The structural, electrical, and magnetic properties of Fe3Si/GaAs(001) hybrid structures with high crystalline and interfacial perfection are studied. The Fe3Si/GaAs(001) hybrid structures are fabricated by molecular beam epitaxy at 200 degreesC. The composition of the films, which can be regarded as a Heusler alloy, is tuned over a wide range of Si content. The high crystalline and interfacial perfection is correlated with the stable Fe3Si phase. The resistivity of the films shows a strong minimum at almost exact stoichiometry which can be explained by the perfection of the ordering of the Si atoms within the Fe3Si phase. The layers are ferromagnetic at room temperature with saturation magnetization values close to bulk Fe3Si. The layers show very small coercive fields which again is correlated with high crystalline and interfacial perfection of the layers within the Fe3Si phase. (C) 2004 American Vacuum Society.