Studies of the physical properties of Co, Cu codoped ZnO powders
Introduction
Diluted magnetic semiconductors (DMS) have been extensively studied over the last decade for their unique semiconducting and magnetic properties [1]. In particular, transition-metal-doped ZnO DMSs have been extensively investigated because theoretical studies have predicted that their Curie temperatures (Tc) should be above room temperature [2]. On the experimental side, controversial results still exist for the Co-doped ZnO system [3], [4], [5], [6], [7]. One question is whether Co substitutes for Zn2+ ions or forms secondary clustering phases, i.e., whether the ferromagnetism is intrinsic or extrinsic. These results challenge our understanding of magnetism in DMSs. The other question concerns the effects of donor and acceptor impurity bands on the magnetic couplings. Recently, Coey et al. proposed that ferromagnetic exchange was mediated by shallow donor electrons that formed bound magnetic polarons (BMPs) in oxide DMSs. They concluded that delocalized electrons provided by shallow donor impurities are absolutely necessary for forming BMPs [8], [9]. However, a clear picture of the mechanism responsible of the ferromagnetic ordering has not yet been established. Lin et al. observed that Li can enhanced the ferromagnetism of Co doped ZnO films [10]. Chakraborti et al. have studied the structural and magnetic properties of Zn0.90Co0.10O prepared by the co-precipitation technique [11]. Hou et al. observed the transition temperature of Zn0.98Cu0.02O to be about 350 K but to decrease to 320 K with nitrogen doping [12].
Even though the effects of Co, Cu codoping of ZnO samples have been studied experimentally, the results are not consistent. Lin et al. supposed that a small amount of additional Cu doping would create additional carriers, and that the magnetization would therefore be greatly enhanced in bulk samples [13]. On the other hand, extra doping with Cu would also be expected to create a secondary phase (CuO) and thereby decrease the carrier concentration with the result that the magnetization would decrease. Chakraborti et al. indicated that itinerant electrons were responsible for the ferromagnetism in the Co, Cu codoped ZnO films [11]. This work showed that Cu has a strong ferromagnetic coupling with the saturation magnetic moment. The authors suggested a mechanism in which a bound magnetic polaron was responsible for magnetic ordering in the Co, Cu codoped films. In order to resolve these issues, we have investigated the magnetic properties of Co, Cu codoped ZnO powders fabricated by the sol–gel method.
Section snippets
Experiment
Zn0.95−xCo0.05CuxO powders with x=0, 0.01, 0.03 and 0.05 were prepared via the sol–gel method. Analytic grade Zn(OAc)2·2H2O, Co(OAc)2·4H2O and Cu(NO3)2·3H2O in stoichiometric quantities were completely dissolved in (CH3)2CHOH. In order to mix the solution uniformly, the solution was stirred for about 2 h at 373 K until a clear solution was obtained. During the stirring, appropriate C2H7NO was added as a sol stabilizer. The solution was subsequently dried at 473 K for 2 h to obtain a dry gel. The
Result and discussion
Fig. 1 shows typical powder XRD patterns for Zn0.95−xCo0.05CuxO with x=0, 0.01, 0.03 and 0.05. When the samples are codoped with Co and Cu the diffraction patterns of Zn0.95−xCo0.05CuxO show the same peaks, which indicates that the wurtzite structure is not changed by doping. No secondary phases or clusters are observed in Zn0.95−xCo0.05CuxO within the detection limits of our XRD system, thus providing evidence for the incorporation of both Co and Cu at the Zn sites. To further check on how Cu
Conclusion
In this paper, we have reported on the structural, electrical, and magnetic properties of Co, Cu codoped ZnO powders prepared by sol–gel method. The magnetic properties indicate that all the samples were ferromagnetic and the moment per Co atom increased with increasing Cu concentration. All the samples have large resistance, as Cu1+ compensate the donors. Cu1+, which acts as an acceptor, appears to offer a good explanation for the enhanced ferromagnetic ordering observed in these powders.
Acknowledgments
This work was supported by National Science Foundation of China (10774037 and 10804026) and Natural Science Foundation of Hebei Province (E2007000280).
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