  The evolutionary scenario of the neutron star magnetic field is examined
assuming a spindown-induced expulsion of magnetic flux originally confined to
the core, in which case the expelled flux undergoes ohmic decay. The nature of
field evolution, for accreting neutron stars, is investigated incorporating the
crustal microphysics and material movement due to accretion. This scenario may
explain the observed field strengths of neutron stars but only if the crustal
lattice contains a large amount of impurity which is in direct contrast to the
models that assume an original crustal field.
