First-principles modeling of interfaces between solids with large lattice mismatch: The prototypical CoO(111)/Ni(111) interface

In this work we investigate the CoO(111)/Ni(111) interface by first-principles calculations, focusing on its structure and stability. To satisfy the approximate 5:6 ratio of the CoO and Ni lattice constants, we construct a supercell with $5\times 5$ Co (O) and $6\times 6$ Ni atoms per layer in the bulk regions. For the interface Ni layer and the adjacent Ni layer we consider different configurations and study the binding energy. We show for an ideal CoO interface terminated by $5\times 5$ O atoms that the structure is more stable if there are $5\times 5$ Ni atoms next to it instead of $6\times 6$ as in the bulk. In addition, we observe that a transition layer with 31 or 33 Ni atoms located between the interface $5\times 5$ Ni and bulk $6\times 6$ Ni layers (which partially reflects the structures of both these layers) enhances the stability of the CoO/Ni interface. The electronic and magnetic modifications induced by the interface formation are discussed.

Figure 1

(a) ($5\times 5$)CoO/($6\times 6$) Ni interface. The vertical lines highlight the Ni and CoO unit cells. The structures of layers 1 and 2 (frist two Ni layers) are unknown with $x_1$ and $x_2$ Ni atoms in these layers. (b) The atoms in layer 1 occupy hollow sites of the adjacent O layer. (c) The atoms in layer 2 occupy hollow sites of the adjacent Ni layer.

Figure 2

(a) The transition layer is located between a top layer with 25 and a bottom layer with 36 close packed atoms. (b)–(f) Possible structures of the transition layer. Yellow (green) background color indicates atoms on hollow (top) sites.

Figure 3

(a) Average binding energy per atom in layer 1 or 2 as calculated for the models of Table . Blue (red) color represents GGA (GGA + $U$) results. (b) Average deviations from the bulk values of Ni-O distances (solid) and of distances between Ni atoms in the two sublayers (dashed).

Figure 4

Distances between nearest neighbor atoms for models (1) to (7): (a) O and Ni, (b) Ni and Ni. Blue (red) color represents GGA (GGA + $U$) results. The vertical solid (dashed) lines give the bulk distances obtained for the GGA (GGA + $U$) method.

Figure 5

Ni DOS: Average over the Ni atoms in the interfaces with (a) 25 and (b) 36 atoms, respectively; (c) bulk NiO; and (d) bulk Ni. Blue (red) color represents GGA (GGA + $U$) results.

Figure 6

Magnetization profile of the Ni atoms for (a) 25 atoms [model (2)] and (b) 36 atoms [model (7)] in the interface. Blue (red) color represents GGA (GGA + $U$) results. The labels of the Ni layers refer to their distance to the interface.