Equilibrium magnetization of a Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub [ital x]] single crystal measured up to 20 T

The reversible magnetization of a high quality Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub [ital x]] 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 ([ital M][sub [ital S]][proportional to]ln[ital B]). For lower fields, we observe a strong dependence of [ital dM][sub [ital S]]/[ital d]ln[ital B] on the magnetic field [ital B]. The results are discussed in terms of quantum and classical fluctuations of pancake vortices.