Imaging of the dynamic magnetic structure in a parallel array of shunted Josephson junctions

A one-dimensional (1D) parallel array of shunted Josephson junctions is one of the basic elements in the family of rapid single-flux quantum logic circuits. It was found recently that current steps always show up in the current-voltage curve of the generator junction when an additional bias current is applied to the edge junction of the array. This effect was found to be due to the self-induced magnetic field produced by the edge current. This nonuniform field divides the array into domains each spanning several unit cells and each containing the same number of flux quanta. We report on experimental results obtained by low-temperature scanning electron microscopy (LTSEM) on the 1D array. The (1-3)-mum spatial resolution achieved by LTSEM enables us to image these domains in scanned measurements where the junctions in the array are heated sequentially. Computer simulations confirm the mechanism of the obtained images and the number of observed domains corresponds to the step position as predicted numerically.