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Structural and ferromagnetic properties of Sn0.95-x(Mn0.05,Lix)O2
Materials Chemistry and Physics 161 (2015) 260e264
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structural and ferromagnetic properties of Sn0.95-x(Mn0.05,Lix)O2 Jianguo Xi a, b, Zhijian Peng a, *, Xiuli Fu b, * a b
School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China
h i g h l i g h t s Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) with well retained host structure was prepared. Sensitivity to Liþ doping level on structure and magnetism evolutions was found. Sn0.95-x(Mn0.05,Lix)O2 is ferromagnetic at 5 K, but paramagnetic at room temperature. The saturation magnetization reaches a maximum of Ms ¼ 1.06 mB/Mn when x ¼ 0.06.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2015 Received in revised form 13 May 2015 Accepted 21 May 2015 Available online 26 May 2015
Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) with well retained host structure was prepared by using conventional solid sintering method. The structure and magnetism evolutions actually show sensitivity to the Li doping level, although the precise position and real content of Li were hard to be determined by powder X-ray diffraction. The measured magnetic susceptibility indicates that Sn0.95-x(Mn0.05,Lix)O2 is ferromagnetic at 5 K, but paramagnetic at room temperature. The saturation magnetization changes with Li doping level, reaching its maximum of Ms ¼ 1.06 mB/Mn when x ¼ 0.06. Li ions can effectively mediate the density of carriers in Sn0.95Mn0.05O2, further enhancing the magnetic moment at low temperature. © 2015 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Magnetic materials Crystal structure Magnetic properties
1. Introduction Diluted magnetic semiconductors (DMSs) are attractive and challenging owing to their potential significance as a kind of multifunctional materials in spintronics [1e3]. The characteristic virtue of DMSs is that they can host both charge and spin degrees of freedom in a single material, justifying them experimentally important semiconducting materials for the wide applications in many new technologies such as spintronics, optoelectronics devices and solar energy conversions [4,5]. As theoretically predicted by Dietl et al. [6], room-temperature ferromagnetism (RTFM) in host nonmagnetic oxide and nitride semiconductors could be induced if they are doped with magnetic transition-metal (TM) elements such as Sc, Ti, V, Cr, Mn, Fe, Co and Ni. Instructed by this, an array of compounds, for example, ZnO [7e11], TiO2 [12,13], and
* Corresponding authors. E-mail addresses: [email protected] (Z. Peng), [email protected] (X. Fu). http://dx.doi.org/10.1016/j.matchemphys.2015.05.050 0254-0584/© 2015 Elsevier B.V. All rights reserved.
SnO2 [14e17], were reported to show FM even above 300 K after being doped. However, despite these significant achievements, the physics of FM in such DMSs has not been completely established yet. To date, the commonly recognized three mechanisms for the origin of FM of DMSs are carrier mediated mechanism [17,18], Fcenter exchange model [19,20], and bound magnetic polaron model [21,22], in which the first one has been discussed more intensively. Moreover, due to its advantages of high dense carriers and wide band gap (~3.6 eV), SnO2 as an oxide semiconductor is a very promising candidate for spintronics. Actually, a large number of studies on TM doped SnO2 exhibiting RTFM have been reported, for example, Co-, Fe-, Ni- and Cr-doped SnO2 [23e26]. However, Mndoped SnO2 is theoretically predicted [27] and experimentally reported [17,28] to be paramagnetic, due to the lack of ferromagnetic coupling between Mn ions in it. Recently, Zhang et al. [29], Jiang et al. [30] and Jayakumar et al. [31] found that the doping of group-I elements is conductive to enhance the RTFM of Zn(TM)O. Kumar et al. [1] revealed that vacancies will control the magnetic properties of ZnNiO films by Li doping that can effectively enhance the density of O vacancies. Gao et al. [5] indicated that Li-doping is
J. Xi et al. / Materials Chemistry and Physics 161 (2015) 260e264
261
capable of lowering the formation energy of the Zn vacancy to produce FM, and the Li ions at the substitutional rather than interstitial sites can even increase the total magnetic moment. Inspired by these reports, in this paper, we have made a systematic effort to explore the structural and magnetic properties of Sn0.95Mn0.05O2 after doping different levels of Li ions.
2. Experimental 2.1. Sample preparation
Fig. 1. XRD patterns of the as-prepared Sn0.95-x(Mn0.05,Lix)O2 with each being marked by the nominal Li doping level.
