- Email: [email protected]
Magnetic behavior of Co–Mn co-doped ZnO nanoparticles
Journal of Magnetism and Magnetic Materials 372 (2014) 37–40
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic behavior of Co–Mn co-doped ZnO nanoparticles Hengda Li n, Xinzhong Liu, Zhigong Zheng The College of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou, Fujian 350108, China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 November 2013 Received in revised form 16 May 2014 Available online 25 July 2014
Here, we report on systematic studies of the magnetic properties of Co and Mn co-doped ZnO nanoparticles prepared by a sol–gel technique. The effect of the concentration of the doping ions on the magnetic properties of Co and Mn co-doped ZnO nanoparticles is presented. X-ray diffraction characterizations (XRD) of co-doped ZnO nanoparticles are all wurtzite structure. The Zn0.96Co0.02Mn0.02O nanoparticles and Zn0.94Co0.02Mn0.04O nanoparticles display ferromagnetic behavior at room temperature. Superconducting quantum interference device (SQUID) magnetometer figures show that with the concentration of the Mn ions increased, the saturation magnetic moment (Ms) increased, and the magnetic is probably due to the co-doping of the Mn ions. Our results demonstrate that the Mn ions doping concentration play an important role in the ferromagnetic properties of Co–Mn co-doped ZnO nanoparticles at room temperature. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO Co-dope Magnetic Nanoparticle
1. Introduction In recent years, transition metal (TM) doped ZnO diluted magnetic semiconductor (DMS) have attracted much scientific interest due to its potential applications in spintronics such as spin-valve transistors, spin light emitting diodes, and nonvolatile storage [1–3]. However, these devices can only be operated at cryogenic temperatures because of the absence of ferromagnetism or the low Curie temperature (TC). A key work to realize spintronics devices is to develop DMS with ferromagnetism above room temperature. The possibility of designing such a DMS is brightened after the advent of transition-metal-doped semiconducting oxides such as ZnO, SnO2 and TiO2. The II–VI semiconducting materials have many novel properties from both fundamental and technological point of view. Among all these oxides, ZnO belongs to the list of the most suitable building block materials for spintronics application due to its abundance and harmonious environment and also due to its potential as a suitable optoelectronic material with a wide band gap (3.37 eV) and a high exciton binding energy (60 meV) [4]. Theoretical calculations have predicted the ZnO-based DMS, such as V-, Cr-, Fe-, Co-, and Ni-doped ZnO compounds can exhibit ferromagnetic (FM) behavior, provided that the transition metal doping produces a partial filled spin-split impurity band [5,6]. Recently, ferromagnetism with a TC higher than RT has also been reported for V-, Fe-, Co-, and Ni-doped ZnO DMS, following the prediction of Dietl et al. [7–12]. Recent experimental work clearly
establishes that the origin of room temperature ferromagnetism in nanocrystalline ZnO is due to free carriers [13,14]. In this work, we use a sol–gel method to fabricate the Co and Mn co-doped ZnO nanoparticles, and made attempts to modify the structures and crystalline properties to obtain the best magnetic effects. For these purposes, an attempt has been made to synthesize room temperature ferromagnetic DMS samples by co-doping Co and Mn ions in ZnO lattice to synthesis Zn0.96Co0.02Mn0.02O and Zn0.94Co0.02Mn0.04O. Magnetic measurements indicated that undoped ZnO was diamagnetic in nature whereas Co and Mn co-doped ZnO samples exhibited ferromagnetic behavior at room temperature, which is possibly related to the substitution of Co and Mn ions for Zn ions in the ZnO lattice, the Mn doping content can affect the ferromagnetic behavior of the nanoparticles effectively.
