Vortex-lattice melting in Bi2Sr2CaCu2O8+δ epitaxial films: Role of the oxygen stoichiometry

We have studied the effects of the oxygen stoichiometry on the magnetic irreversibility line and on the decay of the zero-field-cooled (ZFC) magnetization in Bi2Sr2CaCu2O8+δ epitaxial films. We have found that the irreversibility line is shifted to lower temperatures and fields (relative to the as-grown sample) by reducing thermal treatments, while it is slightly affected by oxidizing treatments. The decay of the ZFC magnetization is also strongly affected by the oxygen content: although at low temperatures (T/Tc0≤0.1) the decay rate of the ZFC magnetization does not depend on the oxygen content, as temperature increases, the relaxation becomes faster with decreasing the oxygen content (above T/Tc0∼0.3 for the reduced sample the time decay is so rapid that it cannot be measured). Such effects are explained taking into account the strong dependence of the effective anisotropy in Bi2Sr2CaCu2O8+δ on the oxygen content. The effective anisotropy is higher in reduced samples, thus shifting the vortex-lattice melting temperature toward lower values. From the analysis of the magnetic irreversibility lines, values of the effective anisotropy factor γ have been obtained for different degrees of oxidation. These results are in good agreement with other estimates obtained by different techniques