Article pubs.acs.org/cm
Ferromagnetism and Crossover of Positive Magnetoresistance to Negative Resistance in Na-Doped ZnO Yiren Wang,† Xi Luo,† Li-Ting Tseng,† Zhimin Ao,‡ Tong Li,§ Guozhong Xing,† Nina Bao,§ Kiyonori Suzukiis,∥ Jun Ding,§ Sean Li,† and Jiabao Yi*,† †
School of Materials Science and Engineering, University of New South Wales, Kensington, 2052, New South Wales Australia Centre for Clean Energy Technology, School of Chemistry and Forensic Science, University of Technology, Sydney, P.O. Box 123, Broadway, Sydney, New South Wales 2007, Australia § Department of Materials Science and Engineering, National University of Singapore, Singapore 119260, Singapore ∥ Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia ‡
S Supporting Information *
ABSTRACT: Na doped ZnO films were fabricated via a hydrothermal process. The films have shown room temperature ferromagnetism and p-type conductivity. Crossover of positive to negative magnetoresistance has been observed with the variation of Na doping concentrations. The positive MR is due to p-p exchange interaction induced Zeeman splitting to suppress the hopping path of holes. The ferromagnetism is attributed to the formation of a Zn vacancy complex. The negative magnetoresistance is due to the minimization of spin-dependent scattering by the applied magnetic field.
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INTRODUCTION Oxide based diluted magnetic semiconductors (DMS) have been of interest due to their possiblility for high Curie temperatures beyond room temperature, predicted by Dietl et al. in 2001.1 Room temperature ferromagnetism (RTF) has been widely reported in oxide based diluted magnetic semiconductors after Dietl’s predication.2−8 However, controversial results have arisen due to the complexity of defects in these oxide semiconductor hosts. Oxide semiconductors doped with magnetic elements with perfect crystalline structure containing very little defects have been found to be nonmagnetic, suggesting that the ferromagnetism in many cases may be associated with the existence of defects, especially in ZnO based DMS.9,10 Room temperature ferromagnetism in many materials is observed only when the samples are prepared under an oxygen deficient environment. Under such conditions, magnetic clusters, secondary phases, and oxygen vacancies are possible as the main contributors to the observed ferromagnetism.9,11−14 In addition, RTF has been observed in nonmagnetic element doped oxides, pure oxides, suggesting that defects may be the origin of ferromagnetism.2,15−21 Oxygen vacancy as the lone contribution has been ruled out by the density functional theory (DFT) calculation and X-ray magnetic circular dichroism (XMCD) characterization in some research.22,23 However, in some other cases, it is thought to trap electrons to form an “F” center, which has exchange interaction with bound magnetic polarons, leading to RTF.18,24 © XXXX American Chemical Society
In addition, oxygen vacancy may play an important role in the RTF via the formation of transition metal−V0 complex.25 However, cation vacancy is considered as the one of the major contributions for the room temperature ferromagnetism in nonmagnetic element doped or pure metallic oxide.14,15,17,18,26−28 Since defects in wide gap semiconductors are possible to induce ferromagnetism, researchers have considered utilizing these defects in engineering the magnetic properties of DMSs. Therefore, nonmagnetic element doping has been purposefully introduced into the oxide host to control the defects, thus generating or enhancing RTF.18,24,27,29−31 Nonmagnetic doping has raised extensive interest since one may avoid the extrinsic ferromagnetism due to magnetic clusters or precipitation of secondary phases associated with the magnetic dopants. In addition, the ferromagnetism induced by defects is of great interest for fundamental physics, since it is different from traditional 3d or 4f element induced ferromagnetism. The ferromagnetism is via the p-p orbital coupling rather than s-p,d exchange coupling.22,27 Furthermore, the magnetic properties can be tailored by the variation of the carrier concentration, showing carrier-mediated ferromagnetism, which is promising for future spintronics devices.27 Received: November 19, 2014 Revised: January 22, 2015
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DOI: 10.1021/cm504261q Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
