Vibrational dynamics of Fe in amorphous Fe-Sc and Fe-Al alloy thin films

Abstract

The atomic vibrational dynamics of 57Fe in 800-A thick amorphous (a-) 57Fe0.25Sc0.75, a-57Fe0.67Sc0.33 and a-57Fe0.14Al0.86 alloy thin films has been investigated at room temperature by nuclear resonant inelastic X-ray scattering (NRIXS) of synchrotron radiation. The amorphous phase has been successfully stabilized by codeposition of Fe and Sc or Al in ultrahigh vacuum onto substrates held at −140 °C during deposition. The amorphous structure of the samples was confirmed by X-ray diffraction and conversion electron Mossbauer spectroscopy. The 57Fe-projected partial vibrational density of states, geE), 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 the reduced density of states, g(E)/E2, versus excitation energy E proves the existence of non-Debye-like vibrational modes (boson peak) with a peak energy, E bp , in the range of 3–7 meV. Both, the boson peak height Hbp and E bp were found to depend on the composition. Above the boson peak, g(E)/E2 exhibits an exponential decrease. Our results demonstrates that the features of the boson peak depend on the amount and type of element M (M = Al, Si, Mg, Sc).