  We perform first principles calculations of the magnetocrystalline anisotropy
energy (MAE) of the L1_{0}-like Fe_{x}Pt_{1-x} samples studied experimentally
by Barmak and co-workers in [J. Appl. Phys. 98 (2005) 033904]. The variation of
composition and long-range chemical order in the samples was studied in terms
of the coherent potential approximation. In accordance with experimental
observations, we find that, in the presence of long-range chemical disorder,
Fe-rich samples exhibit a larger MAE than stoichiometric FePt. By considering
the site- and species-resolved contributions to the MAE, we infer that the MAE
is primarily a function of the degree of completeness of the nominal Fe layers
in the L1_{0} FePt structure.
