Electronic structure, exchange interactions, and Curie temperature of FeCo

Fe-Co alloys in the alpha phase are soft magnetic materials which have high saturation inductions over a wide range of compositions. However, above about 1250 K, an alpha to gamma phase transition occurs. The fcc-based, gamma, high-temperature phase is paramagnetic at this temperature. In this work the low-temperature ordered B2, or alpha', phase, as well as the disordered bcc phase of FeCo alloys, have been studied with first-principles electronic-structure calculations using the layer Korringa-Kohn-Rostoker method. The variation of moment with composition (Slater-Pauling curve) is discussed. For equiatomic FeCo, interatomic exchange couplings are derived from first principles. These exchange interactions are compared to those obtained for pure Fe and Co, and are used within a mean-field theory to estimate the hypothetical Curie temperature of the alpha phase. (C) 1999 American Institute of Physics. [S0021-8979(99)76808-8].