The samples with nominal compositions of Sn0.95-x(Mn0.05,Lix) O2 (0 x 0.08) were synthesized by conventional solid state reaction at 1000 C with the same process as Refs. [32,33] but starting from SnO2 (99.5 wt.%), Mn (98.0 wt.%) and LiNO3 (99.0 wt.%) raw powders. During processing, the raw powders were first mixed and ball-milled for 24 h in de-ionized water using highly wear-resistant ZrO2 balls as a grinding medium. And then, the resultant slurries were dried at 110 C for 12 h in air. After drying, the powder chunks were ground and sieved into fine powders and subsequently pressed into pellets. These pellets were placed in an alumina crucible, and heated up to 1000 C and kept there for 24 h in a Muffle furnace. Finally, the furnace was cooled down to room temperature naturally after heating by shutting off the electricity source.
2.2. Materials characterization
Fig. 2. Evolution of lattice parameters a and c of Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) with respect to the nominal Li doping level.
The phase structure of the samples was identified by powder Xray diffraction (XRD, D/max-2550, Japan) at room temperature with Cu Ka radiation (l ¼ 1.5418 Å) in a continuous scanning mode with a speed of 4 /min. The magnetic properties (magnetization versus applied field, M-H, and magnetization versus temperature, M-T) were recorded by a magnetic property measurement system (MPMS, Quantum Design, USA). To avoid the shielding of magnetization, loose powder of each sample was used for the measurement.
Fig. 3. Isothermal magnetizations for Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) at (a) 5 and (c) 300 K, respectively. (b) and (d) are the enlarged view for the Sn0.95Mn0.05O2 host.
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3. Results and discussion 3.1. Structural analysis Fig. 1 shows the XRD patterns of the as-synthesized Sn0.95(Mn0.05,Lix)O2 (0 x 0.08). Except for the base peaks of rutile SnO2 phase (crystal system: tetragonal, space group: P42/ mnm; JCPDS card: 21e1250), no extra diffraction peaks can be observed within the detection limit of XRD, indicating a single phase for all the investigated specimens [28,34]. Because of the low resolution of powder XRD, however, it is not easy to confirm if Liþ has been well incorporated into SnO2 crystal lattice or not, and to exclude if there exists Mn clusters or other MnxOy second-phase compounds in the SnO2 matrix. Furthermore, Fig. 2 displays the evolution of lattice parameters a and c with the nominal Li doping level, which are calculated according to Scherrer equation on the basis of the (110) and (101) peaks. For the Sn0.95Mn0.05O2 host, the lattice constants a and c are 4.7370(3) and 3.1859(4) Å, respectively, roughly consistent with the
values reported by S. A. Ahmed [35]. The evolution of a and c is apparently isotropic with x varying from 0 to 0.08. When x 0.06, a and c increase separately by approximately 0.04% and 0.06%, to 4.7390(2) Å for a and to 3.1878(3) Å for c at x ¼ 0.06. The identical variation (increase), well obeying Vegard's law, possibly points out that Liþ (ionic radius: 0.76 Å) has occupied the position of Sn4þ (ionic radius: 0.69 Å) partially, i.e., the center of the tetragonal unitcell. On the other hand, as Sn0.95Mn0.05O2 has mixed valences of Mn2þ (ionic radius: 0.67 Å), Mn3þ (ionic radius: 0.65 Å), and Mn4þ (ionic radius: 0.50 Å) [14,35], when Sn4þ is substituted by Liþ, the released holes will naturally result in a valence state transition of partial Mn2þ and Mn3þ into Mn4þ due to the charge reservation. This effect will also play a nontrivial role in the evolution of the lattice parameters, and hence compete with the Li-doping effect [32]. When x < 0.6, the evolution of lattice parameters induced by such valence state transition is likely to be overshadowed by the Lidoping effect, due to the low Li concentration. When x 0.06, the former effect may be superior over the later one, thus a and c decrease by roughly 0.04% from 4.7390(2) to 4.7373(2) and from
Fig. 4. Temperature dependence of magnetization for Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) in 10 kOe: (a) x ¼ 0, (b) x ¼ 0.02, (c) x ¼ 0.04, (d) x ¼ 0.06, and (e) x ¼ 0.08. The inset at x ¼ 0 is the M1 versus H.
