Ab initio and cluster expansion study of surface alloys of Fe and Au on Ru(0001) and Mo(110): Importance of magnetism

We have performed ab initio density functional theory calculations to study freestanding alloy monolayers of Fe and Au in centered rectangular and hexagonal geometries, as well as Fe-Au surface alloys on Ru(0001) and Mo(110) substrates. Though Fe and Au are bulk immiscible, in all four classes of systems we obtain negative formation energies. While the properties of the two classes of freestanding monolayers are roughly similar, with small differences due to the anisotropy and longer bond lengths of the centered rectangular case, the surface alloys on the two substrates behave quite differently. The formation energies on Mo(110) are markedly smaller; we trace this to the fact that magnetism contributes significantly to mixing on Ru(0001) but not on Mo(110). On Ru(0001), there is a very stable (root 3 x root 3) FeAu2 phase, in agreement with experiments. By performing cluster expansion calculations, we show that ordering is not favored on Mo(110), again in accordance with experimental data.