Magnetic properties of selected 3d- and 4f-systems on alpha-Al 2 O 3 substrates

In this work the growth and the magnetic properties of the transition metals molybdenum, niobium, and iron and of the highly-magnetostrictive C15 Laves phases of the RFe2 compounds (R: Rare earth metals: here Tb, Dy, and Tb{0.3}Dy{0.7} deposited on alpha-Al2O3 (sapphire) substrates are analyzed. Next to (11-20) (a-plane) oriented sapphire substrates mainly (10-10) (m-plane) oriented substrates were used. These show a pronounced facetting after high temperature annealing in air. Atomic force microscopy (AFM) measurements reveal a dependence of the height, width, and angle of the facets with the annealing temperature. The observed deviations of the facet angles with respect to the theoretical values of the sapphire (10-1-2) and (10-11) surfaces are explained by cross section high resolution transmission electron microscopy (HR-TEM) measurements. These show the plain formation of the (10-11) surface while the second, energy reduced (10-1-2) facet has a curved shape given by atomic steps of (10-1-2) layers and is formed completely solely at the facet ridges and valleys. Thin films of Mo and Nb, respectively, deposited by means of molecular beam epitaxy (MBE) reveal a non-twinned, (211)-oriented epitaxial growth as well on non-faceted as on faceted sapphire m-plane, as was shown by X-Ray and TEM evaluations. In the case of faceted sapphire the two bcc crystals overgrow the facets homogeneously. Here, the bcc (111) surface is nearly parallel to the sapphire (10-11) facet and the Mo/Nb (100) surface is nearly parallel to the sapphire (10-1-2) surface. (211)-oriented Nb templates on sapphire m-plane can be used for the non-twinned, (211)-oriented growth of RFe2 films by means of MBE. Again, the quality of the RFe2 films grown on faceted sapphire is almost equal to films on the non-faceted substrate. For comparison thin RFe2 films of the established (110) and (111) orientation were prepared. Magnetic and magnetoelastic measurements performed in a self designed setup reveal a high quality of the samples. No difference between samples with undulated and flat morphology can be observed. In addition to the preparation of covering, undulating thin films on faceted sapphire m-plane nanoscopic structures of Nb and Fe were prepared by shallow incidence MBE. The formation of the nanostructures can be explained by a shadowing of the atomic beam due to the facets in addition to de-wetting effects of the metals on the heated sapphire surface. Accordingly, the nanostructures form at the facet ridges and overgrow them. The morphology of the structures can be varied by deposition conditions as was shown for Fe. The shape of the structures vary from pearl-necklet strung spherical nanodots with a diameter of a few 10 nm to oval nanodots of a few 100 nm length to continuous nanowires. Magnetization measurements reveal uniaxial magnetic anisotropy with the easy axis of magnetization parallel to the facet ridges. The shape of the hysteresis is depending on the morphology of the structures. The magnetization reversal processes of the spherical and oval nanodots were simulated by micromagnetic modelling and can be explained by the formation of magnetic vortices.

Supernova detection in a future extension of the IceCube neutrino telescope

Marking the final explosive burning stage of massive stars, supernovae are onernthe of most energetic celestial events. Apart from their enormous optical brightnessrnthey are also known to be associated with strong emission of MeV neutrinos—up tornnow the only proven source of extrasolar neutrinos.rnAlthough being designed for the detection of high energy neutrinos, the recentlyrncompleted IceCube neutrino telescope in the antarctic ice will have the highestrnsensitivity of all current experiments to measure the shape of the neutrino lightrncurve, which is in the MeV range. This measurement is crucial for the understandingrnof supernova dynamics.rnIn this thesis, the development of a Monte Carlo simulation for a future low energyrnextension of IceCube, called PINGU, is described that investigates the response ofrnPINGU to a supernova. Using this simulation, various detector configurations arernanalysed and optimised for supernova detection. The prospects of extracting notrnonly the total light curve, but also the direction of the supernova and the meanrnneutrino energy from the data are discussed. Finally the performance of PINGU isrncompared to the current capabilities of IceCube.

Microstructure of epitaxial thin films of the ferromagnetic shape memory alloy Ni 2 MnGa

