Bragg–Williams Model of CsCl-Type Ordering of the FeCo System in Strong Magnetic Fields

A modified Bragg–Williams (B–W) model of a and a¢ FeCo is extended to estimate the effect of strong magnetic fields on the critical ordering temperature (TORDER) taking into account longrange chemical and magnetic ordering. The model discussed here is generalized from our previous work in which only the larger average exchange per atom in the chemically ordered state was taken into account. A positive shift of critical temperatures for the higher order a fi a¢ order-disorder phase transformation has been predicted in the presence of a strong field. In this work, the experimentally observed dependence of the average magnetic moment of Fe atoms on the degree of chemical order has been accounted for explicitly. The estimated shift in the critical ordering temperature (DTORDER) is larger when the dependence of the Fe moment on the degree of chemical order is taken into account, particularly in the case of Fe-rich compositions (e.g., DTORDER ~ 13 K vs DTORDER ~ 10 K for H ~ 50 T at equiatomic composition). For most compositions, however, the contribution to DTORDER associated with the larger average exchange per atom in the chemically ordered state accounts for the majority of the shift. The estimated effect remains quite small and is only expected to be experimentally observable in static fields larger than currently available in most laboratories (DTORDER is only predicted to be larger than ~2 to 3 K for H>~10 T).