VARIATION IN TC FOR BI2CA1SR2CU2OY DOPED WITH FE AND SUBJECTED TO VACUUM ANNEALING

Substitution of Fe for Cu in Bi2Ca1Sr 2Cu2Oy was found to be more effective in reducing Tc than in YBa2Cu3Oy. The authors discovered that the superconductivity which has been lost in Fe-doped Bi2Ca1Sr2Cu2Oy can be greatly recovered in vacuum at about 400 degrees C. This suggests that the exchange interaction between the magnetic moments and superconducting pairs is not predominantly in pair-breaking mechanisms, leading to the loss of superconductivity in high-Tc ceramics. As the non-doped and Fe-doped Bi2Ca1Sr2Cu2Oy samples were subjected to vacuum annealing, the lattice parameter c was found to increase and Mossbauer data revealed that both the local structure around Fe (or Cu) sites and the relative number of Fe (or Cu) sites associated with different values of quadrupole splittings Delta Q1 and Delta Q2 varied. This implies that the increase in Tc for the annealed samples is related to the local distortion of Cu-O layers. The optimum value of T c ( rho =50%) for Bi2Ca1Sr2Cu 2Oy as high as 94 K can be obtained by annealing the sample in a vacuum at 500 degrees C for 30 min.