesis concludes with a chapter on summary and outlook on all investigations. Finally, the scope of future work is outlined in the last section.

This thesis presents the results of a combined magnetic fabric and palaeomagnetic analysis of lower crustal rocks exposed in the Oman (Semail) ophiolite. This has long been an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution, but magnetic investigations in the ophiolite have been limited. Analyses presented here involve using: (i) magnetic anisotropies as a proxy for magmatic petrofabrics in lower crustal rocks in order to contribute to outstanding questions regarding the mode of accretion of fast-spread oceanic crust; and (ii) classical palaeomagnetic analyses to determine the nature of magnetization in these rocks and gain further insights into the regional-scale pattern of tectonic rotations that have affected the ophiolite. The extensive layered gabbro sequences exposed in the Semail ophiolite have been sampled at a number of key localities. These are shown to have AMS fabrics that are layer-parallel but also have a regional-scale consistency of the orientation of maximum anisotropy axes. This consistency across sites separated by up to 100 km indicates large-scale controls on fabric development and may be due to consistent magmatic flow associated with the spreading system or the influence of plate-scale motions on deformation of crystal mushes emplaced in the lower crust. Detailed analysis of fabrics in a single layer and across the sampled sections are consistent with either magmatic flow during emplacement of a melt layer into a lower crustal sill complex, or traction/drag of such layers in response to regional-scale stresses (e.g. mantle drag). Together, results support formation of the layered gabbros by injection of melt into sill complexes in the lower crust. New anisotropy data from the overlying foliated gabbros sampled at two key localities also provide insights into the style of melt migration at this crustal level. Fabrics are consistent with either focused or anastomosing magmatic upwards flow through this layer, reflecting melt migration beneath a fossil axial melt lens. Previous palaeomagnetic research in lavas of the northern ophiolitic blocks has demonstrated substantial clockwise intraoceanic tectonic rotations. Palaeomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization. Systematic sampling resolves for the first time a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used previously to infer lack of significant rotation of the southern blocks. In contrast, E/ENE-directed remanences in the uppermost gabbros imply a large, clockwise rotation of the southern blocks, of a sense and magnitude consistent with that inferred from extrusive sections in the northern blocks. Hence, without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work were obtained of Cu1-xMxO (M = Fe, Ni, Al e Zn, for 0 ≤ x ≤ 0,1 samples, using the co-precipitation method. These samples were studied using different techniques of characterization structural and magnetic. X-ray diffraction (XRD) results added to the Rietveld refinement analysis of the x-ray powder diffraction data shows that all samples present monoclinic the same structure of CuO with space group C2/c. The magnetization measurements as function of temperature (MvsT) show a fast suppress of TN for Fe-doped CuO samples, in contrast with (Ni, Zn and Al)-doped CuO. Synchrotron radiation XRD data at low temperature performed with show that the Cu0,9Fe0,1O and Cu0,9Ni0,1O samples present slow changes in the cell parameters, which it can be associated with the magnetic transition. Although the X-ray absorption (XAS) measurements performed in Cu K-edge show a variation in the density of unoccupied state in the p sublevel, appreciable changes in the local structure of these ions ban not be confirmed. On the other hand, measurements at the doping K and L2,3-edges confirm the existence of Ni2+ ions and oxygen vacancies and/or defect in your structure, already measurements performed in Cu0,9Fe0,1O sample shown the presence of Fe+2/Fe+3 mixed valence. / Neste trabalho foram obtidas amostras de Cu1-xMxO (M = Fe, Ni, Al e Zn) usando o método químico de co-precipitação. Estas amostras foram estudadas por diferentes técnicas de caracterização estrutural e magnética. Resultados de difração de raios X (DRX) associados às análises de refinamento Rietveld confirmam que todas as amostras de CuO dopadas com Fe, Ni, Al e Zn apresentaram estrutura cristalina similar a estrutura monoclínica do CuO puro, pertencente ao grupo espacial C2/c. As medidas de magnetização em função da temperatura (MvsT) mostram uma rápida supressão da temperatura de Néel (TN) de 210 K para 65 K, na amostras de CuO dopadas com 10% de Fe, em contraste com os sistema dopado com Ni, Zn e Al. Resultados de DRX em baixa temperatura usando radiação síncrotron indicam suaves alterações no parâmetro de rede das amostras de Cu0,9Fe0,1O e Cu0,9Ni0,1O, os quais podem estar associados com a transição magnética. Embora as medidas de absorção de raios X realizadas na borda K do Cu, antes e depois da TN, mostrarem uma variação na densidade de estado desocupado no subnível p, não se verificaram mudanças apreciáveis na estrutura local destes íons. Por outro lado, as medidas de absorção na borda K e L2,3 do dopante confirmam a existência de íons de Ni com estado de oxidação +2 e vacância de oxigênio e/ou defeitos na estrutura monoclínica do CuO dopado com Ni. Já nas medidas realizadas na amostra de Cu0,9Fe0,1O mostra a presença de valência mista de Fe+2/Fe+3 neste composto.