J. Xi et al. / Materials Chemistry and Physics 161 (2015) 260e264
3.1878(3) to 3.1865(2), respectively, due to partial Mn2þ and Mn3þ being transited into Mn4þ. 3.2. Magnetic properties Fig. 3 presents the M-H curves of the Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) samples measured at 5 and 300 K by MPMS under the maximum applied magnetic field of 50 kOe. It can be seen that all the samples exhibit paramagnetic at 300 K and ferromagnetic at 5 K, and the magnetization at 5 K increases with Li doping level, reaching the maximum of ~1.06 mB/Mn when x ¼ 0.06. However, it is necessary to exclude the possibilities of magnetic contaminations, which might produce the ferromagnetism at 5 K. In the present study, even though some MnxOy clusters might exist in the samples, MnxOy clusters cannot be responsible for the ferromagnetism observed from Fig. 3. Because most of MnxOy are antiferromagnetic at low temperature, such as MnO, Mn2O3, and MnO2 with Neel temperatures of 116, 76 and 84 K, respectively [35], their contributions to the ferromagnetism of Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) at 5 K could be excluded. Mn3O4 is an exception, which is ferromagnetic with corresponding Curie temperature (Tc) of 46 K. However, even if all the Mn ions supposedly exist in Mn3O4 in all the samples, they could just contribute ~0.1 mB/Mn to the total magnetic moment, which is much less than the value calculated from Fig. 3. Moreover, no secondary phases are observed within the detection limit of XRD. If Mn3O4 indeed exists in the SnO2 lattice, it must be in a small amount and the magnetic moment from Mn3O4 must be much less than 0.1 mB/Mn. Thus, the Mn-related secondary phases cannot be responsible for the ferromagnetic behavior of Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08). In addition, the increment of Li doping level will lead to the increase in magnetization of Sn0.95x(Mn0.05,Lix)O2 (0 x 0.08). This indicates that Li doping level will positively influence the exchange interaction between Mn2þ ions at the low temperature to some extent. When x ¼ 0.08, the magnetization decreases due to that much Mn2þ ions have been translated into Mn3þ or Mn4þ. The reduction in Mn2þ content will weaken the exchange interaction between Mn2þ through O2, that is, Mn2þ-O2--Mn2þ. The increase in Mn3þ content will enhance the antiferromagnetic super-exchange interaction within Mn3þ ions via O2 ions, i.e., Mn3þ-O2--Mn3þ super-exchange bonds, which will further result in the reduction in the average moment per Mn2þ ion. Fig. 4 illustrates the temperature dependence of magnetization from 5 through 300 K for the as-synthesized Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) in 10 kOe. It can be observed that zero-field-cooling (ZFC) and field-cooling (FC) curves exhibit a distinct splitting recorded at an applied field of 10 kOe up to room temperature in Fig. 4a. This indicates that magnetic domains do not align to a uniform direction under 10 kOe. On the other hand, the weak relationship between M and T shows the occurrence of extremely weak ferromagnetism after consulting from M-H curves at 5 and 300 K in Fig. 3. To achieve more insights into the magnetism in Sn0.95Mn0.05O2, the CurieeWeiss law was used to plot the paramagnetic parts (120 K) of M(T), which is shown by the inset to the corresponding figure (Fig. 4a). The analytical formula was:
MðTÞ ¼
NA m2eff 3kB ðT QW Þ
(1)
where NA is the Avogadro constant, kB the Boltzmann constant, QW the Weiss temperature, and meff the effective magnetic moment. The fit revealed an unusual large QW at about 167 K with an unphysical meff, indicating that the sample is intrinsically paramagnetic, which is consistent with the result derived from the isothermal magnetization characterization. Meanwhile, the splitting between ZFC and FC curves is negligible in the case of Sn0.95-
263
x(Mn0.05,Lix)O2 (0.02 x 0.08) due to the large paramagnetic contribution, which is similar with the result in our previous report [33]. The ZFC and FC curves confirm that with increasing Liþ doping level, no ferromagnetism was observed within the measured temperature range near room temperature, and the M(T) actually presents typical paramagnetic behavior.