Advanced D-8, Germany) was used for the phase characterization and scanning electron microscopy (SEM, XL-30, Philips) was used for the surface morphology analysis. A superconducting quantum interference device (SQUID, XL-5, Quantum Design, U.S.A.) was used for the measurement of magnetic properties and transport properties. X-ray photoelectron spectroscopy (XPS, Kratos AXIS DLD) and time of flight secondary ion mass spectroscopy (TOF-SIMS, ION TOF SIMS IV) was used for identifying the impurities. First-principles calculations were performed using density functional theory from the Vienna ab initio simulation package with a plane wave basis.41 The generalized gradient approximation (GGA) with spinpolarized Perdew−Burke−Ernzerhof (PBE)42,43 scheme was employed for calculating the exchange and correlation functionals. A 3 × 3 × 2 ZnO supercell contained 72 atoms, is used in this calculation. The cutoff energy is 475 eV and Brillouin zone integrations are performed using a 3 × 3 × 2 k-mesh based on the Gamma-centered grid. All calculations are performed with the total energy converged to 1.0 × 10−5 eV/atom and the force on an atom within 0.01ev/Å. The calculated lattice constants for bulk ZnO are a = b = 3.287 Å, c = 5.301 Å, which are in good consistence with the experiment values44 (a = b = 3.25 Å, c = 5.21 Å). The slight overestimation is typical for PBE calculations. Figure 1a is a typical XRD pattern of the film of 1 at % Na−ZnO. Only (002) and (004) peaks are seen in the XRD pattern with a
Magnetoresistance (MR) is one of the most important properties for the application of DMS materials. MR in DMS materials can also be used to probe the s,p-d exchange interaction and spin orbital coupling.32 The small positive MR at low magnetic field indicates spin−orbital coupling, and the negative MR is the suppression of spin scattering, which suggests ferromagnetic behavior of DMS materials.33 In addition, giant positive MR has been observed in magnetic element doped DMSs, which is attributed to the giant Zeeman splitting in the conduction band and the suppression of electron hopping.32,34−36 Furthermore, the study on transition metal doped oxide semiconductors, such as Co- or Cu-doped ZnO has shown different MR behaviors. Cu-doped ZnO only showed negative MR, whereas, positive MR was observed in Co-doped ZnO.37,38 The negative MR is related to the minimization of spin scattering and the positive MR, however, has been well explained by the model of spin split subband.37−39 The MR behavior variation by different element dopings may provide ways for tailoring the magnetic and transport properties in DMS materials. There are many reports on the room temperature ferromagnetism of nonmagnetic property doped oxide semiconductors.19−21,27 However, there are few reports on the MR behavior in a nonmagnetic element doped DMS system.22,27 Since it is found that DMSs doped with nonmagnetic elements can show many magnetic properties similar to those of magnetic element doped DMSs, it may be of interest to understand the behavior of the transport properties, such as the resistivity and magnetoresistance in a nonmagnetic element doped DMSs. In addition, to understand these behaviors may lead to new physics, since this ferromagnetism does not arise from traditional 3d or 4f elements. It is known that Na is one of the most important elements to induce p-type conductivity in ZnO. In this work, we introduce Na in ZnO to fabricate nonmagnetic element doped DMS by a hydrothermal technique. It shows that 1% Nadoped ZnO has room temperature ferromagnetism and p-type conductivity. Crossover of positive to negative magnetoresistance (MR) was observed by the variation of Na doping concentrations. The positive MR is due to p-p exchange interaction induced Zeeman splitting, which suppresses the paths of holes hopping. The negative MR is attributed to the minimization of magnetic scattering. The room temperature ferromagnetism is identified to originate from the formation of the Zn vacancy, which is stabilized by the Na substitution and interstitial to form complex defects. This work has demonstrated that defect engineering can be employed to induce RTF and tune the transport properties. In addition, detailed investigations have indicated that the theory of conventional ferromagnetic materials can be applied in DMSs induced by defect engineering, which may provide great opportunities for searching high quality DMS materials.
