ALPHA''-FE16N2 PHASE EPITAXIALLY GROWN BY SPUTTER BEAM METHOD

Epitaxial Fe-N films have been grown on Fe or Ag(001) single-crystal u nderlayers by the sputter beam method. The Fe-N films pn both underlay ers have abet structure (alpha'-martensite), whose lattice is elongate d along, the [001] direction by nitrogen atoms located in octahedral i nterstices of alpha-Fe. These films possess well-defined crystal orien tation and exhibit a higher saturation magnetization (4 pi M(s)) than that of alpha-Fe for a certain range of various nitrogen content. Post annealing promotes ordering of nitrogen atoms in the bct lattice, resu lting in precipitation of a metastable alpha ''-Fe16N2 phase in Fe-N f ilms. The 4 pi M(s) of the annealed samples tends to increase with the increase of the alpha '' volume fraction, and the Fe-N film with 4 pi M(s) of about 25 kG contains 26-33 vol% alpha '' phase. While both Fe and Ag underlayers can promote epitaxial growth of the Fe-N films, ve ry fast ordering of nitrogen atoms and easily decomposed alpha ''-Fe16 N2 have been observed in the sample on a Ag underlayer. This fact lead s to the fact that the ordering rate and the stability of alpha ''-Fe1 6N2 are very sensitive to its crystal quality. (C) 1996 American Insti tute of Physics.