Sample size effect on the determination of the irreversibility line of high-T_c superconductors

The irreversibility lines of a high-J_c superconducting Bi ₂Sr₂Ca₂Cu₃O_x/Ag tape were systematically measured upon a sequence of subdivisions of the sample. The irreversibility field H_r(T) (parallel to the c axis) was found to change approximately as L^0.13, where L is the effective dimension of the superconducting tape. Furthermore, it was found that the irreversibility line for a grain-aligned Bi₂Sr₂Ca₂Cu ₃O_x specimen can be approximately reproduced by the extrapolation of this relation down to a grain size of a few tens of micrometers. The observed size effect could significantly obscure the real physical meaning of the irreversibility lines. In addition, this finding surprisingly indicated that the Bi₂Sr₂Ca ₂Cu₂O_x/Ag tape and grain-aligned specimen may have similar flux line pinning strength.