Diluted magnetic semiconductors (DMS) have attracted a lot of attention for their potential applications in the field of spin-dependent semiconductor electronics and optoelectronics, or so-called spintronics and optospintronics [1] . Simulations of Sato et al. predicated that the ferromagnetism could also be achieved in V, Cr, Fe, Co, and Ni-doped ZnO [2] . Copper (Cu) is a typical non-magnetic transition metal dopant, because metallic Cu and Cu related oxides are non ferromagnetic materials. Theoretical and experimental studies have confirmed that there is room temperature ferromagnetism (RTFM) in Cu Doped ZnO. For this reason, Cu doped ZnO is considered as an ideal candidate to study the mechanism of ferromagnetism in ZnO based DMS. Room-temperature ferromagnetism in ZnO doped with Fe has been achieved; however, there remain some questions regarding the origin of the magnetic behavior in Fe-doped ZnO materials. It is deemed that additional Cu doping is essential to achieve RTFM in Fe-doped ZnO bulk samples [3] . Howerver, Shim et al. found that the ferromagnetism in Fe- and Cu-codoped ZnO stems from the secondary phase ZnFe₂O₄ [4] .

In the present work, we have introduced additional Cu in Zn_0.98Fe_0.02O bulk samples by a coprecipitation method, and compared the ferromagnetism of Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O system.

Bulk samples with nominal component Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O were prepared by a coprecipitation method. Appropriate proportions of Zn(NO₃)₂∙6H₂O, Cu(NO₃)₂∙3H₂O, and Fe(NO₃)₃∙9H₂O high-purity (99.99%) powders were weighed and mixed according to the desired stoichiometry, the powders were dissolved in distilled water to get homogeneous solution. The mixture were stirred strongly while proper amount of Na(OH) aqueous solution were poured into it, controlling the PH = 7 to deposit all cations of Zn²⁺, Fe³⁺, Cu²⁺ and completely. The obtained precipitate was thoroughly washed with distilled water and dried in air at 200˚C, and then prefired at 400˚C for 8 hours. The prepared powders were ground, palletized, and sintered at 600˚C for 12 hours. To aviod the formation of secondary phase as far as possible, the sintering process was executed in Ar gas atmosphere. X-ray diffraction (XRD, PANalytical B.V.) was used to determine the crystallinity and secondary phase formation. Chemical bonding states and chemical compositions of the samples were analyzed by x-ray photoelectron spectroscopy (XPS, VG Multilab 2000). Physical Properties Measurements System (PPMS, Quantum Design) was used to characterize magnetic behavior of the doped samples.

The crystal structure of the samples was characterized by x-ray diffraction using Cu Kα radiation. Data were collected using a step scan of 0.017˚ in 2θ. Figure 1 presents the typical powder x-ray diffraction patterns for ZnO, Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O. No clear difference in XRD patterns can be found between pure ZnO and doping samples, suggesting that the doping has not changed the structure of ZnO. All the diffraction peaks can be indexed to a wurtzite structure as ZnO, and there is no indication of secondary phase within our detection limit. It suggests that all samples are of single phase and iron, stannum and copper have been incorporated into the lattice structure, forming a solid solution instead of precipitates.

The magnetic properties of Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O were investigated by checking the temperature (T) and magnetic field (H) dependence of the magnetization (M). Figure 2 shows the M as a function of T (M-T) for all samples in an applied field of 1000 Oe from 10 to 300 K. For Zn_0.98Fe_0.02O, M gradually increases with the decrease of T above 25 K, and the curve becomes flat below 25 K, the maximum value of M (M_max) can be estimated to about 0.74 μB/Fe site. The result hints probable low-temperature ferromagnetism in Zn_0.98Fe_0.02O bulk sample. Zn_0.97Fe_0.02Cu_0.01O shows a similar M-T behavior to Zn_0.98Fe_0.02O with an equal value of M_max.

The inverse of M as a function of T (M^-1-T) for Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O was plotted to understand the magnetism, as shown in Figure 3. The solid lines are extrapolation fits to the data in the range of 180 - 300 K for these samples. According to the discussion by Spacek et al. [5] , the Curie-Weiss temperature Θ₀ is evaluated to be 140 K for Zn_0.98Fe_0.02O and 90 K for Zn_0.97Fe_0.02Cu_0.01O. The positive Θ₀ for Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O suggests that ferromagnetic interaction is dominant in the two samples, which confirms the low-temperature ferromagnetism in Zn_0.97Fe_0.02Cu_0.01O and Zn_0.98Fe_0.02O.

