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Two diverse projects illustrate the application of various materials characterization techniques to investigate the structure and properties of nanostructured functional materials formed in both bulk as well as in solutions. In the first project, ordered magnetic nanostructures were formed within polymer matrix by novel metallopolymers. The novel metal-functionalized block copolymers (BCPs) enabled the confinement of cobalt metal ions within nanostructured BCP domains, which upon simple heat treatment resulted in room temperature ferromagnetic (RTFM) materials. On the contrary, cobalt functionalized homopolymer having similar chemical structure and higher loading of metal-ion are unstructured and exhibited superparamagnetic (SPM) behavior at room temperature. Based on a series of detailed investigations, using various materials characterization techniques, it was hypothesized that the SPM cobalt particles within BCP microdomains exhibited a collective behavior due to increased dipolar interactions between them under the nanoconfinement of cylindrical domains in BCP, resulting in RTFM behavior. On contrary, the same SPM cobalt particles formed within homopolymer, without any confinement exhibited SPM behavior either due to lack of interactions or random interactions between them. To further support this hypothesis, a series of BCPs were prepared in which the BCP morphology was varied between the cylindrical, lamellar, and inverted cylindrical phases and their magnetic properties were compared. All these BCPs, which are nanostructured, exhibited RTFM behavior, further supporting the proposed hypothesis. Different dimensionality or degree of nanoconfinement in BCP morphologies affected the magnetization reversal processes in these BCPs, yielding different macroscopic magnetic properties. Most strongly constrained cylindrical morphology has shown best magnetic properties (highest coercivity) among other BCP morphologies. Inverted cylindrical morphology, in which a 3-D matrix is confined between the non-magnetic cylinders, had second highest and lamellar morphology with least confinement among BCPs, exhibited lowest coercivity. The proposed hypothesis was further tested by systematically varying the dipolar interactions between the SPM cobalt nanoparticles by reducing the density of cobalt within the cylindrical domains and varying the dimensions of the cylindrical domains (i.e. diameter). A series of novel ferrocene-cobalt containing block copolymers were developed and cobalt density within the cylindrical domains of BCP was varied by changing the chemical composition of the metal functionalized block. Further, the diameter of the cylindrical domains was varied by varying the molecular weight of the cobalt-containing BCPs. These studies allowed us to understand the fundamental correlations between the self-assembled nanostructures and their macroscopic magnetic properties. In the second part of the thesis, a novel amphiphilic block copolymer (ABC), composed of a hydrophilic zwitterionic block and a hydrophobic PS block, was synthesized by ROMP. The formation of zwitterionic vesicles in an ionic liquid, as well as in PBS buffer, was confirmed by TEM and DLS characterization. The dispersion of vesicles within ionic liquid enabled the usage of conventional, room temperature TEM to visualize them in their solution state. This technique of materials characterization could be extended for the visualization of other hydrophilic soft matter.

An impedance-based core loss measurement technique for power ferrites, the modeling and analysis of mechanisms of high-frequency losses, and design methodology for optimization for high-frequency magnetics are presented. The high-frequency losses of ferrite materials are characterized employing a large-signal impedance measurement technique. The impedance analyzer controlled through an IEEE-488 interface, measures the impedance of the inductor under test under large signal excitation via a power amplifier. The core loss is a form of a parallel resistance is derived from measured impedance characteristics. A wideband impedance probe, enables core loss characterization up to 100 MHz. A comprehensive analysis of all major loss mechanisms in ferrites is presented. A new form of residual losses due to a magnetoelectric effect is postulated to account for losses at high frequencies. Two models of losses in ferrites are proposed, one with emphasis on analysis of loss mechanisms, and the other with an emphasis on the design of high-frequency magnetic components. Both models include the important effect of static bias field, which is the case in many power electronics applications. Magnetic losses due to magnetostriction are measured. Dependence of magnetoelastic resonances on the magnetic bias. core material, core shape and size is studied. The influence of diffusion after-effect on core loss under time-varying bias field is investigated. Thermal stability of high-frequency magnetics is studied. A verification of one- and two- dimensional models of winding losses for solid and litz wire is performed. The optimum design method for high-frequency power transformers and inductors is proposed. / PhD