2. Experiment 2.1. Chemicals zinc acetate (Zn(CH3COO)2 2H2O), copper acetate (Co(CH3COO)2 4H2O), manganese acetate (Mn(CH3COO)2 4H2O) and 2,6-diethylaniline (DEA) were purchased from from Beijing Chemical Co. All the reagents in the experiment were used as received without further purification. Triply distilled water was used for the entire synthesis. 2.2. Instruments
n
Corresponding author. Tel.: þ 86 186 591 70649. E-mail address: [email protected] (H. Li).
http://dx.doi.org/10.1016/j.jmmm.2014.07.006 0304-8853/& 2014 Elsevier B.V. All rights reserved.
The crystal structure of Co–Mn co-doped ZnO nanoparticles (NPs) was characterized by X-ray diffraction (XRD) using a Siemens
38
H. Li et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 37–40
D5005 X-ray powder diffractometer with a Cu Kα (λ ¼1.5418 Å) radiation source at 40 kV and 30 mA. The surface morphology of the samples was measured on a Hitachi H-8100 transmission electron microscope (TEM) operated at an acceleration voltage of 200 kV. Xray photoelectron spectra (XPS) were investigated by using a VG ESCALAB MK II spectrometer with an Mg KR excitation (1253.6 eV). Perkin-Elmer Optima 3300DV Inductive Coupled Plasma Emission Spectrometer (ICP) was used to measure the concentration of the doping ions. Magnetic measurements were carried out in a superconducting quantum interference device (SQUID).
2.3. Sample preparation Co–Mn co-doped ZnO nanocrystalline samples were prepared by the sol–gel method according to the literature [15]. First, an appropriate amount of Zn(CH3COO)2 2H2O (Co(CH3COO)2 4H2O, Mn(CH3COO)2 4H2O) was used as the starting precursors, ethanol solution as a solvent, and DEA as a stabilizing agent. An optimized amount of DEA was added to the solution to enhance the dissolution of the metal ions. After rigorously stirred at 80 1C for 2 h, the sol can be obtained and then dried at 140 1C to get the xerogel. Finally, the xerogel was calcined at 500 1C for 1 h in O2 atmosphere, and we obtained the ZnO, Zn0.98Co0.02O, Zn0.96Co0.02 Mn0.02O, and Zn0.94 Co0.02 Mn0.04O samples.
ZnO Zn0.98Co0.02O Zn0.96Co0.02Mn0.02O Zn0.94Co0.02Mn0.04O
7000
Intensity
6000 5000 4000 3000 2000 1000 0 30
40
50
60
70
2Thera/degree Fig. 1. XRD pattern for the ZnO, Zn0.98Co0.02O, Zn0.96Co0.02Mn Zn0.94Co0.02Mn0.04O samples.
0.02O,
and
3. Results and discussion The single phase of the prepared samples has been confirmed by analyzing the XRD data. Fig. 1 shows the XRD pattern for the ZnO, Zn0.98Co0.02O, Zn0.96Co0.02Mn 0.02O and Zn0.94Co0.02Mn0.04O, samples. The patterns are found to be in a good agreement with the standard peak positions of pure ZnO (JCPDS card no. 36-1451); no peak from Mn2O3 or Co2O3 was found [16]. All the investigated samples are nanocrystalline powder of hexagonal wurtzite structure with space group P63mc. It can be seen from the figure that the Co and Mn co-doped ZnO nanoparticle are all have a good c-axis texture. And we can conclude that the Co and Mn ions are