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Figure 1. (a) XRD spectrum of 1% Na-doped ZnO. The inset is the peak position of (002) of ZnO film doped with various concentrations; (b) EDS spectrum of 1% Na-doped ZnO; (c) is the SEM image in the cross-section; and (d) HRTEM image of 1% Na-doped ZnO.
logarithmic scale in the intensity, indicating that the film was grown with the preferential orientation of [002], which is similar to our previous work with Cu-ZnO film.40 The (002) peaks of the films with different Na doping concentrations in the inset of Figure 1 demonstrate that increasing the doping concentration of Na shifts the peak to the lower angle, suggesting a d-spacing expansion. Since Na+ (0.116 nm) has a larger atomic radius than that of Zn2+ (0.088 nm), such an expansion may be caused by Na ions residing in the interstitial or substitutional Zn sites. The scenario is different from that of Li-doped ZnO, as Li+ (0.076 nm) has a smaller ion radius.27 Figure 1b is the X-ray energy dispersive spectroscopy (EDS) spectrum of 1 at % Na−ZnO, which clearly indicates the existence of Na, Zn, and O elements. The Na concentration analyzed by EDS is approximately 1 at %. In addition, except for the Al peaks from the substrates no other impurities other than Na, Zn, and O can be observed. The inset shows the cross-sectional SEM image of the 1 at% Na−ZnO film, indicating a high quality film with a thickness of around 1 μm. The high resolution TEM image (Figure 1d) shows the lattice fringes of the film (002) plane, demonstrating the good crystallinity of the film. For the nonmagnetic element doped oxide based DMS, one may be concerned with whether there is any impurity of magnetic elements in the films, which may result in the ferromagnetism. In this work, we used SIMS to identify the impurities. The results indicate that there are no impurities other than Na, O, and Zn (Supporting Information, SI, Figure S2a). Since the sensitivity of SIMS is around 0.1 ppm, we can conclude that there is virtually no magnetic impurity in these samples. No magnetic impurities were detected by XPS analysis and Xray energy dispersive spectroscopy either.
EXPERIMENTAL SECTION
Na−ZnO films were fabricated by a hydrothermal method.40 Before the films were grown by the hydrothermal technique, a ZnO seed layer (20 nm) on Al2O3 (001) was first deposited by sputtering. The seed layer had a preferred (002) orientation. The precursor solution is composed of 0.026 M zinc nitrate hexahydrate (Fluka, 99.99%), 0.3125 M ammonium nitrate (Sigma-Aldrich, 99.99%) and different concentrations of sodium nitrate (Fluka, 99.99%) dissolved in deionized water. Ammonia solution (30%) was used to adjust the PH value of the precursor to 7.5. The Al2O3 substrate with seed layer was put into autoclave at 363 K for 24 h. The details of the film deposition can be found in our previous publication.40 XRD (Bruker, B
DOI: 10.1021/cm504261q Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials Table 1. Transport Properties of Na−ZnO Films sample
resistivity (Ω•cm)
mobility (cmV−1S1−)
ZnO 0.2%Na−ZnO 0.6% Na−ZnO 1%Na−ZnO 2%Na−ZnO
0.05 101 20.7 15.8 4.3
2.4 2.6 1.3 0.8 4.7
carriers concentration (/cm3)
type of semiconductivity
× × × × ×
n n p p n
5.2 2.38 2.32 4.9 3.1
Transport measurement shows that the semiconductivity is strongly dependent on the Na doping concentration, as shown in Table 1. For the pure ZnO, it is an n-type semiconductor. When ZnO is doped with a small amount of Na (0.2 at%), the resistivity significantly increases due to the recombination of holes and electron carriers. The resistivity slightly decreases when the doping concentration is 0.6 at %. The conductivity, however, becomes p type. Further increasing the doping concentration to 1 at %, the resistivity becomes smaller, and the film is still p type. If the doping concentration is increased to 2 at %, then the film becomes n type again. Na is one of the most important elements for achieving p type semiconductivity in ZnO.45,46 One substitution of Na with Zn will produce a hole. For the low doping concentration, the produced holes will have recombination with electron carriers, leading to high resistivity. For the doping concentrations higher than 0.6 at%, there will be net holes, which result in p type conductivity. With a further increase in Na doping concentration, some Na atoms may occupy the interstitial site, which leads to n-type carriers. Hence, ptype conductivity changes back to n type due to the large amount of n type carriers. The presence of Na atoms in the interstitial site has been confirmed by XPS analysis (SI Figure S3). Magnetic measurements of the samples demonstrate that some films have room temperature ferromagnetism. The saturation magnetization versus Na doping concentration is plotted in Figure 2a. The film with low doping concentration (i.e., 0.2 at %) does not show ferromagnetism at room temperature. Increasing the doping concentration to 0.6 at %, the ferromagnetism appears, and it reaches the maximum value at 1 at % doping concentration. Further increasing the doping concentration leads to significant decrease in the magnetization, and the ferromagnetism disappears once the doping concentration is beyond 2 at %.
