Large area magneto-optical investigations of YBCO thin films

A new apparatus for magneto-optical investigations of high temperature supe rconducting (HTS) films as large as 20X20 cm(2) is presented. With this equ ipment 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 pre sence of defects. The apparatus was constructed to realize an effective qua lity control of large area HTS thin films used for device fabrication. With this technique, a visualization of flux pattern in superconducting films l arger than 1X1 cm(2) is presented for the first time. The results are compa red to inductive j(c) measurements as well as to micrographs [optical micro scope (OM), scanning electron microscopy (SEM)] and show that also in the l arge area characterization the magneto-optical method is very sensitive to microstructural defects impairing the critical current density, which is th e relevant parameter for an application of the superconducting thin films. Moreover, it could be shown that MOST has appreciable advantages compared t o inductive j(c) scans and microscopy (OM, SEM). In particular it is possib le to observe defects, which are below the optical resolution of the MOST s etup. (C) 1999 American Institute of Physics. [S0034-6748(99)02103-6].