Nanocrystallization and amorphization induced by reactive nitrogen sputtering in iron and permalloy

Thin films of iron and permalloy (Ni₈₀Fe₂₀) were prepared using an Ar+N₂ mixture with a magnetron sputtering technique at ambient temperature. The nitrogen partial pressure during the sputtering process was varied in the range of 0R_N₂100%, keeping the total gas flow at constant. At lower nitrogen pressures (R_N₂33%), both Fe and NiFe first form a nanocrystalline structure, and an increase in R_N₂ results in the formation of an amorphous structure. At intermediate nitrogen partial pressures, nitrides of Fe and NiFe were obtained, while at even higher nitrogen partial pressures, nitrides themselves became nanocrystalline or amorphous. The surface, structural, and magnetic properties of the deposited films were studied using x-ray reflection and diffraction, transmission electron microscopy, polarized neutron reflectivity, and using a dc extraction magnetometer. The growth behavior for amorphous film was found to be different as compared with poly or nanocrystalline films. The soft-magnetic properties of FeN were improved on nanocrystallization, while those of NiFeN were degraded. A mechanism inducing nanocrystallization and amorphization in Fe and NiFe due to reactive nitrogen sputtering is discussed in the present article.