Crystal distortion and the magnetic moment of epitaxially grown α″-Fe₁₆N₂

The magnetic moment of α″-Fe₁₆N₂ has been explored for epitaxial Fe-N thin films grown on Fe and Ag(0 0 1) single-crystal underlayers. Clear evidences for multiple states which have strong correlation with the crystal structure of α″-Fe₁₆N₂ are obtained by the measurements of ferromagnetic resonance, the phase transition temperature, and the lattice constants. The magnetic moment of the films was found to have a strong correlation with the unit cell volume of α″-Fe₁₆N₂. The (α″+α′)-Fe₁₆N₂ with a large unit cell volume around 205.8 angstroms³, which is equal to the value reported by Jack, exhibits very high average magnetic moment of about 2.8 μ_B/Fe atom, and the moment increases with the degree of N site ordering. On the other hand, (α″+α′)-Fe₁₆N₂ with a smaller unit cell volume of around 204 angstroms³ obviously shows lower average magnetic moment of 2.5 μ_B/Fe atom, and the moment is independent. It is highly possible that the large discrepancy of magnetic moment of α″-Fe₁₆N₂ reported so far is due to the difference of unit cell volume of α″-Fe₁₆N₂, which sensitively depends on sample preparation conditions, film thickness and so on. The present study has also clarified the phase transformation process of α″-Fe₁₆N₂ to γ′-Fe₄N, and this process is well explained by the Usikov's model.