In the beginning of the twenty-first century, educational and commercial institutions have driven the demand for cheap and efficient data storage. The storage medium known as magnetic recording media has remained the mainstay for most computer systems due to its large storage capacity per dollar. With the recording media's ever-increasing storage density has come reductions in the magnetic grain size per bit. At the recording bit's density threshold, the magnetic grains become more susceptible to thermal activation, which can render the storage medium unusable. An accurate characterisation of the recording layer's sub-granular structure is essential for understanding the magnetic and thermal mechanisms of high-density recording media. Small-Angle Neutron Scattering (SANS) studies have been performed to investigate the magnetic and physical properties of longitudinal and perpendicular recording grains. The SANS studies of longitudinal magnetic recording media have probed the recording layer's magnetic grain size at a sub-nanometer resolution. In conjunction with these studies, SQUID magnetometry was used to characterise the recording grain's bulk magnetism. Measurements showed that the recording grain was composed of a ferromagnetic hard core (Co-enriched) and a weakly magnetic shell (Cr-enriched). These results provided important information on the grain's magnetic anisotropy, which determines the recording media's magnetic stability. The polarised SANS studies were used to characterise the recording layer's physical granular structure. It was shown that the physical grain size was comparable to its magnetic counterpart. These physical measurements provided insight into the recording grain's chemical composition. The magnetic properties of perpendicular magnetic recording media were studied using SANS and VSM measurements. The neutron scattering studies revealed that the recording grain was composed of a hard ferromagnetic centre enriched with cobalt. The VSM studies showed that the magnetic recording grains exhibited a large perpendicular magnetic anisotropy. These combined studies provided information on the recording grain's ferromagnetic composition and magnetic stability. The polarised SANS measurements showed the physical grain size to be slightly smaller than its magnetic counterpart. This size difference was attributed to the non-magnetic grain boundary composed of SiO2. The boundary thickness determined the degree of inter-granular exchange coupling. Further polarised studies investigated the recording layers switching behaviour, which revealed more information on the grain's magnetic stability.

Este trabalho trata do estudo da eletrodeposição de filmes finos e nanofios magnéticos compostos de cobalto, níquel e molibdênio. Foi avaliada a influência da composição química das soluções utilizadas nas propriedades do material obtido. O uso de uma célula de Hull com eletrodo cilíndrico rotativo permitiu também que fosse estudado com mais detalhe o mecanismo de eletrodeposição da liga CoNiMo. Os filmes finos magnéticos de CoNi e CoNiMo foram eletrodepositados galvanostaticamente utilizando soluções contento citrato ou glicina como aditivo. Composição, microestrutura, morfologia e propriedades magnéticas dos depósitos foram analisados e a influência da composição das soluções e das densidades de corrente aplicadas foi avaliada. Soluções contendo citrato e/ou baixo pH não resultaram em filmes com boas propriedades. O uso de glicina e pH 7 resultou em filmes amorfos e com melhores propriedades magnéticas: magnetização de saturação de 1,2 T e coercividade de 50 Oe. Com o uso da célula de Hull rotativa foi possível avaliar como variava a composição da liga e as densidades de corrente parciais de cada um dos elementos. Notou-se que a deposição de Ni era menor quanto maior a concentração de Co+2 na solução e que o aumento na concentração de glicina favorece a deposição de Co e Mo e prejudica a deposição de Ni. Além disso, a deposição de Mo foi mais influenciada pela concentração de Co+2 do que pela de Ni+2. Tais observações podem ser analisadas com base nos mecanismos já propostos para deposição de Co, Ni e Mo. Nanofios das ligas CoNi e CoNiMo foram preparados através de eletrodeposição potenciostática utilizando membranas comerciais de alumina como molde. Glicina foi utilizada como agente complexante nos eletrólitos. Fios amorfos com diâmetro médio de 200 nm e até 50 µm de comprimento foram obtidos. Em comparação