Electronic and magnetic properties of Co-doped ZnO diluted magnetic semiconductor

Abstract

The effect of low level Co doping (5%) on polycrystalline ZnO samples has been investigated to correlate the observed changes in their magnetic state vis à vis changes in their electronic properties. Rietveld refinement of the XRD patterns confirms single phase crystallization of the samples in the wurtzite type lattice , with no evidence of any secondary phases. The as-synthesized Co-doped sample shows a paramagnetic (PM) state, however, when hydrogenated for ∼6 h, it shows a strong ferromagnetic (FM) ordering. The magnetic moment suppressed significantly when hydrogen ions were evaporated by heating and the sample turned completely paramagnetic upon long heating in air. The Co 2p X-ray photoelectron spectroscopy (XPS) results show that the substituted Co ions are in 2+ oxidation state that incorporate at Zn 2+ sites and no evidence of metallic Co is observed upon hydrogenation. The XRD refinement results and the O 1s XPS results show clear evidence of oxygen depletion upon hydrogenation, followed by a complete regain upon their long heating in air. A plausible explanation for the observed room temperature ferromagnetism (RTFM) is presented in terms of oxygen vacancies, in the framework of bound magnetic polarons model. Our results evidence that the FM ordering can be switched between “on” and “off” by introducing (upon hydrogenation) or removing (by re-heating), respectively, the oxygen vacancies in the Co-doped ZnO matrix.