The reversible magnetization of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ crystals was measured for the H∥c direction. Crystals with different stoichiometry were obtained by quenching from different annealing temperatures between 450 °C and 725 °C, resulting in critical temperatures varying from 84 to 93 K. No large variations in the magnetization with ${\mathit{T}}_{\mathit{c}}$ were found. The apparent increase of ${\mathit{T}}_{\mathit{c}}$ with increasing magnetic field observed in previous work by other authors is not observed, leading to the conclusion that this effect may be caused by inhomogeneities in the oxygen content of the crystals. The magnetization of all specimens approximately followed a logarithmic decrease with increasing magnetic field for a given temperature, allowing the extraction of the magnetic penetration depth λ. The temperature dependence of λ does not follow the empirical [1-(T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{m}}$${]}^{\mathrm{-}0.5}$, m=4 form, but requires values for m of slightly less than 2. The value for the zero-temperature penetration depth is 0.17 μm, independent of specimen ${\mathit{T}}_{\mathit{c}}$. An attempt to fit the magnetization with a Ginzburg-Landau expression using a temperature dependence for ${\mathit{H}}_{\mathit{c}}$ of the type ${\mathit{H}}_{\mathit{c}}$(T)/${\mathit{H}}_{\mathit{c}}$(0)=1-(T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$ with n=2 was not very successful, indicating that a more sophisticated analysis is needed.

• Received 27 December 1991

DOI:https://doi.org/10.1103/PhysRevB.46.6422