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].