Using the high-resolution Faraday effect (HRF) technique, the behavior of magnetic flux in Bi2Sr2CaCu2O8 single crystals is visualized in external magnetic fields up to 1 T. This technique reveals defects in many of these crystals which are probably twin planes. X-ray measurements indicate that two adjacent crystallites have c-axis lattice vectors which deviate from each other by about one degree. As a consequence of these defects, the shielding currents break up in sub-loops, each shielding a defect-free part of the sample. This leads to an underestimate of the critical current values determined by non-local magnetization measurements. Magneto-optical images can be used to calculate the necessary correction.
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