Studies of magnetic vortices in superconductor networks and clusters by scanning SQUID microscopy
We have developed a Scanning SQUID Microscope (SSM) to study the configuration of trapped magnetic vortices in two-dimensional superconductor arrays and clusters. The SSM scans a dc SQUID detector over the array surface, achieving better than 10$\mu$m spatial resolution over a scan range up to 1cm x 1cm and a magnetic flux sensitivity of about 10$\sp{-4}\Phi\sb{\rm O}.$ 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, 2/5, 1/3, 1/4,$\...)$ of the frustration parameter $f\equiv\Phi/\Phi\sb{\rm O},$ 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 trapped vortex-antivortex pairs in arrays and vortex patterns in ensembles of 1 x 1, 2 x 2, and 3 x 3 clusters. The physics of clusters can be explained by statistical processes. Dynamics of vortices in large arrays by current injection have also been investigated. As the array is current biased, most vortices are pinned to the grids with a few vortices hopping perpendicular to the direction of current due to the Lorentz force. Some vortices move in parallel and anti-parallel to the direction of current. Creation and annihilation of vortex-antivortex pairs are also observed.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign