Neutron diffraction and reflection studies of superconductors

Thesis



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.

Authors: Reynolds, J; Reynolds, John M.

• Publication date: 1998
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Magnetic properties; Superconductors