PHASE-TRANSITIONS IN BCC(110) BINARY-ALLOY THIN-FILMS

The order-disorder phase transitions occuring in binary-alloy thin fil ms A(x)B(1-z) with bcc structure are studied. The system is modeled by an Ising Hamiltonian that includes only nearest-neighbor chemical int eractions and it is solved in the mean-held approximation. The interpl ay of surface segregation and chemical order as a function of temperat ure, dim thickness, and surface interactions are studied in detail for films with (110) surface orientation. We find that the order-disorder transition temperature T-c for a thin film with average concentration a: exceeds the bulk T-b of an alloy with the same concentration in tw o cases. One is obtained when there is a strong surface segregation of one of the components, and the second is observed when the surface in teraction is larger than a critical value. Results for the concentrati on and chemical order profiles as a function of temperature and film t hickness are presented.