Surfactant-Assisted Synthesis of Co- and Li-Doped ZnO Nanocrystalline Samples Showing Room-Temperature Ferromagnetism
✍ Scribed by O. D. Jayakumar; I. K. Gopalakrishnan; S. K. Kulshreshtha
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 702 KB
- Volume
- 18
- Category
- Article
- ISSN
- 0935-9648
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✦ Synopsis
Dilute magnetic semiconductor oxides are of current interest because of their potential spintronics applications, where the charge and spin degrees of the freedom of electrons are used simultaneously for novel memory and optical device applications. [1,2] In particular, Co-doped ZnO has attracted considerable interest. [3] There have been a number of reports about the observation of room-temperature ferromagnetism in thin films of Zn 1-x Co x O produced by different techniques. [4][5][6][7] Schwartz et al. [8] also observed ferromagnetism above room temperature in aggregated particles of Co-doped ZnO, by heat treating (below 200 /C) colloidal quantum dots of Co-doped ZnO. However, recent work on well-characterized polycrystalline Zn 1-x Co x O samples indicate that they are not ferromagnetic (FM) at room temperature, [9][10][11][12][13][14] except for the samples in an isolated report by Deka et al. [15] In general, studies on polycrystalline samples have converged to the conclusion that robust room-temperature ferromagnetism (RTF) is not realizable in Co-doped ZnO without additional carrier doping. Sato and Katayama-Yoshida [16] predicted Co-doped ZnO would become FM in the presence of n-type carriers. This was experimentally demonstrated by Schwartz and Gamelin. [17] They were the first to show the reversible cycle from a paramagnetic (PM) to a FM state in Co-doped ZnO spin-coated films, produced from colloidal nanocrystals, by introducing and removing interstitial Zn (Zn i ), a native n-type defect of ZnO. Later, Spaldine, [18] in a computational study, showed that only hole doping promotes RTF in Co-doped ZnO. This is in contrast with RTF observed in electron-doped, Co-doped ZnO by Schwartz and Gamelin. [17] Through an analysis of density functional calculations Sluiter et al. [19] have shown that both electron doping with zinc interstitials and hole doping with zinc vacancies make a ZnO:Co sample strongly FM. Sluiter et al. [19] also showed experimentally that co-doping with Li promotes FM in Co-doped ZnO. They assumed that Li co-doping creates p-type carriers in ZnO.