Cette thèse est consacrée à la réalisation de composites constitués d’inclusions ferromagnétiques dispersées dans une matrice diélectrique en vue d’obtenir des absorbeurs hyper fréquences dans la gamme 1 GHz – 5 GHz. La principale originalité de notre étude repose sur l’utilisation de particules à forte anisotropie géométrique dites sous forme de «pétales». En raison de leurs bonnes propriétés magnétiques (perméabilité et aimantation à saturation élevées), notre choix s’est porté sur l’utilisation d’alliages NiFeMo et FeSiAl pour cette étude. Dans un premier temps, les particules sous forme de pétales sont obtenues par mécanosynthèse. Un ajustement des paramètres de broyage a permis d’obtenir des pétales avec différents rapports de forme (longueur latérale sur épaisseur). Les particules sont ensuite incorporées à une barbotine constituée d’un liant et d’un plastifiant qui est par la suite coulée en bandes selon le procédé Doctor Blade. Les propriétés structurales et magnétiques des particules broyées ainsi que l’orientation des particules dans le composite ont été caractérisées. Dans une seconde partie, les spectres de perméabilité et de permittivité des composites ont été mesurés en cellule monospire et en ligne coaxiale APC7. Les évolutions des spectres en fonction de la quantité de particules dans le composite ainsi qu’en fonction du rapport de forme des particules ont été étudiées. Il a été montré que la transformation en pétales permettait d’obtenir des niveaux élevés de perméabilité et de permittivité. Ainsi, afin d’avoir un absorbeur de qualité, il est nécessaire de limiter les niveaux de permittivité en enrobant les particules. Les pétales NiFeMo ont été enrobés par de la silice selon le procédé Stöber et ceux à base de FeSiAl par oxydation partielle. Les propriétés magnétiques et électriques dynamiques de composites chargées par des pétales enrobés ont également été étudiées. Des modèles analytiques (loi de mélange de Maxwell-Garnett pour la perméabilité et lois d’Odelevsky et de Mclachlan pour la permittivité) ont été utilisés pour reproduire les principales caractéristiques radioélectriques des spectres de perméabilité et de permittivité Enfin, les performances électromagnétiques de ces composites ont été évaluées en considérant le problème générique de la réflexiond’une onde plane sur des revêtements recouvrant un support métallique. Le passage sous forme de pétales a permis de gagner en taux de charge et en épaisseur de couche absorbante par rapport aux composites chargés par des sphères. Cependant, un taux de charge élevé en pétales entraine une augmentation importante de la permittivité réelle et nuit à la condition d’accord. L’enrobage à la silice des pétales NiFeMo a permis de réduire la permittivité et d’atteindre un niveau d’absorption plus important. Ainsi, nous avons pu réaliser des matériaux composites plus légers et plus minces, absorbants dans la gamme de fréquence recherchée / This thesis deals with composites material containing ferromagnetic flaky-shaped particles (high length to thickness ratio) which exhibit improved microwave properties in terms of permeability spectra. It aims at obtaining effective absorbers in the 1 GHz – 5 GHz range. Because of their good magnetic properties (high permeability and saturation magnetization), NiFeMo and FeSiAl alloys were chosen as fillers. Firstly, composite sheets containing flaky-shaped particles were elaborated. Particles with different aspect ratios were obtained through a ball-milling process and then mixed with polymers in a slurry which is casted into sheets by using the Doctor Blade method. Structural and magnetic properties of milled particles and their orientation in the dielectric matrix were investigated. Then, permeability and permittivity spectra measurements were carried out using a single coil permeameter and a standard APC-7 coaxial line. Their evolutions with the particles aspect ratio and the particles volume content in the matrix were studied. It was shown that flaky shaped particles exhibit higher permeability and permittivity levels. Hence, to avoid impedance mismatch, lowering permittivity levels by using an insulating layer is necessary. Consequently,NiFeMo flakes were coated with a silica layer through the Stöber process and FeSiAl flakes with an oxide layer through oxidation. Electromagnetic properties of composites filled with coated flakes were also studied. In addition, permeabilityspectra were fitted combining the Landau-Lifshitz-Gilbert equation and the Maxwell-Garnet mixing rule. Permittivity spectra were fitted using Odelevsky and Mclachlan laws. Finally, their efficiency as a microwave absorber is estimated by calculating the reflection loss of a normal incident electromagnetic wave on an absorbing layer backed by a perfect conductor. Using flaky particles instead of spherical ones allowed the use of lighter and thinner layer. However, it resulted in high permittivity levels and lead to impedance mismatches. Insulating NiFeMo flakes with a silica layer reduced permittivity levels and allowed better absorbing properties. By using flaky shaped particles, we elaborated thinner and lighter microwave absorbers in the 1 GHz – 5 GHz range.