implanted in the ZnO nanoparticle. We have observed that the replacement of Zn ions by Co and Mn ions is expected to decrease the intensity of XRD peaks, indicating that the degree of crystallinity of samples decreases, meanwhile the concentration of the defects increases. Fig. 2 shows the TEM image of the ZnO nanoparticles after calcination at 500 1C. Uniform particle size distribution of Zn0.94Co0.02Mn0.04O samples were identified from these images and Zn0.94Co0.02Mn0.04O sample possess the similar morphology in comparison to pure ZnO sample (Fig. 3). To further examine the character of Co–Mn co-doped ZnO NPs, the states of Co and Mn in ZnO nanoparticles measured by XPS were obtained. The Co 2p1/2 (796.8 eV) and Co 2p3/2 (781.1 eV) peaks had a difference of 15.7 eV, which indicates that Co ions were of þ2 valences when substituting Zn ions in wurtizite ZnO [17]. As for Mn, the binding energy of Mn 2p3/2, Mn 2p1/2 peaks are 641.4 and 653.5 eV, respectively. The Mn2p3/2 peak appears at 641.4 eV, no XPS signals from metallic Mn clusters (637.7 eV) and Mn4 þ ions (642.4 eV). Meanwhile, there is a shake-up peak satellite between Mn 2p3/2 and Mn 2p1/2, which is the typical character of Mn2 þ ions [18]. The results indicated that Co and Mn ions were all þ2 valence when substituting Zn ions in wurtizite ZnO. Further, the content of doping Co and Mn ions has been confirmed by an Inductive Coupled Plasma Emission Spectrometer (ICP) technique (Table 1). As can be seen from Table 1, the final content of Co and Mn ions in the ZnO lattice were almost consistent with the concentration we used in the experiment. From Fig. 4 we can observe that with the increasing of the applied magnetic field, the magnetization is linear increased. So the as-prepared Zn0.98Co0.02O showed paramagnetic character. In this work, we co-doped Mn ions to increase the concentration of
Fig. 2. TEM pictures of ZnO (left) and Zn0.94 Co0.02Mn0.04O (right).
H. Li et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 37–40
39
Mn 2p3/2
Co 2p3/2 Mn 2p1/2
CPS
CPS
Co 2p1/2
660
655
650
645
640
635
Binding Energy (eV)
775
780
785
790
795
800
805
810
Binding Energy (eV)
Fig. 3. The XPS spectra of Zn0.94Co0.02Mn0.04O nanoparticles (a) XPS high-resolution scan of Mn 2p peaks. (b) XPS high resolution scan of Co 2p peaks.
Table 1 Quantitative ICP results of Co and Mn ions concentrations doped in the ZnO nanoparticles. Sample
Stoichiometry
Co concentration
Mn concentration
ZnO
Initial From ICP Initial From ICP Initial From ICP Initial From ICP
0.00 0.00 0.02 0.02134 0.02 0.02031 0.02 0.01994
0.00 0.00 0.00 0.00 0.02 0.01987 0.04 0.04026
Zn0.98Co0.02O Zn0.96Co0.02Mn0.02O Zn0.94Co0.02Mn0.04O
Magnetization (emu/g)
0.08
Fig. 5. Magnetic hysteresis loops for Zn0.96Co0.02Mn0.02O and Zn0.94Co0.02Mn0.04O nanoparticles.
Zn0.98Co0.02O
0.04
0.00 -0.04 -0.08 -60000 -40000 -20000
0
20000
40000
60000
Applied Magnetic Field (Oe) Fig. 4. Magnetic hysteresis loops for Zn0.98Co0.02O nanoparticles.
