The evolution of the in-plane magnetic anisotropies in Fe3O4/GaAs(100) and Fe3O4/MgO/GaAs(100) hybrid spintronic structures has been studied by magneto-optical Kerr effect and ferromagnetic resonance (FMR). The surface and volume contributions to the in-plane cubic and uniaxial anisotropies have been distinguished in Fe3O4/GaAs by fitting the anisotropy constants, measured by FMR, as a function of the magnetic film thickness. It was found that interfacial chemical bonding rather than strain relaxation plays the dominant role in causing the unexpected uniaxial magnetic anisotropy (UMA) in Fe3O4 films grown directly on the GaAs surfaces. In contrast, after MgO barrier insertion, FMR results show that the UMA is greatly reduced, and strain relaxation is found to be the main origin of the UMA in Fe3O4/MgO/GaAs structures.