The dynamics of driven interfaces in the random-field Ising model (RFIM) is investigated by the use of Monte Carlo simulations. Interfaces in the RFIM separate regions of opposite spin orientation. By applying an external field one orientation is energetically favored. This may yield an interface motion which is hindered by the random-field. Without thermal fluctuations the competition between the driving field and the random-field leads to a so-called depinning transition. A permanent interface motion is found only if the driving field exceeds a threshold field H_c. At the transition point the interface velocity vanishes continuously, characterized by a critical exponent beta. The values of beta found in the RFIM for the dimensions d=3,4,5,6 support the assumption that the depinning transition in the RFIM belongs to the universality class of the Edwards-Wilkinson equation with quenched disorder. The energy barriers which cause a pinning of the interface at temperature T=0 can be overcome due to the energy provided by thermal fluctuations. This yields a permanent interface motion. For sufficient small driving fields a so-called creep regime is found in the random-field Ising model. This creep regime is predicted by phenomenological theories, functional renormalization group calculations, and has been observed in experiments. The field dependence of the energy barrier in the RFIM is investigated and the results are compared with those known in the literature. Furthermore, it is investigated whether the influence of temperature on the depinning transition can be understood within the theory of critical phenomena. It is assumed that the interface velocity can be expressed as a generalized homogenous function in the vicinity of the transition point (H=H_c|T=0). This assumption is supported by the results of simulations in the dimensions d=3,4,6, yielding an algebraic decay v(H=H_c) proportional T^(1/psi) with an exponent psi>0. The assumption of the interface velocity being a generalized homogenous function is also validated by simulations of magnetic films. From these simulations it can additionally be concluded that the depinning transition in magnetic films is characterized by the two dimensional exponents. The investigations of the five dimensional model show the occurrence of logarithmic correction revealing that d_c=5 is the upper critical dimension of the depinning transition in the RFIM.

This work presents research on structural and magnetic properties of epitaxial (Fe_t/Pd_4nm)₃₂ multilayers (MLs). The Fe layer thickness is varied from 1nm to 4nm, while all other parameters are kept constant. Structural investigation are made utilizing STM, RHEED, XRD, RBS and (HR)TEM. The RHEED and XRD investigations reveal that thin Fe layers (t _Fe < 2.5nm) exhibit on fcc-like phase with a high magnetic moment of 2.7 µ_B per iron atom. For Fe layers with thicknesses of t_Fe > 2.5nm the magnetic moment of Fe is 2.2 µ_B, which is in agreement with the value of bulk bcc Fe. From RHEED investigations we find that with increasing thickness of the Fe layer the unit cell of Fe is successively compressed in growth direction while its basal plane remains unaltered. In MLs with thin Fe layers with high magnetic moments the atomic volume of iron is comperable to the volume of the high moment phase in fcc Fe, the atomic volume of MLs with thicker Fe layers is close to the equilibrium value of bcc Fe. The mutual interplay of the tetragonal distortion, atomic volume and morphology and its influence on the magnetic properties of the MLs are discussed in this thesis.