4. Conclusions Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) was successfully synthesized via conventional solid state reaction. Structure analysis based on powder X-ray diffraction revealed that typical rutile SnO2 phase was obtained and no secondary phases were detected within the XRD detection limit. Lattice parameters exhibited interesting evolution with Li doping level. Magnetic measurement revealed that Sn0.95Mn0.05O2 is paramagnetic at room temperature and ferromagnetic at low temperature. By Li-doping, the weak ferromagnetism is remarkably enhanced, reaching the magnetic moment of about 1.06 mB/Mn at 5 K when x ¼ 0.06. Acknowledgments The authors would like to thank the financial support for this work from the National Natural Science Foundation of China (Grant nos. 61274015, 11274052 and 51172030), and National Basic Research Program of China (Grant no. 2010CB923200). References [1] E. Senthil Kumar, S. Venkatesh, M.S. Ramachanadra Rao, Appl. Phys. Lett. 96 (2010) 232504. [2] H.S. Kim, F. Lugo, S.J. Pearton, D.P. Norton, Y.L. Wang, F. Ren, Appl. Phys. Lett. 92 (2008) 112108. [3] S.X. Ren, G.W. Sun, J. Zhao, J.Y. Dong, Y. Wei, Z.C. Ma, X. Zhao, W. Chen, Appl. Phys. Lett. 104 (2014) 232406. [4] R. Adhikari, A.K. Das, K. Karmakar, T.V. Chandrasekhar Rao, J. Ghatak, Phys. Rev. B 78 (2008) 024404. [5] H.X. Gao, J.B. Xia, J. Appl. Phys. 111 (2012) 093902. [6] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [7] S. Gautam, P. Thakur, P. Bazylewski, R. Bauer, A.P. Singh, J.Y. Kim, M. Subramanian, R. Jayavel, K. Asokan, K.H. Chae, G.S. Chang, Mater. Chem. Phys. 140 (2013) 130. [8] D.Y. Inamdar, A.D. Lad, A.K. Pathak, I. Dubenko, N. Ali, S. Mahamuni, J. Phys. Chem. C 114 (2010) 1451. [9] R. Viswanatha, D. Naveh, J.R. Chelikowsky, L. Kronik, D.D. Sarma, J. Phys. Chem. Lett. 3 (2013) 2009. [10] E.Z. Liu, J.Z. Jiang, J. Phys. Chem. C 113 (2009) 16116. [11] R. Turgeman, A. Gedanken, Cryst. Growth Des. 6 (2006) 2260. [12] Y. Matsumoto, M. Murakami, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Science 291 (2001) 854. [13] Z.J. Wang, W.D. Wang, J.K. Tang, L.D. Tung, L. Spinu, W.L. Zhou, Appl. Phys. Lett. 83 (2003) 518. [14] A. Espinosa, N. Sanchez, J.S. Marcos, A. de Andres, M.C. Munoz, J. Phys. Chem. C 115 (2011) 24054. [15] S.A. Ahmed, S.H. Mohamed, J. Magn. Magn. Mater. 324 (2012) 812. [16] V. Bilovol, C. Herme, S. Jacobo, A.F. Cabrera, Mater. Chem. Phys. 135 (2012) 334. [17] H. Kimura, T. Fukumura, M. Kawasaki, K. Inaba, T. Hasegawa, Appl. Phys. Lett. 80 (2002) 94. [18] S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, T. Steiner, Semicond. Sci. Tech. 19 (2004) R59. [19] J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, M. Venkatesan, Appl. Phys. Lett. 84 (2004) 1332. [20] C.B. Fitzgerald, M. Venkatesan, A.P. Douvalis, S. Huber, J.M.D. Coey, T. Bakas, J. Appl. Phys. 95 (2004) 7390. [21] R. Adhikari, A.K. Das, D. Karmakar, T.V. Chandrasekhar Rao, J. Ghatak, Phys. Rev. B 78 (2008) 024404. [22] P.A. Wolff, R.N. Bhatt, A.C. Durnst, J. Appl. Phys. 79 (1996) 5196. [23] S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, H.D. Drew, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 91 (2003) 077205. [24] H.S. Kim, L. Bi, G.F. Dionne, C.A. Ross, H.J. Paik, Phys. Rev. B 77 (2008) 214436. [25] N.H. Hong, A. Ruyter, W. Prellier, J. Sakai, N.T. Huong, J. Phys. Condens. Matter 17 (2005) 6533. [26] N.H. Hong, J. Sakai, W. Prellier, A. Hassini, J. Phys. Condens. Matter 17 (2005b) 1697.
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[27] X.L. Wang, Z.X. Dai, Z. Zeng, J. Phys. Condens. Matter 20 (2008) 045214. [28] L.B. Duan, G.H. Rao, J. Yu, Y.C. Wang, G.Y. Liu, J.K. Liang, J. Appl. Phys. 101 (2007) 063917. [29] L.Q. Zhang, Y.Z. Zhang, Z.Z. Ye, J.G. Lu, B. Lu, B. He, J. Appl. Phys. 111 (2012) 123424. [30] J. Jiang, X.T. Wang, L.P. Zhu, Y.Z. Zhang, Z.Z. Ye, B. He, J. Alloys Compd. 513 (2012) 445.
