Surface-induced ordering and disordering in face-centered-cubic alloys: A Monte Carlo study.

Abstract

Using extensive Monte Carlo simulations we have studied phase transitions in a fcc model with antiferromagnetic nearest-neighbor couplings $J$ in the presence of different free surfaces which lead either to surface-induced order or to surface-induced disorder. Our model is a prototype for CuAu-type ordering alloys and shows a strong first-order bulk transition at a temperature $\frac{k{T}_{\mathrm{cb}}}{|J|}=1.738005(50)$. For free (100) surfaces, we find a continuous surface transition at a temperature ${T}_{\mathrm{cs}}>{T}_{\mathrm{cb}}$ exhibiting critical exponents of the two-dimensional Ising model. Surface-induced ordering occurs as the temperature approaches ${T}_{\mathrm{cb}}$ and the surface excess order and surface excess energy diverges logarithmically. For a free (111) surface, the surface order vanishes continuously at ${T}_{\mathrm{cb}}$ accompanied by surface-induced disorder (SID). In addition to a logarithmic divergence of the excess quantities of order and energy, we find further critical exponents which confirm the actual theory of SID and critical wetting and which can be understood in terms of rough interfaces. For both cases of free surfaces, the asymptotic behavior of the squared interfacial width shows the expected logarithmic divergence.