In the last years, there is a rise of interest in investigation and fabrication of nanometer sized magnetic structures due to their various applications (e.g. for data storage or micro sensors). Over the last several decades ion beam implantation became an important tool for the modification of materials and in particular for the manipulation of magnetic properties. Nanopatterning and implantation can be done simultaneously using focused-ion beam (FIB) techniques. FIB implantation and standard ion implantation differ in their beam current densities by 7 orders of magnitude. This difference can strongly influence the structural and magnetic properties, e.g. due to a rise of the local temperature in the sample during ion implantation. In previous investigations both types of implantation techniques were studied separately. The aim of the current research was to compare both implantation techniques in terms of structural changes and changes in magnetic properties using the same material system. Moreover, to separate any possible annealing effects from implantation ones, the influence of temperature on the structural and magnetic properties were additionally investigated. For the current study a model material system which is widely used for industrial applications was chosen: a 50 nm thick non-ordered nano-crystalline permalloy (Ni81Fe19) film grown on a SiO2 buffer layer based onto a (100)-oriented Si substrate. The permalloy films were implanted with a 30 keV Ga+ ion beam; and also a series of as-deposited permalloy films were annealed in an ultra-high vacuum (UHV) chamber. Several investigation techniques were applied to study the film structure and composition, and were mostly based on non-destructive X-ray investigation techniques, which are the primary focus of this work. Besides X-ray diffraction (XRD), providing the long-range order crystal structural information, extended X-ray absorption fine structure (EXAFS) measurements to probe the local structure were performed. Moreover, the film thickness, surface roughness, and interface roughness were obtained from the X-ray reflectivity (XRR) measurements. Additionally cross-sectional transmission electron microscope (XTEM) imaging was used for local structural characterizations. The Ga depth distribution of the samples implanted with a standard ion implanter was measured by the use of Auger electron spectroscopy (AES) and Rutherford backscattering (RBS), and was compared with theoretical TRIDYN calculation. The magnetic properties were characterized via polar magneto-optic Kerr effect (MOKE) measurements at room temperature. It was shown that both implantation techniques lead to a further material crystallization of the partially amorphous permalloy material (i.e. to an increase of the amount of the crystalline material), to a crystallite growth and to a material texturing towards the (111) direction. For low ion fluences a strong increase of the amount of the crystalline material was observed, while for high ion fluences this rise is much weaker. At low ion fluences XTEM images show small isolated crystallites, while for high ones the crystallites start to grow through the entire film. The EXAFS analysis shows that both Ni and Ga atom surroundings have a perfect near-order coordination corresponding to an fcc symmetry. The lattice parameter for both implantation techniques increases with increasing ion fluence according to the same linear law. The lattice parameters obtained from the EXAFS measurements for both implantation types are in a good agreement with the results obtained from the XRD measurements. Grazing incidence XRD (GIXRD) measurements of the samples implanted with a standard ion implanter show an increasing value of microstrain with increasing ion fluence (i.e. the lattice parameter variation is increasing with fluence). Both types of implantation result in an increase of the surface and the interface roughness and demonstrate a decrease of the saturation polarization with increasing ion fluence. From the obtained results it follows that FIB and standard ion implantation influence structure and magnetic properties in a similar way: both lead to a material crystallization, crystallite growth, texturing and decrease of the saturation polarization with increasing ion fluence. A further crystallization of the highly defective nano-crystalline material can be simply understood as a result of exchange processes induced by the energy transferred to the system during the ion implantation. The decrease of the saturation polarization of the implanted samples is mainly attributed to the simple presence of the Ga atoms on the lattice sites of the permalloy film itself. For the annealed samples more complex results were found. The corresponding results can be separated into two temperature regimes: into low (≤400°C) and high (>400°C) temperatures. Similar to the implanted samples, annealing results in a material crystallization with large crystallites growing through the entire film and in a material texturing towards the (111) direction. The EXAFS analysis shows a perfect near-order coordination corresponding to an fcc symmetry. The lattice parameter of the annealed samples slightly decreases at low annealing temperatures, reaches its minimum at about ~400°C and slightly rises at higher ones. From the GIXRD measurements it can be observed that the permalloy material at temperatures above >400°C reaches its strain-free state. On the other hand, the film roughness increases with increasing annealing temperature and a de-wetting of the film is observed at high annealing temperatures. Regardless of the material crystallization and texturing, the samples annealed at low temperatures demonstrate no change in saturation polarization, while at high temperatures a rise by approximately ~15% at 800°C was observed. The rise of the saturation polarization at high annealing temperatures is attributed to the de-wetting effect.

No description available.