the free carrier [19]. It can enhance the spin coupling of the magnetic ions. So we can observe from Fig. 5 that both Zn0.96Co0.02Mn0.02O and Zn0.94Co0.02Mn0.04O samples calcined at 500 1C exhibited ferromagnetic character. It can be seen from the figure that with the concentration of the doping ions increased, the saturation magnetic moment (MS) increased from 0.0018– 0.0098 emu/g. The field dependent magnetization curves for the Zn0.94Co0.02Mn0.04O shows a higher magnetization than that of Zn0.96Co0.02Mn0.02O nanoparticles. It is clear that ferromagnetic ordering at room temperature strongly depends on the concentration of Mn ions doping ions. As discussed above, we can infer that the FM in co-doped ZnO nanoparticles is the intrinsic behavior of Co2 þ and Mn2 þ ions substitution for Zn2 þ in the tetrahedral configuration. According to the RKKY theory, the ferromagnetic interactions in DMS
materials are usually produced due to the exchange interaction between local spin-polarized electrons (such as the electrons of Co2 þ and Mn2 þ ions) and conductive electrons or free carriers [20–22]. Since ferromagnetism in the samples originates from the exchange interaction between free delocalized carriers hole and electron from the valence band, the presence of free carriers is a necessary condition for the appearance of ferromagnetism. Free carriers can be induced by doping, defects, or Co ions in another oxidation state [19]. So in our work, we co-doped the Mn ions to increase the concentration of the free carrier. The increase of the saturation magnetic moment (Ms) may be due to the increasing of the Mn ions. Since from the XRD patterns that there is no other compounds observed in the nanoparticle, so the magnetic cannot come from the Co2O3 or Mn2O3. So it probably comes from the free carrier with the co-doped of the Co and Mn ions, and it is expected to be considered as a promising material for spintronics.
4. Conclusion In this study, we report on the structural, compositional and magnetic characterization of Co–Mn co-doped ZnO DMS nanoparticles of the wurtzite structure synthesized with a simple sol–gel method and detailed structural and magnetic characterizations of Co–Mn co-doped ZnO nanoparticles are investigated. XRD results rules out the presence of clusters and secondary phases as the source of ferromagnetism. We obtained the room temperature ferromagnetic when Mn ions co-doped into the ZnO lattice. Meanwhile, we observed that the co-doping of the Mn2 þ ions in
40
H. Li et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 37–40
the ZnO lattice play an important role in the magnetic behavior, an appropriate amount of the Mn ions doping concentration increased, the saturation magnetic moment increased. Prime novelty statement In this article, we report on the structural, compositional and magnetic characterization of Co–Mn co-doped ZnO DMS nanoparticles of the wurtzite structure synthesized with a simple sol–gel method and detailed structural and magnetic characterizations of Co–Mn co-doped ZnO nanoparticles are investigated. Meanwhile, we observed that the co-doping of the Mn2 þ ions in the ZnO lattice play an important role in the magnetic behavior, an appropriate amount of the Mn ions doping concentration increased, the saturation magnetic moment increased. Acknowledgements This study was supported by the National Natural Science Foundation (Grant no. 10604054) of PR China. References [1] D.P. Joseph, G.S. Kumar, C. Venkateswaran, Mater. Lett. 59 (2005) 2720–2724. [2] P.V. Radovanovic, N.S. Norberg, K.E. McNally, D.R. Gamelin, J. Am. Chem. Soc. 124 (2002) 15192–15193.
[3] C. Liu, F. Yun, H. Morkoc, J. Mater. Sci.—Mater. Electron. 16 (2005) 555–597. [4] H. Zeng, G. Duan, Y. Li, S. Yang, X. Xu, W. Cai, Adv. Funct. Mater. 20 (2010) 561–572. [5] H. Pan, J.B. Yi, L. Shen, R.Q. Wu, J.H. Yang, J.Y. Lin, Y.P. Feng, J. Ding, L.H. Van, J.H. Yin, Phys. Rev. Lett. 99 (2007). [6] N.A. Spaldin, Phys. Rev. B: Condens. Matter 69 (2004). [7] H.O.T. Dietl, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019–1021. [8] N.S. Norberg, K.R. Kittilstved, J.E. Amonette, R.K. Kukkadapu, D.A. Schwartz, D. R. Gamelin, J. Am. Chem. Soc. 126 (2004) 9387–9398. [9] Y.M. Cho, W.K. Choo, H. Kim, D. Kim, Y. Ihm, Appl. Phys. Lett. 80 (2002) 3358–3360. [10] H. Saeki, H. Tabata, T. Kawai, Solid State Commun. 120 (2001) 439–443. [11] H.J. Lee, S.Y. Jeong, C.R. Cho, C.H. Park, Appl. Phys. Lett. 81 (2002) 4020–4022. [12] X. Liu, F. Lin, L. Sun, W. Cheng, X. Ma, W. Shi, Appl. Phys. Lett. 88 (2006). [13] P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M.O. Guillen, B. Johansson, G.A. Gehring, Nat. Mater. 