Total energy calculations based on density functional theory are generally a good approach to obtain the properties of solids. The local density approximation (LDA) is widely used for calculating the ground state properties of electronic systems; for excited states the errors are in general unknown. The important aspects of LDA pertain to the modeling of the exchange-correlation interaction. If the exchange-correlation potential is approximately the same for the ground and excited states, one expects good results from the LDA calculations for excited states. In this thesis, we utilize the total energy technique for numerical computations of the electronic structure of iron in several magnetic phases and crystalline structures. 1. Body-centered-cubic iron in the ferromagnetic and several antiferromagnetic configurations. We use the total energy results to obtain the parameters in a model Heisenberg Hamiltonian. These include the interaction parameters up to 6-th nearest neighbors. Based on this model Hamiltonian we calculate properties such as the critical (Curie) temperature and spin stiffness constant. We assume that the total exchangecorrelation energy functional is the same in the ferromagnetic ground state and the antiferromagnetic excited states. Our model parameters are based directly on ab initio calculations of the electronic structure. Our calculation yields good results compared with experimental values and earlier work. Some other physical quantities, related to the phase transition, and spin waves are also discussed. 2. Face-centered-tetragonal iron. If iron is grown on a proper substrate ( e.g., Cu(100) ), the crystal structure of the thin film displays a face-centered-tetragonal distortion due to the lattice constant misfit between the film and substrate. Therefore, we performed calculations for fct iron in its ferromagnetic, antiferromagnetic, and nonmagnetic phases for a wide range of values of the lattice parameters. In the ferromagnetic calculations, we found two minima in the total energy: one is close to.the bcc structure and the other ( with a lower energy ) is close to fcc. In the antiferromagnetic and nonmagnetic calculations, we found in each case that there is only one minimum near the fcc structure, providing us clear evidence that the antiferromagnetic and nonmagnetic states are (meta)stable near the fcc region and unstable in bcc region. The antiferromagnetic and nonmagnetic states are almost degenerate near the fcc minimum, but the antiferromagnetic phase has the lowest total energy in the whole fct region. Magnetic moments are also calculated for a variety of fct structures. Near the fcc minimum we found that two ferromagnetic phases co-exist, one with a low spin and one with a high spin. These results are consistent with experimental facts and other earlier calculations. Some structural properties, such as the elastic constants and the bulk modulus, are also studied and compared with experimental data and some earlier calculations. / Graduation date: 1992

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New types of quantum interference oscillations in transverse magnetoresistance of single crystals of ultrapure magnesium are reported. These oscillations occur as a function of the angle of rotation when the current is passed along the [112̄0] direction and the magnetic field H(→) is rotated by ≲ 1° from [101̄0] towards the [0001] symmetry axis. In order to characterize the oscillations, extensive qualitative data were taken for fields up to 24 kG and for temperatures in the range of 1.4 K-4.2 K. It is shown that these angle-dependent oscillations have the same origin as the field-dependent interferometer oscillations first reported by Stark and Friedberg. Both types of oscillations arise from the electron quantum states on the coupled orbit network, which is obtained for H(→)∥[1010]. It is shown that high sensitivity of the oscillations and the background magnetoresistance (for H within →1° from [101̄0] to field inhomogeneity and crystal strain yields strong evidence for a new regime of quantum transport. In this regime, quantum phase coherence of the electrons extends over distances of the order of 0.1 mm, and coherence determines not only the oscillation amplitudes but also the background. The "stacked mirror" model of Stark and Reifenberger is not applicable in such a regime of transport. A rudimentary model is presented that seems to be in qualitative agreement with the data. However, band structure calculations firmly establish that the large oscillations arise from the symmetry breaking of the two interferometer lobes as the field is rotated away from [101̄0].

