A theoretical treatment of atomic short-range order and magnetism in iron-rich b.c.c. alloys

Abstract

We use a `first-principles' concentration-wave approach based on a finite-temperature, electronic density-functional, mean-field, grand potential of the random alloy to investigate the atomic short-range order (ASRO) in some FeV and FeAl solid solutions in both ferromagnetic and paramagnetic phases. Thermally induced spin fluctuations are modelled in terms of local moments on the Fe sites. This picture produces satisfactory estimates of all of the alloys' Curie temperatures, . We compare our calculations with the ASRO deduced from neutron and x-ray diffuse scattering measurements on single crystals carried out either in situ at or quenched from temperatures above and below . Our calculations describe the measured ASRO well. Both alloy systems exhibit B2-type ordering correlations in their paramagnetic states which strengthen in the case of FeV and weaken for FeAl as the temperature is lowered into the ferromagnetic state. We extract electronic mechanisms for these effects. The ASROs of the ferromagnetic alloys also show intensity around (1/2, 1/2, 1/2) which is traced to a Fermi surface feature and may be a precursor to the ordering in FeAl. Finally, suggestions for further polarized neutron, in situ measurements are made.