1018 1016 1017 1017 1017
Figure 2b shows the M-H loops of 1 at % Na−ZnO, indicating a saturation magnetization approximately 2 emu/cm3 after deducting the diamagnetic substrate signal, corresponding to 0.41 μB/Na, given that Na is the origin of the ferromagnetism. The M-H curve plotted from the original data is shown in the inset of Figure 2b, and an evident hysteresis loop can be seen. However, 0.2 at % Na-doped ZnO does not show hysteresis, verifying the nonmagnetic behavior, and it also confirms that the ferromagnetism in 1 at % Na−ZnO is not from the magnetic impurities since both samples were fabricated from same precursors, and they should have a similar level of magnetic impurities. In addition, SIMS analysis also showed that there were no magnetic impurities in the 1 at % Na−ZnO film (SI Figure S2). Figure 2c exhibits the temperature dependence of magnetization for the 1 at % Na−ZnO film. The Curie temperature is estimated to be approximately 660 K. The Curie temperature of 0.6 at % Na-doped ZnO cannot be measured due to the low ferromagnetic signal, which is beyond the resolution of the SQUID system in its high temperature operation mode. It is believed that the OH bond on the surface of the ZnO films fabricated by the hydrothermal method may contribute to the ferromagnetism.47 In this case, we removed the OH bonds by annealing the film at 773 K for 30 min in pure O2 atmosphere, and the RTF still remains at both 5 and 300 K, indicating that the ferromagnetism is not induced by the OH bonds (Figure 2d). In addition, we also annealed the film under pure argon atmosphere. The ferromagnetism disappears after the annealing. Comparing the results of Table 1 with those in Figure 2, it can be seen that only Na−ZnO films with p-type conductivity show room temperature ferromagnetism, which is similar to the trend observed in Li-doped ZnO.27 Hence, the ferromagnetism in this work may also originate from the Zn vacancy produced by Na doping, wherein the hole mediation leads to ferromagnetic coupling. It is noteworthy that Na+ (0.116 nm) has a relatively larger atomic radius than that of Li+ (0.076 nm). Hence, Na+ is unlikely to reside in the interstitial sites. In this case, at low doping concentration, Na may mainly occupy the substitution site inducing the Zn vacancies. However, the local moment caused by the Zn vacancies in the ZnO with a low doping concentration may not have long-range ferromagnetic ordering due to low concentrations of holes. Hence, the film with 0.2 at % Na doping is not ferromagnetic. When the doping concentration is increased, more Zn vacancies are produced. The localized magnetic moments mediated by the holes lead to the ferromagnetic ordering at room temperature. For the high doping concentration (i.e., 2 at %), Na+ will reside in both the interstitial and substitutional sites through the distortion of ZnO lattice. This phenomenon has been evidenced by XRD and XPS. In order to further understand the origin of ferromagnetism and the doping mechanism, first principle calculations are employed to calculate the density of states (DOS) of the defects after Na doping. Several kinds of defects are taken into consideration in this work: Zn vacancy (VZn), Na interstitial (Naint), Na substitution with Zn atom (NaZn), Zn interstitial (Zni), Oxygen interstitial (Oi), and Oxygen vacancy (VO). The density of states of the supercell with defects is illustrated in Figure 3. The calculated results show that Zn vacancy prefers a spin polarized state, which is similar to the previously reported works48 and each VZn carries a magnetic moment of about 1.55 μB. NaZn and Zni do not result in magnetic moments, while the Naint has a rather small magnetic moment of 0.0013 μB, which can be neglected. The oxygen vacancy, VO, does not show a magnetic moment either, while Oi prefers a spin polarized state with a magnetic moment of 2.0 μB.
