Scanning Hall probe microscopy of nanostructured superconductors

(H)igh resolution scanning Hall probe microscopy has been used to study flu x structures in various nanostructured superconducting samples. Five micron square YBCO disks exhibit rotationally symmetric flux structures at high t emperatures and after field-cooling to low temperatures. We find that their magnetic response can be almost perfectly compensated by tuning the applie d magnetic field. High temperature measurements of the 'local magnetisation ' above the disks as a function of applied field display pronounced sawtoot h 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 tem peratures. We speculate that we may possibly be observing the formation of Giant Vortex states at high temperatures and multi-vortex states at low tem peratures. A thin Pb film containing a square antidot pinning array has also been stud ied. Close to T-c 'local magnetisation' measurements exhibit pronounced pea ks at rational fractional filling factors reflecting strongly pinned commen surate flux line structures. These special nux structures have been imaged directly at lower temperatures (5.5K) where we find that a maximum of two f lux lines can be pinned at each antidot.