We demonstrate the growth of crystalline Co2FeSi/MgO/GaAs(001) hybrid structures of different MgO interlayer thicknessess by molecular beam epitaxy. The MgO layer thickness was verified by X-ray reflectivity measurements and high-resolution transmission electron microscopy images. Secondary ion mass spectroscopy confirmed that the MgO layer works successfully as a diffusion barrier. Co and Fe defect densities in the GaAs layer could be reduced by three orders of magnitude using a 1.3 nm thick MgO film. High-resolution transmission electron microscopy images together with X-ray diffraction phi scans also revealed an in-plane rotation of the Co2FeSi layer by 45 degrees for samples with MgO interlayers with respect to the GaAs substrate. Investigations of the magnetic properties revealed that the MgO interlayers do not affect the value of the magnetic moment of the Co2FeSi layer. Further, an in-plane rotation of the easy axis of the Co2FeSi layer by 90 degrees for all samples with MgO interlayers was observed and will be discussed.