Figure 2. (a) Saturation magnetization dependence on the doping concentration; (b) M-H loop of 1% Na doped ZnO at 5 and 300 K; The inset is the original M-H curve without deducting substrate signal for 0.2% and 1% Na doped ZnO at 300 K; (c) Curie temperature measurement of 1% Na doped ZnO; (d) M-H loop of 1% Na-ZnO annealed under Ar and O2 atmosphere. C
DOI: 10.1021/cm504261q Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
The calculated formation energy of the supercell with a zinc vacancy is very high, indicating that VZn is difficult to generate under normal circumstances. Naint also has a relatively high formation energy. Hence, Naint is not easy to form either. Unlike VZn and Naint, NaZn can remain stable throughout the whole range of oxygen chemical potential, and it is found to be the most stable defect at low oxygen chemical potential. However, at high oxygen chemical potential, the NaZn + Naint defects complex will have the lowest formation energy, which is attributed to the charge neutrality of the supercell when there is an interstitial and one substitutional Na. However, this complex will only form when the Na concentration is high enough. A significant decrease in the formation energy is observed when the Naint +VZn defect complex exists, lowering the formation energy of almost 5 eV compared to that of a single zinc vacancy. The formation energy can be reduced slightly more when the NaZn + Naint + VZn complex is formed. Therefore, it could be concluded that Na doping in ZnO leads to the decrease in the formation energy of Zn vacancy by forming the complex defect. The complex defect has an overall magnetic moment of 0.5 μB. Whereas, our calculations also indicate that Naint + VZn and NaZn + Naint complex both do not produce magnetic moment. It should be noted that NaZn +VZn complex results in a rather high magnetic moment of about 1.49 μB. However, since the formation energy is very high in the whole range of chemical potential, the complex could not contribute to the room temperature ferromagnetism of the system. Similarly, the formation energies of oxygen interstitial (Oi) and zinc interstitial (Zni) are both very high in all the range of oxygen chemical potential, indicating that oxygen and zinc interstitial are not the origin of room temperature ferromagnetism. Since oxygen interstitial (Oi) has a spin polarized state, we also calculate the formation energy of Naint + Oi, Nasub + Oi and NaZn +Naint + Oi defect complex. The formation energy of Naint + Oi is low, whereas, it is nonmagnetic. The formation energies of both Nasub + Oi and NaZn +Naint + Oi defect complex are very high in the whole range chemical potential (5.39−8.3 eV), confirming that NaZn +Naint +VZn defects complex is the origin of the ferromagnetism. From the afore calculation results of Na−ZnO system, we may know that at low oxygen chemical potential, the dopants mainly substitute the zinc sites, and are likely to form NaZn +Naint defects complex. Therefore, the Na-doped ZnO is nonmagnetic at low oxygen chemical potential since the possible defects are nonmagnetic. At high oxygen chemical potential, Na substitution results in Zn vacancy and holes. The NaZn + Naint + VZn defects complex then forms, which can provide a magnetic moment of 0.5 μB, which is in good agreement with experiment results. As a consequence of the ferromagnetic interactions mediated by the holes, Na−ZnO then shows room temperature ferromagnetism. Experimentally, we have shown that the RTF of the film annealed under O2 atmosphere does not change, whereas, the film annealed under Ar atmosphere does not show ferromagnetism anymore. It may be due to that the low oxygen partial pressure induces oxygen vacancies, which may have recombination with Zn vacancy, leading to the disappearance of the ferromagnetism. In order to confirm the small supercell is valid, we also perform a calculation using a 4 × 4 × 3 