2 (2003) 673–677. [14] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988–990. [15] A. Manivannan, P. Dutta, G. Glaspell, M.S. Seehra, J. Appl. Phys. 99 (2006). [16] H. Zhang, D. Yang, Y.J. Ji, X.Y. Ma, J. Xu, D.L. Que, J. Phys. Chem. B 108 (2004) 3955–3958. [17] Z. Yin, N.F. Chen, C.L. Chai, F. Yang, J. Appl. Phys. 96 (2004) 5093–5096. [18] L.M. Xu, Y.P. Yu, X.J. Xing, X.Y. Wu, S.W. Li, Appl. Phys. A 92 (2008) 361–365. [19] K.R. Kittilstved, D.A. Schwartz, A.C. Tuan, S.M. Heald, S.A. Chambers, D.R. Gamelin, Phys. Rev. Lett. 97 (2006). [20] V.K. Sharma, R. Xalxo, G.D. Varma, Cryst. Res. Technol. 42 (2007) 34–38. [21] D.J. Priour, E.H. Hwang, S. Das Sarma, Phys. Rev. Lett. 92 (2004). [22] D. Wang, S.-J. Xiong, Chin. Phys. Lett. 25 (2008) 1102–1105.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic behavior of Co–Mn co-doped ZnO nanoparticles Hengda Li n, Xinzhong Liu, Zhigong Zheng The College of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou, Fujian 350108, China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 November 2013 Received in revised form 16 May 2014 Available online 25 July 2014
Here, we report on systematic studies of the magnetic properties of Co and Mn co-doped ZnO nanoparticles prepared by a sol–gel technique. The effect of the concentration of the doping ions on the magnetic properties of Co and Mn co-doped ZnO nanoparticles is presented. X-ray diffraction characterizations (XRD) of co-doped ZnO nanoparticles are all wurtzite structure. The Zn0.96Co0.02Mn0.02O nanoparticles and Zn0.94Co0.02Mn0.04O nanoparticles display ferromagnetic behavior at room temperature. Superconducting quantum interference device (SQUID) magnetometer figures show that with the concentration of the Mn ions increased, the saturation magnetic moment (Ms) increased, and the magnetic is probably due to the co-doping of the Mn ions. Our results demonstrate that the Mn ions doping concentration play an important role in the ferromagnetic properties of Co–Mn co-doped ZnO nanoparticles at room temperature. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO Co-dope Magnetic Nanoparticle
1. Introduction In recent years, transition metal (TM) doped ZnO diluted magnetic semiconductor (DMS) have attracted much scientific interest due to its potential applications in spintronics such as spin-valve transistors, spin light emitting diodes, and nonvolatile storage [1–3]. However, these devices can only be operated at cryogenic temperatures because of the absence of ferromagnetism or the low Curie temperature (TC). A key work to realize spintronics devices is to develop DMS with ferromagnetism above room temperature. The possibility of designing such a DMS is brightened after the advent of transition-metal-doped semiconducting oxides such as ZnO, SnO2 and TiO2. The II–VI semiconducting materials have many novel properties from both fundamental and technological point of view. Among all these oxides, ZnO belongs to the list of the most suitable building block materials for spintronics application due to its abundance and harmonious environment and also due to its potential as a suitable optoelectronic material with a wide band gap (3.37 eV) and a high exciton binding energy (60 meV) [4]. Theoretical calculations have predicted the ZnO-based DMS, such as V-, Cr-, Fe-, Co-, and Ni-doped ZnO compounds can exhibit ferromagnetic (FM) behavior, provided that the transition metal doping produces a partial filled spin-split impurity band [5,6]. Recently, ferromagnetism with a TC higher than RT has also been reported for V-, Fe-, Co-, and Ni-doped ZnO DMS, following the prediction of Dietl et al. [7–12]. Recent experimental work clearly
establishes that the origin of room temperature ferromagnetism in nanocrystalline ZnO is due to free carriers [13,14]. In this work, we use a sol–gel method to fabricate the Co and Mn co-doped ZnO nanoparticles, and made attempts to modify the structures and crystalline properties to obtain the best magnetic effects. For these purposes, an attempt has been made to synthesize room temperature ferromagnetic DMS samples by co-doping Co and Mn ions in ZnO lattice to synthesis Zn0.96Co0.02Mn0.02O and Zn0.94Co0.02Mn0.04O. Magnetic measurements indicated that undoped ZnO was diamagnetic in nature whereas Co and Mn co-doped ZnO samples exhibited ferromagnetic behavior at room temperature, which is possibly related to the substitution of Co and Mn ions for Zn ions in the ZnO lattice, the Mn doping content can affect the ferromagnetic behavior of the nanoparticles effectively.
