The Critical Current Density of Superconducting Films: Thickness, Temperature, and Field Dependences

The critical current density (J_c) of pulsed-laser-deposited GdBa₂Cu₃O_7-δ (GdBCO) superconducting films with film thickness (d) ranging from 0.2 to 1.5 μm was studied as functions of temperature (T) and held (H). A thickness dependence of both the self-held and the in-held J_cs at 77 K showed that the J_cs were observed to decrease with increasing film thickness as an expression of J_c ~ d^-1/2, which was predicted by the collective pinning theory of uncorrelated defects. However, the in-held J_c at d ~0.6 μm was found to suddenly increase in the general trend of decreasing J_c-d, which probably suggested a change in the pinning mechanism at that film thickness. The temperature dependence of the in-held J_c in the GdBCO films was specifically studied at the low and the intermediate-held regions. A characteristic held H* separating the held-independent and the held-dependent J_c regions was found to decrease with increasing both T and d. The value of α of the J_c ~ H^α dependence in the intermediate-held region was estimated to be α ~0.48-0.52 for both the thin- and the thick-film regions, which is similar to the theoretical value of α = 0.5 for pinning by sparse point-like defects in REBa₂Cu₃O_7-δ (RE: rare-earth elements) superconducting films. The value of α ~ 0.35 estimated for the 0.6 μm thick GdBCO film might suggest that effective pinning induced via nanosized dislocations formed on the surface of the GdBCO film. Based on the application of the theoretical models, it could be concluded that the dominant pinning mechanism in the GdBCO films is pinning by sparse uncorrelated defects.