We report here the results of a de Haas-van Alphen (dHvA) investigation of the coupled orbit system in magnesium. The data were taken for magnetic fields extending to 52 kG and temperatures down to 0.29° K. The experimental data are interpreted in the light of coupled orbit system theories of Pippard and of Falicov and Stachowiak; these are each reviewed in some detail. The data are found to disagree qualitatively with the predictions of Falicov and Stachowiak. Since this theory has been assumed, for more than a decade, to be equivalent to Pippard's theory, a detailed comparison of these was made. Full spectrum Fourier analysis of Pippard's band structure density of states shows that the two models disagree qualitatively and, therefore, that they are not equivalent. These experimental results, which do not appear to disagree with Pippard's theory, are interpreted to mean that it is fnally possible to obtain crystals of sufficient purity and perfection to make it necessary to use a band structure description of the delocalized electrons on the couple network. Evidence is presented for the existence of a new type of dHvA frequency corresponding to the ΓKM plane cross-section of the Brillouin zone. A proposed explanation for this dHvA frequency involves the field dependent modulation of the zero frequency component of the Fourier transform of the coupled orbit system density of states. Also included are discussions of crystal preparation and handling, cryogenic apparatus, analogue detection apparatus, digital data acquisition and processing hardware based on a microcomputer, and a new software system ideally suited to small computer research environments.

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In this thesis a consistent multipole theory is used to describe light propagation in nonabsorbing magnetic cubic and uniaxial crystals to the order of electric octopoles and magnetic quadrupoles. The first chapter extends Maxwell's equations for a vacuum to their macroscopic form in matter by including bound-source contributions as multipole expansions. From these the corresponding forms for D and H are obtained, which ensure origin-independence of Maxwell's equations. A multipole eigenyalue equation describing light propagation in a source-free homogeneous medium is then derived, which is the basic equation applied in this thesis. In the second chapter it is shown how, from the multipolar form of the propagation equation for transverse waves, expressions can be derived for the various birefringences that may be exhibited in macroscopic platelets of the medium, as introduced by Jones in the formulation of his M-matrix. The following chapter presents the derivation, by means of first-order perturbation theory, of the quantum mechanical expressions for the polarizability tensors which enter the eigenvalue wave equation. The origin independence of the expressions for the various observable quantities is then established. In the fourth chapter the independent components of the polarizability tensors are calculated for two selected crystal point groups, namely 622 and 432, by way of illustration. In chapter five the components calculated in the manner illustrated in the previous chapter are presented in tabular form. The Jones method outlined in chapter two is then applied to the crystal point group 6m2, again as an illustration of the method used to determine the optical effects displayed by this point group. Chapter five concludes with a table containing a listing of the predicted optical effects calculated in this way for all of the magnetic uniaxial and cubic point groups. The thesis concludes with chapter six, in which a summary of the results of the work undertaken is given, together with a discussion of factors influencing measurements of the predicted optical effects. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1997.

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This thesis contains three major areas of work. Chapter 1 describes the various film deposition techniques available for the growth of thin films. These different growth methods, substrate preparation and vacuum techniques, are discussed in the context of the effect of the growth process and deposition environment on film structure and quality. Chapter 2 describes the two film growth chambers. Chapter 3 describes the origin of giant magnetoresistance (GMR) in magnetic multilayered structures, and investigates the magnetoresistive properties of ion-beam sputtered CoCu multilayered thin films. Different film structures and deposition conditions have been investigated in order to maximise the size of negative magnetoresistance seen. Chapter 4 investigates the magnetic and giant magnetoresistive properties of sputtered CoCu and CoAg heterogenous alloys. The magnitude of the GMR effect is found to depend on the Co concentration and Co particle size. Chapter 5 describes an investigation of molecular beam epitaxy (m.b.e.) grown Dy/Y multilayer structures. The temperature dependences of helical magnetic structure in the Dy layers is investigated in two samples of different multilayer structures. Chapter 6 describes an acoustic interferometer which has been used to measure velocity and attenuation changes in an ultrasonic signal at frequencies of order IGHz. Also described is the production of the thin film piezoelectric transducers used in this technique, which are grown directly onto a single crystal of the material under investigation. The acoustic interferometer is used to investigate the dynamics of of a relaxation process between two time-reversed antiferromagnetic states in DyAlG, and a model for domain growth in this system is postulated. Chapter 7 investigates the phase diagram of DyPO₄ using the acoustic interferometry technique. The observations of hysteresis effects are used to define a mixed phase region below the tricritical point.