Analysis of the short-range order of the Au/Fe(001) surface alloy

Scanning tunneling microscopy measurements show that the growth of about half a monolayer Au on Fe(001) at a temperature of $170\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ leads after a postanneal at $700\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ to an island free and homogeneously alloyed surface layer. The surface alloying is in contrast to the bulk where Au and Fe are immiscible. Strong chemical contrast in the images allows us to analyze the short-range order of this surface alloy. Two independent methods are considered: the first method extracts pair occupation probabilities directly from digitized images, the second method makes use of the autocorrelation of filtered images. Both methods lead to the same conclusion: only for the nearest-neighbor sites a strong preference for unlike species is found; for further sites the probabilities follow a random distribution. Scanning tunneling spectroscopy measurements on the alloyed AuFe surface are used to corroborate the interpretation of the chemical contrast. Furthermore, the spectroscopy results show that the Fe(001) surface state shifts to higher energies on areas where pure Fe patches are confined by Au atoms.