Interface magnetism and electronic structure: ZnO(0001)/${\mathrm{Co}}_3{\mathrm{O}}_4$(111)

We have studied the structural, electronic, and magnetic properties of spinel ${\mathrm{Co}}_3{\mathrm{O}}_4$(111) surfaces and their interfaces with ZnO(0001) using density functional theory within the generalized gradient approximation with the on-site Coulomb repulsion term. Two possible forms of spinel surface, containing ${\mathrm{Co}}^{2+}$ or ${\mathrm{Co}}^{3+}$ ions and terminated with either cobalt or oxygen ions, were considered, as well as their interface with zinc oxide. Our calculations demonstrate that ${\mathrm{Co}}^{3+}$ ions attain nonzero magnetic moments at the surface and interface, in contrast to the bulk, where they are not magnetic, leading to the ferromagnetic ordering. Since heavily Co doped ZnO samples can contain a ${\mathrm{Co}}_3{\mathrm{O}}_4$ secondary phase, such magnetic ordering at the interface might explain the origin of the magnetism in such diluted magnetic semiconductors.

Figure 1

Side view of the unit cells of ${\mathrm{Co}}_3{\mathrm{O}}_4\text{/}\mathrm{ZnO}$ interfaces: (a) octahedral B-terminated interface and (b) tetrahedral A-terminated interface. Numbers denote the atomic layers of both interfaces.

Figure 2

Spin density distribution for the octahedral interface, plotted over the vertical $\left(1\overline{1}0\right)$ plane. The scale, shown on the right, has units of ${\mu }_B$. On the left, 1, 2, 3, and 4 stand for the ${\mathrm{Co}}_3{\mathrm{O}}_4$ interface layer numbers. Chemical symbols indicate the positions of corresponding ions.

Figure 3

Layer-resolved DOS of (a) octahedral and (b) tetrahedral spinel (111) surfaces and (111) spinel/(0001) wurtzite interfaces. From top to bottom: top layer of spinel (111) surface with no ZnO cap, layers 1 to 4 of the spinel structure close to the spinel/wurtzite interface, and the layer-resolved DOS for the (111) plane of bulk spinel (see discussions in the text).

Figure 4

Projected LRDOS for the top layer of the octahedral interface and octahedral plane of bulk spinel. The plot demonstrates the dominance of the $p$ and $d$ components of the wave function, while the $s$-state contributions can be neglected.