Realizamos medidas Mossbauer a temperatura ambiente das ligas de SM IND.2 (CO IND.1-XFE IND.X) ind.17, COM X = 0.1 A 0.6, E DAS LIGAS sm (CO IND.0.90-VFE IND.VCU IND.0.08ZR IND.0.02) ind.8.35, Com V variando de 0.23 A 0.27. Estas amostras foram caracterizadas por difração de raio X. elas apresentaram a estrutura romboédrica do tipo th ind.2zn ind.17 (R3barraM) com os átomos de Fe ocupando quatro sítios cristalográficos distintos todos os espectros obtidos apresentaram desdobramento magnético e um nível de complexidade alto, resultado da sobreposição de quatro sítios distintos de átomos de fe. Os espectros foram ajustados e através desse ajuste determinados os parâmetros hiperfinos associados a cada sítio e suas respectivas populações. Os resultados obtidos das populações dos sítios nas series de compostos Sm ind.2 (CO IND.1-XFE IND.X) ind.17 revelaram a forte preferência dos átomos de FE de ocupar os sítios halteres (2c). No modelo proposto por ray, à função do ZR e o de deslocar pares de FE dos sítios halteres, substituindo-os por pares ZR-vacância. Através dos resultados obtidos pudemos também comprovar esta hipótese através da análise das populações dos sítios das duas series de amostras. / Mössbauer measurements were carried out at room temperature for Sm2(Co1-xFex)17 alloys with x=0,1 to 0,6 and Sm2(Co0.9-vFevCu0.08Zr0.02)8.35, with v=0.23 to 0.27. All samples were characterized by X-ray diffraction. They showed rhombohedral structure of th2Zn17 type (R3m), where Fe atoms occupy four crystallographic sites. All spectra showed magnetic splitting and a high complexity resulting from the superposition of four Fe sites. Hyperfine parameters and site populations were obtained by least-squares fitting of the spectra. For Sm2(Co1-xFex)17 alloys, Fe atoms show a strong preference for 2c (dumbbell) sites. In the Zr-containing alloys, on the other hand, this tendency is significantly reduced, confirming A. E. Rays hypothesis on the role of Zr in the improvement of the magnetic properties of these alloys through metallurgical treatments.

Iron-cobalt alloys have been extensively studied as potential hard disk drive write head materials due to their potentially high saturation flux densities (~2.4T), low coercivities and ease of deposition. Iron-cobalt plating solutions have, however, been shown to have stability issues, necessitating that they be used at low pH or that a stabilizing agent be added to the solution. The purpose of this thesis is to evaluate the stability of a dibasic ammonium citrate plating solution and to characterize the deposits which result from its use. The plating solutions are found to be less stable than previously claimed. The solutions are oxidized by dissolved oxygen, which leads to a valence change in the iron ions and eventually the formation of iron oxide/hydroxide precipitates. These effects are exacerbated by heating or the application of a voltage across the solution. Deposits plated from the solution are fine grained (<40nm) and compact through their thickness. While normally deposited as the equilibrium BCC phase, metastable phases are deposited at elevated temperatures, high pH or in the absence of a stabilizing agent. A metastable phase which is isomorphous to α-Mn is deposited at elevated temperatures. This phase transforms to the BCC phase when annealed at >174ºC and is highly textured. Its presence is detrimental to deposit coercivity. / Materials Engineering

