THEORETICAL-STUDY OF ORDERING IN FE-AL ALLOYS BASED AN A DENSITY-FUNCTIONAL GENERALIZED-PERTURBATION METHOD

We present a theoretical study of ordering in Fe-Al alloys assuming di fferent underlying magnetic structures: paramagnetic, ferromagnetic, a nd disordered local moments (DLM's). We calculate the effective pair ( chemical) interactions using the generalized perturbation method (GPM) in the linear muffin tin orbital basis. The reference medium for the GPM is chosen as the completely disordered state of the alloy, with it s electronic structure described via the coherent potential approximat ion. The effective pair interactions are used to obtain the ordered su perlattice structures and to estimate the order-disorder transition te mperatures. The tendency of primary ordering to the B2 structure and s econdary ordering to the DO3 structure is examined as a function of Fe concentration. We find that the tendency to B2 (CsCl) ordering decrea ses in sequence from the paramagnetic to the DLM's and to the ferromag netic model. The tendency to secondary ordering in the DO3 structure i s strongest in the ferromagnetic model and is found to increase with F e concentration due to enhanced spin polarization. Factors such as lat tice relaxation, charge transfer, and alloy volume (per atom) are foun d to be much more important for secondary than for primary ordering. A lthough the model provides a way to study the ordering tendency in the alloy based on an ab initio electronic structure calculation, it is d eficient in capturing all the intricacies of the interplay between mag netic and chemical structure. Effects of spin fluctuations on the orde r-disorder transition are also neglected.