Nanoscale Phase Separation in Fe,O,(lll) Films on Sapphire(OOO1) and Phase Stability of Fe,O,(OOl) Films on MgO(001) Grown by Oxygen-Plasma-Assisted Molecular Beam Expitaxy

Nanoscale phase separation in Fe304(l 11) films on sapphire(0001) and phase stability of Fe304(001) films on MgO(001) grown by oxygen-plasma-assisted molecular beam epitaxy We report a phase instability in oxygen-plasma-assisted molecular beam epitaxy of Fe304 films on sapphire (0001) substrates. Under a wide range of growth conditions, Fe304 (111) films phase separate, on a nanometer length scale, into Fe304, Fe0 and metallic Fe, which is attributed to formation of the thermodynamically unstable phase Fe0 in the initial stages of (111) growth. In contrast, Fe304 (001) films, grown simultaneously on MgO(001) substrates, do not exhibit this phase instability. We specify growth conditions for which single-phase, epitaxial Fe304 (111) films can be grown by plasma-assisted molecular beam epitaxy or by reactive evaporation of Fe in molecular oxygen. Film orientation and phase separation strongly influence magnetic properties. Single-phase Fe304 (111) films are much more difficult to magnetize than Fe304 (001) films and phase separation makes the films even more difficult to magnetize.