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

Thin films of iron and permalloy $\left({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}\right)$ were prepared using an $\mathrm{Ar}+{\mathrm{N}}_2$ mixture with a magnetron sputtering technique at ambient temperature. The nitrogen partial pressure during the sputtering process was varied in the range of $0⩽R_{{\mathrm{N}}_2}⩽100\%$, keeping the total gas flow at constant. At lower nitrogen pressures $(R_{{\mathrm{N}}_2}⩽33\%)$, both Fe and NiFe first form a nanocrystalline structure, and an increase in $R_{{\mathrm{N}}_2}$ 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.

Figure 1

Grazing incidence x-ray-diffraction pattern of FeN thin films prepared with different nitrogen partial pressure.

Figure 2

TEM planar view of the sample prepared at 10% nitrogen partial pressure. The inset of the picture shows the electron-diffraction pattern.

Figure 3

(Color online) Interatomic spacing as a function of nitrogen partial pressure in FeN and NiFeN.

Figure 4

Grazing incidence x-ray-diffraction pattern of NiFeN thin films prepared with different nitrogen partial pressure.

Figure 5

Grazing incidence x-ray-diffraction pattern of FeN film prepared with 2% (a), 5% (b), 10% (c), and 20% (d), in the as-deposited state and after vacuum annealing at various temperatures. The films were annealed isochronally for $1\mathrm{h}$.

Figure 6

Grazing incidence x-ray-diffraction pattern of NiFeN thin film prepared with 83% nitrogen partial pressure after annealing at different temperatures.

Figure 7

X-ray reflectivity pattern of FeN thin films prepared with different nitrogen partial pressure. Scattered data (gray-shaded) represent experimental data and solid lines correspond to the theoretical model.

Figure 8

(Color online) X-ray reflectivity pattern of pure Fe film (a), FeN film with 10% N (b), and FeN film with 83% nitrogen partial pressure (c). The evolution of film roughness with film thickness (d).

Figure 9

(Color online) Magnetization measurements of FeN thin films prepared at different nitrogen partial pressures. The inset shows a variation in coercivity as a function of nitrogen partial pressure during sputtering.

Figure 10

(Color online) Magnetization measurements of FeN thin film prepared at 10% nitrogen partial pressure as a function of annealing temperature.

Figure 11

(Color online) Magnetization measurements of NiFeN thin films prepared at different nitrogen partial pressures.

Figure 12

(Color online) Polarized neutron reflectivity measurements of FeN thin films prepared at different nitrogen partial pressures. Closed circles represents the data corresponding to spin-up reflectivity $R+$ and open circles to spin-down reflectivity $R-$. The solid line represents fitting to the experimental data.

Figure 13

(Color online) Magnetic moment as obtained with the bulk magnetization measurements and polarized neutron reflectivity.

Figure 14

(Color online) Distribution of the interstitial site in bcc and fcc structures.

Figure 15

Microstrain obtained from the XRD data (see Fig. 1 and Fig. 4) for FeN and NiFeN samples prepared with different nitrogen partial pressures. The solid and dotted lines are a guide to the eye.