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

Abstract

We have studied the structural, electronic and magnetic properties of spinel Co 3 O 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 Co 2+ or 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 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 Co 3 O 4 secondary phase, such a magnetic ordering at the interface might explain the origin of the magnetism in such diluted magnetic semiconductors (DMS). PACS numbers: 73.20.-r, 75.70.-i Acknowledgments 16References 17 I. INTRODUCTIONMagnetic semiconductors (MS) and diluted magnetic semiconductors (DMS) exhibit both ferromagnetic and semiconducting properties. Therefore, they are promising materials for spintronics, which utilizes for information processing not only the electron charge but also its spin. Historically, the first DMS with a high Curie temperature up to about 200 K was GaAs doped with Mn ions. 1,2 In that compound, the ferromagnetism is promoted by hole carriers, which align along the local Mn magnetic moments and called carrier-induced ferromagnetism or Zener p − d exchange. It is crucial for this mechanism that Mn at the Ga site becomes Mn 2+ instead of Ga 3+ , thus providing at the same time a local spin and a hole charge carrier. Extension of the mechanism, proposed in a very influential paper 3 of Dietl and co-workers, allows a prediction that the above room-temperature ferromagnetism in ZnO:Co and GaN:Mn is due to the same carrier-induced mechanism. This would be responsible for the ferromagnetism with a sufficiently high number of hole charge carriers.First experiments after that prediction 4 seemed to confirm the mechanism proposed and has also been supported by ab-initio calculations. 5 However, it soon turned out that the Co impurity is in fact isovalent to the Zn ion 6 and provides no charge carriers at all, while the situation in GaN:Mn is similar. 7We are going to concentrate here on ZnO:Co, where the experimental reports demon-2 strate that the above room-temperature ferromagnetism in ZnO:Co persist. Even though its origin is still not clarified, there are clear indications in more recent experiments that the magnetism in the ZnO:Co system is attributed to the formation of the Co 3 O 4 phase in ZnO. 8-12 Therefore, we will focus here on the role of the Co 3 O 4 phase, although several attempts to explain the mechanism of the ferromagnetism in realistic ZnO:Co systems exist including, for instance, spinodal decomposition 13 or Lieb-Mattis ferrimagnetism, 14 to cite just two ideas. The typical doping level of Co in ZnO can be relatively high (in the range between 10% and 30%). This leads to the secondar...

Self-Power Broadband Photodetection Enhancement via Co3O4/ZnCo2O4/ZnO Heterojunction with DFT Analysis

“…This hybridization lowers the energy of electronic states, necessitating electrons to be excited with higher energy to follow their original pathways, thereby influencing the optical absorption properties of Co 3 O 4 . Furthermore, ZnO nanorods can enhance electron transport along the c -axis, facilitating the effective movement of excitons in Co 3 O 4 away from the interface region . This phenomenon may exacerbate changes in the electronic structure of Co 2+ 3d orbitals, potentially contributing to the anomalous transparency observed in the Co 3 O 4 /ZnO/ZnO film between 400 and 598 nm.…”