  We report that special care is needed when longitudinal magnetic
susceptibility is computed in a magnetically ordered phase, especially in
metals. We demonstrate this by studying static susceptibility in both a
ferromagnetic and an antiferromagnetic state in the random phase approximation
to the two-dimensional Hubbard model on a square lattice. In contrast to the
case in the disordered phase, a first derivative of the chemical potential (or
the density) with respect to a magnetic field does not vanish in a magnetically
ordered phase when the field is applied parallel to the magnetic moment. This
effect is crucial and should be included when computing magnetic susceptibility
in the ordered phase, otherwise an unphysical result would be obtained. In
addition, consequently the magnetic susceptibility becomes different when
computed at a fixed density and a fixed chemical potential in the ordered
phase. In particular, we cannot employ magnetic susceptibility at a fixed
chemical potential to describe a system with a fixed density even if the
chemical potential is tuned to reproduce the correct density.
