Order-disorder phase transitions in the surface layers of fcc (001) AB alloy thin films

The order-disorder phase transitions of the fcc AB alloy thin films in (001) direction are studied numerically by the cluster variation method. The computation results show that the order-disorder phase transitions of thin films are determined by the parity of the layer number of films. The thin films with different parity will show different phase structures and thermodynamic properties. When the surface field is weak, for films with even number of layers, in the chemical potential region corresponding to the bulk concentration x = 0.5, there appears the middle-temperature phase as well as the wetting phenomenon due to the missing of symmetry of the films. While for films with odd number of layers, the order-disorder phase transitions are similar with that of the bulk materials. When the surface field is strong, for films with odd number of layers, in the chemical potential region corresponding to the bulk concentration x = 0. 5, there is an AB(AB)(n) A phase which does not show order-disorder phase transition with the increase of temperature due to the combined effect of the surface field and finite size.