CRITICAL PHENOMENA AT THE FE3AL(1(1)OVER-BAR-O) SURFACE - A GLANCING ANGLE X-RAY-SCATTERING STUDY

We describe the details of a glancing angle X-ray scattering study of the critical behaviour of the order parameter and the order parameter fluctuations at the Fe3Al(110BAR) surface when the alloy approaches the bulk critical temperature. The temperature dependence of the X-ray scattering at and around the (111) superlattice reflection has been measured in a surface sensitive scattering geometry and analyzed in view of the predictions of modern renormalization theories which account for the presence of a surface. It is found that the equilibrium surface displays two different regimes: the top three layers are strongly affected by surface segregation phenomena and display a complicated thermodynamic behaviour, the observed critical phenomena in the subsequent layers are governed by the new surface exponents beta1 = 0.75 +/- 0.06, gamma11 = - 0.33 +/- 0.06, eta(parallel-to) = 1.52 +/- 0.04 and nu = 0.60 +/- 0.15. It is demonstrated by this study that on the one hand the presence of the surface alters the character of the order-disorder transition near the surface in a significant manner, and that on the other hand the observed surface critical scattering and, consequently, the surface free energy and the surface-dominated pair correlation function still show scaling behaviour. In particular the scaling laws 2beta1 = nu(1 + eta(parallel-to)) and gamma11 = nu(1 - eta(parallel-to)) are confirmed by the experiment.