First-principles study of the adsorption of MgO molecules on a clean Fe(001) surface

The adsorption of MgO molecules on a Fe(001) surface was studied using density functional theory and projector augmented-wave methods. The energetically most favored configurations for different adsorption sites considered were identified. The most preferable adsorption geometry is when the MgO molecules are parallel to the surface, with Mg in the interstitial site and O in the on-top site of the Fe atom. During the adsorption of subsequent MgO molecules in this geometry, a sharp, nonoxidized interface is formed between the MgO adlayer and the Fe(001) surface. The adsorption of MgO perpendicular to the surface, with oxygen incorporated in the topmost Fe layer, is less probable, but it may lead to the formation of the FeO layer when stabilized with an excess of oxygen atoms. Structural, electronic, and magnetic properties of both interface types were examined for the MgO coverage from 1/9 to 1 monolayer (ML). Electronic and magnetic properties are sensitive to the MgO coverage. For lower coverage of MgO, clear hybridization between the Fe $3d$ and O $2p$ states is shown. The average magnetic moment of the surface Fe atoms is reduced with coverage, achieving $2.78{\mu }_{\mathrm{B}}$ for 1 ML of MgO.

Figure 1

Adsorption sites (crosses) for MgO molecules in the $3\times 3$ surface unit cell representing the Fe(001) surface: on-top (ot), interstitial (int), bridge (bri) site. The brown and light brown balls correspond to the first and second Fe layers, respectively.

Figure 2

Coverage dependence of adsorption energy (a), Mg-O distance (b), and work function (c) for the most favorable overall int-ot orientation of MgO molecules adsorbed on the Fe(001) surface. For comparison, the adsorption energy plot in (a) also presents the results for the most favorable perpendicular O-int orientation of the molecules.

Figure 3

Side and top (right column) views of the most stable final int-ot configuration of MgO molecules adsorbed on the Fe(001) surface, with the Mg atom (red) in the interstitial (int) and the O atom (blue) on-top (ot) of the Fe atom. Medium (brown) balls represent Fe atoms. In the top view, Fe atoms of the second layer are a lighter shade.

Figure 4

Top and side (bottom panels) views of isosurfaces of the valence charge density difference, $\mathrm{\Delta }\rho \left(\mathbf{r}\right)$, depending on the MgO coverage. Electron charge gain (loss) is drawn in yellow (blue). The isosurface density level is 0.005 $e/{\mathrm{bohr}}^3$. Red, blue, and brown balls represent Mg, O, and Fe atoms, respectively. The black lines in the top view panels mark the plane cuts for the side views shown in the lower row panels.

Figure 5

Bader charge difference ($\mathrm{\Delta }Q$) on the surface atoms of the top layers of the MgO/Fe(001) system induced by different degrees of MgO coverage.

Figure 6

Partial densities of states (PDOSs) of the clean Fe(001) surface and for different degrees of MgO coverage adsorbed in the most stable int-ot configuration. Positive and negative densities correspond to the spin-up and spin-down states, respectively. Energy zero corresponds to the Fermi level. The plots for Mg states are multiplied by 5.

Figure 7

Coverage dependence of the average magnetic moment on the Fe atoms in the topmost surface layer.