We have investigated several topics in the area of manganites including oxygen disorder, grain boundaries, low field magnetoresistance, magnetoresistive anisotropy and magnetic properties. Studied materials were in the form of polycrystalline samples and epitaxial thin films. The studied compounds were Sm(1-x)Sr(x)MnO3 (SSMO) and La(1-x)Ca(x)MnO3 (LCMO). 1-We have studied the effects of oxygen disorder and grain boundary disorder in the SSMO system close to half hole doping level. The temperature dependencies of resistivity and magnetoresistance were measured as a function of the vacuum annealing time. We observed a logarithmic increase of the resistivity as a function of vacuum annealing time. We have shown that an increasing grain boundary disorder softens the magnetic phase transition from a first order phase transition into a second order transition. Furthermore, the peaks in the resistivity and specific heat are broadened and there is an increase in the charge-carrier scattering rates in the metallic state. On the other hand, the polaronic hopping activation energies in the insulating state changed slightly as a function of grain boundary disorder. The origin of these phenomena is discussed. Magnetoresistive anisotropy has been studied as a function of the grain size. Results showed a strong grain size dependence of anisotropic electrical transport in granular samples of manganites. 2-We investigated the anisotropic magnetoresistance (AMR) in ultrathin LCMO films grown on various substrates. It was found that depending on the strain state, the AMR in some of these systems exceeds 100% and can even change sign. These changes are dramatic when compared to the few percent change in AMR in conventional ferromagnets. The mechanism behind these changes in the AMR is discussed. We have also studied the effects of strain on resistive peak broadening with a simple percolation model. We have shown that strain associated with a lattice mismatched substrate in thin films can cause new electronic behavior, not found in bulk materials or thicker films of the same chemical composition. Resistivity of the ultra thin films exhibit strong relaxation effects when measured as a function of time in a constant magnetic field.

In this thesis I present a new numerical scheme for 2.5-D/3-D direct current resistivity modelling in heterogeneous, anisotropic media. This method, named the ‘Gaussian quadrature grid’ (GQG) method, co-operatively combines the solution of the Variational Principle of the partial differential equation, Gaussian quadrature abscissae and local cardinal functions so that it has the main advantages of the spectral element method. The formulation shows that the GQG method is a modification of the spectral element method and does not employ the constant elements and require the mesh generator to match the earth’s surface. This makes it much easier to deal with geological models having a 2-D/3-D complex topography than using traditional numerical methods. The GQG technique can achieve a similar convergence rate to the spectral element method. It is shown that it transforms the 2.5-D/3-D resistivity modelling problem into a sparse and symmetric linear equation system, which can be solved by an iterative or matrix inversion method. Comparison with analytic solutions for homogeneous isotropic and anisotropic models shows that the error depends on the Gaussian quadrature order (abscissae number) and the sub-domain size. The higher order or smaller the subdomain size employed, the more accurate the solution. Several other synthetic examples, both homogeneous and inhomogeneous, incorporating sloping, undulating and severe topography are presented and found to yield results comparable to finite element solutions involving a dense mesh. The thesis also presents for the first time explicit expressions for the Fréchet derivatives or sensitivity functions in resistivity imaging of a heterogeneous and fully anisotropic earth. The formulation involves the Green’s functions and their gradients, and is developed both from a formal perturbation analysis and by means of a numerical (finite element) method. A critical factor in the equations is the derivative of the electrical conductivity tensor with respect to the principal conductivity values and the angles defining the axes of symmetry; these are given analytically. The Fréchet derivative expressions are given for both the 2.5-D and the 3-D problem using both constant point and constant block model parameterisations. Special cases like the isotropic earth and tilted transversely isotropic (TTI) media are shown to emerge from the general solutions. Numerical examples are presented for the various sensitivities as functions of the dip angle and strike of the plane of stratification in uniform TTI media. In addition, analytic solutions are derived for the electric potential, current density and Fréchet derivatives at any interior point within a 3-D transversely isotropic homogeneous medium having a tilted axis of symmetry. The current electrode is assumed to be on the surface of the Earth and the plane of stratification given arbitrary strike and dip. Profiles can be computed for any azimuth. The equipotentials exhibit an elliptical pattern and are not orthogonal to the current density vectors, which are strongly angle dependent. Current density reaches its maximum value in a direction parallel to the longitudinal conductivity direction. Illustrative examples of the Fréchet derivatives are given for the 2.5-D problem, in which the profile is taken perpendicular to strike. All three derivatives of the Green’s function with respect to longitudinal conductivity, transverse resistivity and dip angle of the symmetry axis (dG/dσ₁,dG/dσ₁,dG/dθ₀ ) show a strongly asymmetric pattern compared to the isotropic case. The patterns are aligned in the direction of the tilt angle. Such sensitivity patterns are useful in real time experimental design as well as in the fast inversion of resistivity data collected over an anisotropic earth. / Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2009

Bibliography: leaves 91-95. / x, 155 leaves ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This thesis contains methods of solution for a number of different problems within the area of the elasticity of anisotropic materials. The first problem concerns the calculation of stresses and strains within a concentric arrangement of cylindrical shells, where each shell has a differing set of anisotropic properties. This has immediate application to the design of yacht masts, and the particular example of the "Moth" yacht mast is considered. The second problem considered is the uncoupled thermo-elastic problem, where a boundary element method is derived for solving the class of boundary value problems governing plane thermo-elastic deformations of isotropic and anisotropic materials. The final class of problems deals with mixed boundary value problems in which the stresses become singular at some points, for instance in elastic problems containing cracks. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied Mathematics, 1998

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 44-46). Also available on the World Wide Web.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 123-126).

Thesis (Ph.D.)--McMaster University (Canada), 1993. / Source: Dissertation Abstracts International, Volume: 54-12, Section: B, page: 6281. Adviser: T. Timusk.

Diss. (sammanfattning) Stockholm : Karol. inst., 2002. / Härtill 5 uppsatser.

Thesis (Ph. D.)--Missouri University of Science and Technology, 2009. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 29, 2009) Includes bibliographical references (p. 119-131).

Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (leaves [193]-203).

Thesis (Ph. D.)--Leiden Rijksuniversiteit, 1971. / Vita. Summaries in English and Dutch. Includes bibliographical references.