MAGNETIC ANOMALY IN A PBISR2Y1-XCAXCU3O8-CRYSTAL(Y SINGLE)

We present a detailed study of the fish-tail anomaly in the magnetizat ion hysteresis curve of a crystal of Pb2Sr2Y1-xCaCu3O8+y. We also pres ent measurements of field-cooled and zero-field cooled magnetization, and of time decay in the remanent state. We show that these data suppo rt the suggestion of Daeumling et al that the fish-tail results from a microscopic distribution of a second phase and oxygen defects which p rovide additional pinning at low fields, but develop into normal regio ns at high fields.