IMAGING OF MAGNETIC VORTICES IN SUPERCONDUCTING NETWORKS AND CLUSTERS BY SCANNING SQUID MICROSCOPY

We have imaged the configuration of trapped magnetic vortices in two-dimensional superconductor arrays and clusters using a scanning SQUID microscope (SSM). The SSM scans a dc SQUID detector over the array surface, achieving better than 10 mum spatial resolution over a scan range up to 1 cm X 1 cm and a magnetic flux sensitivity of about 10(-4)PHI0. We have obtained images of large square arrays cooled at different values of applied magnetic flux per cell PHI. For low rational values (1/2,1/3,1/4,...) of the frustration parameter f = PHI/PHI0, we find regions of periodically arranged vortices separated by domain walls; at other values of the field, the vortex pattern is disordered. We also present observations of vortex patterns in ensembles of 3 X 3 clusters.