Preserving the Insulating Character and Tuning the Magnetic Properties through the Formation of Defects at NiFe2O4(001) Surfaces
Résumé
We performed a numerical investigation of possible atomic and electronic reconstructions at NiFe2O4(001) surfaces. Using ab initio thermodynamics calculations, we identified that NixFeyO8-terminated surfaces can be stabilized in oxygen-rich conditions with growth temperatures below 950 K. The stabilization of the Ni2Fe2O8 termination obtained from a truncation of the bulk structure ends up in a slight reduction of the surface magnetization because of the transfer of holes at the surface, as a consequence of the polar character of the NiFe2O4(001) layer. The associated surface states are expected to present a small band gap of 0.21 eV at the Fermi level, resulting from a charge ordering, which could disappear at high temperature, to the benefit of the appearance of a spin-polarized two-dimensional hole gas. However, we demonstrated that the appearance of such a two-dimensional hole gas is subject to the conditions that the cation ordering in octahedral sites is not modified and that the stoichiometry is preserved. In particular, we highlighted that the formation of oxygen vacancies is expected to be easier at the surface and may help to recover the surface insulating state. According to our results, the surfaces stabilized in iron-rich or oxygen-poor conditions should present a higher magnetization.