Phonon confinement in ultrathin nickel films

We present an interpretation of the recently obtained data related to the measurements of the sound velocity in ultrathin nickel films. The interpretation is based on the phonon frequency spectrum size quantization, which in turn leads to a decreasing of the phonon group velocity in the confinement direction as the film thickness decreases. A good agreement with the experimental data is obtained for all film thicknesses for which the sound velocity has been measured.