  A model of the ferromagnetic origin of magnetic fields of neutron stars is
considered. In this model, the magnetic phase transition occurs inside the core
of neutron stars soon after formation. However, owing to the high electrical
conductivity the core magnetic field is initially fully screened. We study how
this magnetic field emerges for an outside observer. After some time, the
induced field that screens the ferromagnetic field decays enough to uncover a
detectable fraction of the ferromagnetic field. We conjecture that weak fields
of millisecond pulsars of 10^8-10^9 G could be identified with ferromagnetic
fields of unshielded fraction f=10^-4 resulting from the decay of screening
fields by a factor 1-f in 10^8 yr since their birth.
