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 μm spatial resolution over a scan range up to 1 cm×1 cm and a magnetic flux sensitivity of about 10 ^-4Φ₀. We have obtained images of large square arrays cooled at different values of applied magnetic flux per cell Φ. For low rational values (1/2,1/3,1/4,.) of the frustration parameter f=Φ/Φ₀, 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 3×3 clusters.