Structure and growth mode of thin Co films on Fe(001): comparison of purely thermal and ion-assisted deposition

The growth of epitaxial Co layers on Fe(001) by simultaneous thermal evapor ation of Co atoms and ion bombardment with low-energy (300-1000 eV) Ar ions has been investigated for a wide range of ion/atom flux ratios (up to 0.5) and compared with growth by a purely thermal deposition procedure. Co nomi nal thickness was in the 2-50 monolayer (ML) range. At low Co nominal thick ness (2-10 monolayers), the thermally grown films tend to form islands, whi le the ion beam-assisted growth results in the formation of a continuous Co layer. Structural characterisation of the Co layer was performed by exploi ting the dependence of the Auger emission intensity on the angle of inciden ce of the primary exciting electron beam (primary-beam diffraction modulate d electron emission, PDMEE). For purely thermal deposition, transition from the cubic phase to the equilibrium h.c.p. phase occurs at a critical nomin al thickness of 15 ML, both structures showing a significant strain (4% con traction and 6% expansion compared to the ideal b.c.c. and h.c.p. phases, r espectively). Ion assistance has proved to be effective in lowering both th e critical thickness of the cubic phase and strain in the hexagonal Co film . The relevant parameter which comes into play in determining the effective ness of ion assistance has been found to be the average energy deposited pe r atom. (C) 2001 Elsevier Science B.V. All rights reserved.