I have studied two magnetic properties of superconductors using different neutron scattering techniques. Due to their magnetic moment and lack of electrical charge, neutrons can provide an extremely sensitive probe into magnetism on a small scale. Unusual magnetic structures have previously been observed in the non-superconducting PrBa₂Cu₃O₆+_δ, in particular showing a coupling between the rare-earth and the Cu-O₂ planes. For comparison, I have used elastic neutron scattering to measure the temperature dependent magnetic structure of the high-T_c superconductor NdBa₂Cu₃O₇, and its nonsuperconducting reduced form NdBa₂Cu₃O₆. The magnetic structures of both the Nd and Cu sub-lattices are presented. Measurements are shown for single crystals of both the oxygenated and reduced form, and also for a non-stoichiometric single crystal with Nd substituted on the Ba sites. I present a quantitative analysis of the magnetic moments, with account taken of the instrumental resolution including sample mosaicity and intrinsic peak shapes. No evidence was found for any coupling of the type seen in PrBa₂Cu₃O₆+_δ. Many of the underlying properties of superconductors are evidenced in the way magnetic fields are excluded in the different states. I have used polarized neutron reflectometry to measure the magnetic field profile in thin film superconductors. I present measurements for pure niobium and preliminary measurements for YBa₂Cu₃O₇. The samples studied are much thicker than those previously measured, to provide a closer match to the bulk superconductor properties and allow meaningful measurements of the higher field states. For niobium, the applied field dependence is measured for fields covering the Meissner and mixed states, and models are compared for these states and the surface superconductivity state.

Graduation date: 1996

Graduation date: 2000

Graduation date: 1995

No description available.

A series of modelling and experimental studies of the magnetic properties of bulk, single grain high temperature superconductors (HTS), such as RE-Ba-Cu-0 [(RE)BCO] where RE refers to a rare earth element, have been performed using a modelling framework developed in this study. This modelling framework can simulate the various magnetisation processes of bulk HTS in an attempt to understand the generation of relatively large magnetic fields by these materials for their potential use in a number of high field, 'quasi-permanent' magnet applications. Chapters I and 2 introduce bulk HTS based on their magnetic properties. More specifically, Chapter I describes the background for Type II bulk superconductors as a group of electrical materials whose various electromagnetic applications originate from their unique magnetic properties, and in particular from their ability to trap magnetic field by flux pinning ( often described by the Bean model) and their Jc-B (the Kim model) and E-J characteristics (the flux flow resistivity model and the power law model). Chapter 2 relates the properties of bulk superconductors, and especially those relating Jc, to the major research areas within bulk superconductivity of fabrication, magnetisation and modelling. State-of-the-art techniques for each area are discussed within the context of delivering effectively the full potential of Jc of a bulk superconductor in order to produce the maximum possible trapped field. Chapter 3 describes the modelling framework developed and used throughout this thesis, and presents three representative examples to demonstrate its capability in understanding the magnetic behaviour of bulk HTS during various magnetisation processes. The modelling framework solves Campbell's equation (which describes the force-displacement relation of magnetic flux lines) and the heat equation simultaneously using the finite element method (FEM) in the commercial software package FlexPDE. Two modes of simulation (timeindependent or time-dependent) are investigated, which are applicable to both static and pulsed field magnetisation (PFM) processes. This study extends significantly the research into PFM of bulk HTS, which has been limited generally to experimental techniques to date . Chapter 4 focuses on modelling the magnetisation of bulk HTS using split-coil arrangements, which serves as a comprehensive example of a direct application of the modelling framework developed. Split-coil arrangements are viewed as a preferred, but less understood, alternative to conventional solenoidal coils for practical magnetisation processes, and pa1ticularly for in situ PFM processes. Two major questions have been answered regarding the nature of split-coil magnetisation using the modelling framework developed: the geometrical conditions for designing an effective split-coil magnetisation ainngement have been established; the mechanisms of a split-coil magnetisation process, which consist of two distinct regimes of flux penetration, are understood. Both regimes are completely different from those observed in solenoidal-coil magnetisation processes. Finally, the association between the geometrical conditions and the mechanisms has been established Chapter 5 reports the development of a novel modelling-aided, non-destructive method of measuring Jc and the flux flow resistivity Pv (regarded as the key parameter of the flux flow resistivity model) in bulk HTS, which is generally considered impossible using common experimental techniques. This combination with experiment represents a second application of the modelling framework. The experimental part of this method involves magnetising a bulk HTS using a specific profile of external field, during which the induced voltage within the pick-up coil wound around the sample is measured and used to calculate key fields for the purposes of comparison. The modelling part of the method establishes good agreement between the measured and simulated fields using estimated values of Jc and Pv� Chapter 6 summarises all the research presented in this thesis from the perspective of the development and the application of the modelling framework for studying the magnetic properties of bulk HTS. It improves considerably the understanding of the mechanisms of magnetisation processes and the magnetic behaviour of these technologically important materials during their magnetisation. It also serves as a cost-effective tool for designing practical magnetisation arrangements and related processes in order to achieve the full capability of a bulk HTS effectively. Finally the framework plays a core potential role in the field of modelling-aided, non-destructive characterisation of the magnetic properties of (RE)BCO and other bulk superconductors.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Physics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / by William J. Gallagher. / Ph.D.

Graduation date: 1997

No description available.

PrBa₂Cu₃O₆₊_x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa₂Cu₃O₆₊_x:it is the only compound which has the same structure as YBa₂Cu₃O₆₊_x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa₂Cu₃O₆₊_x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O₂ planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa₂Cu₃O₆₊_x, I have used instead the compounds PrO₂ and Y_0.3Pr_0.7Ba₂Cu₄O₈, which contain praseodymium in a chemically similar environment. Y_0.3Pr_0.7Ba₂Cu₄O₈ shows the same broadening effects as PrBa₂Cu₃O₆₊_x; PrO₂ shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.