Surface-induced disordering at first-order transitions in body-centered cubic binary alloys: A Monte-Carlo simulation

Surface effects on the phase transition from theDO ₃ phase to the disordered phase are studied for a bcc Ising antiferromagnet with nearest and next-nearest neighbor exchange interactions in a magnetic field. This model can also be considered to represent binary alloys such as the FeAl-system; missing interactions near the surface translate then into surface magnetic fields. The change of the local magnetization near the surface then corresponds to surface enrichment of one component. For a plausible choice of parameters surface-induced disordering is found and the associated critical behavior is studied. Varying the bulk fieldH near the transition fieldH _c , we find that the thickness of the disordered layer increases proportional to ln(H-H _c ) while the local order parameter at the surface vanishes as (H-H _c )^1/2. These results are consistent with mean field predictions for the single order parameter case, although several ordering fields occur (sublattice magnetizations and magnetization).