It is shown that intrinsic and extrinsic structural defects have an enormous influence on the field and current distributions of superconductors which depend normally only on the geometry of the sample. The flux penetration and the current flow along various defects found in high-Tc superconductors are analysed in detail. The defects can be classified by means of the flux pattern and of the corresponding shape of the current lines around the defect. In the case of a high defect density the penetration of vortices into the sample will lead to a change of the length scale of the shielding currents flowing in the Meissner area and of the critical currents flowing within the flux front, and current loops of a smaller length scale are formed. This formation of current loops is directly observed. This so-called magnetically induced granularity will lead to an underestimation of magnetization in integral measurements such as magnetometry. Magneto-optical flux visualization can be used to obtain the necessary corrections. It is shown that flux visualization is a powerful tool to test the quality of superconducting samples.
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