Investigating the ferromagnetic exchange interaction in Co-doped ZnO magnetic semiconductors

We propose a defect-complex model to clarify the origin of ferromagnetism in Co-doped ZnO magnetic semiconductors, based on first-principles calculations. The spin-polarized defect states of oxygen vacancy (VO) induced by the charge-transfer process are essential to the ferromagnetism. Depending on the charge state of VO and carrier concentration, (VO2+ + zinc interstitial) and (VO0 + zinc vacancy) defect complexes are predicted to control the ferromagnetic exchange interaction between the Co ions for insulating and conductive samples, respectively.