Order-disorder phenomena in binary alloy thin films

We consider the order-disorder phase transition in thin films of the binary, AB₃ type, alloys. The theory is based on the model consisting of pairwise interactions between nearest neighbour atoms belonging to the sublattices in the form of the monoatomic layers parallel to the surface. This model, the so-called Valenta's model is well-known in literature. It is essential to take it into account for the description of thin films while the model of continuum can be successfully used for semi-infinite samples. The numerical results only slightly differ for both the models in the case when the film thickness tends to infinity. In this context, the present paper brings the new, original results for the influence of the surface on the properties of long-range ordering in alloy thin films. The long-range order parameter and the concentration distribution, mutually dependent, depend on the film thickness and the boundary conditions at the surfaces. First of all, the interesting phenomenon, surface disordering, is discussed in analogy to the surface melting. Moreover, an interpretation of the disordering kinetics type is proposed in connection with the behaviour of disorder which appears as layer by layer process starting from the surface planes.