Browsing by Subject "Anisotropy"

Now showing 1 - 6 of 6
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    Form from function: generalized anisotropic inverse mechanics for soft tissues.
    (2011-08) Raghupathy, Ramesh
    Elastography, the imaging of soft tissues on the basis of elastic modulus has gained popularity in the last few decades and holds promise for application in many medical areas. Most of the attention has focused on heterogeneous materials that are locally isotropic, the intent being to detect a stiff tumor within a compliant tissue. Many tissues of mechanical interest, however, are anisotropic, so a method capable of determining material anisotropy would be attractive. This work presents a method, named GAIM (Generalized Anisotropic Inverse Mechanics), to determine the mechanical anisotropy of heterogeneous, anisotropic tissues, by directly solving the finite-element representation of the stress balance in the tissue. GAIM divides the sample into subdomains assumed to have uniform properties and determines the material constants in each subdomain. Use of a linear material model led to rapid computation with statistical confidence levels as performance metrics. Multiple tests, asymmetric loading and strain heterogeneity are needed to address the ill-posedness of the inverse problem, and represent a paradigm shift in the testing of soft tissues. Simulated experiments of fibrous soft-tissues demonstrated the ability of the method to capture anisotropy qualitatively even though only a linear model is used. Results from the tests on soft-tissue analogs demonstrated the success in identifying regional differences in anisotropy based on full-field displacements and boundary forces obtained from multiple biaxial extension tests. The method’s success in capturing regional anisotropic changes associated with growth and remodelling in fibroblastpopulated cruciforms is a significant achievement, and holds promise for determining structural information of tissues from the mechanical response, since the structural and mechanical anisotropy are correlated. The linear GAIM model can be extended by a second step for nonlinearity with a fiber-based constitutive model. A closedform solution for the latter was developed and provides rapid results for nonlinear regression. In summary, this work has built a novel exploratory tool to extract regionspecific anisotropic properties on intact tissue samples. GAIM can be applied to provide information on the mechanical function of healthy tissue subjected to complex physiologic loads, identify regions within a tissue that exhibit irregular mechanical behavior (possibly due to disease or damage), and provide structural information from the mechanical function of tissues that are not amenable to structural tests.
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    Magnetic and physical characteristics of magnetite associated with deformation and exsolution.
    (2011-10) Till, Jessica Lynn
    This thesis contains a collection of laboratory-based studies designed to characterize the magnetic properties and physical aspects of magnetite that result from deformation or high temperature growth. In Chapter 2, a detailed rock magnetic characterization of rocks containing nanoscale magnetite exsolved from volcanic glass identifies the location of domain-state thresholds through distinct transitions in remanence and susceptibility properties. This unique material is an excellent candidate for standard material to be used in studies of magnetite granulometry. In Chapter 3, theoretical timescales for the growth of sub-microscopic magnetite needles during exsolution from plagioclase are calculated using results of diffusion experiments. Measured diffusivities are modeled to calculate the amount of diffusion-limited growth possible under different conditions of nucleation temperature and cooling rate. In Chapters 4 and 5, the development and evolution of magnetic fabrics are investigated through deformation experiments on synthetic rock-analogues at high temperatures and ductile conditions. Stress-induced changes in rock magnetic properties after deformation are significant. Examination of deformation-induced remagnetization demonstrates that a primary remanence can survive conditions equivalent to moderate metamorphism in certain cases and that petrofabric can play an important role in determining the remanence stability. High-temperature deformation experiments result in a pattern of anisotropy development that indicates plastic deformation of magnetic grains, which is distinct from anisotropy development resulting from different magnetite strain responses. Experimental data are combined with theoretical magnetic anisotropy models and used to estimate effective magnetite strains and strain partitioning from magnetic fabric data in deformed samples. Finally, observations of strong shape-preferred orientation and deformation-induced microstructures in magnetite grains from high-temperature shear experiments indicate plastic deformation of magnetite. Microstructural observations place constraints on the rheological behavior of magnetite and the conditions in which dislocation creep is dominant. These observations prompt a re-examination of the previously established magnetite flow laws which are modified and used to construct new deformation mechanism maps.
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    Magnetic Anisotropies and Damping in Epitaxial Iron Thin Films
    (2021-08) Etheridge, James
    In the research presented in the following thesis, the magnetic properties of a setof Fe/InAs(001) heterostructures with thicknesses ranging from 1.4 nm to 39.0 nm are investigated through the use of ferromagnetic resonance, x-ray diffraction, and magnetometry. The magnetic anisotropy results are heavily dependent on Fe thickness. The ferromagnetic resonance data point to an anisotropic relaxation of the Fe film that induces a shear strain in the Fe lattice. The shear strain produces an extra term of magnetoelastic origins in the free energy density resulting in several interesting results including a rotation of the uniaxial easy axis. Several x-ray diffraction experiments were performed to confirm the anisotropic relaxation of the Fe film. The magnetic damping of the samples were also investigated, yielding results that were anisotropic. The cause of the damping results can most likely be attributed to two-magnon scattering.
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    Mantle dynamics, composition, and state in regions associated with active and ancient subduction.
    (2008-05) Courtier, Anna Mahr
    Abstract not available.
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    Micromagnetic study of heat assisted magnetic recording using renormalized media cells
    (2014-05) Huang, Pin-Wei
    Perpendicular magnetic recording is currently very near to its physical limit, making it difficult for researchers to keep the pace of the growth of areal density of hard disk drives. Heat-assisted magnetic recording (HAMR) is considered to be the next generation technology for magnetic recording beyond 1 Tb/in2. Complete understanding of HAMR processes is necessary to optimize the design parameters. In this thesis, current state-of-the-art modelling methods are developed aiming at HAMR recording. First a simple torque-based method for calculating the transient behavior of temperature-dependent magnetic anisotropy is introduced. By using this method several physical quantities at finite temperature including effective anisotropy, anisotropy field, and their fluctuations are obtained. A composite grain that includes a high Curie temperature soft layer can reduce the anisotropy fluctuations. Then a new scheme for the simulation of HAMR that systematically includes fluctuating material properties above a predefined length scale, while retaining magnetostatic interactions is introduced. Renormalized media parameters, Ms, Ku, Aex and &alpha, suitable for useful length scales, are found numerically. These renormalized parameters are then used to model the Voronoi-cell-composed medium in the HAMR simulation. Transition jitters are obtained under various conditions. The results show that moderate maximum temperature of the heat spot, intergranular exchange coupling, media thickness of at least 10 nm, nonzero canting angle of the head field, relatively low head velocity, and large head-field strength are helpful for a successful recording. This scheme of HAMR simulation is used to find the dependencies of recording performance on the grain size and damping. The simulated results are used to compare with an experimental demonstration. Finally, composite FeRh/FePt for HAMR media is investigated with micromagnetic simulation. It is found to potentially lower recording temperature, while retaining high anisotropy field gradient. The transition width is predicted to depend on the media cooling rate. The thickness of the FeRh layer and the applied field can significantly affect the switching time of FePt layer, and therefore alter recording performance. Applied field magnitudes and angles are identified that allow successful switching within 100 pS. It is shown that by using up to 15 nm of FeRh with 6 nm of FePt, the jitter for 5.6 nm grains can be nearly equal to the grain-size limited value, for head velocities as high as 20 m/s.
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    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.