  Ultra-thin FeAs is of interest both as the active component in the newly
identified pnictide superconductors, and in spintronic applications at the
interface between ferromagnetic Fe and semiconducting GaAs. Here we use
first-principles density functional theory to investigate the properties of
FeAs/GaAs heterostructures. We find that the Fermi surface is modified from
that characteristic of the pnictide superconductors by interactions between the
FeAs layer and the As atoms in the GaAs layers. Regardless of the number of
FeAs layers, the Fe to As ratio, or the strain state, the lowest energy
magnetic ordering is always antiferromagnetic, suggesting that such
heterostructures are not promising spintronic systems, and offering an
explanation for the failure of spin injection across Fe/GaAs interfaces.
