DEFECTS IN YBA2CU3O7-X CRYSTALS AND THE QUANTIZATION OF MAGNETIC-FLUX

YBa2CU3O7-x single crystals of high quality have been prepared from Ba O/Cu melts. Careful and sensitive magnetization measurements at fields lower than 1 mT, and at LHe temperature, have been performed on sever al crystals. In two cases, the magnetization of the sample was hystere tic, and the magnetic flux inside the sample changed like a regular st aircase on changing the applied field. The data are explained by the q uantization of the magnetic flux enclosed into one, or perhaps a few, cavities closed by Josephson junctions.