The molecular beam epitaxy growth of Fe on MnAs/GaAs(001) leads to the formation of a new phase of the FeMnAs compound at the Fe/MnAs interface. We investigated the structural and magnetic properties of this interfacial layer by high angle annular dark field imaging in a scanning transmission electron microscope (HAADF-STEM). We determined that the epitaxial FeMnAs layer presents an unusual orthorhombic structure, with vacancy ordering. We completed our study by ab initio calculations, and we foresee an antiferromagnetic ground state for this structure.