Submonolayers of Adsorbates on Stepped Co/Cu(100):} Switching of the Easy Axis

We report a striking sensitivity of the easy magnetization axis in epitaxial Co films upon adsorption of submonolayers of Cu, Ag, Fe, and O if the Cu substrate is a stepped surface slightly miscut from the (100) orientation. All adsorbates cause a switch of the easy axis by $90^{\circ}$ within the plane. The key role of this switching phenomenon is played by the atoms at the step sites and their influence on the magnetic surface anisotropy.