Atomic vibrational dynamics of amorphous Fe–Mg alloy thin films

The atomic vibrational dynamics of 57Fe in 800-Å thick amorphous FexMg1−x alloy thin films (0.3≤x≤0.7) has been investigated at room temperature by nuclear resonant inelastic X-ray scattering (NRIXS) of 14.4125 keV synchrotron radiation. The amorphous phase has been successfully stabilized by codeposition of Fe and Mg in ultrahigh vacuum onto a substrate held at −140 °C during deposition. The amorphous structure of the samples was confirmed by X-ray diffraction and conversion electron Mössbauer spectroscopy. The 57Fe-projected partial vibrational density of states, g(E), has been obtained from the measured NRIXS vibrational excitation probability, together with thermodynamic quantities such as the probability of recoilless absorption (f-factor), the average kinetic energy per Fe atom, the average force constant, and the vibrational entropy per Fe atom. A plot of g(E)/E2 versus E proves the existence of non-Debye-like vibrational excitations with a peak at Ebp∼3–5 meV (boson peak). Both the boson peak height and Ebp were found to depend linearly on the composition x. Above the boson peak, g(E)/E2 exhibits an exponential decrease.