com os filmes finos estudados previamente, os nanofios apresentaram maiores coercividade e cristalinidade. A coercividade medida foi de 220 Oe com o campo magnético aplicado em paralelo aos fios e de 350 Oe com o campo aplicado perpendicularmente. A presença do molibdênio não afetou as características magnéticas dos nanofios. / This work focuses on the electrodeposition of CoNi and CoNiMo thin films and nanowires. The influence of the chemical composition of several tested solutions over the properties of the material were evaluated. A rotating cylinder Hull cell allowed a more detailed study of the deposition mechanism. The thin films were galvanostatic electrodeposited from solutions containing either citrate or glycine as additives. The composition, microstructure, morphology and magnetic properties of the deposits were analyzed and related with the different bath compositions used and the applied current densities. Baths containing citrate and/or at low pH are not suitable conditions to produce magnetic films with reasonable good properties to be used in magnetic devices. Generally, use of glycine in the bath and pH 7 yielded better films. Magnetic saturation values around 1.2 T and coercivities as low as 50 Oe were obtained for films prepared using baths containing glycine. Films electrodeposited with the citrate containing baths showed higher coercivity: 125 Oe. The investigation of a wide variation of parameters the electrodeposition of the CoNiMo alloys was performed using a rotating cylinder Hull cell. Alloy composition, current efficiency and partial currents of each metal were analyzed. The nickel deposition rate decreased by increasing Co ions and glycine in the electrolyte. The latter also resulted in na augmented concentration of Ni-Gly complexed species. Molybdenum induced codeposition was verified for both excess Ni and excess Co electrolytes. The results indicate that Mo reduction was affected by the Co/Ni ratio in the electrolyte. With an excess Ni in solution, Mo wt. % increased with an increase in cobalt ion electrolyte concentration. On the other hand, with an excess of cobalt in solution, Mo wt. % was not significantly affected by nickel ion concentration. These results were analysed based on the current proposed mechanisms for Co, Ni and Mo deposition. CoNi and CoNiMo nanowires were electrodeposited using commercial alumina templates and a pH 7 glycine-ammonia electrolyte. The resulting magnetic properties and composition were compared with thin film counterparts. The nanowires had larger coercivity (220 Oe) and more crystallinity than the thin. The presence of molybdenum had no significant influence over the coercivity and remanence in the nanowires, unlike thin films.

O objetivo deste trabalho foi tentar correlacionar microestrutura e propriedades magnéticas de ímãs permanentes de alta energia do tipo Sm(CoFeCuZr)z para aplicações em temperaturas acima de 300°C. Avaliou-se a possibilidade de que o constituinte matriz da microestrutura dos ímãs para altas temperaturas fosse diferente do constituinte matriz dos ímãs convencionais. Com este propósito foram estudados três conjuntos de amostras: i) Dois ímãs comerciais para aplicações na temperatura ambiente, um da Electron Energy e outro da Vacuumschmelze. Eles foram caracterizados magneticamente e a sua microestrutura foi analisada por microscópio eletrônico de varredura dotado de análise química, com o objetivo de avaliar se os mesmos apresentavam dois microconstituintes presentes em algumas ligas de Sm(CoFeCuZr)z. ii) Uma liga comercial da Johnson Matthey (JM) que é utilizada para produzir ímãs permanentes de Sm(CoFeCuZr)z. Essa liga foi utilizada para o estudo e avaliação de ciclos térmicos como solubilização, tratamento isotérmico, rampa de resfriamento lento e tratamento térmico a 400°C, e o seu efeito nas propriedades magnéticas e na micro e nano estruturas. iii) Duas séries de ligas com seis amostras cada série produzidas em forno a arco voltaico no LMM-IFUSP, com o propósito de estudar o efeito do teor de cobre e samário nas propriedades magnéticas, na microestrutura e na nanoestrutura dessas ligas. A caracterização magnética em campo de 9T permitiu determinar a polarização máxima e o campo coercivo das amostras. A caracterização magnética em campos de até 2T permitiu determinar o campo coercivo e a polarização remanente das amostras em temperaturas na faixa de 200 a 500°C. Três ligas resultaram em campo coercivo Hci > 398kA/m (Hci > 5kOe) a 450°C, que segundo a literatura, podem ser utilizadas em aplicações em temperaturas elevadas. Uma caracterização detalhada da microestrutura e da composição química foi feita utilizando-se Microscopia Eletrônica de Varredura (MEV) com VI análise química de praticamente todas as amostras. No diagrama de fases pseudo-ternário foi possível definir o campo de fases das