Magnetic and Mössbauer studies of single-crystal Fe₁₆N ₂ and Fe-N martensite films epitaxially grown by molecular beam epitaxy (invited)

Single-phase, single-crystal Fe₁₆N₂(001) films and Fe-11 at. %N martensite films of 200-900 Å thickness have been epitaxially grown on In_0.2Ga_0.8As(001) substrates by evaporating Fe in an atmosphere of mixed gas of N₂ and NH₃, followed by annealing. The saturation magnetizations 4πM_s's for Fe ₁₆N₂ and Fe-N martensite films have been measured to be around 29 and 24 kG at room temperature, respectively, and almost constant in the above thickness range by using a vibrating sample magnetometer. 4πM _s for Fe-N martensite films has been increased with ordering of N atoms caused by annealing and finally reached around 29 kG for Fe ₁₆N₂. Mössbauer spectra have been measured for those films. The spectrum for Fe-N martensite films was a superposed one with hyperfine fields of 360, 310, and 250 kOe, similar to those previously reported for martensite. While the spectrum became simpler with ordering, finally reaching a single hyperfine field of 330 kOe for Fe₁₆N₂. 4πM_s of 29 kG for Fe₁₆N₂ (3.2 μ_B/Fe atom) and 4πM_s of 24 kG for martensite (2.6 μ_B/Fe atom) has not been explained based on the conventional band theory of 3d metal magnetism. Behaviors of Mössbauer spectra could not be understood based on the conventional concept either. Thus a new physical concept is likely to be needed for clarification of giant magnetic moments and Mössbauer spectra for Fe₁₆N₂ and Fe-N martensites.