Zhejiang Univ, Dept Mat Sci & Engn, Lab New Structured Mat, Hangzhou 310027, Peoples R China
; Natl Inst Mat Sci, Superconducting Mat Ctr, New Mat Grp, Tsukuba, Ibaraki 3050044, Japan
; Forschungszentrum Karlsruhe, Inst Nanotechnol, D-76021 Karlsruhe, Germany
; Forschungszentrum Karlsruhe, Inst Nucl Waste Disposal, D-76021 Karlsruhe, Germany
; Forschungszentrum Karlsruhe, Inst Solid State Phys, D-76021 Karlsruhe, Germany
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
The atomic structure and the magnetic properties of 5 and 10 at. % Co-doped ZnO samples prepared by a high-pressure and high-temperature method have been investigated by various techniques, including x-ray diffraction with Rietveld refinements, scanning electron microscopy, high-resolution transmission electron microscopy, nanometer-sized element mapping, x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and extended x-ray absorption fine structure and magnetization measurements. It is found that Co ions with a valence of 2+ substitute Zn ions in the wurtzite ZnO structure. No metallic or oxidic Co-rich clusters were detected in the samples. If Co2+ ions substitute Zn2+ ions, no intrinsic ferromagnetism is observed in ZnO at doping levels up to 10 at. % Co even down to 5 K. A paramagnetic behavior with partially antiferromagnetic interactions is observed. Thus, in bulk Co-doped ZnO system any observation of ferromagnetism is probably due to small clusters of second phase materials. However, in thin films the situation might be more complex, since other extrinsic influences, such as strain or proximity effects, may modify the electronic and magnetic properties.