Atomic and electronic structure of polar Fe${}_2$O${}_3$(0001)/MgO(111) interfaces

We present a first-principles investigation of the structural, electronic, and magnetic properties of ultrathin Fe${}_2$O${}_3$(0001) films on a polar MgO(111) substrate. The results imply that the heterointerface is atomically abrupt with oxidelike stacking for film thicknesses between $\sim$1.5 and 8.5 Å. The Fe-Fe bilayer (nominal separation of 0.59 Å in Fe${}_2$O${}_3$) at the interface collapses into an “Fe${}_2$” monolayer. Both electronic polarization and structural relaxations effectively screen the dipole field of the polar interface system. The structural relaxations—consisting of interpenetration, separation, and merger of Fe and oxygen planes—are particularly drastic in the three- and four-bilayers-thick films, giving rise to barrierless movement of oxygen towards the surface and the formation of an “Fe${}_2|$FeO${}_3$” layer structure not seen in hematite. Comparisons to calculations of unsupported polar Fe${}_2$O${}_3$(0001) slabs demonstrate that these unusual changes in stacking sequence and electronic structure are associated with the polar nature of this oxide heterointerface.

Figure 1

Structure of bulk hexagonal $\alpha$-Fe${}_2$O${}_3$. The green (light) and purple (dark) spheres represent the O and Fe atoms, respectively. (a) Bulk unit cell in a $\langle 1\overline{1}00\rangle$ projection: $+$ and $-$ indicate the relative magnetic ordering of Fe atoms. (b) $\langle 11\overline{2}0\rangle$ crystal projection showing the ABAC stacking sequence with Fe atoms in $A$ sites and O atoms alternating between $B$ and $C$ sites.

Figure 2

Side view of the (a) unrelaxed and (b) relaxed structures for the three-Fe-bilayer hematite film on MgO(111). Purple (dark), green (light), and orange (medium dark) spheres represent Fe, O, and Mg, respectively.

Figure 3

(a) Atomic positions along the surface normal for atoms in the three-bilayer film as a function of the relaxation steps. The in-plane atomic positions are indicated on the left and the final layer structure on the right. (b) Top view of the hexagonal plane. Red and purple arrows represent the ($1\times 1$) MgO plane and the Fe${}_2$O${}_3$(0001)/MgO(111) ($\sqrt{3}\times \sqrt{3}$) in-plane lattice vectors, respectively. The possible Fe sites ($A_{-1}$, $A_0$, $A_1$) and one set of the oxygen sites ($C$) are indicated; the shaded gray triangle shows the local structure of the FeO${}_3$ plane.

Figure 4

Atomic positions along the surface normal as a function of relaxation step, for an initial geometry consisting of the relaxed three-Fe-bilayers structure of Fig. 3 with a fourth Fe${}_2$O${}_3$ unit added by (a) an Fe bilayer followed by a 3O plane after 15 relaxation steps, or (b) a complete 3(O-Fe-Fe) unit at the start of the calculation. The sequential approach is also more appropriate on the layer-by-layer growth.

Figure 5

Comparison of the planar positions of the GGA and GGA + $U$ ($U-J=4$ eV) relaxed three-bilayer structures. The positions relative to the dotted lines represent the stacking sites; atomic sites are labeled according to their positions in the film. The Fe and O spin-resolved local density of states for the different atoms are shown in the top and bottom panels; the GGA results are given as filled regions while the GGA + $U$ results are given as solid lines.

Figure 6

Planar-averaged (a) Coulomb and (b) ionic potential for calculated (solid) and fitted (dashed) charges for the relaxed three-Fe-bilayers-thick Fe${}_2$O${}_3$(0001) film on MgO(111). Positions of Fe (diamond), O (triangle), and Mg (square) atoms are also shown. (c) Dipole potentials across the unrelaxed (solid circle) and relaxed (solid triangle) hematite films as a function of the number of Fe bilayers. The hashed circle and triangle show the dipole potentials for an unrelaxed and relaxed four-Fe-bilayers film, respectively, initiated with an O${}_3$ termination. The dipole potential $V_d$ is calculated as the difference between the average ionic potential in the MgO substrate and the vacuum zero.