This thesis presents the study of organofunctional alkoxysilane coatings to prevent high temperature oxidation of Sm-Co powders. Sm-Co are important permanent magnetic alloys, owing to their high Curie temperature and large values of magnetocrystalline anisotropy. They possess stable magnetic properties in the temperature range -40 to 120 °C which makes them very attractive candidates for automobile’s electric motors. However, the environmental conditions for such applications are a sum of high temperatures, humidity, fuels and salts which provide perfect breeding ground for corrosion. In this study we report the high temperature oxidation resistance of Sm2Co17 powders coated with four common commercially available organofunctional silanes; (3-aminopropyl)trimethoxysilane (APTMS), (3-aminopropyl)triethoxysilane (APTES), methyltrimethoxysilane (MTMS) and (3-glycidyloxypropyl)trimethoxysilane (GPTMS). The as received powder was a multimodal mixture of many sizes and shapes which represented a typical ball milling product. The thermal analyses of the powders suggested that the powders without surface coatings had profound affinity towards oxidation. The thermal properties of sieved uncoated powders revealed that the small powders were more susceptible to oxidation than the large powders due to their large specific surface area. The isothermal properties of coated powders revealed that the powders coated with silanes had at least 10 times higher resistance to oxidation as compared to uncoated powders heated at 400 °C for 10 h. The non-isothermal tests conducted from room temperature to 500 °C also revealed that the uncoated powders gained 6 times more mass as compared to the powders coated with an ideal (MTMS) silane. The microstructural analysis of the uncoated powders heated from 400 °C to 550 °C revealed diffusion of oxygen, instable intermetallic phases which resulted in a redistribution of alloying elements, precipitation of alloying elements and formation of a featureless shell (approximately 20 µm in thickness) that surrounded the unreacted core. The coated powders on the other hand showed homogenous distribution of alloying elements, stable intermetallic phases and limited the shell thickness (1 µm). The thermo-magnetic properties of Sm-Co powders showed that the thermal instability also affected the magnetic properties adversely. It was found that the magnetic properties were deteriorated with a decrease in powder size. The energy dispersive spectroscopic (EDS) analyses showed that the small powders contained higher oxygen content than the large powders. Moreover XRD analysis also revealed that the small powders contain higher residual strains and smaller crystallite size which can play their role in deteriorating magnetic properties. It was found that surface modification by silanization improve the thermo-magnetic properties by effectively shielding the powder surfaces from surface oxidation. The rheological properties Sm-Co/PA12 composites revealed that the viscosity of the composites was increased with decreasing powder size due to the presence of rough surfaces and sharp corners in small powders. The rheological properties of the melts containing coated powders revealed that the silane layer acted as a lubricant and decreased the melt viscosity. It was found that coating the powders with silanes not only improve the rheological properties but also improve the other physical properties such as glass transition temperature the loss modulus by modifying the interfacial layer between the polymer matrix (PA12) and the powder. It results in a decrease in viscosity, a broadening of the glass transition temperature and a change in the damping properties of the composites. The dynamic mechanical properties of Sm-Co/PA12 composites showed that the storage modulus was increased with decreasing powder size. The results were expected as the rough surfaces act as local welding points between the powder and the polymer matrix. It was found that the surface modification improve the storage modulus. It is assumed that the silanes modify the interfacial properties which not only resulted in increasing the storage modulus but also broadened the glass transition temperature, Tg and damping, tanδ peaks. From the thermogravimetric, microstructural, rheological and magnetic analyses it can be concluded that the silanes are the effective coatings in preventing high temperature oxidation, stabilizing microstructure, enhancing mechanical properties, and improving rheological and magnetic properties. / <p>QC 20121205</p>

Studying magnetic properties of disordered alloys is important both for the understanding of phase transformations in alloys and from the point of view of fundamental issues of magnetism in solids. Disorder in a magnetic system can result in unconventional magnetic structures, such as spin glass, which have rather peculiar features. In this Thesis, a rather general approach to studying disordered magnetic alloys from first principles is presented. Phase transformations and magnetic behavior of crystalline substitutional alloys are considered. This approach is exemplified by calculations of an archetypical spin-glass material: the CuMn alloy. First, a general theoretical framework for the description of the thermodynamics of disordered magnetic alloys is given. It is shown that under certain conditions, a complex magnetic system can be reduced to an effective system containing no magnetic degrees of freedom. This substantially simplifies the investigation of phase transformations in magnetic alloys. The effective model is described in terms of material-specific interaction parameters. It is shown that interaction parameters can be obtained from the ground-state property of a disordered alloy which are in turn calculated from first principles by means of highly accurate up-to-date numerical techniques based on the Green's function method. The interaction parameters can subsequently be used in thermodynamic Monte-Carlo simulations to produce the atomic and magnetic structures of an alloy. An example of calculations for the Cu-rich CuMn alloy is given. It is demonstrated that the atomic and magnetic structure of the alloy obtained by the presented approach agrees very well with the results of neutron-scattering experiments for this system. Moreover, numerical simulations enable one to predict the ground state structure of the alloy, which is difficult to observe in experiment due to large atomic diffusion barriers at temperatures close to the temperature of the phase transformation. A general description of a spin glass is given, and difficulties of modeling this type of magnetic systems are discussed. To overcome the difficulties, improved Monte-Carlo methods, such as parallel tempering, overrelaxation technique, and finite-size scaling method of analysis, are introduced. The results for the CuMn alloy are presented.

No description available.