[31] O.D. Jayakumar, C. Sudakar, A. Vinu, A. Asthana, A.K. Tyagi, J. Phys. Chem. C 113 (2009) 4814. [32] J.G. Xi, Z.J. Peng, X.L. Fu, J. Magn. Magn. Mater. 358e359 (2014) 128. [33] J.G. Xi, Z.J. Peng, X.L. Fu, Mater. Lett. 137 (2014) 471. [34] C.V. Komen, A. Thurber, K.M. Reddy, J. Hays, A. Punnoose, J. Appl. Phys. 103 (2008) 7D141. [35] S.A. Ahmed, Solid State Commun. 150 (2010) 2190.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structural and ferromagnetic properties of Sn0.95-x(Mn0.05,Lix)O2 Jianguo Xi a, b, Zhijian Peng a, *, Xiuli Fu b, * a b
School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China
h i g h l i g h t s Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) with well retained host structure was prepared. Sensitivity to Liþ doping level on structure and magnetism evolutions was found. Sn0.95-x(Mn0.05,Lix)O2 is ferromagnetic at 5 K, but paramagnetic at room temperature. The saturation magnetization reaches a maximum of Ms ¼ 1.06 mB/Mn when x ¼ 0.06.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2015 Received in revised form 13 May 2015 Accepted 21 May 2015 Available online 26 May 2015
Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) with well retained host structure was prepared by using conventional solid sintering method. The structure and magnetism evolutions actually show sensitivity to the Li doping level, although the precise position and real content of Li were hard to be determined by powder X-ray diffraction. The measured magnetic susceptibility indicates that Sn0.95-x(Mn0.05,Lix)O2 is ferromagnetic at 5 K, but paramagnetic at room temperature. The saturation magnetization changes with Li doping level, reaching its maximum of Ms ¼ 1.06 mB/Mn when x ¼ 0.06. Li ions can effectively mediate the density of carriers in Sn0.95Mn0.05O2, further enhancing the magnetic moment at low temperature. © 2015 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Magnetic materials Crystal structure Magnetic properties
1. Introduction Diluted magnetic semiconductors (DMSs) are attractive and challenging owing to their potential significance as a kind of multifunctional materials in spintronics [1e3]. The characteristic virtue of DMSs is that they can host both charge and spin degrees of freedom in a single material, justifying them experimentally important semiconducting materials for the wide applications in many new technologies such as spintronics, optoelectronics devices and solar energy conversions [4,5]. As theoretically predicted by Dietl et al. [6], room-temperature ferromagnetism (RTFM) in host nonmagnetic oxide and nitride semiconductors could be induced if they are doped with magnetic transition-metal (TM) elements such as Sc, Ti, V, Cr, Mn, Fe, Co and Ni. Instructed by this, an array of compounds, for example, ZnO [7e11], TiO2 [12,13], and
* Corresponding authors. E-mail addresses: [email protected] (Z. Peng), [email protected] (X. Fu). http://dx.doi.org/10.1016/j.matchemphys.2015.05.050 0254-0584/© 2015 Elsevier B.V. All rights reserved.
SnO2 [14e17], were reported to show FM even above 300 K after being doped. However, despite these significant achievements, the physics of FM in such DMSs has not been completely established yet. To date, the commonly recognized three mechanisms for the origin of FM of DMSs are carrier mediated mechanism [17,18], Fcenter exchange model [19,20], and bound magnetic polaron model [21,22], in which the first one has been discussed more intensively. Moreover, due to its advantages of high dense carriers and wide band gap (~3.6 eV), SnO2 as an oxide semiconductor is a very promising candidate for spintronics. Actually, a large number of studies on TM doped SnO2 exhibiting RTFM have been reported, for example, Co-, Fe-, Ni- and Cr-doped SnO2 [23e26]. However, Mndoped SnO2 is theoretically predicted [27] and experimentally reported [17,28] to be paramagnetic, due to the lack of ferromagnetic coupling between Mn ions in it. Recently, Zhang et al. [29], Jiang et al. [30] and Jayakumar et al. [31] found that the doping of group-I elements is conductive to enhance the RTFM of Zn(TM)O. Kumar et al. [1] revealed that vacancies will control the magnetic properties of ZnNiO films by Li doping that can effectively enhance the density of O vacancies. Gao et al. [5] indicated that Li-doping is
J. Xi et al. / Materials Chemistry and Physics 161 (2015) 260e264
261
capable of lowering the formation energy of the Zn vacancy to produce FM, and the Li ions at the substitutional rather than interstitial sites can even increase the total magnetic moment. Inspired by these reports, in this paper, we have made a systematic effort to explore the structural and magnetic properties of Sn0.95Mn0.05O2 after doping different levels of Li ions.
