EVIDENCE FOR A DEPINNING TRANSITION IN THE VORTEX QUASI-LATTICE OF BI2SR2CACU2O8+DELTA

Magnetization measurements have been performed on several crystals of Bi2Sr2CaCu2O8+delta as a function of both temperature and magnetic fie ld. A pinning crossover for vortices is identified at fields above the arrowhead second peak transition but below the irreversibility line i n agreement with recent Hall-array measurements. In addition, a small but clear feature is apparent in the derivative of the magnetic moment as a function of temperature at fields below that of the arrowhead at an almost field-independent temperature of about 32 K. These features define a pinning-related transition that crosses continuously from th e disordered into the ordered vortex solid phase. [S0163-1829(97)02641 -6].