Neste trabalho, calculam-se o calor específico e a entropia do Modelo de Anderson simétrico de uma impureza usando o Grupo de Renormalização Numérico (GRN). O método é baseado na discretização logarítmica da banda de condução do metal hospedeiro a qual a impureza está acoplada. Porém, esta discretização introduz oscilações nas propriedades termodinâmicas. Esta inconveniência, inerente ao método, é contornável para a suscetibilidade magnética, mas é crítica para o calor específico, restringindo o alcance do GRN. Para sobrepor essa dificuldade, é usado o novo procedimento denominado intercalado que foi desenvolvido para o cálculo da suscetibilidade magnética de modelos de duas impurezas. Para reduzir as matrizes e o tempo computacional, é usado, também, o operador carga axial, recentemente definido no contexto do Modelo de Kondo de duas impurezas, e que é conservado pelo Hamiltoniano de Anderson simétrico. As curvas obtidas são comparadas com resultados exatos obtidos por ansatz de Bethe e pelo Modelo de Nível Ressonante. / The specific heat and the entropy of the one-impurity symmetric Anderson Model are calculated using the Numerical Renormalization Group (NRG). The heart of the method is the logarithmic discretization of the metal conduction band where the impurity is coupled. However, this discretization, inherent in the method, introduces oscillations in the thermodynamical properties. For the susceptibility it is not so critical but for the specific heat the usual calculation is prohibitive. To overcome this difficulty, we use the new procedure called interleaved that was developed to calculate the susceptibility of two-impurity models. In order to reduce the matrices and computation time, use is made of the axial charge operator recently defined in the two-impurity Kondo Model context and that is conserved by the symmetric Anderson Hamiltonian. The curves obtained are compared with exacts results of Bethe ansatz and Resonant Level Model.

Neste trabalho, calculam-se o calor específico e a entropia do Modelo de Anderson simétrico de uma impureza usando o Grupo de Renormalização Numérico (GRN). O método é baseado na discretização logarítmica da banda de condução do metal hospedeiro a qual a impureza está acoplada. Porém, esta discretização introduz oscilações nas propriedades termodinâmicas. Esta inconveniência, inerente ao método, é contornável para a suscetibilidade magnética, mas é crítica para o calor específico, restringindo o alcance do GRN. Para sobrepor essa dificuldade, é usado o novo procedimento denominado intercalado que foi desenvolvido para o cálculo da suscetibilidade magnética de modelos de duas impurezas. Para reduzir as matrizes e o tempo computacional, é usado, também, o operador carga axial, recentemente definido no contexto do Modelo de Kondo de duas impurezas, e que é conservado pelo Hamiltoniano de Anderson simétrico. As curvas obtidas são comparadas com resultados exatos obtidos por ansatz de Bethe e pelo Modelo de Nível Ressonante. / The specific heat and the entropy of the one-impurity symmetric Anderson Model are calculated using the Numerical Renormalization Group (NRG). The heart of the method is the logarithmic discretization of the metal conduction band where the impurity is coupled. However, this discretization, inherent in the method, introduces oscillations in the thermodynamical properties. For the susceptibility it is not so critical but for the specific heat the usual calculation is prohibitive. To overcome this difficulty, we use the new procedure called interleaved that was developed to calculate the susceptibility of two-impurity models. In order to reduce the matrices and computation time, use is made of the axial charge operator recently defined in the two-impurity Kondo Model context and that is conserved by the symmetric Anderson Hamiltonian. The curves obtained are compared with exacts results of Bethe ansatz and Resonant Level Model.

This thesis discusses two different magnetic materials: epitaxial yttrium iron garnet (YIG) and heteromorphous CoFeB-SiO2 films. YIG films were grown by pulse laser deposition (PLD) techniques onto gadolinium gallium garnet (GGG) substrates of (111) and (001) crystal orientations. Using stoichiometric and overstoichiometric ablative targets, we developed two types of YIG submicron films. The films grown from overstoichiometric targets have magnetic properties slightly different from standard liquid phase epitaxy (LPE) YIGs. They also demonstrate good substrate matching and approximately 6% nonstoichiometry. In contrary, films grown from stoichiometric targets posses surprisingly high values of uniaxial anisotropy, meanwhile cubic anisotropy is reduced several times. These films also reveal strong lattice distortions and nonstoichiometry around 17%. Employing Weiss molecular field theory and single-ion anisotropy model we determined the preferential occupancy of the octahedral [a] positions in the YIG cubic lattices by Fe3+ vacancies. The vacancies were found to be preferentially oriented along the growth direction perpendicular to the film surface. We called this effect “deformation blockade”. Different magnetostatic surface wave (MSSW) filters were also demonstrated. The filters employ high uniaxial anisotropy in YIG submicron films with magnetic losses ΔH ~ 1 Oe.  Heteromorphous CoFeB-SiO2 films were deposited onto glass substrates employing carrousel magnetron sputtering. This novel technique allows amorphous films fabrication with record high in-plane anisotropy. The induced anisotropy fields here are approximately dozen times greater the values achieved using conventional growth technique when external bias field is applied during deposition process. Interesting observations were made studying CoFeB-SiO2 magnetization dynamics in the wide frequency range from 500 kHz up to 15 GHz.  Two different anomalies of the magnetic susceptibility were found at the field of in-plane anisotropy Hp and critical field Hcr (0 &lt; Hcr &lt; Hp). We explained the anomalies appearance by sequence of the domain walls transformations so that Néel-Bloch-Néel domain wall transition stands for the instability at H = ±Hcr and transition from the uniformly magnetized state to the domain state with Néel domain wall and vice versa is responsible for the instability at H = ±Hp. / QC 20111122