composições 2:17R com baixo e alto Sm. As amostras da liga comercial JM e três das doze ligas produzidas em forno a arco voltaico apresentaram os microconstituintes claro e escuro e estes parecem não afetar as propriedades magnéticas das amostras. Foi observado que com o aumento do teor de Sm na liga a polarização máxima diminui. A polarização máxima Jmáx também diminui com o aumento do teor de Cu na liga, porém de forma menos acentuada. Foi possível observar a estrutura nanométrica, bem como a presença e a formação das nanocélulas nessas ligas nanocristalinas com elétrons secundários via microscopia eletrônica de varredura com fonte de emissão de campo MEV/FEG, que é um diferencial neste trabalho uma vez que essas análises são típicas em microscopia eletrônica de transmissão. Verificou-se que as nanocélulas são observáveis com FEG em amostras cuja coercividade são superiores a 159 kA/m (2kOe). Foi possível aplicar o modelo de Stoner-Wohlfarth modificado por Callen Liu e Cullen à curva de histerese para determinar o campo de anisotropia magnetocristalina, a magnetização de saturação e o coeficiente de interação de campo médio 1/d. A difração de Raios-X aliada ao refinamento Rietveld permitiram determinar as fases formadas em altas temperaturas após o tratamento de homogeneização a 1175°C/4h. Verificou-se que há a formação de duas fases romboédricas, uma rica em cobre e a outra rica em ferro. Este dado é inédito na literatura. / The main objective of this study was to correlate microstructure and magnetic properties of high-energy permanent magnets of the Sm(CoFeCuZr)z type for applications at temperatures above 300°C. The possibility that the matrix constituents of the microstructure of high temperature magnets are different from the matrix constituents of the conventional magnets is investigated with three sets of samples: i) Two magnets for commercial applications at room temperature, one produced by Electron Energy and the other by Vacuumschmelze. ii) A Sm(CoFeCuZr)z as-cast alloy synthesized by Johnson Matthey (JM) for permanent magnets production was used to study the effect of heat treatment steps as follows: (a) solutionizing at 1175°C for 4h, b) followed by quenching in water, (c) reheating at 820°C for 7h, d) followed by slow cooling down at 1°C/min to 400°C, and (e) heat treatment at 400°C for different durations and their effects on its magnetic properties and its micro and nanostructure. iii) Two series of Sm(CobalFe0.15CuxZr0.023)7 and Sm(CobalFe0.15CuxZr0.023)8.5 alloys with x=0.058, 0.088, 0.108 were prepared by arc melting technique, in order to study the effect of Copper and Samarium contents on its microstructure and magnetic properties. The characterization in magnetic fields up to 9T allowed to determine the maximum polarization and the intrinsic coercivity of the samples. The characterization in magnetic field up 2T allowed to determine the coercive field and remanence of the samples at temperatures between 200°C and 500°C. Three alloys resulted in coercive field Hci > 398kA/m (Hci > 5kOe) at 450°C, which according to the literature, can be used in applications at elevated temperatures. A detailed characterization of the microstructure and chemical composition was performed using scanning electron microscopy (SEM) with EDAX chemical analysis of virtually all samples. The JM samples and three of the twelve alloys produced in the electric arc furnace showed the light and dark microconstituents, here identified as two 2:17 rombohedral phases with low and VIII high Sm. In a pseudo-ternary phase diagram it was possible to define the two phase field of 2:17R with low and high Sm. No correlation could be found with the magnetic properties of the samples. It was observed that by increasing the Sm content in the alloy the maximum induction Jmax decreases. The maximum induction Jmax also decreases with increasing Cu content in the alloy, but to a lesser extent. It was possible to observe the nanocrystalline microstructure as well as the presence and formation of nanocrystalline cells with scanning electron microscopy with field emission source SEM-FEG. Those characteristics are usually only observed by transmission electron microscopy. It was found that the nanocrystalline cells are observable with SEM-FEG on samples whose coercivity is above 159 kA/m (2kOe). It was possible to apply the Stoner-Wohlfarth model modified by Callen Liu and Cullen in the hysteresis loop to determine the magnetocrystalline anisotropy field, the saturation magnetization and the mean field interaction 1/d. The X-ray diffraction coupled with a Rietveld refinement were used to determine the phases formed at high temperatures after the solutinizing heat treatment at 1175°C/4h. It was found that there are two kinds of 2:17 rhombohedral phases formation. One is rich in copper and the other rich in iron.

