The order-disorder phase transitions occuring in binary-alloy thin films ${\mathit{A}}_{\mathit{x}}$${\mathit{B}}_{1\mathrm{-}\mathit{x}}$ with bcc structure are studied. The system is modeled by an Ising Hamiltonian that includes only nearest-neighbor chemical interactions and it is solved in the mean-field approximation. The interplay of surface segregation and chemical order as a function of temperature, film thickness, and surface interactions are studied in detail for films with (110) surface orientation. We find that the order-disorder transition temperature ${\mathit{T}}_{\mathit{c}}$ for a thin film with average concentration x exceeds the bulk ${\mathit{T}}_{\mathit{b}}$ of an alloy with the same concentration in two cases. One is obtained when there is a strong surface segregation of one of the components, and the second is observed when the surface interaction is larger than a critical value. Results for the concentration and chemical order profiles as a function of temperature and film thickness are presented.

• Received 8 December 1994

DOI:https://doi.org/10.1103/PhysRevB.52.5392