Onset of magnetism in B2 transition-metal aluminides

Ab initio calculation results for the electronic structure of disordered bcc FexAl1-x(0.4 < x < 0.75), CoxAl1-x, and NiAl1-x (x = 0.4;0.5;0.6) alloys near the 1:1 stoichiometry, as well as of the ordered B2 (FeAl, CoAl, NiAl) phases with point defects are presented. The calculations were performed using the coherent potential approximation within the Korringa-Kohn-Rostoker method for the disordered case and the tight-binding linear muffin-tin orbital method for the intermetallic compounds. We studied in particular the onset of magnetism in Fe-Al and Co-Al systems as a function of the defect structure. We found the appearance of large local magnetic moments associated with the transition metal (TM) antisite defect in FeAl and CoAl compounds, in agreement with the experimental findings. Moreover, we found that any vacancies on both sublattices enhance the magnetic moments via reducing the charge transfer to a TM atom. Disordered Fe-Al alloys are ferromagnetically ordered for the whole range of composition studied, whereas Co-Al becomes magnetic only for Co concentration greater than or equal to 0.5. [S0163-1829(99)00910-8].