Nenhuma / A espectroscopia Mössbauer de 57Fe é uma técnica nuclear, não-destrutiva, utilizada na investigação das propriedades estruturais, magnéticas e hiperfinas de diferentes materiais sólidos. Trata-se, portanto, de uma poderosa ferramenta de caracterização de materiais na física, metalurgia, geologia e biologia, particularmente em materiais magnéticos, ligas metálicas e minerais contendo Fe. Na atualidade, a Espectroscopia Mössbauer de Elétrons de Conversão (CEMS) é amplamente utilizada no estudo de filmes finos e ultrafinos, bem como outros materiais nanoestruturados. No caso de nanoestruturas magnéticas, estudos a baixas temperaturas (LT) são particularmente importantes devido à possibilidade de efeitos oriundos do fenômeno do superparamagnetismo. Neste trabalho foi desenvolvido um sistema de medição CEMS a baixas temperaturas (< 20 K) utilizando um Channeltron (multiplicador de e-) e um Criostato ótico (Modelo SVT-400 da Janis Research Co, EUA) tendo como base um detector originalmente desenvolvido no Departamento de Física Aplicada / Espectroscopia Mössbauer da Universidade de Duisburg-Essen, Alemanha. O detector LTCEMS foi construído, testado e aplicado com sucesso num estudo preliminar de caracterização de bicamadas Fe/(EuxPb1-x)Te(111) com uso de camada sonda de 57Fe de apenas 15Å, obtendo resultados a temperaturas de até 8 K na amostra. / The 57Fe Mössbauer spectroscopy is a nuclear, non-destructive technique used for the investigation of structural, magnetic and hyperfine properties of several materials. It is a powerful tool in characterizing materials in physics, metallurgy, geology and biology field areas, especially magnetic materials, alloys and minerals containing Fe. Lately, the Conversion Electron Mössbauer Spectroscopy (CEMS) is widely used in making studies on ultra-thin magnetic films, as well as other nanoestructured materials. In case of magnetic nanostructures, low temperature (LT) studies are especially important due to the possibility of dealing with superparamagnetic effects. In this work it was developed a CEMS measurement system for low temperatures (< 20K) based on a solid-state electron multiplier (Channeltron) and an optical cryostat (Model SVT-400, Janis Research Co, USA), from which the project was originally conceived at the Applied Physics / Mössbauer Spectroscopy Department from University of Duisburg-Essen, Germany. The LT-CEMS system was fully built, tested and successfully applied in a preliminary characterization of Fe/(EuxPb1-x)Te(111) bilayers with use of a 15 Å, 57Fe probe layer, with reasonable results at sample temperatures as low as 8 K.

Made available in DSpace on 2016-12-23T14:20:01Z (GMT). No. of bitstreams: 1 EMC_Heusler- parte 1.pdf: 1976712 bytes, checksum: 54cbe3c62f71ef7121b79c6f4e1efaa0 (MD5) Previous issue date: 2008-03-28 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work, magnetic entropy change of nonstoichiometric Ni54[Mn(1-x)Fex]19Ga27 Heusler alloys was experimentally verified by partial substitution (up to 50%) of the Mn atoms by magnetic Fe atoms. Such study was mainly concentrated in the magneto-structural transformation region, at low temperatures. The analysis of X-ray diffraction patterns indicate that the partial substitution of Mn atoms by Fe atoms causes predominance of the L21 -phase, however accompanied by spurious phases. The magnetization measurements as a function of the temperature in the low magnetic field reveals that, in all concentration range (0 x 0.5), the system presents a magnetic transition (Ferromagnetic Paramagnetic) at a temperature, TC, near the room temperature, when the material lay in the austenitic phase. Moreover, as many others Heusler alloys, the material undergoes a martensitic structural transition at low temperatures, TM. As the Fe concentration increases, the Curie temperature, TC, undergoes a little variation, increasing around 5%, while TM decreases slowly and monotonically. The magnetic entropy change, for a field of 5T, presents a maximum SM = - 9,3 J/kg.K, for x = 0.1, at a temperature of 250K, and than decreases for x 0.3, changing linearly with the maximum applied field. / Neste trabalho, verificou-se experimentalmente a variação da entropia magnética nas ligas Heusler não-estequiométricas Ni54[Mn(1-x)Fex]19Ga27, substituindo-se parcialmente (até 50%) os átomos de Mn por átomos de Fe. Tal estudo focalizou-se principalmente na região de transformação magneto-estrutural a baixas temperaturas. A análise dos difratogramas de raios-X indica que a substituição parcial de Mn por Fe nessa faixa de concentração, ocasiona uma predominância da fase- L21, porém acompanhada de outras fases espúrias. As medidas de magnetização em função da temperatura realizadas a campos baixos revelam que, em toda a faixa de concentração estudada (0 x 0,5), o sistema apresenta uma transição magnética (Ferromagnético Paramagnético) ao atingir a temperatura de Curie, TC, um pouco acima da temperatura ambiente, quando o material se encontra na fase austenítica. E, assim como muitas outras ligas Heusler, o material sofre uma transição estrutural ao ser resfriado a temperaturas inferiores à temperatura de transição martensítica, TM. À medida que se aumenta a concentração de Fe, a temperatura de Curie, TC, sofre uma pequena variação, aumentando em torno de 5%, enquanto que TM diminui lenta e monotonicamente. A variação da entropia, para um campo de 5T, apresenta um máximo SM = - 9,3 J/kg.K para x = 0,1, a uma temperatura T = 250K e, então, diminui para x 0,3, variando linearmente com o campo aplicado.

