The main objective of this research is the study of the magnetization of ferromagnetic spheres in the presence of external magnetic fields. The exact analytical solutions derived in this thesis are benchmark solutions, valuable in testing the correctness and accuracy of various approximate models and numerical methods. The total scalar magnetic potential outside the spheres, related to the magnetic field intensity, is obtained by the superposition of the potentials due to all spheres and the potential corresponding to the external field. The translational addition theorems for scalar Laplacian functions are used to solve boundary value by imposing exact boundary conditions. The scalar magnetic potential inside each sphere, related to the magnetic flux density, also satisfies the Laplace equation, which is solved by imposing the boundary conditions known from the solution of the outside field. Finally, the expressions derived are used to generate numerical results of controllable accuracy for field quantities.

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

Since the discovery of the high temperature superconductor YBa2Cu3O7-δ (YBCO), with transition temperature (Tc = 77 K), above liquid nitrogen point in 1987 many research projects have been dedicated to enhancing the high field performance of this material for practical applications. The 2nd generation YBCO-based coated conductors are believed to be the most promising approach for commercial applications including power transmission, motors, generators, and high field magnets. With the advances of nanotechnologies, different nanoengineering methods have been demonstrated to enhance the performance of YBCO thin films, include doping with 0-dimensional (0-D) self-assembled nanoparticles, 1-dimensional (1-D) nanorods, and 2-dimensional (2-D) nanolayers. Furthermore, dopants with ferromagnetic properties are also reported to provide enhanced pinning effects by Lorentz force, especially under high-applied magnetic fields. The principle of these methods is to generate high-density defects at the heterogeneous interfaces as artificial pinning centers in an effort to improve the flux-pinning properties. The morphology and dimensions of the nanoinclusions play an important role in pining enhancement. Optimized pinning structures are likely to be located at energetically favorable vortex cores, which form a triangular lattice with dimensions close to the YBCO coherence length ξ (ξab ~ 4 nm; ξc ~ 0.5 nm at 77 K.) However, it is challenging to achieve small dimensional nanodopants in the vapor deposited YBCO thin films. The purpose of this research is to utilize nanoengineering methods to produce optimized pinning structure in YBCO thin films. In this thesis, we systematically study the effects of different nanoinclusions on the flux-pinning properties of YBCO thin films. The 0-D ferromagnetic Fe2O3 and CoFe2O4 nanoparticles, 2-D CeO2 multilayers, and tunable vertically aligned nanocomposites (VAN) of (Fe2O3)x:(CeO2)1-x and (CoFe2O4)x:(CeO2)1-x systems are introduced into the YBCO matrix as artificial pinning centers. Results suggest that all nanoinclusions showed significant enhancement in the superconducting properties of YBCO. The ferromagnetic pinning centers dominate at high field and low temperature regimes, however, the defect pinning centers dominate at low field and high temperature regimes. The uniquely arranged VAN structure of alternating magnetic and non-magnetic nanophases, which incorporates both high defect density and tunable distribution of magnetic dopants, is believed to be an ideal solution for flux-pinning enhancement.

The procedure for milling micrometre scale cantilevers of lutetium iron garnet using a focused ion beam microscope was developed. The infrastructure to study these cantilevers using rotational hysteresis loops and ferromagnetic resonance experiments was set up. The cantilevers were shown to remain magnetic after milling, and the origin of their hysteresis loops investigated with a variant of the Stoner-Wohlfarth model. Ferromagnetic resonance in the cantilevers was demonstrated as the first step towards studying magnetomechanical coupling.

Thermally activated shape memory polymers are a desirable material for use in dynamic structures due to their large strain recovery, light weight, and tunable activation. The addition of ferromagnetic susceptor particles to a polymer matrix provides the ability to heat volumetrically and remotely via induction. Here, remote induction heating of magnetite filler particles dispersed in a thermoset matrix is used to activate shape memory polymer as both solid and foam composites. Bulk material properties and performance are characterized and compared over a range of filler parameters, induction parameters, and packaging configurations. Magnetite filler particles are investigated over a range of power input, in order to understand the effects of particle size and shape on heat generation and flux into the matrix. This investigation successfully activates shape memory polymers in 10 to 20 seconds, with no significant impact of filler particles up to 10wt% on mechanical properties of shape memory foam. Performance of different particle materials is dependent upon the amplitude of the driving magnetic field. There is a general improvement in heating performance for increased content of filler particles. Characterization indicates that heat transfer between the filler nanoparticles and the foam is the primary constraint in improved heating performance. The use of smaller, acicular particles as one way to improve heat transfer, by increasing interfacial area between filler and matrix, is further examined.

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

Thesis (Ph.D.)--McMaster University, 2004. / Advisor: David Venus. Includes bibliographical references (leaves 138-144). Also available via World Wide Web.

