Cobalt-vacancy complexes and origin of ferromagnetism in Zn1-xCoxO dilute magnetic semiconductors

The field of dilute magnetic semiconductors is gaining increasing interest for the potential technological applications in spintronic devices, in which the spin degree of freedom of the charge carriers can be used to enhance the capabilities of conventional electronics, combining semiconducting and magnetic properties. Among dilute magnetic semiconductors, compounds based on ZnO (Zn1-xCoxO and Zn1-xMnxO) are especially appealing since they exhibit ferromagnetism at room temperature and have low toxicity. Despite this great potential, the origin of ferromagnetism in dilute ferromagnetic oxides has been mysterious for years. In this present work, we investigated the local structure of ferromagnetic Zn1-xCoxO epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio density functional theory calculations of selected defect structures. This approach allowed us to perform a full quantitative investigation of the local environment of Co atoms and the three-dimensional structure of the defect complexes. We gave clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration [1]. We also established the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results led to the conclusion that cobalt-oxygen vacancy complexes play an important role in originating the high temperature ferromagnetism of Zn1-xCoxO. These results provided significant contribution to the understanding of the origin of ferromagnetism in dilute ferromagnetic oxides.