La thèse s'inscrit dans le cadre du projet GENOME « Gestion Optimisée de l'Energie » dont l'enjeu majeur est la conception d'un avion plus électrique. L'augmentation de l'efficacité énergétique et de la compacité des systèmes électriques de ces avions entraîne de fortes sollicitations en température. Ces sollicitations sont liées à la compacité des systèmes (réduction de masse et de volume) ainsi qu'à leur localisation par rapport aux sources chaudes (réacteur d'avion par exemple). De ce fait, les matériaux magnétiques des nouveaux convertisseurs électriques doivent pouvoir fonctionner sous des conditions de hautes températures, supérieures à 200°C. Il s'agit du polycristallin FeCoV dédié à la fabrication des tôles du stator et du rotor des génératrices rapides (situées à proximité des réacteurs) et le nanocristallin FeCuNuSiB dédié à la conception des inductances et transformateurs des convertisseurs statiques. Ce manuscrit s'intéresse à l'étude du vieillissement thermique de ces deux familles de matériaux magnétiques. Ces matériaux, fournis par la société APERAM, se déclinent sous plusieurs nuances et finitions. L'étude du vieillissement consiste en l'application de plusieurs essais de vieillissement continus sous différentes températures (jusqu'à 300 °C pour les FeCoV et 240 °C pour les nanocristallins). Plusieurs grandeurs macroscopiques magnétiques, électriques et mécaniques (pour les FeCoV) sont mesurées à chaque intervalle de vieillissement. Grâce à ces mesures macroscopiques et à des mesures complémentaires effectuées à l'échelle microscopique, des analyses sont faites et des hypothèses sont proposées afin d'expliquer les mécanismes de vieillissement de ces deux familles de matériaux et dans le but de proposer des modèles phénoménologiques fiables / The thesis takes part of the project GENOME “Gestion Optimisée de l’Energie” whose major issue is the design of the more electrical aircraft. The increase in the energy efficiency and the compactness of the electrical systems of these aircrafts lead to high temperature stresses. These thermal stresses are related to the compactness of the systems (reduction of mass and volume) as well as their location with respect to the hot sources (aircraft engine for example). Thus, the magnetic materials of the new electrical converters must be able to operate under conditions of high temperatures, above 200 °C. Typically, the FeCoV polycrystalline materials are dedicated to the fabrication of the stator and rotor sheets of the fast generators (located near the aircraft engine) and the FeCuNbSiB nanocrystalline materials are dedicated to the design of inductors and transformers of the static converters.This manuscript concerns the thermal ageing study of these two magnetic material families. These materials, supplied by the company APERAM, are available in several shades. The ageing study consists on applying several continuous ageing treatments at different temperatures (up to 300 °C for FeCoV and 240 °C for FeCuNbSiB). At each ageing step, several macroscopic properties namely: magnetic, electrical and mechanical (for the FeCoV materials) properties are measured. Using these macroscopic properties and complementary measurements carried out on a microscopic scale, analyses are made and hypotheses are proposed in order to explain the ageing mechanisms of these magnetic material families. The understanding of the magnetic ageing mechanisms is necessary towards establishing of phenomenological ageing models

