Formation of an interfacial alloy and epitaxial bcc Cr layers on Au(100)

We have used angle-resolved photoemission, low-energy electron diffraction, and work-function measurements to probe the crystallographic and electronic structure of the Cr/Au(100) interface formed at temperatures in the 300--500-K range. The first Cr monolayers (ML) diffuse inward and occupy Au lattice sites, forming a random substitutional Au_100-x{Cr}ₓ$ fcc alloy with a Cr concentration increasing gradually up to {~}50% with increasing Cr coverage. The alloy formation process is diffusion limited and the total amount of Cr involved in the reaction varies from 2--3 ML at 300 K to 4 ML or more at temperatures above 400 K. This metastable epitaxial interfacial alloy exhibits a high structural quality and a large local magnetic moment ({~}4μB) on Cr. Upon further Cr deposition at temperatures below 400 K a disordered Cr-rich phase is first formed in the 4--6-ML range but at 7--8 ML or higher the interfacial alloy behaves as an excellent template for epitaxial growth of well-ordered bcc Cr films. In particular, in the 10--60-ML range the valence-band photoemission spectra are identical to those measured on a single-crystal Cr (100) surface as far as bulk transitions are concerned. Distinct differences visible in the surface electronic structure can be traced back to Au segregation at the film surface as well as a substantial surface roughness. In the 7--10-ML range, important deviations from bulk Cr electronic structure become apparent and the results show a narrowing of the d bands and an evolution towards a two-dimensional band structure.