Growth mode of ultrathin Co films on Fe(001) prepared by low energy ion-assisted deposition

Co films were epitaxially grown on Fe(001) by simultaneous thermal evaporation of Co atoms and ion bombardment with low-energy (300-1000 eV) Ar ions in a wide range of ion-to-atom flux ratio (0.02-0.5). The 0-50 ML coverage range was investigated, Transition from island growth to a continuous layer growth occurs on passing from purely thermal to ion-assisted deposition procedure. Structural characterization was performed by Primary-beam Diffraction Modulated Electron Emission (PDMEE). For purely thermal deposition, transition from the bcc phase to the equilibrium, hcp phase has been observed at a critical coverage of about 15 ML, both structures showing a significant strain (7% contraction and 5% expansion with respect to the ideal bcc and hcp phase, respectively). Ion assistance has proved to be effective in lowering both the strains in the Co film and the critical thickness of the bcc phase.