Evidence of Cobalt-Vacancy Complexes in Zn1-xCoxO Dilute Magnetic Semiconductors

We investigate the local structure of ferromagnetic Zn1-xCoxO epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio calculations of selected defect structures. We give clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration. We also establish the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results lead to the conclusion that oxygen vacancies play an important role in originating the high temperature ferromagnetism of Zn1-xCoxO.