Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Magnetic Reversal of Artificial Kagome Ice(2013) Daunheimer, Stephen Allen; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Geometric frustration is a phenomenon where a crystalline material cannot satisfy all of its competing interactions, which can drastically change the behavior of a material. When water freezes into solid ice, the hydrogen atom positions are geometrically frustrated due to different interactions among neighboring oxygen atoms. Frustration is not limited to electrostatic interactions, though. Magnetic spin ice mimics the crystal structure and, therefore, the frustration of water ice. However, a problem with the spin ices is that the details of the magnetic state cannot be imaged which makes the dynamics difficult to probe. In 2006, a model system known as “artificial” spin ice was created to alleviate these problems. The artificial spin ices are also geometrically frustrated, but they are easier to fabricate, and the interactions in the system can be tailored to suit the experiment. They are made of lithographically defined arrays of interacting ferromagnetic elements, and the entire sample may be imaged to view the details of the magnetic state through a dynamic process. The research presented here focuses on artificial spin ices with a honeycomb shape known as artificial kagome ice. Low disorder samples are created to study the dynamics of the magnetic reversal process to better understand the statistics and kinetics of the reversal process of frustrated materials. Results indicate reversals are defined by a complex avalanche process with a randomness that can be tuned by crystal geometry and reversal angle. Magnetically reversing samples at field angles of 180°, 100°, and 120° allows us to directly measure the disorder in our samples. Many 180° reversals were performed to allow us to measure the randomness of the reversals. Reversals at 180° are highly random, whereas at 100° and 120° they are much less so. There appears to be a change in the nature of the reversals at an angle of θ = 130° where avalanches start appearing in the reversals. As the angle is increased, large avalanches spanning the entire crystal start to dominate the reversal process. The detailed image sequences of an artificial spin kagome ice sample allow us to simply model the behavior of the crystal as the motion of magnetic monopoles. Also, we make connections to the well-studied science of Barkhausen noise in magnetic materials noting that our samples exhibit the power law behavior typical of Barkhausen experiments.Item QUANTUM SIMULATIONS OF THE ISING MODEL WITH TRAPPED IONS: DEVIL'S STAIRCASE AND ARBITRARY LATTICE PROPOSAL(2013) Korenblit, Simcha; Monroe, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A collection of trapped atomic ions represents one of the most attractive platforms for the quantum simulation of interacting spin networks and quantum magnetism. Spin-dependent optical dipole forces applied to an ion crystal create long-range eective spin-spin interactions and allow the simulation of spin Hamiltonians that possess nontrivial phases and dynamics. We trap linear chains of 171Yb+ ions in a Paul trap, and constrain the occupation of energy levels to the ground hyperne clock-states, creating a qubit or pseudo-spin 1/2 system. We proceed to implement spin-spin couplings between two ions using the far detuned Mlmer-Srenson scheme and perform adiabatic quantum simulations of Ising Hamiltonians with long-range couplings. We then demonstrate our ability to control the sign and relative strength of the interaction between three ions. Using this control, we simulate a frustrated triangular lattice, and for the first time establish an experimental connection between frustration and quantum entanglement. We then scale up our simulation to show phase transitions from paramagnetism to ferromagnetism for nine ions, and to anti-ferromagnetism for sixteen ions. The experimental work culminates with our most complicated Hamiltonian - a long range anti-ferromagnetic Ising interaction between 10 ions with a biasing axial field. Theoretical work presented in this thesis shows how the approach to quantum simulation utilized in this thesis can be further extended and improved. It is shown how appropriate design of laser elds can provide for arbitrary multidimensional spin-spin interaction graphs even for the case of a linear spatial array of ions. This scheme uses currently existing trap technology and is scalable to levels where classical methods of simulation are intractable.Item Properties of Metallic Helimagnets(2012) Ho, Kwan-yuet; Kirkpatrick, Theodore R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates various aspects of helimagnets. Helimagnets are magnets with spins aligned in helical order at low temperatures. It exists in materials of crystal structure lacking the spatial inversion symmetry. The helical order is due to the Dzyaloshinskii-Moriya (DM) mechanism. Examples of helimagnets include MnSi, FeGe and Fe1-xCoxSi. A field theory appropriate for such magnets is used to derive the phase diagram in a mean-field approximation. The helical phase, the conical phase, the columnar phase and the non-Fermi-liquid (NFL) region in the paramagnetic phase are discussed. It is shown that the orientation of the helical vector along an external magnetic field within the conical phase occurs via two distinct phase transitions. The columnar phase, believed to be a Skyrmion lattice, is found to exist as Abrikosov Skyrmions near the helimagnetic phase boundary, and the core-to-core distance is estimated. The Goldstone modes that result from the long-range order in the various phases are determined, and their consequences for electronic properties, in particular, the specific heat, single-particle relaxation rate and the electrical conductivity, are derived. In addition, Skyrmion gases and lattices in helimagnets are studied, and the size of a Skyrmion in various phases is estimated. For isolated Skyrmions, the long distance tail is related to the magnetization correlation functions and exhibits power-law decay if the phase spontaneously breaks a continuous symmetry, but decays exponentially otherwise. The size of a Skyrmion is found to depend on a number of length scales. These length scales are related to the strength of DM interaction, the temperature, and the external magnetic field.Item Development of Magnetic Field Sensors Using Bismuth - Substituted Garnets Thin Films with In-Plane Magnetization(2006-04-24) NISTOR, IULIAN; Mayergoyz, Isaak D.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, the use of magnetic single crystal Bismuth-substituted Iron Garnet thin-films with giant magneto-optical effect as optical sensors for measuring low intensity magnetic fields over a high frequency range (up to 1GHz) is discussed. The advantages of these optical sensors are high intrinsic sensitivity and the possibility of tailoring the field range of the sensor. Such sensors could find applications in various industry and research fields where high sensitivity and electric isolation are required, such as power industry, vehicle detection, and read heads for recording magnetic media with high-density and high transfer rates. The thesis has three major components that correspond, in order, to the following topics: garnet growth, characterization and actual device design. First, the liquid phase epitaxy method is discussed for the growth of single crystal epitaxial garnet thin films of high optical quality. Second, the garnet thin films are fully characterized using various magnetic and optical techniques. Novel optical techniques are suggested, that allow the local measurement of properties such as magnetostriction constants and magnetic anisotropy of garnets. The results of these extensive measurements allow for the identification of melt compositions and growth conditions that yield thin garnet films with in-plane magnetization, giant Faraday rotation per unit length, large negative uniaxial anisotropies and small cubic anisotropy, as required for the sensing applications. In the end, the design of magnetic field sensors based on single and multi-layer garnet thin films is demonstrated, and devices are built for measurements of response and noise equivalent fields. Under the category of sensors, another sensing application is included, that utilizes garnet thin films for direct imaging of two-dimensional fringing magnetic fields with sub-micron resolution.