SURFACE SEGREGATION-INDUCED CRITICAL PHENOMENA AT FECO(001) SURFACES

We present an x-ray scattering study of the continuous B2-A2 order-dis order transition in semi-infinite FeCo using synchrotron radiation, wh ich gives access to the weak one-electron contrast associated with the B2 ordering phenomena in this alloy. The asymptotic (001) Bragg scatt ering profiles disclose an oscillating Fe-Co layering profile, which a lters the surface critical phenomena in a characteristic way: The surf ace-related order parameter persists above the bulk critical temperatu re and exhibits a temperature dependence which provides unambiguous ev idence for the existence of a surface field (h(l)) which couples to th e surface susceptibility.