  Both DC and AC magnetization measurements were performed on the
NdFeAsO0.88F0.12 superconductor to investigate the influence of magnetism on
the superconducting properties of this system. The crossover of the ZFC and FC
magnetic susceptibility curves under 7.5KOe was observed. The imaginary
component of the first harmonics of the AC magnetic susceptibility, increases
with the increasing DC field below 10K and shows frequency dependency under
7.5KOe at low temperature. The paramagnetism of Nd3+ ions tilts the magnetic
hysteresis loops and broadens the hysteresis width. After correction for the
paramagnetism, the field and temperature dependence of intrinsic Jcm was
obtained and compared with the experimentally obtained total Jcm. The origin of
the abnormal behavior of magnetization was investigated and attributed to a
magnetic background, which was speculated to be caused by the spin-glass state.
However, this magnetic background does not affect the flux pinning properties
in this sample. The related mechanism was discussed.
