The magnetic properties of superconductors

It has long been known that a type-ll superconductor (a class which includes high-Tc materials) allows an applied magnetic field to penetrate as flux lines. This thesis is based on the study of these flux lines and the lattice structure they form. The techniques of neutron scattering and muon spin rotation have been used to investigate this system. Neutron scattering gives information on the structure and long range order of the flux line lattice; Muon spin rotation gives information on the local field distributions inside the sample. Used together these two techniques provide a very powerful tool for probing the low field portion of the B-T phase diagram of a high-Tc superconductor. In this thesis two very different samples of the highly anisotropic material Bi2.15Sr1.85CaCu2O8+δ are investigated. The first sample is a high quality single crystal (known as the pristine sample). The second is a mosaic of crystals which have had columnar defects of normal material produced through them. In the first sample, at low fields the melting line seen is consistent with a 3-D lattice coupled mainly by electromagnetic interactions. As the field increases above ∼100mT the melting line becomes almost independent of field, at a temperature of about 20K. The competition between thermal and static disorder in the system has an interesting consequence of re-aligning the vortices along the c-axis. The second sample shows very different behaviour due to the increased pinning. The 3-D portion of the phase diagram seems to extend to ∼600mT. There is a large amount of disorder over the range of fields investigated, it is not as great as the disorder predicted by simulations however. This is due to the separation of the defects, and hence the relative inter-vortex repulsion being different. Also looked at is the less anisotropic superconductor YBa2Cu3O7-δ. This system is 3-D in nature over the whole range of fields investigated. For the first time a sample which is not dominated by twin planes has been studied by neutron scattering. A number of interesting features have been seen; including an, as yet unexplained, realignment of the flux lattice at a field of ∼3T. Other work presented includes an investigation of SrRu2O4, Organic superconductors, the Borocarbide system, low κ superconductors and the characterisation of one of the instruments used. (author)