2. Experiment 2.1. Chemicals zinc acetate (Zn(CH3COO)2 2H2O), copper acetate (Co(CH3COO)2 4H2O), manganese acetate (Mn(CH3COO)2 4H2O) and 2,6-diethylaniline (DEA) were purchased from from Beijing Chemical Co. All the reagents in the experiment were used as received without further purification. Triply distilled water was used for the entire synthesis. 2.2. Instruments
n
Corresponding author. Tel.: þ 86 186 591 70649. E-mail address: [email protected] (H. Li).
http://dx.doi.org/10.1016/j.jmmm.2014.07.006 0304-8853/& 2014 Elsevier B.V. All rights reserved.
The crystal structure of Co–Mn co-doped ZnO nanoparticles (NPs) was characterized by X-ray diffraction (XRD) using a Siemens
38
H. Li et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 37–40
D5005 X-ray powder diffractometer with a Cu Kα (λ ¼1.5418 Å) radiation source at 40 kV and 30 mA. The surface morphology of the samples was measured on a Hitachi H-8100 transmission electron microscope (TEM) operated at an acceleration voltage of 200 kV. Xray photoelectron spectra (XPS) were investigated by using a VG ESCALAB MK II spectrometer with an Mg KR excitation (1253.6 eV). Perkin-Elmer Optima 3300DV Inductive Coupled Plasma Emission Spectrometer (ICP) was used to measure the concentration of the doping ions. Magnetic measurements were carried out in a superconducting quantum interference device (SQUID).
2.3. Sample preparation Co–Mn co-doped ZnO nanocrystalline samples were prepared by the sol–gel method according to the literature [15]. First, an appropriate amount of Zn(CH3COO)2 2H2O (Co(CH3COO)2 4H2O, Mn(CH3COO)2 4H2O) was used as the starting precursors, ethanol solution as a solvent, and DEA as a stabilizing agent. An optimized amount of DEA was added to the solution to enhance the dissolution of the metal ions. After rigorously stirred at 80 1C for 2 h, the sol can be obtained and then dried at 140 1C to get the xerogel. Finally, the xerogel was calcined at 500 1C for 1 h in O2 atmosphere, and we obtained the ZnO, Zn0.98Co0.02O, Zn0.96Co0.02 Mn0.02O, and Zn0.94 Co0.02 Mn0.04O samples.
ZnO Zn0.98Co0.02O Zn0.96Co0.02Mn0.02O Zn0.94Co0.02Mn0.04O
7000
Intensity
6000 5000 4000 3000 2000 1000 0 30
40
50
60
70
2Thera/degree Fig. 1. XRD pattern for the ZnO, Zn0.98Co0.02O, Zn0.96Co0.02Mn Zn0.94Co0.02Mn0.04O samples.