[pt] Nanopartículas são importantes ferramentas utilizadas em medicina, tanto para diagnóstico como para tratamento de diversas doenças. Seus tamanhos podem ser controlados, variando de dezenas até centenas de nanômetros, tornando-as menores ou comparáveis às dimensões de células, bactérias e vírus. As nanopartículas magnéticas possuem um núcleo de material magnético recoberto por camadas de diferentes materiais, incluindo sílica ou um polímero. Esta cobertura é responsável pela funcionalização, de forma que elas realizem tarefas específicas, seja para funcionar como um marcador com fins diagnósticos e/ou como um transportador de fármacos. É muito importante no processo de fabricação e utilização das nanopartículas o conhecimento de suas propriedades magnéticas. Com este objetivo, construímos um magnetômetro baseado em um criorefrigerador com capacidade para medir propriedades magnéticas em função da temperatura desde ambiente até 6 K. Como sensor magnético utilizamos um elemento Hall de GaAs de baixo custo. O magnetômetro construído tem uma configuração diferente dos magnetômetros Hall tradicionais, já que neste caso a amostra se movimenta na região do sensor. De forma a aumentar a exatidão do momento magnético obtido, foi desenvolvido um modelo que leva em consideração a geometria da amostra. A resolução está limitada pelo sensor utilizado em 10-7 Am2. O magnetômetro foi calibrado de forma independente e seu desempenho foi comparado a magnetômetros de amostra vibrante (VSM) comerciais, apresentando erros menores que 2 porcento na magnetização obtida de diversas amostras. Todos os equipamentos envolvidos na operação do magnetômetro a baixas temperaturas são controlados utilizando a linguagem LabVIEW. Na versão atual do programa, curvas M x H e ZFC-FC podem ser obtidas. Como exemplo de aplicação, fabricamos nanopartículas magnéticas com núcleo de oxido de ferro pelo processo de coprecipitação em meio alcalino e recobrimos com surfactantes e SiO2. As propriedades magnéticas das nanopartículas foram obtidas utilizando o magnetômetro construído. As nanopartículas apresentaram comportamento superparamagnético e grande potencial para liberação controlada de drogas. / [en] Nanoparticles are important tools used in medicine, for diagnosis as well as for treatment of various diseases. Their sizes can be controlled, ranging from tens to hundreds of nanometers, enabling them to interact with cells, bacteria, and viruses. Magnetic nanoparticles have a core of magnetic material coated with layers of different materials, including silica or a polymer. This coating is responsible for their functionalization, so they can carry out specific tasks serving as a marker for diagnostic purposes and / or as a carrier for drugs. The knowledge of the magnetic properties of nanoparticles is very important in the manufacturing process and their use. With this aim, we built a magnetometer based on a cryorefrigerator capable of measuring their magnetic properties as a function of temperature from room temperature to 6 K. We used a low cost GaAs Hall element as its magnetic sensor. The magnetometer built has a different configuration from the traditional Hall magnetometers, since in this case the sample moves in the region of the sensor. A model which takes into consideration the geometry of the sample was developed in order to increase the accuracy of the magnetic moment obtained. The magnetometer resolution is limited by the Hall sensor used in 10-7 Am2. The magnetometer was calibrated independently and its performance was compared to commercial vibrating sample magnetometers (VSM) showing errors smaller than 2 percent in the magnetization obtained from various samples. All the equipment involved in the operation of magnetometers at low temperatures is controlled by using the LabVIEW language. The M x H e ZFC-FC curves can be obtained in the current version. We manufactured the core with magnetic nanoparticles of iron oxide by coprecipitation process in an alkaline medium, coated with surfactants and SiO2. The magnetic properties of the nanoparticles were obtained using the magnetometer built. The nanoparticles showed superparamagnetic behavior and great potential for controlled drug release.