Extrinsic origin of ferromagnetism in ZnO and Zn0.9Co0.1O magnetic semiconductor films prepared by sol-gel technique

We report on the origin of ferromagnetism in ZnO and Co-doped ZnO magnetic semiconductor films prepared by sol-gel technique on Si(100) and SiO2 substrates. X-ray diffraction measurements have shown that ZnO and Co-doped ZnO have the expected wurtzite structure and a preferential orientation along the c axis. Optical transmittance measurements revealed that Co is incorporated into the lattice, as Co2+ substituting Zn2+ ions. Magnetization measurements have shown room-temperature ferromagnetism for both ZnO and ZnO:Co films. After annealing, the magnetization curves recorded on free substrates show the same ferromagnetic behavior as the ZnO:Co film indicating that the ferromagnetism is extrinsic and due to external pollution. (c) 2006 American Institute of Physics.