REAL-TIME IMAGING OF THE MAGNETIC-FLUX DISTRIBUTION IN SUPERCONDUCTING YBA2CU3O7-DELTA

THE magnetic response of the high-transition-temperature oxide superconductors to an applied field is related to two technologically important issues in these materials: their finite resistivities well below the superconducting transition and their critical currents. In addition, the scientifically interesting issues of pinning, melting, vortex glass behaviour and vortex liquids 1 can be probed by studying the time dependence of the magnetization. Most techniques used to probe the magnetization of superconductors represent averages over the entire volume of the sample. This precludes the study of defects, and also raises the question of whether the observed behaviour is intrinsic or is dominated by defects. On the other hand, spatially resolved techniques such as magnetic decoration or scanned probe microscopy are usually too slow to allow studies of the dynamics of these systems. Here we demonstrate a technique for real-time, spatially resolved imaging of flux dynamics in superconductors with a wide variety of sample morphologies. Following earlier work 2-6, we place a wafer of a magneto-optical material-in this case, a doped garnet film-in contact with a superconducting sample in a polarizing light microscope. The spatial and temporal resolution afforded by the high-quality garnet film allows us to investigate the effect of defects (specifically, twin planes) on the magnetic response of the superconductor.