Chin. Phys. B Vol. 20, No. 2 (2011) 027103 Effect of oxygen vacancy defect on the magnetic properties of Co-doped ZnO * Weng Zhen-Zhen(翁臻臻) a)b) , Zhang Jian-Min(张健敏) c) , Huang Zhi-Gao(黄志高) c) † , and Lin Wen-Xiong(林文雄) a) ‡ a) Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China b) College of Physics and Information Engineering, Fuzhou University, Fuzhou 350108, China c) School of Physics and OptoElectronics Technology, Fujian Normal University, Fuzhou 350007, China (Received 25 July 2010; revised manuscript received 13 September 2010) The influence of oxygen vacancy on the magnetism of Co-doped ZnO has been investigated by the first-principles calculations. It is suggested that oxygen vacancy and its location play crucial roles on the magnetic properties of Co-doped ZnO. The exchange coupling mechanism should account for the magnetism in Co-doped ZnO with oxygen vacancy and the oxygen vacancy is likely to be close to the Co atom. The oxygen vacancy (doping electrons) might be available for carrier mediation but is localized with a certain length and can strengthen the ferromagnetic exchange interaction between Co atoms. Keywords: Co-doped ZnO, oxygen vacancy, ferromagnetism PACS: 71.15.Mb, 71.55.Gs, 75.50.Pp DOI: 10.1088/1674-1056/20/2/027103 1. Introduction Transition metal (TM)-doped diluted magnetic semiconductors (DMSs) have become a highly desir- able goal in the last years because of their poten- tial applications in the field of spintronics. [1] Among DMSs II–VI compounds doped with TM, ZnO have attracted great attention due to their abundance and environment-friendly nature and also due to their po- tential as a suitable optoelectronic material with a wide-gap (3.3 eV) and high exciton binding energy of 60 meV. Moreover, since Dietl et al. have pre- dicted that a high-Curie temperature (T c ) DMSs could be obtained in p-type Mn-doped ZnO, [2] great in- terests in TM-doped ZnO have been generated. In particular, the Co-doped ZnO system is promising for applications requiring ferromagnetism above room temperature. [3,4] For practical applications, enhanc- ing the T c of DMSs to above room temperature is a key issue. Despite a great deal of studies focusing on the Co-doped ZnO, there have been many contra- dictory reports on its magnetic states. Theoretically, Sato et al. predicted that Co-doped ZnO might be ferromagnetic (FM) without any carrier doping [5] and some other computational results also predicted the existence of high T c for it. [6-8] Experimentally, Ueda et al. reported ferromagnetic behaviour of Co-doped ZnO synthesized by pulsed-laser deposition and with a T c above 300 K; [9] Janisch et al. showed that mag- netic moments of Zn 1-x Co x O films grown by various techniques spread over a relatively wide range from 0.56 to 2.6 μ B /Co. [10] However, other reports claimed no trace of ferromagnetism in these systems. [11,12] Re- cently, it has been found that the magnetic proper- ties of DMSs are sensitive to the growth methods and growth conditions. [13-17] Hsu et al. have investigated the FM dependence of oxygen vacancies in the Co doped ZnO films by x-ray near edge spectroscopy. It was found that the enhancement (suppression) of fer- romagnetism is strongly correlated with the increase (decrease) of the oxygen vacancies in ZnO. [18] Also, Seghier et al. have suggested that the oxygen vacancy (V O ) is likely to be a main mediator for the ferro- magnetism in the Co-doped ZnO. [19] Therefore, it is necessary to investigate the effect of V O on the mag- netic properties of Co-doped ZnO. In this paper, we report the effect of V O on the electronic structure and magnetic coupling for Co- doped ZnO with first-principles method. We first cal- * Project supported by the National Key Project for Basic Research of China (Grant No. 2005CB623605), the Fund of National Engineering Research Center for Optoelectronic Crystalline Materials (Grant No. 2005DC105003) and the National Natural Science Foundation of China (Grant No. 60876069). † Corresponding author. E-mail: zghuang@fjnu.edu.cn ‡ E-mail: wxlin@fjirsm.ac.cn c  2011 Chinese Physical Society and IOP Publishing Ltd http://www.iop.org/journals/cpb http://cpb.iphy.ac.cn 027103-1