Layer relaxation and intermixing in Fe/Cu(001) studied by surface x-ray diffraction

The structure of Fe films thermally deposited on Cu(001) was analyzed using surface x-ray diffraction in the coverage range between 6 and 8 monolayers. Based on the analysis of crystal truncation rod data measured at 120 and 300 K, i.e., below and above transition temperatures reported for ferro- and antiferromagnetic ordering, no changes of the interlayer spacings larger than about {+-}0.015 A are found. Within the Fe film these correspond to fcc Fe (1.78 A), while the top-layer spacing is expanded by 3-5 % in agreement with previous low-energy electron diffraction studies. Lateral disorder of surface atoms as described by the Debye parameter indicates displacements of the top-layer positions up to 0.23 A corresponding to zigzag displacements observed in the p2mg (2x1) superstructure. The inherent large penetration depth of the x rays also allowed the study of the structure and composition of the buried Fe/Cu interface. The data indicate Fe-Cu intermixing, where nearly 50% of a Fe (Cu) monolayer are exchanged. Four layers across the interface are significantly affected. About 30% of the first Fe (Cu) and up to 15% of the second Fe (Cu) layer is alloyed by Cu (Fe)