Made available in DSpace on 2014-12-17T15:42:08Z (GMT). No. of bitstreams: 1 RegiaCP_TESE.pdf: 5580865 bytes, checksum: b0d574167afcc48aece0c780848733c2 (MD5) Previous issue date: 2009-12-22 / Universidade Estadual de Roraima / Were synthesized systems Ni0,5Zn0,5Fe2O4, i0,2Zn0,5Mn0,3Fe2O4, Mn0,5Zn0,5Fe2O4, Ni0,5Mg0,5Fe2O4, Ni0,2Cu0,3Zn0,5Fe2O4 and Ni0,2Cu0,3Zn0,5Mg0,08Fe2O4, the precursors citrate method. The decomposition of the precursors was studied by thermogravimetric analysis and spectroscopy in the infrared region, the temperature of 350?C/3h. The evolution of the phases formed after calcinations at 350, 500, 900 and 1100?C/3h was accompanied by X-ray diffraction using the Rietveld refinement to better identify the structures formed. The materials were also analyzed by scanning electron microscopy, magnetic measurements and analysis of the reflectivity of the material. The samples calcined at different temperatures showed an increase of crystallinity with increasing calcination temperature, verifying that for some compositions at temperatures above 500?C precipitates of second phase such as hematite and CuO. The compositions of manganese present in the structure diffusion processes slower due to the ionic radius of manganese is greater than for other ions substitutes, a fact that delays the stabilization of spinel structure and promotes the precipitation of second phase. The compositions presented with copper precipitation CuO phase at a temperature of 900 and 1100?C/3h This occurs according to the literature because the concentration of copper in the structure is greater than 0.25 mol%. The magnetic measurements revealed features of a soft ferrimagnetic material, resulting in better magnetic properties for the NiZn ferrite and NiCuZnMg at high temperatures. The reflectivity measurements showed greater absorption of electromagnetic radiation in the microwave band for the samples calcined at 1100?C/3h, which has higher crystallite size and consequently the formation of multi-domain, increasing the magnetization of the material. The results of absorption agreed with the magnetic measurements, indicating among the ferrites studied, those of NiZn and NiCuZnMg as better absorbing the incident radiation. / Foram sintetizados os sistemas Ni0,5Zn0,5Fe2O4, Ni0,2Zn0,5Mn0,3Fe2O4, Mn0,5Zn0,5Fe2O4, Ni0,5Mg0,5Fe2O4, Ni0,2Cu0,3Zn0,5Fe2O4 e Ni0,2Cu0,3Zn0,5Mg0,08Fe2O4, pelo m?todo dos citratos precursores. A decomposi??o dos precursores foi estudada por an?lise termogravim?trica e espectroscopia na regi?o do infravermelho, na temperatura de 350?C/3h. A evolu??o das fases formadas ap?s as calcina??es a 350, 500, 900 e 1100?C/3h foi acompanhada por difra??o de raios X utilizando o refinamento de Rietveld, para melhor identifica??o das estruturas formadas. Os materiais foram tamb?m analisados por microscopia eletr?nica de varredura, medidas magn?ticas e an?lise da refletividade do material. As amostras calcinadas em diferentes temperaturas indicaram um aumento da cristalinidade com o aumento da temperatura de calcina??o, verificando-se que, para algumas composi??es, em temperaturas acima de 500?C ocorre a precipita??o de segunda fase, como hematita e CuO. As composi??es com mangan?s na estrutura apresentaram processos de difus?o mais lentos devido ao raio i?nico do mangan?s ser maior em rela??o aos outros ?ons substituintes, fato que retarda a estabiliza??o da estrutura espin?lio e favorece a precipita??o de segunda fase. As composi??es com cobre apresentaram precipita??o da fase CuO na temperatura de 900 e 1100?C/3h, fato que ocorre segundo a literatura porque a concentra??o de cobre na estrutura ? maior que 0,25 mol%. As medidas magn?ticas revelaram racter?sticas de um material ferrimagn?tico macio, obtendo-se melhores ar?metros magn?ticos para as ferritas de NiZn e NiCuZnMg em altas temperaturas. As medidas de refletividade mostraram maior absor??o da radia??o eletromagn?tica na faixa de microondas para as amostras calcinadas a 1100?C/3h, que tem maior tamanho do cristalito e em conseq??ncia forma??o de multidom?nios, aumentando a magnetiza??o do material. Os resultados de absor??o concordaram com as medidas magn?ticas, indicando dentre as ferritas estudadas, as de NiZn e NiCuZnMg como melhores absorvedoras da radia??o incidente.

