We measured the vibrational density of states (DOS) of nanocrystalline $\alpha -\mathrm{Fe}$ by nuclear inelastic scattering of synchrotron radiation. Specimens prepared by inert gas condensation with different crystallite sizes and oxide contents have been investigated. An enhanced population of low-energy vibrational modes arises from the high fraction of interfacial sites connected with the small crystallite size. Oxidation similarly contributes to the low-energy DOS but additionally brings about stiff modes above the high-frequency cutoff of bulk $\alpha -\mathrm{Fe}.$ Broadening of the DOS peaks due to reduced phonon lifetime in the nanometer-sized crystallites is observed. All measured samples exhibit a Debye-like behavior with the low-energy tail of the DOS being essentially quadratic in energy.

• Received 13 May 2002

DOI:https://doi.org/10.1103/PhysRevB.66.073410