EQUILIBRIUM MAGNETIZATION OF A BI2SR2CACU2OX SINGLE-CRYSTAL MEASURED UP TO 20-T

The reversible magnetization of a high quality Bi2Sr2CaCu2Ox crystal has been measured by torque magnetometry at fields up to 20 T and temperatures ranging from 40 to 100 K. For fields above 7 T the superconducting diamagnetic moment follows a simple London-like behavior (Ms is-proportional-to ln B). For lower fields, we observe a strong dependence of dM(S)/d ln B on the magnetic field B. The results axe discussed in terms of quantum and classical fluctuations of pancake vortices.