DIRECT OBSERVATION OF ANISOTROPIC FLUX-PENETRATION IN TWINNED YBA2CU3O7-X SINGLE AND POLYCRYSTALS

We report microscopic maps of magnetic induction in YBa2Cu3O7-x crysta ls which directly show a correlation of flux penetration with twin bou ndary orientation and twin density. These maps were obtained by means of a recently-improved magneto-optical imaging technique. Pinning was lowest in untwinned regions and increased with increasing twin density . Even at low temperatures (6 K), twin boundaries led to a large aniso tropy in flux pinning. This anisotropy, defined as the ratio of the ma gnetic induction gradient across twin boundaries to that along twin bo undaries, was 10 at 17 K; this ratio increased with increasing tempera ture. In polycrystals, twin boundaries also had a strongly anisotropic effect on flux flow into a grain from a grain boundary.