Epitaxial 80-nm-thick films of Fe were grown at room temperature by molecular beam epitaxy on GaAs(001) and on a reacted layer composed of a mixture of (Fe2As+Fe3Ga1.8As0.2) resulting from the solid-state interdiffusions in a (20 nm Fe)/GaAs(001) structure annealed at 450 °C. The partially relaxed strains induced by the lattice mismatch were studied using ion channeling, Rutherford backscattering, and x-ray diffraction. Both channeled angular scans and x-ray diagrams showed that the 80 nm Fe films are not fully relaxed. The average crystalline quality of the Fe films is better on GaAs(001) (χmin=3%) than on the reacted layer (χmin=7%) mainly because of the mosaic structure of the reacted layer. The strains parallel to the surface in the Fe films were found to be compressive in the Fe/GaAs(001) heterostructure and tensile in the Fe/(Fe2As+Fe3Ga1.8As0.2)/GaAs(001) one. Therefore, our work shows that the presence of a reacted layer at the interface in a Fe/GaAs structure does not impede the epitaxial growth of Fe but can invert the kind of strain appearing in the Fe film. © 1999 American Institute of Physics.