Molecular beam epitaxy of ferromagnetic γ′-Fe₄N thin films on LaAlO₃(1 0 0), SrTiO₃(1 0 0) and MgO(1 0 0) substrates

We have attempted to grow a-axis-oriented γ′-Fe₄N epitaxial films with smooth surfaces at around 400 °C on LaAlO ₃(LAO)(1 0 0), SrTiO₃(STO)(1 0 0) and MgO(1 0 0) substrates by molecular beam epitaxy using solid Fe and a radio-frequency NH₃ plasma as the Fe and N sources, respectively. The lattice mismatch of these substrates to γ′-Fe₄N is 0%, 3% and 11%, respectively. Epitaxial growth of γ′-Fe₄N films was successfully achieved on the STO(1 0 0) and LAO(1 0 0) substrates. It was found from reflection high-energy electron diffraction and X-ray diffraction that the a-axis orientation of γ′-Fe₄N was degraded with increasing lattice mismatch. High-angle annular dark-field scanning transmission electron microscopy and electron energy-loss spectroscopy measurements showed that amorphous AlO layers were present at the γ′-Fe₄N/LAO interface.