Magnetization anomalies above the vortex lattice melting transition

A weak irreversible behavior was found in magnetization hysteresis curves i n pristine as well as heavy-ion-irradiated Bi2Sr2CaCu2O8+delta Single cryst als measured by a SQUID magnetometer both above the vortex lattice melting transition in lower fields and the vortex-glass transition in higher fields . The onset of this anomaly on magnetic field H-F strongly depends on sampl e disorder level, and begins to appear in the middle of the first-order pha se transition. H-F systematically shifts to higher temperature and field re gion with increasing the number of columnar defects. The results are interp reted by a disorder-induced pinning, which is effective below H-F and becom es ineffective above H-F (C) 2000 Published by Elsevier Science B.V, All ri ghts reserved.