DFT insight into the structural and magnetic properties of MgFe₂O₄ ferrite: Bulk and surface

Abstract

We present a Density Functional Theory (DFT) based ab initio study of the structural, electronic, magnetic, and hyperfne properties of the semiconducting spinel-ferrite MgFe₂O₄. Calculations were performed using the all-electron Full-Potential Linearized Augmented Plane Wave (FP-LAPW) and the pseudopotential and plane-wave (PW-PP) methods in the frameworks of the Generalized Gradient Approximation (GGA) and GGA + U. To determine the lowest-energy structural and magnetic configuration of MgFe2O4, we examined various spin arrangements and cation distributions of Mg and Fe in the spinel lattice. Our results indicate that MgFe₂O₄ adopts an inverted antiferromagnetic structure. We also show that the energy necessary to reduce the inversion degree from 1 to 0.875 is smaller than those provided by the thermal treatments usually applied for the growth of thesamples, showing that the formation of metastable phases with inversion degrees smaller than 1 is plausible, in agreement with the experimental results. Hyperfine interactions properties at Fe sites were obtained and compared to Mossbauer spectroscopy measurements at 4 K and 300 K, enabling a detailed characterization of the local Fe environments and gives support to our structural and magnetic model for MgFe2O4. Based on the results obtained for the bulk calculations, different (001) MgFe2O4 surface terminations were examined and we found that, after a strong surface reconstruction, the most stable termination is the one that exposes Mg, Fe, and O atoms. We show that the surface inversion degree predicted by DFT is lower than the bulk value, a result that is consistent with experimental results obtained in nanoparticles and that cationic inversion in the superficial layers plays a crucial role in the magnetic response of MgFe2O4 nanoparticles and thin films.