0.02O,
and
3. Results and discussion The single phase of the prepared samples has been confirmed by analyzing the XRD data. Fig. 1 shows the XRD pattern for the ZnO, Zn0.98Co0.02O, Zn0.96Co0.02Mn 0.02O and Zn0.94Co0.02Mn0.04O, samples. The patterns are found to be in a good agreement with the standard peak positions of pure ZnO (JCPDS card no. 36-1451); no peak from Mn2O3 or Co2O3 was found [16]. All the investigated samples are nanocrystalline powder of hexagonal wurtzite structure with space group P63mc. It can be seen from the figure that the Co and Mn co-doped ZnO nanoparticle are all have a good c-axis texture. And we can conclude that the Co and Mn ions are implanted in the ZnO nanoparticle. We have observed that the replacement of Zn ions by Co and Mn ions is expected to decrease the intensity of XRD peaks, indicating that the degree of crystallinity of samples decreases, meanwhile the concentration of the defects increases. Fig. 2 shows the TEM image of the ZnO nanoparticles after calcination at 500 1C. Uniform particle size distribution of Zn0.94Co0.02Mn0.04O samples were identified from these images and Zn0.94Co0.02Mn0.04O sample possess the similar morphology in comparison to pure ZnO sample (Fig. 3). To further examine the character of Co–Mn co-doped ZnO NPs, the states of Co and Mn in ZnO nanoparticles measured by XPS were obtained. The Co 2p1/2 (796.8 eV) and Co 2p3/2 (781.1 eV) peaks had a difference of 15.7 eV, which indicates that Co ions were of þ2 valences when substituting Zn ions in wurtizite ZnO [17]. As for Mn, the binding energy of Mn 2p3/2, Mn 2p1/2 peaks are 641.4 and 653.5 eV, respectively. The Mn2p3/2 peak appears at 641.4 eV, no XPS signals from metallic Mn clusters (637.7 eV) and Mn4 þ ions (642.4 eV). Meanwhile, there is a shake-up peak satellite between Mn 2p3/2 and Mn 2p1/2, which is the typical character of Mn2 þ ions [18]. The results indicated that Co and Mn ions were all þ2 valence when substituting Zn ions in wurtizite ZnO. Further, the content of doping Co and Mn ions has been confirmed by an Inductive Coupled Plasma Emission Spectrometer (ICP) technique (Table 1). As can be seen from Table 1, the final content of Co and Mn ions in the ZnO lattice were almost consistent with the concentration we used in the experiment. From Fig. 4 we can observe that with the increasing of the applied magnetic field, the magnetization is linear increased. So the as-prepared Zn0.98Co0.02O showed paramagnetic character. In this work, we co-doped Mn ions to increase the concentration of
Fig. 2. TEM pictures of ZnO (left) and Zn0.94 Co0.02Mn0.04O (right).
H. Li et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 37–40
39
Mn 2p3/2
Co 2p3/2 Mn 2p1/2
CPS
CPS
Co 2p1/2
660
655
650
645
640
635
Binding Energy (eV)
775
780
785
790
795
800
805
810
Binding Energy (eV)
Fig. 3. The XPS spectra of Zn0.94Co0.02Mn0.04O nanoparticles (a) XPS high-resolution scan of Mn 2p peaks. (b) XPS high resolution scan of Co 2p peaks.
Table 1 Quantitative ICP results of Co and Mn ions concentrations doped in the ZnO nanoparticles. Sample
Stoichiometry
Co concentration
Mn concentration
ZnO
Initial From ICP Initial From ICP Initial From ICP Initial From ICP
0.00 0.00 0.02 0.02134 0.02 0.02031 0.02 0.01994
0.00 0.00 0.00 0.00 0.02 0.01987 0.04 0.04026
Zn0.98Co0.02O Zn0.96Co0.02Mn0.02O Zn0.94Co0.02Mn0.04O
Magnetization (emu/g)
0.08
Fig. 5. Magnetic hysteresis loops for Zn0.96Co0.02Mn0.02O and Zn0.94Co0.02Mn0.04O nanoparticles.