In dieser Arbeit werden magnetische Vortex-Dreifachlagen-Systeme untersucht. Mittels Magnetfeld, Strom und Röntgenzirkulardichroismus kann erstmals die magnetische Konfiguration der Vortexlagen mit dem simultan gemessenen Magnetowiderstand verglichen werden. Die senkrecht mit Strom durchflossenen Kobalt-Kupfer-Permalloy Scheiben werden in einem Mehrschrittprozess mittels Elektronenstrahllithographie auf einer Membran hergestellt, um mit Rastertransmissions-Röntgenmikroskopie untersuchbar zu sein. Die Auswertung der STXM-Bilder zeigt das gleiche Verhalten wie die Widerstandsmessungen und erlaubt eine eindeutige Zuordnung. Um auch die kleinsten scheibenförmigen Dreifachlagensysteme mittels ferromagnetischer Resonanz zu messen, wurde die Mikroresonator FMR optimiert. Damit können bereits etwa 2.3*10^7 Kobaltatome gemessen werden, wobei die Empfindlichkeit bis zu 4*10^6 Atomen ausreichend sein sollte. Durch 6-fache Mittelung lässt sich ein Kobaltwürfel mit einer Kantenlänge von 12,5nm detektieren. Dabei sind nicht nur die uniforme Mode, sondern auch lokal angeregte Moden sichtbar. Mittels mikromagnetischer Simulationen lassen sich den Resonanzen Modenbilder zuordnen. Die scheibenförmige Dreifachlage wird mit den FMR-Messungen sowohl mit verringertem Durchmesser, als auch mit reduzierter Zwischenschicht untersucht.

La recherche de nouveaux matériaux absorbant les ondes électromagnétiques dans le domaine des hyperfréquences est devenue un enjeu majeur avec le développement croissant des moyens de communication et, corrélativement, le besoin de se prémunir efficacement contre la « pollution » électromagnétique (protection des équipements et des personnes). Dans ce manuscrit, nous nous intéressons à un matériau composite anisotrope constitué d'une matrice polymère chargée en particules ferromagnétiques aplaties appelées pétales. Ce type de composite suscite actuellement un intérêt grandissant dans la communauté grâce aux nombreux avantages qu'il présente (ajustement de la bande d'absorption en fonction de la nature et du rapport de forme des charges, faible épaisseur du revêtement). Le champ de cette étude s'est voulu relativement large afin d'avoir une vue d'ensemble des problématiques liées à ces composites à base de pétales et cela à plusieurs échelles. Dans un premier temps, une étude originale a été menée sur le pétale seul. Après des travaux préliminaires portant sur l'élaboration des pétales et leurs propriétés morphologiques, des caractérisations magnétiques statiques et dynamiques ont été réalisées sur un pétale unique. Le spectre de perméabilité obtenu met en évidence trois résonances. Une étude complémentaire menée sur des objets de mêmes dimensions élaborés par PVD et structurés par FIB a permis, par analogie, d'attribuer ces trois résonances respectivement aux domaines, aux parois et au vortex joignant les parois. Dans un second temps, une étude approfondie a été menée sur le composite complet. L'adaptation de la technique de perméamétrie par perturbation de spire a rendu possible la caractérisation fine et complète de matériaux composites épais avec ou sans champ magnétique statique appliqué et pour différentes directions de pompage. Celle-ci a permis de confirmer l'origine des différentes résonances malgré le désordre structurel caractéristique des milieux composites. Pour finir, une étude à vocation plus applicative a été réalisée établissant le lien fort entre la microstructure (porosité, orientation et taille des particules) et les propriétés électromagnétiques du composite. Le potentiel industriel, tant d'un point de vue de l'élaboration que des performances, a pu être démontré. / Research on new microwave absorbing materials has become a major issue with the growing development of wireless communications and, accordingly, the need to shield effectively from the electromagnetic "pollution" (protection of equipment and persons).In this manuscript, we focus on an anisotropic composite consisting of aligned ferromagnetic flakes embedded in a nonmagnetic matrix. There is currently a growing interest for this kind of composite in the community due to the many benefits it provides (adjustment of the absorption band as a function of both nature and flakes aspect ratio, thin composite coating). The scope of this study was deliberately relatively large in order to get an overview of issues related to these flakes composites at several scales. At first, an original study was conducted on a single flake. After preliminary work on the development of flakes and their morphological properties, static and dynamic magnetic characterizations were performed on a single flake. The permeability spectrum obtained shows three resonances. A complementary study on objects of the same size, produced by PVD and structured by FIB has, by analogy, attributed these three resonances to respectively, domains, domains wall and an embedded vortex (vortex linked with several domains walls).In a second step, the evolution of the shorted microstrip permeametry technique made possible the detailed and complete characterization of thick composites with or without a static magnetic field applied in different directions and for various pumping directions. This confirmed the origin of the different resonances despite the characteristic structural disorder of composite media.Finally, an applicative study was conducted establishing the strong link between the microstructure (porosity, particle size and orientation) and the electromagnetic properties of the composite. Taking into consideration both elaboration and performance point of views, the industrial potential of this composite has been demonstrated.

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