Large area magneto-optical investigations of YBCO thin films

A new apparatus for magneto-optical investigations of high temperature superconducting (HTS) films as large as 20x20 cm2 is presented. With this equipment flux penetration of an external magnetic field into YBCO thin films has been studied by scanning the samples through an inhomogeneous magnetic field (magneto-optical scanning technique, MOST). The normal penetration of magnetic flux into a superconductor will be changed drastically in the presence of defects. The apparatus was constructed to realize an effective quality control of large area HTS thin films used for device fabrication. With this technique, a visualization of flux pattern in superconducting films larger than 131 cm2 is presented for the first time. The results are compared to inductive jc measurements as well as to micrographs (optical microscope (OM), scanning electron microscopy (SEM)) and show that also in the large area characterization the magneto-optical method is very sensitive to microstructural defects impairing the critical current density, which is the relevant parameter for an application of the superconducting thin films. Moreover, it could be shown that MOST has appreciable advantages compared to inductive jc scans and microscopy (OM, SEM). In particular it is possible to observe defects, which are below the optical resolution of the MOST setup