Magnetically induced crystal structure and phase stability in Fe 1 2 c Co c

We present anab initio determination of the crystallographic phase stability of Fe-Co alloys as a function of concentration, using the coherent potential approximation. A bcc → hcp phase transition is found at a concentration of 85 at. % of Co, in good agreement with the experimental phase diagram. We demonstrate that for the Fe-rich random alloys magnetism stabilizes the bcc phase relative to the close-packed fcc and hcp phases. Magnetism also favors the partially ordered a8 phase relative to the random bcc alloy. This unique relation between magnetism and phase stability for the Fe-Co alloys is analyzed by a spin-polarized canonical d-band model.@S0163-1829 ~96!09326-5#