Zn0.98Co0.02O
0.04
0.00 -0.04 -0.08 -60000 -40000 -20000
0
20000
40000
60000
Applied Magnetic Field (Oe) Fig. 4. Magnetic hysteresis loops for Zn0.98Co0.02O nanoparticles.
the free carrier [19]. It can enhance the spin coupling of the magnetic ions. So we can observe from Fig. 5 that both Zn0.96Co0.02Mn0.02O and Zn0.94Co0.02Mn0.04O samples calcined at 500 1C exhibited ferromagnetic character. It can be seen from the figure that with the concentration of the doping ions increased, the saturation magnetic moment (MS) increased from 0.0018– 0.0098 emu/g. The field dependent magnetization curves for the Zn0.94Co0.02Mn0.04O shows a higher magnetization than that of Zn0.96Co0.02Mn0.02O nanoparticles. It is clear that ferromagnetic ordering at room temperature strongly depends on the concentration of Mn ions doping ions. As discussed above, we can infer that the FM in co-doped ZnO nanoparticles is the intrinsic behavior of Co2 þ and Mn2 þ ions substitution for Zn2 þ in the tetrahedral configuration. According to the RKKY theory, the ferromagnetic interactions in DMS
materials are usually produced due to the exchange interaction between local spin-polarized electrons (such as the electrons of Co2 þ and Mn2 þ ions) and conductive electrons or free carriers [20–22]. Since ferromagnetism in the samples originates from the exchange interaction between free delocalized carriers hole and electron from the valence band, the presence of free carriers is a necessary condition for the appearance of ferromagnetism. Free carriers can be induced by doping, defects, or Co ions in another oxidation state [19]. So in our work, we co-doped the Mn ions to increase the concentration of the free carrier. The increase of the saturation magnetic moment (Ms) may be due to the increasing of the Mn ions. Since from the XRD patterns that there is no other compounds observed in the nanoparticle, so the magnetic cannot come from the Co2O3 or Mn2O3. So it probably comes from the free carrier with the co-doped of the Co and Mn ions, and it is expected to be considered as a promising material for spintronics.
4. Conclusion In this study, we report on the structural, compositional and magnetic characterization of Co–Mn co-doped ZnO DMS nanoparticles of the wurtzite structure synthesized with a simple sol–gel method and detailed structural and magnetic characterizations of Co–Mn co-doped ZnO nanoparticles are investigated. XRD results rules out the presence of clusters and secondary phases as the source of ferromagnetism. We obtained the room temperature ferromagnetic when Mn ions co-doped into the ZnO lattice. Meanwhile, we observed that the co-doping of the Mn2 þ ions in
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H. Li et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 37–40
the ZnO lattice play an important role in the magnetic behavior, an appropriate amount of the Mn ions doping concentration increased, the saturation magnetic moment increased. Prime novelty statement In this article, we report on the structural, compositional and magnetic characterization of Co–Mn co-doped ZnO DMS nanoparticles of the wurtzite structure synthesized with a simple sol–gel method and detailed structural and magnetic characterizations of Co–Mn co-doped ZnO nanoparticles are investigated. Meanwhile, we observed that the co-doping of the Mn2 þ ions in the ZnO lattice play an important role in the magnetic behavior, an appropriate amount of the Mn ions doping concentration increased, the saturation magnetic moment increased. Acknowledgements This study was supported by the National Natural Science Foundation (Grant no. 10604054) of PR China. References [1] D.P. Joseph, G.S. Kumar, C. Venkateswaran, Mater. Lett. 59 (2005) 2720–2724. [2] P.V. Radovanovic, N.S. Norberg, K.E. McNally, D.R. Gamelin, J. Am. Chem. Soc. 124 (2002) 15192–15193.
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