Thickness dependence of pinning properties in Bi-2212 superconductor

The effect of the thickness of Bi-2212 superconducting thin films and single crystals on the critical current property is investigated in the range of 0.1–10 µm. The peak effect of critical current density at low temperatures disappears for films thinner than the critical thickness in the range of 0.5–1.0 µm. On the other hand, the irreversibility field increases with increasing thickness over the entire range of investigation at low temperatures, suggesting the critical thickness for the irreversibility field is larger than 10 µm. This behaviour is different from that in three-dimensional RE-123 superconductors in which the two critical thicknesses are the same. While the critical thickness for the irreversibility field is given by the pinning correlation length in both types of superconductors, the critical thickness for the peak effect in Bi-2212 seems to be given by the interlayer coupling length. On the other hand, the irreversibility field in the same Bi-2212 specimens is found to decrease with increasing thickness at high temperatures, which is completely opposite to the dependence at low temperatures. This behaviour appears to contradict the theoretical prediction of the flux creep model. However, a detailed investigation reveals that this agrees with the theoretical prediction under the principle of irreversible thermodynamics which describes how the transverse order of flux lines is developed so as to reduce the energy dissipation due to the flux creep for extremely thin films.