STRUCTURAL PHASE-TRANSFORMATION UNDER REVERSED STRAIN - A COMPARATIVE-STUDY OF IRON ULTRATHIN-FILM GROWTH ON NICKEL AND COPPER(100)

The lattice rearrangement processes of the fcc-->bcc phase transformat ion in ultrathin iron films depend on the sign of the applied epitaxia l strain. Pseudomorphic iron films grown on nickel and copper (100) su bstrates are under about 1% strain, which is compressive in the case o f Ni and tensile in the case of Cu. The phase transformation in the ir on:copper films starts by forming misfit dislocation-like ridge struct ures oriented along [011] directions at about 4.6 ML, which soon devel op into bcc phase precipitations with indications of a Pitsch orientat ion. No such dislocation-like structures have been observed for all gr owth stages of iron on nickel, where the fcc-->bcc transformation star ts at 5.5 ML by forming bcc island chains along [001] directions with a significant misfit accommodating effect. Above 10 ML, the step heigh t of the iron films on nickel increases by 11% to about 2.1 Angstrom a s a consequence of the fcc-->bcc transformation. The drastic differenc es between the observed transformation phenomena in the two systems ar e discussed in terms of the specific applied misfit strains and seem t o be consistent with former observations of thicker bicrystal systems as well as with continuum theoretical approaches. (C) 1998 Elsevier Sc ience B.V. All rights reserved.