2. Experimental 2.1. Sample preparation
Fig. 1. XRD patterns of the as-prepared Sn0.95-x(Mn0.05,Lix)O2 with each being marked by the nominal Li doping level.
The samples with nominal compositions of Sn0.95-x(Mn0.05,Lix) O2 (0 x 0.08) were synthesized by conventional solid state reaction at 1000 C with the same process as Refs. [32,33] but starting from SnO2 (99.5 wt.%), Mn (98.0 wt.%) and LiNO3 (99.0 wt.%) raw powders. During processing, the raw powders were first mixed and ball-milled for 24 h in de-ionized water using highly wear-resistant ZrO2 balls as a grinding medium. And then, the resultant slurries were dried at 110 C for 12 h in air. After drying, the powder chunks were ground and sieved into fine powders and subsequently pressed into pellets. These pellets were placed in an alumina crucible, and heated up to 1000 C and kept there for 24 h in a Muffle furnace. Finally, the furnace was cooled down to room temperature naturally after heating by shutting off the electricity source.
2.2. Materials characterization
Fig. 2. Evolution of lattice parameters a and c of Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) with respect to the nominal Li doping level.
The phase structure of the samples was identified by powder Xray diffraction (XRD, D/max-2550, Japan) at room temperature with Cu Ka radiation (l ¼ 1.5418 Å) in a continuous scanning mode with a speed of 4 /min. The magnetic properties (magnetization versus applied field, M-H, and magnetization versus temperature, M-T) were recorded by a magnetic property measurement system (MPMS, Quantum Design, USA). To avoid the shielding of magnetization, loose powder of each sample was used for the measurement.
Fig. 3. Isothermal magnetizations for Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) at (a) 5 and (c) 300 K, respectively. (b) and (d) are the enlarged view for the Sn0.95Mn0.05O2 host.
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J. Xi et al. / Materials Chemistry and Physics 161 (2015) 260e264
3. Results and discussion 3.1. Structural analysis Fig. 1 shows the XRD patterns of the as-synthesized Sn0.95(Mn0.05,Lix)O2 (0 x 0.08). Except for the base peaks of rutile SnO2 phase (crystal system: tetragonal, space group: P42/ mnm; JCPDS card: 21e1250), no extra diffraction peaks can be observed within the detection limit of XRD, indicating a single phase for all the investigated specimens [28,34]. Because of the low resolution of powder XRD, however, it is not easy to confirm if Liþ has been well incorporated into SnO2 crystal lattice or not, and to exclude if there exists Mn clusters or other MnxOy second-phase compounds in the SnO2 matrix. Furthermore, Fig. 2 displays the evolution of lattice parameters a and c with the nominal Li doping level, which are calculated according to Scherrer equation on the basis of the (110) and (101) peaks. For the Sn0.95Mn0.05O2 host, the lattice constants a and c are 4.7370(3) and 3.1859(4) Å, respectively, roughly consistent with the
values reported by S. A. Ahmed [35]. The evolution of a and c is apparently isotropic with x varying from 0 to 0.08. When x 0.06, a and c increase separately by approximately 0.04% and 0.06%, to 4.7390(2) Å for a and to 3.1878(3) Å for c at x ¼ 0.06. The identical variation (increase), well obeying Vegard's law, possibly points out that Liþ (ionic radius: 0.76 Å) has occupied the position of Sn4þ (ionic radius: 0.69 Å) partially, i.e., the center of the tetragonal unitcell. On the other hand, as Sn0.95Mn0.05O2 has mixed valences of Mn2þ (ionic radius: 0.67 Å), Mn3þ (ionic radius: 0.65 Å), and Mn4þ (ionic radius: 0.50 Å) [14,35], when Sn4þ is substituted by Liþ, the released holes will naturally result in a valence state transition of partial Mn2þ and Mn3þ into Mn4þ due to the charge reservation. This effect will also play a nontrivial role in the evolution of the lattice parameters, and hence compete with the Li-doping effect [32]. When x < 0.6, the evolution of lattice parameters induced by such valence state transition is likely to be overshadowed by the Lidoping effect, due to the low Li concentration. When x 0.06, the former effect may be superior over the later one, thus a and c decrease by roughly 0.04% from 4.7390(2) to 4.7373(2) and from
Fig. 4. Temperature dependence of magnetization for Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) in 10 kOe: (a) x ¼ 0, (b) x ¼ 0.02, (c) x ¼ 0.04, (d) x ¼ 0.06, and (e) x ¼ 0.08. The inset at x ¼ 0 is the M1 versus H.