In the present thesis the relaxor properties of the tungsten-bronze ferroelectric material strontium-barium niobate Sr_1-wBa_wNb₂O₆(SBN) are investigated. SBN is a very good experimental realization of the three-dimensional random-field Ising model (3d <i>RFIM</i>). The quenched random fields (<i>RF</i>) originate from charge disorder and can be enhanced by doping with Ce³⁺. They are responsible for the formation of polar nanoclusters in the paraelectric phase. In order to study the critical behavior of SBN, linear birefringence (<i>LB</i>) and second harmonic generation (<i>SHG</i>) have been measured as a function of temperature. Within an Ornstein-Zernike analysis of the <i>LB</i> data the autocorrelation function, <<i>P</i>²>, was determined, whereas the dipolar correlation length was obtained from the <i>SHG</i> data. They suggest that, due to its intrinsic disorder, pure SBN does not belong to the 3d Ising universality class. Doping with Ce³⁺ions, which seem to generate <i>RF</i>s, enhances the relaxor properties. The critical exponents <i>v</i> and <i>y</i> of SBN:Ce shift against those of the 3d <i>RFIM</i>. The domain morphology of SBN:Ce has been investigated by piezoelectric force microscopy (<i>PFM</i>). Fractal-like shaped zero-field cooled nanodomains are observed. Their size distribution can be described by a power law with exponential cutoff in accordance with prediction for the <i>RFIM</i>. It was measured for the first time in a <i>RF</i> system. The temperature and field induced evolution of natural and written domains has been studied with <i>PFM</i>, <i>LB</i> and <i>SHG</i> measurements. It reveals a very slow relaxation from a macrodomain into a depolarized multidomain state (and vice versa) even above <i>T_C</i>. This hints at strong pinning forces due to quenched <i>RF</i>s. They are also responsible for the observed aging in poled SBN and the field induced cluster percolation above <i>T_C</i> measured with <i>SH</i>-hysteresis. The domains can be considered as a "thick" phase grating for <i>SH</i>-diffraction. The Bragg-regime diffraction efficiency reflects the temperature and field induced change of the averaged domain sizes in the system.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Rev. doct. diss., Hebrew University of Jerusalem, 2005. / Bibliogr.: p. [289]-307.

Non comminiqué

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

No description available.

Le rôle de l'environnement sur l'évolution des galaxies n'est pas encore entièrement connu. Pour quantifier et mettre des limites aux rôles joués par les processus externes, on doit identifier un échantillon de galaxies isolées. Le projet AMIGA ``Analyse du Milieu interstellaire des galaxies isolées'' fait une étude multi-longueur d'ondes d'un grand échantillon de galaxies isolées pour examiner leur milieu interstellaire et l'activité de formation d'étoiles.<br /><br />Nous avons étudié 950 galaxies en provenance du Catalogue de Galaxies Isolées (Karachentseva, 1973) et évalué leur isolation au moyen d'une procédure de classification automatique de séparation étoile/galaxie (jusqu'à M_B = 17.5) sur de larges champs digitalisés POSS-I autour de chaque galaxie isolée. Nous avons défini, comparé et discuté différents critères pour quantifier le degré d'isolation de ces galaxies, comme la révision du critère de Karachentseva, la densité de surface locale, l'estimation des forces de marées externes affectant chaque galaxie isolée. Nous trouvons des galaxies n'obéissant pas au critère de base de Karachentseva et nous définissons différents sous-échantillons de galaxies selon leurs degrés d'isolation. De plus nous avons cherché les redshifts des galaxies centrales ainsi que ceux de leurs compagnons pour avoir accès à la dimension radiale et ainsi une image en trois dimensions de l'environnement. Enfin, nous avons appliqué nos procédures aux triplets, groupes compacts et amas de galaxies et interprété la population de galaxies isolées à la lumière de ces échantillons de contrôle.<br /><br />La formation d'étoiles est connue pour être affectée par l'environnement local des galaxies mais le taux de formation d'étoiles dépend aussi grandement des caractéristiques intrinsèques du milieu interstellaire. Séparer ces deux effets reste un problème difficile. Pour solutionner, nous avons observé et compilé des données photométriques pour 200 galaxies spirales issues du Catalogue des Galaxies Isolées qui sont par définition dans des régions de faible densité. Ensuite, nous avons étudié l'aspect de la morphologie en Halpha des 45 galaxies les plus grandes et les moins inclinées. En utilisant les techniques de Transformation de Fourier Rapide, nous nous focalisons sur les modes des bras spiraux. Nous quantifions la force des barres et nous donnons les couples entre les étoiles nouvellement formées et la matière optique. Nous interprétons les diverses barres et morphologies Halpha observées en termes d'évolution séculaire subie par les galaxies isolées. La fréquence observée des modèles morphologiques particuliers apporte des contraintes sur la durée de vie des barres, et les temps de destruction associés. En utilisant des simulations numériques, l'essai d'adapter les distributions Halpha apporte des contraintes sur la loi de formation d'étoiles, qui est susceptible de différer d'une simple loi de Schmidt.

Texte remanié de: Tesis de maestría--México--Escuela nacional de antropología e historia, 1989. / Bibliogr. p. 155-176.