α″-Fe₁₆N₂ phase epitaxially grown by sputter beam method

Epitaxial Fe-N films have been grown on Fe or Ag (001) single-crystal underlayers by the sputter beam method. The Fe-N films on both underlayers have a bct structure (α′-martensite), whose lattice is elongated along the [001] direction by nitrogen atoms located in octahedral interstices of α-Fe. These films possess well-defined crystal orientation and exhibit a higher saturation magnetization (4πM_s) than that of α-Fe for a certain range of various nitrogen content. Postannealing promotes ordering of nitrogen atoms in the bct lattice, resulting in precipitation of a metastable α″-Fe₁₆N₂ phase in Fe-N films. The 4πM_s of the annealed samples tends to increase with the increase of the α″ volume fraction, and the Fe-N film with 4πM_s of about 25 kG contains 26-33 vol% α″ phase. While both Fe and Ag underlayers can promote epitaxial growth of the Fe-N films, very fast ordering of nitrogen atoms and easily decomposed α″-Fe₁₆N₂ have been observed in the sample on a Ag underlayer. This fact leads to the fact that the ordering rate and the stability of α″-Fe₁₆N₂ are very sensitive to its crystal quality.