Study of the multicritical point in the vortex phase diagram (H parallel to c) of a weakly pinned crystal of YBa2Cu3O7-delta - art. no. 132505

dc magnetization studies on a high quality YBa2Cu3O7-delta crystal show tha t the Aux-line lattice melting line extends beyond the notional "tricritica l point'' in the intermediate field region (similar to 10 T), which broadly agrees with a recent theoretical proposal. The magnetization hysteresis da ta reveal the presence of both the peak effect (PE) and the fishtail effect in isothermal scans in the temperature interval 66-75 K. The broad anomaly in the magnetization hysteresis observed at lower temperatures (T<50 K), s plits into two well separated peaks above 65 K. The upper peak closely rese mbles the classical PE phenomenon and the locus of the peak fields shows a turnaround behavior. On the other hand, the lower peak progressively moves to smaller fields as T --> T-c.