A microwave reflection method has been used to measure the spin excitations corresponding to the translational mode of magnetic vortices in samples containing either one or two vortices. Experimental findings are complemented by micromagnetic simulations. One-vortex systems are investigated in micron-sized circular and elliptical cylinders. For ellipses, the resonance frequency can effectively be tuned by applying static magnetic fields and the field dependence of the frequency is significant for fields applied along the short axes but negligible when applied along the long axes of the ellipses. This is contrary to the circular case, where virtually no field dependence was found. This can be understood by considering the shape of the vortex potential well. Further, it is found that the resonance frequency is independent on the direction of the excitation field for the one-vortex systems. Ellipses containing two interacting vortices are also investigated. It is shown that the relative vortex core polarizations dominate the vortex translational mode and cause, in the case of opposite polarizations, a dependence on the excitation field direction. For parallel core polarizations, no dependence on the excitation field direction is found. The dependence of the resonance frequencies on applied static fields along the long and short axes are also experimentally mapped out and compared with micromagnetic simulations, where the possible eigenmodes are determined. Another section of the thesis introduces the dawning of a device based on patterned magnetic elliptical elements for the manipulation and movement of magnetic particles on a surface. The controlled movement and separation of individual particles are successfully demonstrated. Contributions to micromagnetic standard problems and simulations on magnetization switching in nanoscale particles have also been performed. The standard problems highlight some important aspects of choosing the discretization cell sizes and the finite temperature simulations show that thermal fluctuations can alter the magnetization reversal paths.

In this study, prealloyed austenitic stainless steel and premixed soft magnetic Ni-Fe permalloy compacts were consolidated through microwave and conventional sintering routes at combinations of various sintering temperatures and compaction pressures. Sintered alloys were characterized in terms of their densification, microstructural evolution as well as mechanical and magnetic properties. The effect of sintering method in terms of the applied sintering parameters on the final properties of the compacts were investigated in a comparative manner. It was determined that microwave sintered permalloys are superior compared to their conventionally sintered counterparts in densification response, microstructural characteristics such as pore shape and distribution as well as mechanical properties for both austenitic stainless steel and permalloy compacts. However, permeability of the microwave sintered permalloys was inferior to their conventionally sintered counterparts in some cases due to microstructural refinement associated with microwave sintering route.

The morphology, composition, and magnetic properties of NiFe thin films were characterized. Films with thicknesses up to 137 nm were deposited in an RF induction evaporator at high vacuum (10^-8 mbar). Time resolved magneto-optic Kerr effect microscopy (TR-MOKE) was used to measure the Gilbert damping constant, an important dynamic magnetic property with applications to magnetic data storage. The composition of each film was measured with energy-dispersive X-ray (EDX) microscopy and used to determine the weight percent of Ni and Fe in each film. A trend of increased damping with increased thickness was found, in agreement with published results. Magnetic properties and roughness were found to differ significantly from previous films grown in the same vacuum chamber by Rudge, and are attributed to different growth modes produced by differing deposition conditions. However, the weight percent of Ni in each film was found to be inconsistent, deviating by up to 7% from the Ni80Fe20 evaporation source. Inconsistent composition, caused by the inability to control deposition parameters, prevents insight into Gilbert damping from being drawn from the analysis. / Graduate

Thesis (M.S.)--Miami University, Dept. of Physics, 2009. / Title from first page of PDF document. Includes bibliographical references (p. 38).

Title from first page of PDF document. Includes bibliographical references (p. 62-63).

The dynamic process of how fast a spintronic device can be switched from one state to another is characterized by the Gilbert damping parameter. It has been found that the Gilbert damping along with other intrinsic properties in permalloy, can be tuned by different dopants and doping concentration. Therefore, a study of intrinsic magnetic properties with emphasis on the dependence of the Gilbert damping parameter from first principles calculations is investigated. It is aimed at to give an insight of the microscopic understanding originated from electronic structure and to provide a guideline in the practical spintronic design. The topic of the present thesis is to investigate, by means of first principle calculations, how the variation of the Gilbert damping parameter depends upon the electronic structure of pure and doped permalloy. We show that the Gilbert damping has a monotonic increase with the doping concentration due to an increasing amount of scattering processes. The dopants of the 5d transition metal give rise to a much larger impact than the 4d, as the spin orbit coupling effect is more pronounced in the heavy elements. Our results are in satistying agreement with experiment. / <p>QC 20150629</p>

Master´s thesis is focus on the proces of mechanical alloying. It is the proces of modifying a hetegeneous mixture of powder materials into a homogeneous composite powder. Experiments are focus on three types of composite materials. A magnetic soft alloy Permalloy, ODS steel based on commercially available powder steel 434 LHC and low-activation high-chrome ODS steel 14Cr-2W. On composite powders are made a series of mechanical tests and chemicel analysis. Based on this tests and analysis it was possible to confirm the milling time needed to create fully homogeneous composite powder. Next step is compaction of composite powder into compact volume and another mechanical tests ana analysis of microstructure. In these analyzes to determine whether i tis necessary to use protective atmosphere during mechanical alloying. All three type of materials succesfull prepared by mechanical alloying. It was found that for created of a fully homogeneous composite powder is necessary to perform mechanical alloying for 24 hours. When processing of corrosion resistant materials, i tis possible to perform mechanical alloying in an air atmosphere. During mechanical alloying materials which are subject to oxidation, i tis necessary to use protective atmosphere.