DETERMINATION OF THE ONSET OF BULK PINNING AND THE LOW-TEMPERATURE-IRREVERSIBILITY LINE IN BI2SR2CACU2O8+DELTA

We report the results of dc magnetization measurements of Bi2Sr2CaCu2O 8+delta single crystals as a function of temperature. The evolution of the hysteresis loops with temperature is examined and the irreversibi lity line extracted. These loops can be scaled onto each other from 4. 2 to 20 K, while the lower held part of the loops for the virgin curve s continue to scale up to T-c. At intermediate temperatures, the scali ng is broken by the appearance of the second peak or ''arrowhead'' whi ch appears at around 20 K but gradually disappears above about 30 K. W hile the behavior is dominated by significant bulk pinning at low temp eratures, the hysteresis and irreversibility fields at high temperatur es are considerably modified by geometrical and surface barrier effect s. We identify the onset, with increasing field, of bulk pinning and s ignificant critical current from the emergence of the ''arrowhead'' fe ature from the background surface barrier-determined magnetization cur ve. Scaling in the low-temperature regime, and extrapolation, enables the irreversibility line at 0 K to be estimated at about 110 T. We pro pose a phase diagram up to high magnetic fields over a wide range of t emperature.