Gegenstand dieser Arbeit ist die Präparation und die ausführliche Charakterisierung epitaktischer Dünnschicht-Proben der Heusler Verbindung Ni2MnGa. Diese intermetallische Verbindung zeigt einen magnetischen Formgedächtnis-Effekt (MFG), der sowohl im Bezug auf mögliche Anwendungen, als auch im Kontext der Grundlagenforschung äußerst interessant ist. In Einkristallen nahe der Stöchiometrie Ni2MnGa wurden riesige magnetfeldinduzierte Dehnungen von bis zu 10 % nachgewiesen. Der zugrundeliegende Mechanismus basiert auf einer Umverteilung von kristallographischen Zwillings-Varianten, die eine tetragonale oder orthorhombische Symmetrie besitzen. Unter dem Einfluss des Magnetfeldes bewegen sich die Zwillingsgrenzen durch den Kristall, was eine makroskopische Formänderung mit sich bringt. Die somit erzeugten reversiblen Längenänderungen können mit hoher Frequenz geschaltet werden, was Ni2MnGa zu einem vielversprechenden Aktuatorwerkstoff macht. rnDa der Effekt auf einem intrinsischen Prozess beruht, eignen sich Bauteile aus MFG Legierungen zur Integration in Mikrosystemen (z.B. im Bereich der Mikrofluidik). rnrnBislang konnten große magnetfeldinduzierte Dehnungen nur für Einkristalle und Polykristalle mit hoher Porosität („foams") nachgewiesen werden. Um den Effekt für Anwendungen nutzbar zu machen, werden allerdings Konzepte zur Miniaturisierung benötigt. Eine Möglichkeit bieten epitaktische dünne Filme, die im Rahmen dieser Arbeit hergestellt und untersucht werden sollen. Im Fokus stehen dabei die Optimierung der Herstellungsparameter, sowie die Präparation von freitragenden Schichten. Zudem werden verschiedene Konzepte zur Herstellung freistehender Mikrostrukturen erprobt. Mittels Röntgendiffraktometrie konnte die komplizierte Kristallstruktur für verschiedene Wachstumsrichtungen verstanden und die genaue Verteilung der Zwillingsvarianten aufgedeckt werden. In Verbindung mit Mikroskopie-Methoden konnte so die Zwillingsstruktur auf verschiedenen Längenskalen geklärt werden. Die Ergebnisse erklären das Ausbleiben des MFG Effekts in den Proben mit (100) Orientierung. Andererseits wurde für Schichten mit (110) Wachstum eine vielversprechende Mikrostruktur entdeckt, die einen guten Ausgangspunkt für weitere Untersuchungen bietet.rnDurch die spezielle Geometrie der Proben war es möglich, Spektroskopie-Experimente in Transmission durchzuführen. Die Ergebnisse stellen den ersten experimentellen Nachweis der Änderungen in der elektronischen Struktur einer metallischen Verbindung während des martensitischen Phasenübergangs dar. Durch Messen des magnetischen Zirkulardichroismus in der Röntgenabsorption konnten quantitative Aussagen über die magnetischen Momente von Ni und Mn getroffen werden. Die Methode erlaubt überdies die Beiträge von Spin- und Bahn-Moment separat zu bestimmen. Durch winkelabhängige Messungen gelang es, die mikroskopische Ursache der magnetischen Anisotropie aufzuklären. Diese Ergebnisse tragen wesentlich zum Verständnis der komplexen magnetischen und strukturellen Eigenschaften von Ni2MnGa bei.rn

Software tools and efficient algorithms for the feature detection, feature tracking, event localization, and visualization of large sets of atmospheric data

Data sets describing the state of the earth's atmosphere are of great importance in the atmospheric sciences. Over the last decades, the quality and sheer amount of the available data increased significantly, resulting in a rising demand for new tools capable of handling and analysing these large, multidimensional sets of atmospheric data. The interdisciplinary work presented in this thesis covers the development and the application of practical software tools and efficient algorithms from the field of computer science, aiming at the goal of enabling atmospheric scientists to analyse and to gain new insights from these large data sets. For this purpose, our tools combine novel techniques with well-established methods from different areas such as scientific visualization and data segmentation. In this thesis, three practical tools are presented. Two of these tools are software systems (Insight and IWAL) for different types of processing and interactive visualization of data, the third tool is an efficient algorithm for data segmentation implemented as part of Insight.Insight is a toolkit for the interactive, three-dimensional visualization and processing of large sets of atmospheric data, originally developed as a testing environment for the novel segmentation algorithm. It provides a dynamic system for combining at runtime data from different sources, a variety of different data processing algorithms, and several visualization techniques. Its modular architecture and flexible scripting support led to additional applications of the software, from which two examples are presented: the usage of Insight as a WMS (web map service) server, and the automatic production of a sequence of images for the visualization of cyclone simulations. The core application of Insight is the provision of the novel segmentation algorithm for the efficient detection and tracking of 3D features in large sets of atmospheric data, as well as for the precise localization of the occurring genesis, lysis, merging and splitting events. Data segmentation usually leads to a significant reduction of the size of the considered data. This enables a practical visualization of the data, statistical analyses of the features and their events, and the manual or automatic detection of interesting situations for subsequent detailed investigation. The concepts of the novel algorithm, its technical realization, and several extensions for avoiding under- and over-segmentation are discussed. As example applications, this thesis covers the setup and the results of the segmentation of upper-tropospheric jet streams and cyclones as full 3D objects. Finally, IWAL is presented, which is a web application for providing an easy interactive access to meteorological data visualizations, primarily aimed at students. As a web application, the needs to retrieve all input data sets and to install and handle complex visualization tools on a local machine are avoided. The main challenge in the provision of customizable visualizations to large numbers of simultaneous users was to find an acceptable trade-off between the available visualization options and the performance of the application. Besides the implementational details, benchmarks and the results of a user survey are presented.