Grazing-incidence x-ray scattering on theCu3Au(111) phase transition

We report the results of a grazing-incidence x-ray study on the order-disorder phase transition of a (111)-oriented ${\mathrm{Cu}}_3$Au film (0.7 μm) epitaxially grown on an ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ substrate with a Nb buffer layer. By varying the incident beam angles to the surface, penetration depths between 22 and 650 Å were probed. While the bulk disorders at $T_c^b$=665 K through a first-order transition, the surface region appears to exhibit an enhanced ordering with a slightly higher transition temperature. Close to $T_c^b$ pronounced short-range-order scattering is seen in the deeper layers, whereas this effect is much reduced closer to the surface.