В работе представлены результаты исследования влияния состояния поверхности ленты на распределение намагниченности и магнитную проницаемость аморфного магнитомягкого сплава Co–Ni–Fe–Cr–Mn–Si–B, а также неоднородности магнитных характеристик в исходном (закаленном) состоянии, Такие исследования имеют важное значение для оптимизации магнитных характеристик и способствуют повышению качества изделий из этого сплава. Состояние поверхности ленты изменяли при помощи термообработок на воздухе в интервале температур 90–380 оС, обработкой водой и ацетоном, формированием полимерного покрытия. Исследования показали, что ленты аморфного сплава Co–Ni–Fe–Cr–Mn–Si–B в закаленном состоянии обладают существенной неоднородностью магнитных характеристик, которая носит закономерный характер и может быть связана с технологией изготовления ленты. Обработка водой приводит к окислению и гидрированию поверхности ленты и ее влияние зависит от знака константы магнитострикции (λs). В состоянии с λs<0 способствует увеличению объема доменов с ортогональной намагниченностью и остроты магнитной текстуры в плоскости ленты, определяемой отношением объемов доменов с планарной намагниченностью, ориентированной вдоль и поперек оси ленты. В состоянии с λs>0 вызывает уменьшение объема доменов с ортогональной намагниченностью и остроты магнитной текстуры. Обработка ацетоном вследствие его окисления и гидрирования содержащимися в поверхности ионами водорода и кислорода оказывает влияние на распределение намагниченности в ленте противоположное воздействию воды, которое также зависит от знака константы магнитострикции. Увеличение длительности обработки ацетоном приводит к усилению ее влияния на изменение объема доменов с ортогональной намагниченностью и ослаблению влияния на перераспределение намагниченности в плоскости ленты. Низкотемпературный отжиг в интервале температур 90 –130оС не способствует изменению максимальной магнитной проницаемости, объема доменов с ортогональной намагниченностью и знака λs, но способствует перераспределению намагниченности в плоскости ленты в результате в результате взаимодействия с атмосферным паром. Наблюдаемое сильное уменьшение максимальной магнитной проницаемости при температурах выше 180 оС связано с ростом объема доменов с ортогональной намагниченностью и снижением остроты магнитной текстуры в плоскости ленты. При 250оС обнаружено формирование поперечной магнитной анизотропии. Полимерное покрытие, формируемое на ленте в закаленном состоянии при 90 –130оС, способствует повышению максимальной магнитной проницаемости и уменьшению объема доменов с ортогональной намагниченностью во всем интервале температур. Полученные результаты находят объяснение в рамках влияния анизотропных напряжений, индуцируемых в ленте в результате взаимодействия ее поверхности с химически активными средами, магнитоактивным полимерным покрытием и анизотропной поверхностной кристаллизацией. / The paper presents the results of the influence of the ribbon surface state on the magnetization distribution and magnetic permeability of amorphous magnetically soft Co-Ni-Fe-Cr-Mn-Si-B alloy, as well as heterogeneity of magnetic characteristics in the initial (as-quenched) state. The state of the ribbon surface was changed by heat treatment in the air in the temperature range 90-380oC, treatment with water and acetone, formation of a polymer coating. The studies showed that the Co-Ni-Fe-Cr-Mn-Si-B amorphous alloy ribbons in the as-quenched state have a significant heterogeneity of magnetic characteristics, which has a natural character and can be associated with the tape manufacturing technology. The water treatment leads to oxidation and hydrogenation of the strip surface and its influence depends on the sign of the magnetostriction constant (λs). In the state with λs<0 the volume of domains with orthogonal magnetization and the sharpness of the magnetic texture in the tape plane, determined by the ratio of the volumes of domains with planar magnetization oriented along and across the tape axis, are increased. In the condition with λs>0 the volume of domains with orthogonal magnetization and the sharpness of the magnetic texture decrease. Treatment with acetone due to its oxidation and hydrogenation by hydrogen and oxygen ions contained in the surface affects the magnetization distribution in the tape opposite to that of water, which also depends on the sign of the magnetostriction constant. Increasing the duration of treatment with acetone leads to increasing its effect on the change in the volume of domains with orthogonal magnetization and to a weaker effect on the redistribution of magnetization in the tape plane. Low-temperature annealing in the temperature range 90 - 130oC does not contribute to changes in the maximum magnetic permeability, the volume of domains with orthogonal magnetization and the sign of λs but contributes to the redistribution of magnetization in the tape plane because of interaction with atmospheric vapor. The observed strong decrease in the maximum magnetic permeability at temperatures above 180oC is associated with an increase in the volume of domains with orthogonal magnetization and a decrease in the sharpness of the magnetic texture in the tape plane. The formation of transverse magnetic anisotropy was detected at 250oC. The polymer coating formed on the tape in the quenched state at 90 - 130oC increases the maximum magnetic permeability and reduces the volume of domains with orthogonal magnetization throughout the temperature range. The obtained results find an explanation in the influence of anisotropic stresses induced in the tape because of interaction of its surface with chemically active media, magnetically active polymer coating and anisotropic surface crystallization.