The growth of epitaxial iron nitrides by gas flow assisted MBE

There is a strong interest in obtaining epitaxial layers of the α′′ and γ′ nitride phases of iron, because of their special ferromagnetic properties. The fact that these nitrides have a small fcc or bcc unit cell allows epitaxial growth without domain formation in contrast to the case of, for instance, Fe3O4. We tried three different routes to grow epitaxial iron nitride layers on MgO(0 0 1). The substrate temperature was varied between 150°C and 300°C in all cases. We found that it is impossible to grow nitride layers using a gas flow of mixtures of NH3, N2 and H2 as a nitrogen source, as was reported by a group of Hitachi. Only if this flow is led over a hot (1000–1200°C) iron surface, the formation of nitrides is observed. The nitrides were successfully grown using a home-built radio frequency atomic nitrogen source. The phases and structures of the layers were investigated with RBS, Mössbauer spectroscopy, X-ray diffraction and elastic recoil detection. It was observed that the presence of H2 and the substrate temperature during growth are crucial to obtain epitaxial layers of γ′ iron nitride. A mono-crystalline layer of γ′ was grown directly on a MgO substrate at a temperature of 200°C. A highly disordered (amorphous) ε-like layer was grown at room temperature of a MgO substrate.