NMR-based microprobes for magnetic field measurements

The thesis considers the problem of measuring magnetic fields to high accuracy using the preferred method of nuclear magnetic resonance (NMR). In chapter one, a brief summary of diverse methods of field measurement is given, and the basic principles of NMR spectroscopy are explained, concentrating on the topics of signal reception, relaxation, line width and magnetic field inhomogeneity. In chapter two, the use of a microprobe for highly-localised NMR measurements of field strength is introduced. Perturbations to the field by the probe, that destroy the accuracy of the measurements, are analysed and the contributions of the various probe components to this problem are discussed. In chapter three, an initial, simple probe design is presented that exploits the symmetries inherent in a description of static magnetic fields to minimise field perturbations. The loss of signal-to-noise ratio associated with the design—long wires parallel to the field direction—is discussed and experimental results are presented that validate the concept. The venerable use of zero-susceptibility wire, with a coaxial structure of metals of opposite magnetic susceptibility, is introduced in chapter four, and the simple first-order recipe, currently used by manufacturers to determine the relative thicknesses of metals, is derived. Deficiencies in current methods of producing such wire are also highlighted. The novel manufacture of zero-susceptibility, copper-plated aluminium conductor is then described in detail. This particular combination of metals is shown to be advantageous and measurements on a long sample confirmed the advantage. However, results obtained with a probe constructed of such wire were mediocre, as described in chapter five, and suspicion is directed toward the simplistic theory currently used. A summary and references conclude the thesis.