We have established an optimized growth temperature range, namely, 150 degreesC<T-G<250 degreesC, where ferromagnetic Fe3Si/GaAs(001) hybrid structures with high crystalline and interfacial quality can be fabricated by molecular-beam epitaxy. The composition of the Fe3Si layers, which can be regarded as a Heusler alloy, was tuned within the stable Fe3Si phase. The layers show high magnetic moments with a value of 1050 emu/cm(3), which is close to that of bulk Fe3Si. (C) 2003 American Institute of Physics.