J. Xi et al. / Materials Chemistry and Physics 161 (2015) 260e264
3.1878(3) to 3.1865(2), respectively, due to partial Mn2þ and Mn3þ being transited into Mn4þ. 3.2. Magnetic properties Fig. 3 presents the M-H curves of the Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) samples measured at 5 and 300 K by MPMS under the maximum applied magnetic field of 50 kOe. It can be seen that all the samples exhibit paramagnetic at 300 K and ferromagnetic at 5 K, and the magnetization at 5 K increases with Li doping level, reaching the maximum of ~1.06 mB/Mn when x ¼ 0.06. However, it is necessary to exclude the possibilities of magnetic contaminations, which might produce the ferromagnetism at 5 K. In the present study, even though some MnxOy clusters might exist in the samples, MnxOy clusters cannot be responsible for the ferromagnetism observed from Fig. 3. Because most of MnxOy are antiferromagnetic at low temperature, such as MnO, Mn2O3, and MnO2 with Neel temperatures of 116, 76 and 84 K, respectively [35], their contributions to the ferromagnetism of Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) at 5 K could be excluded. Mn3O4 is an exception, which is ferromagnetic with corresponding Curie temperature (Tc) of 46 K. However, even if all the Mn ions supposedly exist in Mn3O4 in all the samples, they could just contribute ~0.1 mB/Mn to the total magnetic moment, which is much less than the value calculated from Fig. 3. Moreover, no secondary phases are observed within the detection limit of XRD. If Mn3O4 indeed exists in the SnO2 lattice, it must be in a small amount and the magnetic moment from Mn3O4 must be much less than 0.1 mB/Mn. Thus, the Mn-related secondary phases cannot be responsible for the ferromagnetic behavior of Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08). In addition, the increment of Li doping level will lead to the increase in magnetization of Sn0.95x(Mn0.05,Lix)O2 (0 x 0.08). This indicates that Li doping level will positively influence the exchange interaction between Mn2þ ions at the low temperature to some extent. When x ¼ 0.08, the magnetization decreases due to that much Mn2þ ions have been translated into Mn3þ or Mn4þ. The reduction in Mn2þ content will weaken the exchange interaction between Mn2þ through O2, that is, Mn2þ-O2--Mn2þ. The increase in Mn3þ content will enhance the antiferromagnetic super-exchange interaction within Mn3þ ions via O2 ions, i.e., Mn3þ-O2--Mn3þ super-exchange bonds, which will further result in the reduction in the average moment per Mn2þ ion. Fig. 4 illustrates the temperature dependence of magnetization from 5 through 300 K for the as-synthesized Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) in 10 kOe. It can be observed that zero-field-cooling (ZFC) and field-cooling (FC) curves exhibit a distinct splitting recorded at an applied field of 10 kOe up to room temperature in Fig. 4a. This indicates that magnetic domains do not align to a uniform direction under 10 kOe. On the other hand, the weak relationship between M and T shows the occurrence of extremely weak ferromagnetism after consulting from M-H curves at 5 and 300 K in Fig. 3. To achieve more insights into the magnetism in Sn0.95Mn0.05O2, the CurieeWeiss law was used to plot the paramagnetic parts (120 K) of M(T), which is shown by the inset to the corresponding figure (Fig. 4a). The analytical formula was:
MðTÞ ¼
NA m2eff 3kB ðT QW Þ
(1)
where NA is the Avogadro constant, kB the Boltzmann constant, QW the Weiss temperature, and meff the effective magnetic moment. The fit revealed an unusual large QW at about 167 K with an unphysical meff, indicating that the sample is intrinsically paramagnetic, which is consistent with the result derived from the isothermal magnetization characterization. Meanwhile, the splitting between ZFC and FC curves is negligible in the case of Sn0.95-
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x(Mn0.05,Lix)O2 (0.02 x 0.08) due to the large paramagnetic contribution, which is similar with the result in our previous report [33]. The ZFC and FC curves confirm that with increasing Liþ doping level, no ferromagnetism was observed within the measured temperature range near room temperature, and the M(T) actually presents typical paramagnetic behavior.
