Nanoscale phase separation in Fe[sub 3]O[sub 4](111) films on sapphire(0001) and phase stability of Fe[sub 3]O[sub 4](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 Fe[sub 3]O[sub 4] films on sapphire (0001) substrates. Under a wide range of growth conditions, Fe[sub 3]O[sub 4](111) films phase separate, on a nanometer length scale, into Fe[sub 3]O[sub 4], FeO and metallic Fe, which is attributed to formation of the thermodynamically unstable phase FeO in the initial stages of (111) growth. In contrast, Fe[sub 3]O[sub 4](001) films, grown simultaneously on MgO(001) substrates, do not exhibit this phase instability. We specify growth conditions for which single-phase, epitaxial Fe[sub 3]O[sub 4](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 Fe[sub 3]O[sub 4](111) films are much more difficult to magnetize than Fe[sub 3]O[sub 4](001) films and phase separation makes the films even more difficult to magnetize. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]