Browsing by Subject "Noise"

Now showing 1 - 9 of 9
  • Thumbnail Image
    Item
  • Thumbnail Image
    Item
    Effects of noise on fast mapping and word learning scores in preschool children with and without hearing loss.
    (2010-01) Blaiser, Kristina M.
    This study examines the fast mapping and word learning abilities of three- to five-year old children with and without hearing loss, in quiet and noise conditions. Nineteen children with hearing loss (HL) and 17 normal hearing peers (NH) participated in this study. Children were introduced to eight novel words in each condition. Children's ability to `fast map' (i.e., comprehend or produce new words after minimal experience) was measured in the first session (Time 1). `Word learning' (the comprehension or production of previously unfamiliar words following additional exposures) was measured following three individual training sessions (i.e., Time 2). Results indicated that children in the HL group performed similarly to NH peers on fast mapping and word learning measures in quiet. In noise, the HL group performed significantly poorer at the fast mapping time point than the NH group. However, at Time 2 there were no significant between-group differences in the noise condition. A series of correlation and regression analyses was used to investigate variables associated with fast mapping and novel word learning in quiet and noise conditions. Age was significantly correlated to fast mapping and word learning performance in quiet and noise in the NH group, but not in the HL group. Age fit with hearing aids was the only traditional hearing loss factor that was correlated with fast mapping performance in noise for the HL group. Results showed that age was a significant predictor of fast mapping performance in noise for the NH group, but not the HL group. Word learning in quiet was a significant predictor for word learning in noise for the NH group, fast mapping in noise was a significant predictor for the HL group. In addition, performance in quiet significantly predicted fast mapping and word learning scores in noise for the NH group; however, there was no significant correlation between performance in quiet and noise for the HL group.
  • Thumbnail Image
    Item
    Emergent 1/f noise in systems of oscillating nanomagnetic dots
    (2016-08) Costanzi, Barry
    The observation of noise signals with a $\frac{1}{f}$ power spectral density dependence on frequency \emph{f} is both ubiquitous in quantitative measurements across fields, and not entirely well understood. So-called ``$\frac{1}{f}$" spectra have been observed in systems spanning the realm of physics, and in other disciplines as well. Van der Ziel's model of $\frac{1}{f}$ noise as a composite of Lorentizian noise signals is the most widely accepted explanation for $\frac{1}{f}$, but experiments have for the most part only implicitly confirmed the result thus far. In this thesis, an explicit bottom-up approach to the Van der Ziel model is presented by combining random telegraph noise signals in square magnetic dots. Square dots made of the iron-nickel alloy Permalloy were fabricated to be 250 nm on a side and $\sim$ 10 nm thick. The configurational anisotropy of the dots is small enough to reduce energy barriers between adjacent magnetic states to approximately thermal energies through the application of an external field, causing two-state thermal hopping of the magnetization. This magnetization was measured through the anisotropic magnetoresistance of the dots. The random telegraph signals generate Lorentizan spectra when transformed to the frequency domain, and are shown to combine to form $\frac{1}{f}$ spectra when multiple dots are measured in series. The energy landscape of the dots is determined through easy-axis coercivity measurements, and the distribution of energy barriers predicts a range of applied fields where individual noise signals should combine to produce $\frac{1}{f}$ noise by the Van der Ziel model. Experiment shows good agreement with the predicted range of these ``noise fields" for two different series of samples with different coercivity distributions. Measurements of both individual dots and aggregate multi-dot signals show that the number of individual oscillating dots necessary to produce an aggregate $\frac{1}{f}$ signal is lower than might be expected, with $\frac{1}{f}$ observed in collections of fewer than ten oscillating dots, and in some cases as few as two. Additionally, while the statistics over multiple samples agree with the Van der Ziel model, individual collections of dots exhibiting $\frac{1}{f}$ noise can either vary signifcantly from the ideal Van der Ziel distribution, or defy the distribution description altogether when the number of dots becomes too few. This suggests that the Van der Ziel model is a sufficient but not necessary condition for observing $\frac{1}{f}$ noise in a collection of Lorentizan oscillators, and that the actual requirements to generate $\frac{1}{f}$ noise are much looser than Van der Ziel's. In systems with any type of distribution of Lorentizan signals, $\frac{1}{f}$ noise is likely due to combination of those signals. This result is relevant other systems exhibiting magnetic noise, as well as non-magnetic systems displaying both RTN and $\frac{1}{f}$ noise.
  • Thumbnail Image
    Item
    Noise detection and transport measurements of spin valve systems.
