  Temperature dependent resistivity of the iron-based superconductor
NdFeAsO0.88F0.12 was measured under different applied fields and excitation
currents. Arrhenius plot shows an anomalous tail effect, which contains obvious
two resistivity dropping stages. The first is caused by the normal
superconducting transition, and the second is supposed to be related to the
weak-link between the grains. A model for the resistivity dropping related to
the weak-link behavior is proposed, which is based on the Josephson junctions
formed by the impurities in grain boundaries like FeAs, Sm2O3 and cracks
together with the adjacent grains. These Josephson junctions can be easily
broken by the applied fields and the excitations currents, leading to the
anomalous resistivity tail in many polycrystalline iron-based superconductors.
The calculated resistivity dropping agrees well with the experimental data,
which manifests the correctness of the explanation of the obtained anomalous
tail effect.
