High-energy phonon confinement in nanoscale metallic multilayers

The Fe-projected vibrational density of states $g(E)$ in nanoscale ⁵⁷Fe/M multilayers, where M=Cr, Co, Cu, Pd, or Ag was measured by nuclear resonant inelastic x-ray scattering. With decreasing Fe thickness, the high-energy phonon peak of Fe near 36 meV is suppressed for the ``soft'' metals Ag, Pd, and Cu, but much less so for the ``hard'' metals Co and Cr. This effect is attributed to Fe phonon confinement and interface localization due to an energy mismatch between $g(E)$ of M and of Fe.