Non-equilibrium effects in nanoparticulate assemblies, bond-disordered ferromagnets, and collections of two-level subsystems

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dc.contributor.supervisor Roshko, Roy M. (Physics & Astronomy) en
dc.contributor.author Viddal, Candice April Harder
dc.date.accessioned 2009-01-21T15:03:20Z
dc.date.available 2009-01-21T15:03:20Z
dc.date.issued 2009-01-21T15:03:20Z
dc.identifier.uri http://hdl.handle.net/1993/3123
dc.description.abstract The central concern of this thesis is the study of non-equilibrium behaviour in magnetic materials and its interpretation within the framework of the Preisach model of hysteresis. Comprehensive experimental characterizations of the field and temperature and time dependence of a suite of standard magnetic response functions have been performed on a variety of magnetic materials, including a naturally occurring mineral of nanodimensional titanomagnetite particles embedded in volcanic glass, a compressed powder of nanodimensional magnetite particles immobilized in an organic binder, a thin film of nanodimensional Fe particles embedded in alumina, and a series of sintered, bond-disordered CaxSr1-xRuO3 ferromagnets. The measurements were compared with numerical simulations based on a model Preisach ensemble of thermally activated two-level subsystems, characterized individually by a double well free energy profile in a two-dimensional configuration space, an elementary moment reversal, a dissipation field and a bias field, and characterized collectively by a distribution of these characteristic fields. Our efforts were concentrated on two principal spheres of investigation. (1) By performing detailed numerical simulations of the relaxation response of model Preisach collections of two-level subsystems under the same field and temperature protocols used to probe experimentally the relaxation dynamics of spin glasses, we have been able to show that aging, memory and rejuvenation effects are ubiquitous features of all materials which possess a broad distribution of free energy barriers which block the approach to thermal equilibrium. (2) We propose a general strategy for isolating and quantifying the two principal mechanisms, thermal fluctuations and barrier growth, which are jointly responsible for shaping the measured temperature dependence of the magnetic properties of all magnetic materials which exhibit a history dependent response to an external field excitation, and is based on the analysis of viscosity isotherms and, in particular, on a plot of T ln(tr/0) versus Ha , where tr is the time at which a viscosity isotherm measured in a field Ha at temperature T reverses sign. When thermal activation dominates barrier growth, this plot will yield a universal curve while, in the opposite limit the plot fractures into a family of isothermal curves. The strategy is applied to the analysis of each magnetic material listed above. en
dc.format.extent 41123478 bytes
dc.format.mimetype application/pdf
dc.language.iso en_US
dc.subject hysteresis en
dc.subject non-equilibrium en
dc.subject nanodimensional en
dc.subject thermal fluctuations en
dc.subject bistable subsystems en
dc.subject viscosity isotherms en
dc.subject spin glass en
dc.subject aging en
dc.subject memory en
dc.subject rejuvenation en
dc.subject barrier growth en
dc.subject energy landscape en
dc.title Non-equilibrium effects in nanoparticulate assemblies, bond-disordered ferromagnets, and collections of two-level subsystems en
dc.degree.discipline Physics and Astronomy en
dc.contributor.examiningcommittee Williams, Gwyn (Physics & Astronomy) Hawthorne, Frank C. (Geological Sciences) Stancu, Alexandru (Department of Physics, "Alexandru Ioan Cuza" University, Romania) en
dc.degree.level Doctor of Philosophy (Ph.D.) en
dc.description.note February 2009 en

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