ZnO supercell contains 96 Zn atoms and 96 O atoms. The results indicates that NaZn + Naint + VZn has a stable magnetic moment of 1.24 μB with a formation energy as low as −2.2 eV at high oxygen chemical potential, confirming the validity of afore calculations. Transport properties are of importance for DMSs. Figure 5 shows the temperature dependence of resistivity and magnetoresistance of Na doped ZnO films. The temperature dependence of resistance of all the samples is plotted in Figure 5a. The resistance of the film decreases with increasing the doping concentration. It is well-known that ZnO is an n-type semiconductor due to its intrinsic defects, such as Zn interstitial or oxygen vacancies. Alkaline element doping can induce holes for p-type conductivity.45,46 However, low doping concentration may lead to the recombination between holes and electrons for the intrinsic n type behavior of pure ZnO. Hence, the resistance may significantly increase due to the recombination (i.e., 0.2 at% Na-doped ZnO). Further increasing the doping concentration may lower the resistivity as the hole concentration increases, given that the structure
Figure 3. Density of states of Zn vacancy, Na interstitial, Na substitution, Zn interstitial, oxygen interstitial, and oxygen vacancies, respectively. The insets are the corresponding DOS in a narrow scale. To confirm which defect is the major origin of the ferromagnetism, we calculate the formation energy as well as the relationship between the formation energy and oxygen chemical potential of these defects, as shown in Figure 4. There are diverse factors that contribute to
Figure 4. Formation energy versus oxygen chemical potential.
chemical potentials, for instance, growing environment or partial pressure. Thus, the range of chemical potential of Na-doped ZnO is related to molecular oxygen and metallic sodium. To prevent the formation of pure Na in the calculation, the bulk energy of Na is the upper limit of Na chemical potential, while the lower boundary is taken from the chemical potential of Na in sodium oxide. It should be noted that sodium oxide is rather active. Consequently, the actual value of the chemical potential can be much lower. The formation energy of ZnO with defects in the neutral state can be computed as follows: ΔEf = Edefect − Esupercell −
∑ Ni·μi
where Edefect is the total energy of supercell with defects, and Esupercell is the total energy for the equivalent ZnO supercell. Ni stands for the number of atoms of type i that have been added to (Ni > 0) or removed from (Ni < 0) the supercell when the defect is created, and μi represents the relevant chemical potentials of these atoms. D
DOI: 10.1021/cm504261q Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials Inσ ∝ T −1/2
The fitted curves are shown in the inset of Figure 5d−h. The curvature plots of all the films confirm that Mott VRH plays an important role in the low temperature conductivity. It is noteworthy that in oxide magnetic semiconductors, the bound magnetic poloron (BMP) is one of the main mechanisms for the origin of ferromagnetism.53−55 The ferromagnetism is via the overlapping of magnetic polorons or the exchange interaction between magnetic polorons and magnetic impurities. In our work, the concentration of magnetic moments from Zn vacancies is relatively low. In addition, there are no magnetic impurities. Furthermore, the doping concentration of Na significantly influences the ferromagnetic ordering. Both 0.2 and 2% Na-doped ZnO do not have ferromagnetic ordering. Hence, both formation mechanisms in our work may not be possible, confirming the carrier mediated ferromagnetism. Figure 5b shows magnetoresistance (MR) behavior of the Na−ZnO with different doping concentrations at 5 K. It is interesting to note that each film has an individual MR character. For the 0.2 at % Na doped ZnO films, no MR can be observed, and this is consistent with