First principles calculations of the O surfaces of Co-ZnO show that substitutional Co ions develop large magnetic moments which long-range order depends on their mutual distance. The local spin polarization induced at the O atoms is 3 times larger at the surface than in the bulk, and the surface stability is considerably reinforced by Co. Moreover, a robust ferromagnetic state is predicted at the O (0001) surface even in the absence of magnetic atoms, correlated with the number of p holes in the valence band of the oxide, and with a highly anisotropic distribution of the magnetic charge even in the absence of spin-orbit coupling.

The adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface has been investigated by density functional ab-initio calculations. An on top H(1 × 1) ordered overlayer with genuine H-Zn chemical bonds is shown to be energetically favorable. The H covered surface is metallic and spinpolarized, with a noticeable magnetic moment at the surface region. Lower hydrogen coverages lead to strengthening of the H-Zn bonds, corrugation of the surface layer and to an insulating surface. Our results explain experimental observations of hydrogen adsorption on this surface, and not only predict a metal-insulator transition, but primarily provide a method to reversible switch surface magnetism by varying the hydrogen density on the surface.PACS numbers: 71.20. Tx, 71.70.Ej ZnO is one of the most technologically important metal oxides, which holds great promise for applications as a wide band gap semiconductor: it shows the largest charge-carrier mobility among oxides, extraordinary catalytic properties and the unusual coexistence of transparency and conductivity [1]. The recently reported high temperature (HT) ferromagnetism in ZnO-based thin layers and nanostructures [2] turns it additionally into a potential HT semiconducting ferromagnet, which could be used in magnetoelectric and magnetotransport devices tuning simultaneously charge and spin [3]. However, at present, and despite numerous experimental and theoretical studies, the mechanism behind the HT magnetic order in ZnO is still under debate [4,5,6,7]. Particular attention deserves the role of H doping in ZnO. Hydrogen is a very reactive element that exhibits qualitatively different behaviour in different media. It is amphoteric, can act either as donor (H+) or acceptor (H-), can occupy different lattice sites and even modify the host structure, and in general counteracts the conductivity of the host. Isolated hydrogen has been found to act as a shallow donor in bulk ZnO, and thus it has been attributed as a source of the unintentional n-type conductivity exhibited by ZnO [8].At surfaces, the interaction with ambient H is almost unavoidable even in ultra-high-vacuum conditions. Moreover, the presence of hydrogen has a pronounced influence which can even change the surface electronic properties. Understanding the interaction of hydrogen with the ZnO surfaces is crucial in order to control the properties of ZnO-based low-dimensional structures. In recent years, systematic investigations of the different ZnO low-indexed surfaces have been performed [9]. There is agreement in the formation of an ordered hydrogen overlayer on both the non-polar ZnO (1010) and the polar O-ended ZnO (0001) surfaces. Much few work has been devoted to the Zn-terminated ZnO(0001) surface, where the interaction of H atoms is thought to be the weakest among the ZnO surfaces, since the binding energy of ZnH pairs should be considerably weaker than that of OH bonds. An experimental study revealed that, exposing this surface to atomic hydrogen, an ordered (1 × 1) overlayer consisting of Zn-hydri...

SnO 2 and Mn-doped SnO 2 thin films have been grown by radio frequency sputtering in two different atmospheres (Ar and Ar/O 2 ) on Si(100) and Al 2 O 3 (R-cut) at room temperature (RT) and at 500 °C. The RT films are amorphous; those grown at 500 °C are polycrystalline or epitaxial, depending on the substrate. All the films, undoped or Mn-doped, present a paramagnetic signal, and ferromagnetism is not observed, regardless of the growth conditions or their structure. The measured magnetization systematically decreases when the films are grown in oxygen-free atmosphere, thus indicating that magnetism is not promoted by oxygen vacancies, and no correlation is found between conductivity and magnetism. We compare the experimental results with ab initio density functional calculations and demonstrate that the Sn vacancies are the origin of the measured magnetization in SnO 2 undoped films. Oxygen vacancies contribute to neither the magnetic moment nor the conductivity of the samples. The localized nature of the defect-induced electronic levels prevents a collective magnetic or conducting behavior.