Effect of ion irradiation on the structural and the magnetic properties of Zn0.75Co0.25O magnetic semiconductors

We have used magnetron co-sputtering to grow Zn0.75Co0.25O magnetic semiconductors on SiO2/Si substrate at 400degreesC. X-ray diffraction analysis has shown that the film is textured with c-axis of the wurtzite structure along the growth direction. Optical absorption measurements have been performed and showed three absorptions bands that are due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ substituting Zn2+ ions. Magnetization measurements have been performed at room temperature and have shown the presence of the ferromagnetism with a saturation magnetization of about 0.2 mu(B)/Co. Irradiation with He ions at 520degreesC have been performed on the as-deposited film. For a dose of 6 x 10(16) cm(-2) a huge increase of the magnetic moment up to 0.37 mu(B)/Co has been observed and accompanied with an improvement of the crystalline quality. However, for higher doses the magnetic moment is strongly reduced down to 0.13mu(B)/Co. This is attributed to the change in the free carriers density in the ZnO host. (C) 2004 Elsevier B.V. All rights reserved.