Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item POLARIZATION-PRESERVING WAVEGUIDE FILTER FOR ASTRONOMICAL APPLICATIONS(2005-11-29) Vanin, Felice Maria; Zaki, Kawthar; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As part of NASA's Beyond Einstein Program, the characterization of the Cosmic Background polarization has become a high priority for NASA. Since 2002, the efforts in which NASA is engaged are the first steps in the development of the instrumentation to achieve this vision. The critical systems that have previously caused the most losses have to be replaced. Microwave waveguide components are ideal to replaces old systems, improving accuracy of measurement and reducing the losses. In this work, a novel microwave waveguide filter and transformer are presented and discussed. The filter has been designed with 30% fractional bandwidth, guaranteeing extreme stop-band performances by using a quadruple ridge waveguide cross section. The waveguide component exhibits four-fold-symmetry preserving the dual polarization state of the electromagnetic field and resulting ideal for astronomical detections. Moreover, the important suppression of the fundamental mode re-resonance by using attenuation zeros is also discussed.Item NONLINEAR SPIN DYNAMICS AND ULTRA-FAST PRECESSIONAL SWITCHING(2005-04-20) Dimian, Mihai; Mayergoyz, Isaak D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis is intended to provide a theoretical analysis of magnetization dynamics in nanometer scale structures over picosecond time scales. This research has been motivated by promising technological applications in the area of magnetic data storage, as well as by pure scientific quest for ultra-fast spin dynamics in nanostructures. The present paradigm of magnetic data storage is approaching its fundamental limits for areal storage density, as well as for speed in data processing. As a result, there is an urgent need for reliable alternatives to current magnetic recording media, which are based on longitudinal thin film, and to the conventional mechanism of magnetization reversal, based on damping switching. In this dissertation, faster modes of magnetization reversals, using precessional magnetization motion, are analyzed in traditional longitudinal media and in its promising alternatives: perpendicular and patterned media. This analysis uses multi-spin description of magnetic nanoparticles and continuum micromagnetics for thin film media. The spins dynamics in both discrete and continuum versions is modeled by Landau Lifshitz type equations. These models are introduced in Chapter 2, subsequent to an overview of magnetic recording media offered in Chapter 1. The analytical study of precessional switching in perpendicular thin film media is presented in Chapter 3. The features of precessional magnetization switching and conventional magnetization reversal are compared, and the design of magnetic field pulses that guarantee precessional switching is discussed. In Chapter 4, the study of precessional magnetization switching in longitudinal thin film media is undertaken. After a short summary of the research studies on this topic, the inverse problem approach to the analysis of precessional switching in these media is presented. This approach leads to explicit expressions for the magnetic field pulses that guarantee the precessional switching. The study of surface anisotropy effects on magnetization reversals in nanoparticles is presented in Chapter 5. The expressions for critical magnetic fields that guarantee the quasi-static and precessional reversals are analytically derived for the case of very strong exchange and weak surface anisotropy. These analytical results are also used to test the numerical approach, which is applied to the general case of the problem.