  For a ultrathin NiO(001) film of 4 monolayers (ML) thickness grown on
Ag(001), the vibrational properties have been determined by high-resolution
electron energy loss spectroscopy (HREELS). For the well-ordered
pseudomorphically grown film, nine phonon modes have been identified and their
dispersions have been revealed along the Gamma-X high-symmetry direction. The
comparison with phonon data for a 25 ML thick NiO(001) film shows that the NiO
(001) phonon properties are already fully developed at 4 ML. Significant
differences are found for the surface-localized phonon S6 which has an
increased dispersion for the ultrathin film. The dipole-active Fuchs-Kliewer
phonon-polariton exhibits a narrower lineshape than the mode found for a
single-crystal surface, which might hint to a reduced antiferromagnetic
coupling in the ultrathin film.