A dependência com a temperatura dos tempos de relaxação nuclear e da forma da linha de ressonância dos núcleos F0, nos antiferromagnetos diluidos Fex Zn1-x F2 e Mnx Zn1-x F2, foram estudados na faixa TN&#8804;T&#8804;300K. Os tempos de relaxação spin, rede (1/T1) dos núcleos F0, os quais não estão acoplados aos spins Fe (ou Mn) através da interação hiperfina transferida, foram medidos e calculados teoricamente em função da concentração x. Os resultados teóricos apresentam uma boa concordância com os dados experimentais para concentrações na faixa 0.1&#8804;x&#8804;0.8. A dependência com a temperatura de 1/T1, para TN<T&#8804;300K foi entendido qualitativamente em ambos sistemas. Os resultados obtidos para 1/T1, em temperaturas próximas de TN foram utilizados para estudar os \"Efeitos de Campo Aleatório\" no comportamento crítico do Mn.65Zn.35 F2 com o campo aplicado paralelamente e perperdicularmente ao eixo fácil (C). Encontrou-se neste caso, que a temperatura de transição TN decresce substancialmente com o aumento da intensidade do campo somente quando H0 || C. Os resultados experimentais obtidos estão de acordo com as teorias do Efeito de Campos Aleatórios em antiferromagnetos anisotrópicos diluídos. A divergência crítica do segundo momento da linha de ressonância não homogeneamente alargada do F0 foi estudada acima de TN. Os resultados experimentais concordam com os cálculos de Heller para o alargamento não homogêneo causado por Efeitos de Campos Aleatórios. Observou-se que a forma da linha se altera na região crítica. Longe de TN ela é Gaussiana e para t&#8804;10-2 ela mostra uma tendência a se tornar Lorentziana. Abaixo de TN a sua meia largura aumenta, seguindo qualitativamente o aumento da magnetização das sub-redes. / The temperature dependence of the nuclear relaxation rates and line shapes of the F0 resonance in the diluted antiferromagnet Fex Zn1-x F2 and Mnx Zn1-x F2 are studied over a large temperature range TN&#8804;T&#8804;300K. The high (room) temperature spinlattice relaxation rates (1/ T1) of the F0 nuclei, which are not transfer hiperfine coupled to the Fe (or Mn) spins, have been measured and calculated as a function of the concentration x. Good agreement with experiment is found for the theoretical results, which have been obtained in the range 0.1&#8804;x&#8804;0.8. The temperature dependence of TN<T&#8804;300K was qualitatively understood \"Random Field Effects\" on the critical behavior of Mn.65Zn.35 F2, for fields applied parallel and perpendicular to the easy (C) axis. It was found that the transition temperature TN depressed substantially with field only for H0 || C. The experimental results are in general accord with the theory for Random Field Effects in desordered, anisotropic antiferromagnets. The critical divergence of the in homogeneously broadened F0 NMR was studied in Fe.6Zn.4F2 above TN. The experimental results agree with Heller\'s calculation of the NMR line broadening by Random Field Effects. With H0 || C the line shape changes from Gaussian towards Lorentzian for t&#8804;10-2 and below TN its line width increase qualitatively following increase in the sublattice magnetization.