Phonons in Ultrathin Oxide Films: 2D to 3D Transition in FeO on Pt(111)

The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (MLs) were studied using the nuclear inelastic scattering of synchrotron radiation. A distinct evolution of vibrational characteristics with thickness, revealed in the phonon density of states (PDOS), shows a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low-energy part of the PDOS followed a linear ∝E dependence in energy that is characteristic for two-dimensional systems. This dependence gradually transforms with thickness to the bulk ∝E^{2} relationship. Density-functional theory phonon calculations perfectly reproduced the measured 1-ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. The calculations show that the 2D PDOS character is due to a weak coupling of the FeO film to the Pt(111) substrate. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long-range magnetic order and stabilization of an unusual structural phase.

Lattice dynamics of Fe-doped CoO from first principles

The vibrational dynamics of substitutional Fe and its influence on the vibrational properties of the host CoO matrix have been investigated using ab initio calculated Hellmann-Feynman forces. Corrections for the strong on-site interaction have been taken into account via the Hubbard potential U and the local exchange interaction J. Calculations were performed with constant U on Co and variable U on Fe. It was found that Fe impurities exhibit higher values of effective force constant than the original Co. New localized modes are created in the host CoO matrix due to force constant defect between Fe and Co. Iron impurities affect the optical phonon vibrations, while the long wavelength acoustic phonons do not experience changes upon the doping. Mean-squared vibrational amplitudes of cations and anions in the ideal and Fe-doped CoO are compared to the available experimental data. The calculated mean-squared displacements of Fe remain in very good agreement with those measured by Mössbauer spectroscopy.