Structural, electronic, and magnetic properties of nanometer-sized iron-oxide atomic clusters: Comparison between GGA and GGA plus U approaches

We perform spin-polarized density-functional theory simulations within the generalized gradient approximation (GGA) and GGA+U method on nanometer-sized iron-oxide atomic clusters in different stoichiometries. By comparing total energies of structures with different symmetries and selected collinear magnetic configurations we find that low symmetry and, in general, ferrimagnetic structures exhibiting low total magnetic moment are energetically favorable. For the oxygen-rich Fe25O30 cluster we obtain a cagelike geometry with a few ions within the cage that seem to stabilize the structure. Considering the Fe33O32 cluster of nanometer-size we propose the formation of a rocksalt type structure, which is characteristic of bulk FeO. Based on data of iron d shell electron occupancies, we exclude double exchange from possible magnetic interactions between iron ions, and we point to a competition between direct exchange and superexchange, where the dominant interaction is determined by the cluster topology. For the smaller Fe13O8 cluster we find ferromagnetic energetically favorable geometries of lower symmetry than previously reported. Our results demonstrate the importance of going beyond GGA, in particular, physical properties obtained within GGA + U description are found to be remarkably different from those using GGA. In order to confirm our theoretical predictions, cluster experiments in this size regime are desirable.