    (2011-08) Guo, Feng
    Electronic noise not only limits the performance of magnetic devices in practical applications but also provides valuable physical insights into these devices. The first part of this thesis discusses how the low frequency noise in magnetic tunnel junctions and giant magnetoresistance devices can be used to understand the fundamental noise sources. Previously, the low frequency noise in these systems has been reported to have an enormously large magnitude when the magnetization switches. This was attributed to magnetic fluctuations. An alternative mechanism of a slow drift in the device resistance is discussed, and we show how it produces noise spectra that are similar to those in previous reports. We conclude that this resistance drift causes a measurement artifact and the low frequency magnetic noise is not present in the measured samples within measurement error. As a second part of the thesis, we discuss a pronounced voltage dependent conductance feature present at nonzero bias in some magnetic tunnel junctions. The presence of this feature depends upon the oxidation condition for creating the barrier, and this effect is found to be interfacial in nature. We describe how the electronic structures and density of states at the barrier interfaces could be responsible for this effect, and possibility of utilizing the conductance measurement to probe the interfacial states.
  • Thumbnail Image
    Item
    Noise-Induced Effects on Discrete Breathers in a Nonlinear Electrical Lattice
    (2018-08) Edlund, Connor
    A universal concern in lattice structures, whether they be naturally occurring or engineered, is exactly how energy can and does move within them. A significant phenomenon that has been shown theoretically, numerically, and experimentally to affect the behavior of energy in lattice structures is that of discrete breathers, also known as energy localizations or intrinsic localized modes. Discrete breathers affect the energy distribution in a lattice or array by concentrating it in localized and oscillatory fashion. While these have been known to occur in linear lattices with defects, they also occur in perfect (translationally invariant) nonlinear lattices of sufficient anharmonicity. This thesis seeks to further the study of the latter case, specifically by investigating how white and Lévy stable noise can be used to manipulate and create discrete breathers in a macro-scale nonlinear electric lattice. Using a dynamical model from literature and the Euler-Maruyama method of numerical integration, the effects of both additive white noise and Lévy stable noise on breathers in this array are investigated. These breathers are first initialized in the array using small randomized initial voltage conditions and a driving frequency below the system's natural frequency. It is found that additive white noise can in fact affect the number of breathers present by causing some of them to combine. Additionally, noise can be used to shorten the transient time before breather appearance. Importantly, to garner these effects, the noise must be applied during this transient phase. Applying it after will have little effect as already-formed breathers are too robust. The results are mostly the same for Lévy stable noise, although the large flights from the mean characteristic of this noise type require that the noise intensity used be reduced significantly. Next the ability of white noise to generate discrete breathers is investigated. The initial conditions are set to zero, meaning that no breathers will form in the system without some kind of intervention. Applying noise for either the entire simulation duration or only the first half both resulted in the consistent and reliable creation of discrete breathers. Additionally, it is found that a temporary burst of noise across the array can be used to create breathers on command at any time. The breathers created will exhibit some noisy behavior, but this can be minimized by applying the necessary noise to only one cell. Thus, additive noise can serve as a useful means of breather creation, a key result of this thesis. These results testify to the significance of the interactions between the discrete breathers in this nonlinear electric line and noise. Noise can influence the number of breathers present, and even create breathers on command. The latter result is of particular significance and represents a promising area for future work.
  • Thumbnail Image
    Item
    The origin of magnetic noise in nanoscale square dots
    (2014-05) Endean, Daniel E.
    Magnetic fluctuations, also referred to as magnetic noise, in very small (sub-micron) magnetic systems are important both in studying the fundamental physics of statistical mechanics and in technology. Thermal fluctuations of the magnetization define the ultimate limit of magnetic storage densities and sensing technologies but may be useful in some biomedical applications. At high frequencies (>100 kHz), fluctuations of the magnetization about the internal field are the dominant form of magnetic noise. At lower frequencies, 1/f and random telegraph noise have been observed in many magnetic systems. Yet these noise sources are challenging to reproduce due to their origin in defects and, thus, identification of the physical mechanism which produces them is difficult. Further progress in studying magnetic noise requires a model system where the fluctuations are reproducible and the physical origin is known. In this thesis, random telegraph noise is identified in square magnetic dots and shown to originate from a configurational anisotropy associated with the square dot geometry. The square dots were fabricated from thin (10 nm) Permalloy films with side lengths ranging from 200 nm to 1000 nm, and the magnetization was measured via the anisotropic magnetoresistance. It is first shown, through measurements unaffected by the noise in these samples, that the square dot geometry exhibits a preference for the magnetization to align parallel to an edge of the square. A model of this four-fold configurational anisotropy explains the behavior of the magnetization and provides two methods to characterize the strength of the anisotropy.It is then shown that when a field is applied along the dot diagonal, the configurational anisotropy barrier in this direction is lowered, which allows thermal switching of the magnetization between low-energy magnetic states. The telegraph state lifetimes are quantified and shown to vary with applied field magnitude, field direction, and temperature as expected. The switching rate obeys an Arrhenius law and the energy barriers measured in the noise data agree well with those measurements independent from the noise. In addition, micromagnetic simulations of the Landau-Lifshitz-Gilbert equation reproduce the observed behavior and confirm the explanation of the magnetic noise in these samples.
