  Among the ferromagnetic binary alloys, body centered cubic (bcc) Fe-Co is the
one showing the highest magnetization. It is known experimentally that ordered
Fe-Co structures show a larger magnetization than the random solid solutions
with the same Co content. In this work, based on density functional theory
(DFT) studies, we aim at a quantitative prediction of this feature, and point
out the role of the orbital magnetic moments. Then, we introduce a DFT-based
analytical model correlating local magnetic moments and chemical compositions
for Co concentrations ranging from 0 to 70 at.%. It is also extended to predict
the global magnetization of both ordered and disordered structures at given
concentration and chemical short range orders. The latter model is particularly
useful for interpreting experimental data. Based on these models, we note that
the local magnetic moment of a Fe atom is mainly dictated by the Co
concentration in its first two neighbor shells. The detailed local arrangement
of the Co atoms has a minor effect. These simple models can fully reproduce the
difference in magnetization between the ordered and disordered Fe-Co alloys
between 30% and 70% Co, in good agreement with experimental data. Finally, we
show that a similar model can be established for another bcc binary Fe alloy,
the Fe-Ni, also presenting ferromagnetic interactions between atoms.
