Magnetic flux penetration and distribution in Bi₂Sr₂CaCu₂O_y single crystals by magnetooptical measurements

Magnetic flux penetration and flux density distributions in Bi2Sr2CaCu2Oy single crystals have been investigated by the magneto-optical technique. In as-grown crystals, preferential flux penetration along narrow bands in the a-direction is observed, independent of the geometry of the crystals. On annealing in flowing oxygen, flux penetration becomes more uniform but significantly anisotropic. Comparison with ac susceptibility measurements near Tc, suggests that inhomogeneous flux penetration in the as-grown crystals may be associated with inhomogeneous oxygen stoichiometry.