  The magnetic properties of various iron pnictides are investigated using
first-principles pseudopotential calculations. We consider three different
families, LaFePnO, BaFe$_2$Pn$_2$, and LiFePn with Pn=As and Sb, and find that
the Fe local spin moment and the stability of the stripe-type antiferromagnetic
phase increases from As to Sb for all of the three families, with a partial gap
formed at the Fermi energy. In the meanwhile, the Fermi-surface nesting is
found to be enhanced from Pn=As to Sb for LaFePnO, but not for BaFe$_2$Pn$_2$
and LiFePn. These results indicate that it is not the Fermi surface nesting but
the local moment interaction that determines the stability of the magnetic
phase in these materials, and that the partial gap is an induced feature by a
specific magnetic order.