the nonmagnetic behavior of the film. For the 0.6 at % Na-doped ZnO, a positive MR at the low applied magnetic field and a negative MR at the high field are observed. Spin−orbit coupling may be attributed to the positive MR at low magnetic field. The negative MR in the relatively high magnetic field is due to the suppression of spin scattering caused by the localized magnetic Zn vacancies.56 Further increasing the doping concentration to 1 at % Na, the MR becomes positive at the magnetic field up to 3 T. As shown in Figure 2a, 1% Na−ZnO has the strongest magnetization among all the films. The strong Zeeman splitting of the conduction band by the s-p interaction may result in a strong positive MR.35,57−59 As aforementioned, the Zn vacancy is the origin of the magnetic moment, which is induced by the polarized O surrounded around Zn vacancy.60 The ferromagnetism ordering is formed by the exchange interaction between hole carriers and localized magnetic moments (Zn vacancies). Therefore, the coupling between carriers and O 2p induced p-p exchange may lead to the spin splitting of conduction band (Zeeman splitting). Increasing external magnetic field results in the large Zeeman splitting, which suppresses the hole’s hopping path. Therefore, the resistivity increases, resulting in the positive MR.57−59 For the 2 at % Na-doped ZnO, many Na ions enter interstitial sites to induce electron carries, which recombine with the holes. Hence, the ferromagnetic ordering through the holes mediation is weakened, thus resulting in the decrease in p-p exchange interaction. In this situation, the positive MR will strongly decrease and eventually disappear with a further increase in Na-doping concentration.61 Certainly, the magnetic scattering suppression by the applied magnetic field then dominates, producing the negative MR, as shown in Figure 5b. Similarly, for the 1 at % Na−ZnO sample, the positive MR is observed at 100 and 5 K. Crossover of the positive MR to negative MR is observed at room temperature (Figure 5c). The phenomena were also observed in Co doped ZnO single crystals, Co doped ZnO thin films, as well as in Mn doped ZnO thin films.38,51,58This work has shown that defects induced ferromagnetic ordering has a similar phenomenon to that ferromagnetic ordering induced by the magnetic element in DMSs (i.e., Mn-ZnO and Cu doped ZnO). The disappearance of positive MR at room temperature is due to the decreasing p-p exchange interaction. The minimization of spin scattering from the localized magnetic moments due to spin alignment by the applied magnetic field is again attributed to the negative MR observed at room temperature. It should be noted that the anomalous Hall effect (AHE) has been observed for 1 at % Na− ZnO at 5 K. However, no AHE can be observed at room temperature, which is similar to that of ref 27. In summary, we have successfully fabricated high quality Na−ZnO thick films using the hydrothermal method. Room temperature ferromagnetism has been observed for the 1 at % Na-doped ZnO. The possible contribution from the magnetic impurities to the observed phenomena has been excluded. The ZnO vacancy, which is induced by the Na substitution and stabilized by both Na substitution and Na interstitial, has been identified as the origin of room temperature
Figure 5. (a) R-T curve of Na−ZnO with a variety of doping concentrations; (b) MR curve of Na−ZnO with a variety of doping concentrations at 5 K; and (c) temperature dependence of MR curve of 1% Na−ZnO. does not change too much. Whereas, increasing the doping concentration further may lead to n type conductivity again due to the large amount of interstitial Na ions (i.e., 2 at % Na−ZnO). Figure 5a also shows that the resistivity at low temperature is much higher than that at room temperature, indicating a typical semiconductor behavior. For the magnetic ZnO film with dopants, the conductivity at low temperature (
