Scanning hall probe microscopy of nanostructured superconductors

Abstract

High resolution scanning Hall probe microscopy has been used to study flux structures in various nanostructured superconducting samples. Five micron square YBCO disks exhibit rotationally symmetric flux structures at high temperatures and after field-cooling to low temperatures. We find that their magnetic response can be almost perfectly compensated by tuning the applied magnetic field. High temperature measurements of the ‘local magnetisation’ above the disks as a function of applied field display pronounced sawtooth structures with very sharp jumps. In contrast asymmetric flux structures are observed after cooling in H=0 and the application of a field at low temperatures. We speculate that we may possibly be observing the formation of Giant Vortex states at high temperatures and multi-vortex states at low temperatures. A thin Pb film containing a square antidot pinning array has also been studied. Close to T c ‘local magnetisation’ measurements exhibit pronounced peaks at rational fractional filling factors reflecting strongly pinned commensurate flux line structures. These special flux structures have been imaged directly at lower temperatures (5.5K) where we find that a maximum of two flux lines can be pinned at each antidot.