Fe diffusion in amorphous and nanocrystalline alloys studied using nuclear resonance reflectivity

It is demonstrated that nuclear resonance reflectivity from isotopic multilayers can be used as a sensitive technique to study self-diffusion of a Mössbauer isotope (${}^{57}\mathrm{Fe}$ in the present case). In the case of isotopic multilayers, in which alternate layers have the same chemical composition and differ only in the abundance of ${}^{57}\mathrm{Fe}$, nuclear resonance scattering causes x-ray scattering contrast between adjacent layers, resulting in the appearance of Bragg peaks corresponding to the bilayer periodicity. Diffusion of ${}^{57}\mathrm{Fe}$ across the isotopic interface results in a decrease in the scattering contrast and thus a decrease in the intensity of the Bragg peak, making it possible to measure diffusion lengths of the order of $0.1\mathrm{nm}$ in chemically homogeneous films. The technique has been used to study self-diffusion of Fe in amorphous FeZr and nanocrystalline FeN alloys. In $\mathrm{a}\text{−}\mathrm{Fe}\mathrm{Zr}$, measurements yield activation energy for Fe diffusion $E=0.42\pm 0.05\mathrm{eV}$ and the pre-exponent factor $D_0=\exp (-39\pm 1){\mathrm{m}}^2∕\mathrm{s}$. In nanocrystalline ${\mathrm{Fe}}_{60}{\mathrm{N}}_{40}$, variation in diffusivity due to structural relaxation at temperatures as low as $393\mathrm{K}$ could be observed. Measurements in the structurally relaxed state yield $E=0.8\pm 0.2\mathrm{eV}$ and $D_0=\exp (-28\pm 4){\mathrm{m}}^2∕\mathrm{s}$.

Figure 1

(Color online) Conversion electron Mössbauer spectra of the amorphous ${\mathrm{Fe}}_{70}{\mathrm{Zr}}_{30}$ film as a function of isochronal annealing at different temperatures. Continuous curves represent fit to the experimental data.

Figure 2

X-ray reflectivity of the ${}^{57}\mathrm{Fr}_{70}{\mathrm{Zr}}_{30}∕{\mathrm{Fe}}_{70}{\mathrm{Zr}}_{30}$ multilayer taken using Cu $K\alpha$ radiation.

Figure 3

(Color online) Nuclear resonance reflectivity of the ${}^{57}\mathrm{Fr}_{70}{\mathrm{Zr}}_{30}∕{\mathrm{Fe}}_{70}{\mathrm{Zr}}_{30}$ multilayer taken at the Mössbauer resonance energy of ${}^{57}\mathrm{Fe}$ $\left(14.413\mathrm{keV}\right)$, as a function of isochronal annealing at different temperatures. The Bragg peak around $q=0.9{\mathrm{nm}}^{-1}$ corresponds to the isotopic periodicity of the multilayer.

Figure 4

Arrhenius behavior of diffusion coefficient with isochronal annealing temperature for a FeZr multilayer.

Figure 5

(Color online) XPS spectra of FeNZr film covering the binding energy ranges corresponding to Zr $3d_{5∕2}$, N $1s$, and Fe $2p_{3∕2}$ levels.

Figure 6

(Color online) XRD pattern of nanocrystalline FeNZr film as a function of isochronal annealing at different temperatures. The solid curves represent Gaussian fit to the line profile, which has been used to estimate the crystallite size.

Figure 7

Conversion electron Mössbauer spectra of nanocrystalline FeNZr film as a function of isochronal annealing at different temperatures. Continuous curves represent fit to the experimental data.

Figure 8

Nuclear resonance reflectivity of the ${}^{57}\mathrm{Fe}\mathrm{N}\mathrm{Zr}∕\mathrm{Fe}\mathrm{N}\mathrm{Zr}$ multilayer taken at the Mössbauer resonance energy of ${}^{57}\mathrm{Fe}$ $\left(14.413\mathrm{keV}\right)$, as a function of annealing time at $393\mathrm{K}$.

Figure 9

Annealing time dependence of the diffusion length in a FeNZr film at various temperatures.

Figure 10

Arrhenius behavior of diffusion coefficient with isothermal annealing temperature for FeNZr multilayer.

Figure 11

Plot of $\ln D_0$ vs $E$ for various amorphous and crystalline alloys showing correlation between the two quantitites. Amorphous and crystalline systems exhibit distinctly different $\ln D_0-E$ correlations. Data has been taken from Refs. 23, 25, 7. For comparision, data for $a\text{−}{\mathrm{Fe}}_{70}{\mathrm{Zr}}_{30}$ and $n\text{−}\mathrm{Fe}\mathrm{N}\mathrm{Zr}$ as obtained in the present work have also been plotted.