Neutron scattering studies of magnetic properties of superconductors

The magnetic properties of conventional and high-temperature superconductors have been investigated using several neutron scattering techniques. Elastic-neutron diffraction measurements have been performed upon a single crystal of PrBa₂Cu₃O₇-δ to explore the nature of the magnetic order. At Tɴ₁, which is above room temperature for all 8, the Cu spins in the CuO₂ planes order in a simple antiferromagnetic arrangement with the spins lying within the a-b plane. At a lower temperature, at TN₂, the chainer Cu spins also order antiferromagnetically in the a-b plane forcing an overall non- collinear configuration of Cu spins. TN₂ was found to be strongly dependent on the oxygen concentration of the crystal. No long-range magnetic ordering of the Pr spins was observed down to 1.9K, although strong two-dimensional short-range correlations of the Pr spins were observed at temperatures up to 20K, with the Pr spins aligning antiferromagnetically along the c direction. The failure of the onset of long-range magnetic order is attributed to frustration in the crystal caused by impurities and substitutional defects. These strong two-dimensional short-range correlations, at uncharacteristically high temperatures, provide possible evidence of the hybridisation of the Pr 4f electrons with the conduction electrons of the CuO₂ planes. The magnetisation density induced by an applied field in PrBa₂Cu₃O₇-δ has been measured by polarised-neutron diffraction. The magnetisation density distribution at the Pr site is found to differ strongly from a spherical free-ion distribution with the magnetisation density extending towards the Cu sites in the CuO₂ planes. This distortion from a free-ion density distribution again supports the hypothesis of the hybridisation of the Pr 4f electrons with the conduction electrons of the CuO₂ planes. Polarised-neutron reflectrometry has been used to study the evolution of the magnetic induction profile at the surface of a superconducting Pb film with increasing applied field. In the bulk superconducting state the spin- dependent reflectivity profiles are consistent with a pure exponential decay of the magnetic induction with a penetration depth of (390 ± 10) A. In the surface superconducting region the reflectivity profiles are accurately described over the whole range of applied fields by the Ginsburg-Landau theory if the Ginsburg-Landau parameter, K, is allowed to vary