A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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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 Dependence of the perpendicular anisotropy in Co/Au multilayers on the number of repetitions(American Institute of Physics, 2003-05-15) Gubbiotti, G.; Carlotti, G.; Albertini, F.; Casoli, F.; Bontempi, E.; Depero, L. E.; Mengucci, P.; Di Cristoforo, A.; Koo, H.; Gomez, R. D.The correlations between structure and magnetism in [Co(0.9 nm)/Au(5 nm)]XN multilayer films with different number of repetitions N510, 30, and 50, have been studied by the combined use of complementary structural and magnetic techniques, such as x-ray reflectivity, x-ray diffraction, and transmission electron microscopy, alternating gradient force magnetometry, magnetic force microscopy and Brillouin light scattering. On increasing the value of N, an overall improvement of the multilayer quality is observed which corresponds to a change in the micromagnetic structure and to an enhancement of the perpendicular anisotropy. These effects have been attributed to a reduction of the magnetostatic energy associated with the formation of perpendicular magnetic domains in multilayers with increasing number of layers repetitions.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.