Interface magnetism and electronic structure: ZnO(0001)/Co3O4(111)

We have studied the structural, electronic and magnetic properties of spinel $\rm Co_3O_4$(111) surfaces and their interfaces with ZnO (0001) using density functional theory (DFT) within the Generalized Gradient Approximation with on-site Coulomb repulsion term (GGA+U). Two possible forms of spinel surface, containing $\rm Co^{2+}$ and $\rm Co^{3+}$ ions and terminated with either cobalt or oxygen ions were considered, as well as their interface with zinc oxide. Our calculations demonstrate that $\rm Co^{3+}$ ions attain non-zero 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 $\rm Co_3O_4$ secondary phase, such a magnetic ordering at the interface might explain the origin of the magnetism in these diluted magnetic semiconductors (DMS).