We have combined x-ray diffraction studies with first-principles calculations to study the interplay between segregation and ordering at the (110) surface of ${\mathrm{Ni}}_{90}{\mathrm{Al}}_{10}$. We find a ${L1\mathrm{}}_2$-ordered monolayer at the surface. The observed ordering as well as recently reported Al segregation at the surface are explained in a consistent picture. A delicate competition between the tendency for Al segregation and ordering in the Ni-Al system induced by the symmetry break at the surface stabilizes a long-range ordered surface in the entire concentration range $c_{\mathrm{Ni}}>0.75\mathrm{}$.

• Received 23 July 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.236102