This paper reports on the preparation and characterization of Co-GaAs contact likely to be used for a magnetoelectronics device: the spin field effect transistor. In particular, we investigate the efficiency of an intermediate oxide layer to improve the magnetic properties of ultrathin Co layers grown on GaAs. We show that the oxide layer improves the structural quality and increases the related anisotropy coefficient of Co indeed. Although the easy magnetization axis is not perpendicular to the plane of the layers, it is possible to perpendicularly magnetize the Co layers by applying a high enough magnetic field.