Structural, Vibrational, and Magnetic Properties of FeCoOn0/+ (n = 1–6) Bimetallic Oxide Clusters

Abstract

FeCo nanostructures are very interesting for storage media, sensing, and biomedical applications. To learn how an oxidizing environment may affect the physical and chemical properties of FeCo nanoparticles in the molecular limit, we investigated, by means of density functional theoretic calculations, the structural, electronic, and magnetic properties of neutral and ionized oxides of the magnetic dimer FeCo as a function of the oxygen content (FeCoOn and FeCoOn+ with n = 1-6). Our aim was to rationalize the structural pattern, energetics, effects of oxidation on the local and total magnetic moments and magnetic couplings, and effect of the ionization on all those properties. The binding energy shows saturation for an oxygen content in the range of n = 4-6. The metal-metal bond weakens with increasing oxygen content. This is reflected in the metal-to-oxygen charge transfer although not systematically in the magnetic properties because the total spin moment oscillates as a function of n between high-spin states (characterized by parallel magnetic couplings) and low-spin states (characterized by antiparallel couplings). Oxide clusters in the high-spin state retain the same total moment as the bare FeCo dimer because of the direct contribution of the oxygen atoms. Upon ionization, the weakening of the metal-metal bond is less marked and the overall magnetic moment decreases because of the increasing tendency toward antiparallel couplings. We calculated the vibrational frequencies and IR intensities of certain isomers of different geometries and spin states, from which future IR spectroscopy experiments could confirm the structural pattern and, indirectly, the magnetic state.