Figure 4 shows the M-H curves of Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O taken at 300 K, the inset gives a partial enlarged detail. All samples show a room-temperature ferromagnetic behavior with a modest hysteresis loops, which suggests that their Curie temperature (TC) are higher than RT. This result is consistent to the theoretical predictions [2] [6] and essential for practical application in spintronics. From the inset, the Coercive force (HC) and residual magnetization (MR) can be estimated to about 90 Oe and 0.005 μB/Fe site for Zn_0.98Fe_0.02O, 100 Oe and 0.006 μB/Fe site for Zn_0.97Fe_0.02Cu_0.01O. Simultaneously, the saturation magnetization (MS) also can be estimated to about 0.08 μB/Fe site for Zn_0.98Fe_0.02O, 0.085 μB/Fe site for Zn_0.97Fe_0.02Cu_0.01O from Figure 4. From the above, we can see that additional Cu doping has not induced remarkable change in magnetic properties at 300 K. This result is very different to the previous results [4] , in there, a small amount of additional Cu doping in Zn_0.95Fe_0.05O bulk sample caused a drastic change in M, the MS at room temperature of the sample with 1% Cu doping becomes 30 times larger than that of the sample without Cu.

The M-H curves of Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O also have been measured at 10K, as shown in Figure 5, a partial enlarged detail also has been given in the inset. Both Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O show “S” shaped hysteresis loops. The MS of Zn_0.98Fe_0.02O and Zn_0.97Fe_0.02Cu_0.01O is about 1.5 μB/Fe site and 1.85 μB/Fe site, respectively. It seems that additional Cu doping causes an enhancement of ferromagnetism at low temperature of 10 K.

The observation of RT FM in Zn-Fe-O system is consistent with the prediction by theory, it has been proved

that the RT FM in this system is intrinsic, in accordance with the previous work [7] . It should be noted that, additional Cu doping has not induced obvious change in ferromagnetism of Zn-Fe-O system at 300 K, but rather caused a small increase in M only at 10 K. It means that additional Cu doping has a little effect on ferromagnetism of Zn-Fe-O system, but not very crucial as mentioned in [4] .

The most popular mechanisms relative to carrier proposed to explain ferromagnetic ordering in DMSs are RKKY interaction, double-exchange mechanism, and the bound magnetic polaron (BMP) model. A quantitative calculation of the carrier concentration is very helpful to understand this issue by measuring the Hall effect of these samples, but we have failed to obtain the carrier concentration due to the considerable Hall voltage created by the large bulk resistivity higher than 106 Ω∙cm at room temperature. Nevertheless, from this we can conclude that most of carriers are localized in these samples and these samples are insulating. So both RKKY-type and double-exchange mechanism can be eliminated because that there are not enough free carriers to mediate RKKY-type interaction and/or double-exchange interaction. It seems that the bound magnetic polaron (BMP) [8] [9] model is an alternative theory for the FM at RT observed in this study. For an insulating DMS system with a quite low carrier density to exhibit ferromagnetism, the BMP model provides a mechanism whereby holes that are located spatially at or near the transition-metal ion are responsible for mediating ferromagnetism [10] . So appropriate hole concentration is necessary in order to induce ferromagnetic ordering. Additional Cu doping will increase the hole density of the system, thus increases the number of BMPs, and then results in an enhanced ferromagnetism. However, the increase of hole density caused by additional Cu doping is quite limited because it is very difficult to realize heavy acceptor doping in ZnO matrix. Therefore, additional Cu doping has induced no significant change in ferromagnetism of Zn_0.98Fe_0.02O, just only at 10 K, caused a light increase of M.

In conclusion, the magnetic properties of Fe-doped ZnO bulk samples doped with additional Cu were comparatively investigated. All doping samples are single phase with a wurtzite structure characterized by XRD. The results of magnetic measurement suggest that Cu doping has enhanced the ferromagnetism of Zn_0.98Fe_0.02O at 10 K to some extent. This is consistent with the bound magnetic polaron model relative to hole, in which bound holes mediate the ferromagnetic ordering.

This work was supported by the National Science Foundation of China under Grant No. 51002144.