HIGH-RESOLUTION MAGNETOOPTICAL IMAGING OF CRITICAL CURRENTS IN YBA2CU3O7-DELTA THIN-FILMS

Magneto-optical investigation of flux penetration into type-II superco nductors allows the determination of the local critical current densit y j(c), by inversion of the Biot-Savart law. Due to the required compu tational effort, in the past, this method was limited to low spatial r esolution of the current density. In this paper, we present a fast inv ersion scheme using Fast-Fourier-Transformation, which allows high spa tial resolution imaging of current distributions. Due to the nonlocal relation between magnetic field and current density, it is necessary t o use a method that images field and current distribution of supercond uctors as a whole, and enables high resolution at the same time. To de monstrate the power of our method, the local current density in a YBa2 Cu3O7-delta square and disk are imaged with high resolution and descri bed in detail for increasing and decreasing external magnetic fields. At low fields, the dependencies of j(c)(B) on the local magnetic field B and of the average j(c) (magnetic moment) on the applied magnetic f ield show significant differences. Finally, we directly image the loca l current density near macroscopic defects in a square and a disk. The observed current distributions near defects significantly differ from the predictions of an extended Bean model. (C) 1998 Elsevier Science B.V.