COMPLETE INVERSION OF THE MEISSNER EFFECT IN BI2SR2CACU2O8+DELTA CERAMICS AT ULTRALOW FIELDS

Using high-resolution superconducting quantum interference device magn etometry, the field-cooled magnetization M(FC) of Bi-2212 ceramics wit h bulk transition temperatures between 62 and 93 K has been investigat ed from 0.5 mOe to 5 Oe. Upon diamagnetic backgrounds of different mag nitude and temperature variation, for all materials a positive contrib ution to M(FC) appears, which increases to a formally inverted Meissne r effect at the lowest fields, M(+) = H. The temperature variations of the diamagnetic and inverted responses indicate a common effect of a thermally activated process on both signals.