Theoretical study of ordering in Fe-Al alloys based on a density-functional generalized-perturbation method

We present a theoretical study of ordering in Fe-Al alloys assuming different underlying magnetic structures: paramagnetic, ferromagnetic, and 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 its electronic structure described via the coherent potential approximation. The effective pair interactions are used to obtain the ordered superlattice structures and to estimate the order-disorder transition temperatures. The tendency of primary ordering to the B2 structure and secondary ordering to the ${\mathrm{DO}}_3$ structure is examined as a function of Fe concentration. We find that the tendency to B2 (CsCl) ordering decreases in sequence from the paramagnetic to the DLM's and to the ferromagnetic model. The tendency to secondary ordering in the ${\mathrm{DO}}_3$ structure is strongest in the ferromagnetic model and is found to increase with Fe concentration due to enhanced spin polarization. Factors such as lattice relaxation, charge transfer, and alloy volume (per atom) are found to be much more important for secondary than for primary ordering. Although the model provides a way to study the ordering tendency in the alloy based on an ab initio electronic structure calculation, it is deficient in capturing all the intricacies of the interplay between magnetic and chemical structure. Effects of spin fluctuations on the order-disorder transition are also neglected.