4. Conclusions Sn0.95-x(Mn0.05,Lix)O2 (0 x 0.08) was successfully synthesized via conventional solid state reaction. Structure analysis based on powder X-ray diffraction revealed that typical rutile SnO2 phase was obtained and no secondary phases were detected within the XRD detection limit. Lattice parameters exhibited interesting evolution with Li doping level. Magnetic measurement revealed that Sn0.95Mn0.05O2 is paramagnetic at room temperature and ferromagnetic at low temperature. By Li-doping, the weak ferromagnetism is remarkably enhanced, reaching the magnetic moment of about 1.06 mB/Mn at 5 K when x ¼ 0.06. Acknowledgments The authors would like to thank the financial support for this work from the National Natural Science Foundation of China (Grant nos. 61274015, 11274052 and 51172030), and National Basic Research Program of China (Grant no. 2010CB923200). References [1] E. Senthil Kumar, S. Venkatesh, M.S. Ramachanadra Rao, Appl. Phys. Lett. 96 (2010) 232504. [2] H.S. Kim, F. Lugo, S.J. Pearton, D.P. Norton, Y.L. Wang, F. Ren, Appl. Phys. Lett. 92 (2008) 112108. [3] S.X. Ren, G.W. Sun, J. Zhao, J.Y. Dong, Y. Wei, Z.C. Ma, X. Zhao, W. Chen, Appl. Phys. Lett. 104 (2014) 232406. [4] R. Adhikari, A.K. Das, K. Karmakar, T.V. Chandrasekhar Rao, J. Ghatak, Phys. Rev. B 78 (2008) 024404. [5] H.X. Gao, J.B. Xia, J. Appl. Phys. 111 (2012) 093902. [6] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [7] S. Gautam, P. Thakur, P. Bazylewski, R. Bauer, A.P. Singh, J.Y. Kim, M. Subramanian, R. Jayavel, K. Asokan, K.H. Chae, G.S. Chang, Mater. Chem. Phys. 140 (2013) 130. [8] D.Y. Inamdar, A.D. Lad, A.K. Pathak, I. Dubenko, N. Ali, S. Mahamuni, J. Phys. Chem. C 114 (2010) 1451. [9] R. Viswanatha, D. Naveh, J.R. Chelikowsky, L. Kronik, D.D. Sarma, J. Phys. Chem. Lett. 3 (2013) 2009. [10] E.Z. Liu, J.Z. Jiang, J. Phys. Chem. C 113 (2009) 16116. [11] R. Turgeman, A. Gedanken, Cryst. Growth Des. 6 (2006) 2260. [12] Y. Matsumoto, M. Murakami, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Science 291 (2001) 854. [13] Z.J. Wang, W.D. Wang, J.K. Tang, L.D. Tung, L. Spinu, W.L. Zhou, Appl. Phys. Lett. 83 (2003) 518. [14] A. Espinosa, N. Sanchez, J.S. Marcos, A. de Andres, M.C. Munoz, J. Phys. Chem. C 115 (2011) 24054. [15] S.A. Ahmed, S.H. Mohamed, J. Magn. Magn. Mater. 324 (2012) 812. [16] V. Bilovol, C. Herme, S. Jacobo, A.F. Cabrera, Mater. Chem. Phys. 135 (2012) 334. [17] H. Kimura, T. Fukumura, M. Kawasaki, K. Inaba, T. Hasegawa, Appl. Phys. Lett. 80 (2002) 94. [18] S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, T. Steiner, Semicond. Sci. Tech. 19 (2004) R59. [19] J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, M. Venkatesan, Appl. Phys. Lett. 84 (2004) 1332. [20] C.B. Fitzgerald, M. Venkatesan, A.P. Douvalis, S. Huber, J.M.D. Coey, T. Bakas, J. Appl. Phys. 95 (2004) 7390. [21] R. Adhikari, A.K. Das, D. Karmakar, T.V. Chandrasekhar Rao, J. Ghatak, Phys. Rev. B 78 (2008) 024404. [22] P.A. Wolff, R.N. Bhatt, A.C. Durnst, J. Appl. Phys. 79 (1996) 5196. [23] S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, H.D. Drew, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 91 (2003) 077205. [24] H.S. Kim, L. Bi, G.F. Dionne, C.A. Ross, H.J. Paik, Phys. Rev. B 77 (2008) 214436. [25] N.H. Hong, A. Ruyter, W. Prellier, J. Sakai, N.T. Huong, J. Phys. Condens. Matter 17 (2005) 6533. [26] N.H. Hong, J. Sakai, W. Prellier, A. Hassini, J. Phys. Condens. Matter 17 (2005b) 1697.
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