K.W. Geng

Luminescence and Raman scattering properties of Ag-doped ZnO films

Ag-doped ZnO films were prepared by direct current reactive magnetron sputtering using a zinc target with various Ag-chips attached. The influence of Ag doping on the microstructure, photoluminescence and Raman scattering of ZnO films were systematically investigated. The results indicate that ZnO films doped with Ag can still retain a wurtzite structure, although the c-axis as preferred orientation is decreased by Ag doping. The near band edge emission of ZnO film can be enhanced by Ag doping with a concentration of 1.6–2.8 at.% and quench with a further increase in the Ag concentration. A local vibrational mode at 411 cm−1 induced by Ag dopant can be observed in the Raman spectra of the Ag-doped ZnO films, which might be used as an indication of Ag incorporation into the ZnO lattice.

Local Fe structure and ferromagnetism in Fe-doped ZnO films

The local Fe structure and corresponding ferromagnetism are different for various concentrations of Fe-doped ZnO (Zn1?xFexO, x = 0?0.07) films, which are prepared on LiNbO3(104) substrates by reactive magnetron sputtering. X-ray photoelectron spectroscopy and x-ray absorption near-edge structure (XANES) reveal that, when x?0.04, Fe is in the 2+ state and is incorporated into the wurtzite lattice of ZnO, and as x increases further, a second phase Fe3O4 is induced. Furthermore, full multiple-scattering substitution ab?initio calculation of Fe?K-edge XANES is used to confirm the local structure of Fe in films with different x. The single-phase Fe-doped ZnO films (x?0.04) exhibit ferromagnetism above room temperature and the mechanism of bound magnetic polarons (BMPs) is proposed to discuss the magnetic properties. The presence of the second phase is responsible for the strong ferromagnetism for higher Fe concentration.

Transition from diluted magnetic insulator to semiconductor in Co-doped ZnO transparent oxide

Epitaxial growth of Co-doped ZnO films has been prepared on Al2O3 (001) substrate by an efficient and low cost method, i.e., direct current reactive magnetron cosputtering. The correlation among local Co structure and magnetic, electric, and optical properties of Co:ZnO films has been carefully studied. Taking advantage of decreasing the oxygen partial pressure, we have demonstrated the transition from diluted magnetic insulators (DMIs) to semiconductors (DMSs) in Zn0.96Co0.04O films. Full multiple-scattering ab initio calculations of Co K-edge x-ray-absorption near-edge structure clearly provide a structure fingerprint to determine Co2+ replacing Zn2+ and a slight dissimilarity of local Co structure between DMIs and DMSs. Co:ZnO insulating films exhibiting more robust room temperature ferromagnetism than semiconducting films reveal that structure defects are seen to be responsible for the ferromagnetic ordering in Co:ZnO transparent oxide. Furthermore, optical transmittance measurements show a large reds...

Magnetoresistance flipping of Fe∕Ru multilayers prepared by electron-beam evaporation

Fe∕Ru multilayers were prepared by electron-beam evaporation. The magnetoresistance (MR) properties of the multilayer switched the MR sign with the Fe and Ru layer thickness. When the thickness of magnetic Fe layers was fixed at 1.2 nm, the MR effects of the multilayer transformed from a negative to a positive one, with the thickness of nonmagnetic Ru layer changed from 1.2 to 5.0 nm. While when the thickness of nonmagnetic Ru layers was fixed, the transformation of the MR effect is inversed, i.e., transformed from a positive to a negative one with the thickness of the magnetic Fe layer increased. The origin of the MR variation is analyzed. The inverse giant magnetoresistance is related to the Fe∕Ru interface layer, in which the scattering spin asymmetry is less than 1. The MR dependence on the Fe and Ru layer thickness reveals the competition between two mechanisms of normal MR and inverse MR