Abstract
Polycrystalline NdFeAsO0.88F0.12 samples were prepared by both high pressure (HP) and ambient pressure (AP) methods. Magnetic hysteresis loops (MHLs) as well as magnetic relaxation were measured to investigate the vortex dynamic properties of the two samples. Magnetic relaxation rate S combined with effective pinning barrier energies U eff were calculated as a function of temperature and magnetic field. The results suggest that: (1) The samples with different connectivity display different properties of MHLs, indicating the coexistence of bulk superconductivity and granularity; (2) The different S(T) behaviors between sample AP and sample HP result from the transition from globality to granularity; (3) Field dependent S shows that the pinning mechanism of NdFeAsO0.88F0.12 can not be explained by collective pinning theory; (4) The anomalous temperature and magnetic field dependence of effective barrier energies as well as magnetic relaxation rate may be evoked by the competition between Bean-Livingstone (BL) surface pinning and bulk pinning.
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