VORTEX-LATTICE MELTING IN BI2SR2CACU2O8-FILMS - ROLE OF THE OXYGEN STOICHIOMETRY(DELTA EPITAXIAL)

We have studied the effects of the oxygen stoichiometry on the magneti c irreversibility line and on the decay of the zero-field-cooled (ZFC) magnetization in Bi2Sr2CaCu2O8+delta epitaxial films. We have found t hat the irreversibility line is shifted to lower temperatures and fiel ds (relative to the as-grown sample) by reducing thermal treatments, w hile it is slightly affected by oxidizing treatments. The decay of the ZFC magnetization is also strongly affected by the oxygen content: al though at low temperatures (T/T(c0) less-than-or-equal-to 0.1) the dec ay rate of the ZFC magnetization does not depend on the oxygen content , as temperature increases, the relaxation becomes faster with decreas ing the oxygen content (above T/T(c0) approximately 0.3 for the reduce d 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+delta on the oxygen content. Th e effective anisotropy is higher in reduced samples, thus shifting the vortex-lattice melting temperature toward lower values. From the anal ysis of the magnetic irreversibility lines, values of the effective an isotropy factor gamma have been obtained for different degrees of oxid ation. These results are in good agreement with other estimates obtain ed by different techniques.