  Within the framework of density functional theory we investigate the nature
of magnetism in various families of Fe-based superconductors. (i) We show that
magnetization of stripe-type antiferromagnetic order always becomes stronger
when As is substituted by Sb in LaOFeAs, BaFe$_2$As$_2$ and LiFeAs. By
calculating Pauli susceptibilities, we attribute the magnetization increase
obtained after replacing As by Sb to the enhancement of an instability at
$(\pi,\pi)$. This points to a strong connection between Fermi surface nesting
and magnetism, which supports the theory of the itinerant nature of magnetism
in various families of Fe-based superconductors. (ii) We find that within the
family LaOFe$Pn$ ($Pn$=P, As, Sb, Bi) the absence of an antiferromagnetic phase
in LaOFeP and its presence in LaOFeAs can be attributed to the competition of
instabilities in the Pauli susceptibility at $(\pi,\pi)$ and $(0,0)$, which
further strengthens the close relation between Fermi surface nesting and
experimentally observed magnetization. (iii) Finally, based on our relaxed
structures and Pauli susceptibility results, we predict that LaOFeSb upon
doping or application of pressure should be a candidate for a superconductor
with the highest transition temperature among the hypothetical compounds
LaOFeSb, LaOFeBi, ScOFeP and ScOFeAs while the parent compounds LaOFeSb and
LaOFeBi should show at ambient pressure a stripe-type antiferromagnetic
metallic state.
