Amorphization in iron nitride thin films prepared by reactive ion-beam sputtering

Reactive ion-beam sputtering has been used to prepare iron nitride over a wide composition range. It is found that the samples deposited at room temperature exhibit amorphous phase in the composition range from $12\mathrm{at}.\%$ nitrogen to $23\mathrm{at}.\%$ nitrogen. For samples deposited at liquid nitrogen temperature the system is amorphous up to $35\mathrm{at}.\%$ nitrogen. Amorphization can be understood in terms of a frustration in the system due to a competition between $\alpha$ and $\epsilon$ phases. Kinetic constraints are also found to play a role in the amorphization process. Mössbauer measurements suggest that the local order in the amorphous phase consists of a mixture of $\alpha \text{−}\mathrm{Fe}$–like and $\epsilon \text{−}{\mathrm{Fe}}_3\mathrm{N}$–like short-range orders. On the iron rich side the amorphous phase exhibits two-step crystallization, with a primary $\alpha \text{−}\mathrm{Fe}$ phase precipitating out in the first step. Around $22\mathrm{at}.\%$ nitrogen the system exhibits a single step isomorphous transformation to $\epsilon$ phase. Thus, amorphous iron nitride phases exhibit behavior very similar to the conventional transition metal-metalloid amorphous alloys. In the remaining composition range nanocrystalline phases are formed. The amorphous magnetic iron nitride phases are expected to have distinct advantages over their crystalline counterparts in terms of soft magnetic applications.

Figure 1

Nitrogen concentration in the sample as a function of nitrogen flow rate for the samples deposited at room temperature and liquid nitrogen temperature. The error bar comes mainly from composition variation in the film.

Figure 2

(Color online) XRD patterns of the pristine samples deposited at (a) room temperature and (b) liquid nitrogen temperature with various nitrogen concentration.

Figure 3

Plot of average $\mathrm{Fe}\text{−}\mathrm{Fe}$ distance as a function of Fe concentration.

Figure 4

(Color online) Some representative room temperature Mössbauer spectra as a function of nitrogen concentration.

Figure 5

Magnetic hyperfine values of the as-deposited samples at room temperature in pristine state.

Figure 6

Gibbs free energy of $\alpha$, $\epsilon$, and $\gamma$ phase of iron nitride as a function of nitrogen concentration, as taken from Ref. 27.

Figure 7

Hyperfine field distribution in amorphous $\mathrm{Fe}\text{−}\mathrm{N}$ samples as obtained from fitting of the conversion electron Mössbauer spectra.

Figure 8

(Color online) XRD pattern of sample $S_{R}15$ as a function of isochronal annealing at various temperatures.

Figure 9

(Color online) Conversion electron Mössbauer spectra of sample $S_{R}15$ as a function of isochronal annealing at various temperatures.

Figure 10

(Color online) XRD patterns of sample $S_{R}23$ as a function of isochronal annealing at various temperatures.

Figure 11

Conversion electron Mössbauer spectra of sample $S_{R}23$ as a function of isochronal annealing at various temperatures.