In this article we describe a study of the magnetic semiconductors of Fe/GaAs, undertaken theoretically and experimentally. We discuss the structural advantage of body-centered-cubic-Fe structure lattice matching to GaAs (001). Theoretical calculations using the self-consistent linear augmented-plane-wave method indicate the existence of an energy state of the quantum well in Fe layers in a GaAs/Fe/GaAs double heterostructure. We then present the preparation of Fe/GaAs heterostructures by using molecular beam epitaxy. alpha-Fe and delta-Fe could be grown epitaxially on (001) GaAs at a substrate temperature of 290 and 580 degrees C, respectively, which was confirmed from the results of reflection high energy electron diffraction and x-ray diffraction measurements. We also found that, in order to obtain alpha-Fe/GaAs, a low-temperature GaAs growth must be induced before the Fe growth can occur. Differences in magnetic properties were observed in the magnetic measurements, indicating that alpha-Fe dominates the ferromagnetic state, while the delta-Fe shows relatively slight ferromagnetic behavior.