High resolution scanning Hall probe microscopy and Hall magnetometry have been used to investigate the magnetic properties of single crystal samples of Co-doped Sr-122 and Ba-122 iron- based superconductors and high quality MgB2 thin films. We have made a quantitative analysis of the evolution of the profiles of well-isolated vortices as a function of temperature, and used a fitting procedure to extract the temperature-dependent magnetic field penetration depth, . This, in turn, allowed us to infer the temperature-dependent superfluid density which has been compared with -model results for a two band superconductor. Fit parameters yield insights into the symmetry of the order parameter at the electron and hole pockets as well as the relative contributions of the bands to the superfluid density in the iron-based crystals. Vortex imaging and ‘local’ magnetisation measurements, with a static Hall probe parked just above the sample surface, also yielded important information about the distribution of pinning sites, the strength of vortex pinning and the possible presence of material inhomogeneities in all samples. Finally, we have investigated vortex spatial distributions over large areas in high quality MgB2 thin films using our mK SHPM. A careful statistical analysis of vortex positions in these films shows unusual anisotropies, with evidence of a second weak peak in the nearest neighbour vortex-vortex bond length distribution at small distances. This result was completely unexpected as one would predict such ‘type 1.5 superconductivity’ effects to be suppressed by strong scattering in MgB2 thin films.
|Date of Award||19 Jul 2013|
|Supervisor||Simon Bending (Supervisor)|