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

Abstract

It is demonstrated that nuclear resonance reflectivity from isotopic multilayers can be used as a sensitive technique to study self-diffusion of a M\"ossbauer 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 $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in chemically homogeneous films. The technique has been used to study self-diffusion of Fe in amorphous FeZr and nanocrystalline FeN alloys. In $\mathrm{a}\text{\ensuremath{-}}\mathrm{Fe}\mathrm{Zr}$, measurements yield activation energy for Fe diffusion $E=0.42\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and the pre-exponent factor ${D}_{0}=\mathrm{exp}(\ensuremath{-}39\ifmmode\pm\else\textpm\fi{}1)\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$. In nanocrystalline ${\mathrm{Fe}}_{60}{\mathrm{N}}_{40}$, variation in diffusivity due to structural relaxation at temperatures as low as $393\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ could be observed. Measurements in the structurally relaxed state yield $E=0.8\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${D}_{0}=\mathrm{exp}(\ensuremath{-}28\ifmmode\pm\else\textpm\fi{}4)\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$.