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 gamma;′-Fe ₄ N 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 -Fe. Conversion electron Mössbauer spectroscopy demonstrated that the highest-magnetization film consists mixtures of gamma;′-Fe ₄ N 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 -Fe which is in a heterogeneous state with gamma;′-Fe ₄ α-Fe N for films deposited at a certain nitrogen pressure.