The structure of the c(2x2) Mn/Ni(001) surface alloy by photoelectron diffraction

Surface alloys are two-dimensional metallic systems that can have structures that are unique to the surface, and have no counterpart in the bulk binary phase diagram. A very unusual structure was reported for the Mn-Ni system, based on a quantitative LEED structure determination, which showed that the Mn atoms were displaced out of the surface by a substantial amount. This displacement was attributed to a large magnetic moment on the Mn atoms. The structure of the Mn-Ni surface alloy was proposed to be based on a bulk termination model. Magnetic measurements on Mn-Ni, however, showed conclusively that the magnetic structure of these surface alloys is distinct from the bulk alloy analogs. For example, bulk MnNi is an antiferromagnet, whereas the surface alloy is ferromagnetic. This suggests that the proposed structure, based on bulk termination, is not complete. Photoelectron diffraction techniques were used to investigate this structure, using both a comparison to multiple scattering calculations and photoelectron holography.