THICKNESS AND ROUGHNESS DEPENDENCE OF MAGNETIC-FLUX PENETRATION AND CRITICAL-CURRENT DENSITIES IN YBA2CU3O7-DELTA THIN-FILMS

As a result of the island growth mode all epitactic YBa2Cu3O7-delta th in films show surface roughness. To investigate a possible surface pin ning mechanism, combined magneto-optical and atomic force microscopy s tudies were carried out. Measurement of the spatial distribution of ma gnetic flux density on the surface of thin YBa2Cu3O7-delta films by me ans of the magneto-optical Faraday effect (MOFE) under application of external magnetic fields allows an accurate determination of critical current densities j(c). In order to have a quantitative comparison bet ween the Bean model for thin films and experiment, a new nonlinear cal ibration technique for the flux densities was developed. For determini ng the thickness and roughness dependence of j(c), samples with YBaCuO -strips of different thickness and roughness were patterned from one f ilm. With the roughness determined experimentally by afm measurements, a satisfactory agreement between the measured and calculated thicknes s dependence of j(c) is achieved. Surface pinning is found to cause be tween 10%-30% of the critical current densities of epitactic YBa2Cu3O7 -delta thin films. Additionally microscopic deviations of the flux pro files from the Bean model are detected. Evidence for matching effects of the vortex line distribution with the density of surface pins is gi ven.