Phase transitions in bcc(110) binary-alloy thin films

The order-disorder phase transitions occuring in binary-alloy thin films AxB1-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 Tc for a thin film with average concentration x exceeds the bulk Tb 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. © 1995 The American Physical Society.

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