Anomalous Magnetization Behavior of Fe-N Films Deposited by Reactive Pulsed Laser Deposition

A series of Fe-N films with thickness of 200 nm have been prepared by reactive pulsed laser deposition (PLD) at 25°C and at 150°C in various nitrogen pressures. Both series of samples consist of mixtures of γ^´-Fe ₄N and α-Fe phase. The amount of α-Fe decreases with the increase of nitrogen pressure. Higher remnant ratio and lower coercivity are observed for the films deposited at 150°C compared to the films deposited at 25°C. An anomalously large magnetization is observed for both series of films deposited at a nitrogen pressure of 8 mTorr. The average magnetization is as high as or even larger than that of α-Fe. Conversion electron Mössbauer spectroscopy demonstrated that the highest-magnetization film consists mixtures of γ^´-Fe₄N and α-Fe phase at a mole ratio of 1:6. The origin of this intriguing high moment is ascribed to a certain lattice constant of the α-Fe which is in a heterogeneous state with γ^´-Fe₄N for films deposited at a certain nitrogen pressure.