  • Thumbnail Image
    Item
    Probing Spin Glass Energy Landscapes with 1/f Noise
    (2021-01) Harrison, David
    The spin-glass transition is a dynamical phase transition, similar in nature to that of structural glasses and other systems. Upon quenching from above the transition temperature Tg to a measurement temperature T < Tg, experiments and simulations have unveiled an underlying length scale, the spin-glass correlation length, that grows very slowly with time. The time-dependent growth correlation length is key to the observed dynamics and to the understanding of the underlying energy barrier distribution. Analysis of our measurements provides the first explicit determination of the evolution of the spin-glass energy barrier distribution as a function of film thickness and temperature. By fabricating samples of multiple thicknesses, and allowing the correlation length to grow to the sample thickness on experimental timescales, it becomes possible to extract information about the length dependence of the energy barriers. We have made measurements of the 1/f noise in the resistance of spin-glass films of five thicknesses. These results are consistent with the limited earlier thin film measurements, despite the use of a different cooling protocol. A recent analysis, based on simulations of mesoscale samples with a number of spins comparable to those under experimental study, provided an explanation for the barrier growth in the earlier measurements, but suggested that our cooling protocol would have produced very different dynamics, consistent with activation over a single, temperature-independent barrier distribution, which we do not observe. This suggests that either the growth of in-plane correlations are playing a role, or that the explanation for the barrier growth in the earlier measurements applies to our measurements in spite of our cooling protocol.
  • Thumbnail Image
    Item
    Superconducting vortex noise measurements on niobium films with periodic pinning sites
    (2012-09) Schulz, Tanner Franz
    Vortex noise in superconductors is due to the motion of quantized flux (vortices) across the surface of the superconductor. This changing magnetic flux gives rise to a voltage which we detect as a noise signal. Previous measurements show that the vortex noise is related to the interaction of the vortices with intrinsic pinning sites which limit vortex motion. These studies show that the noise signal is composed of voltage bursts related to the vortices moving in fits and starts due to the random location and strength of the intrinsic vortex pinning sites. Our samples are composed of a periodic triangular array of holes that serve as vortex pinning sites. These samples show periodic minima in resistance when the vortex/pinning site densities are commensurate. We find a vortex noise signal that is field, current and temperature dependent, arising from the vortices being depinned. We measure the vortex noise to determine if there is a change from uncorrelated vortex depinning below the first matching field where unoccupied pinning sites exist, to highly correlated vortex depinning at the first matching field when the vortexes are commensurate with the pinning lattice. Our measurements show that there is no field dependence in the vortex noise signal, indicating that the vortex depinning is correlated as the lattice is driven by a DC current. We explain this result in terms of the long range order of our pinning lattice and the strength of the vortex-vortex interaction in our 100 nm pinning lattice. Our findings suggest that the vortex noise is dominated by the vortex-vortex not vortex-pinning site interactions.
  • Thumbnail Image
    Item
    Synthetic jet flow and heat transfer for electronics cooling
    (2014-05) Huang, Longzhong
    The progressive increase of heat dissipation from modern electronics requires more and more powerful cooling systems. Various cooling technologies have been developed such as liquid cooling, micro-channel cooling, and active cooling. The present study focuses on applying a unique device called a synthetic jet to cool electronics. A synthetic jet is able to generate an unsteady flow with a simple structure that makes it effective in convective heat transfer. This study provides both practical and fundamental view of synthetic jets in the application of electronics cooling. A mock-up synthetic jet is fabricated to study heat transfer and fluid mechanics of synthetic jet cooling. The scaled synthetic jet is geometrically and dynamically similar to the actual jet. The heat transfer performance characteristics of a synthetic jet impinging on a fin are tested with different operating frequencies and with different orifice shapes. Flow visualizations and detail flow field measurements of the impinging synthetic jet flow are documented to support the heat transfer experiment. The optimized parameters obtained from the scaled experiment are applied to the actual synthetic jet design. The actual synthetic jet is realized using a piezoelectric stack and applied on a cooling system based on a full-sized heat sink module. The cooling performance of the whole system is documented. The noise characteristics of the actual synthetic jet is tested and analyzed. A muffler with optimized parameters is found and used for noise reduction. Numerical simulation is used to find the optimal design for the synthetic jets. The computation is realized by the commercial software ANSYS Fluent. The numerical model is verified by comparing the computational results with experimental results. A parametric study of heat transfer performance of synthetic jet cooling is documented.