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Enhanced ferromagnetism by lithium doping in Sn0.95Fe0.05O2
Journal of Magnetism and Magnetic Materials 358-359 (2014) 128–131
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Enhanced ferromagnetism by lithium doping in Sn0.95Fe0.05O2 Jian-guo Xi a,b, Zhi-jian Peng a,n, Xiu-li Fu b,n 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
art ic l e i nf o
a b s t r a c t
Article history: Received 24 March 2013 Received in revised form 31 December 2013 Available online 30 January 2014
Lithium doping effects on Sn0.95Fe0.05O2 were studied in terms of structural and magnetic properties. The nominal lithium doping level in Sn0.95 x(Fe0.05,Lix)O2 was achieved to x ¼0.06 with well retaining the host structure. Although the precise position and real content of lithium were hard to be determined by powder X-ray diffraction, the structure parameters, as well as the magnetic properties, actually show sensitivity to the doping. The most interesting observation herein is the remarkably enhanced ferromagnetism, possibly due to that the doped lithium favors the ferromagnetic exchange interaction in Sn0.95Fe0.05O2 through tuning the concentration of Fe3 þ ions responsible for the magnetic interaction. The observation is very similar with those cases in other group-I elements doped diluted magnetic semiconductors, indicating possibly generic phenomena being significant for spintronics. & 2014 Elsevier B.V. All rights reserved.
Keywords: Diluted magnetic semiconductor SnO2 Li þ ion doping Magnetic property
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted considerable interest owing to the significance for application as a kind of multifunctional material in spintronics [1]. The characteristic virtue of DMSs is that they host both charge and spin degrees of freedom in single material, and the delicate interplay between them thereby gives rise to exotic properties. As proposed by Dietl et al., room-temperature ferromagnetism (FM) 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 [2]. Instructed by this, an array of compounds, for examples, ZnO [3–7], TiO2 [8,9], and SnO2 [10–13], was reported to show FM even above 300 K after being doped. Despite these significant achievements, the physics of FM in such DMSs has not been completely established yet. Regarding the sofar major viewpoints about the origin, such as the carrier mediated mechanism through tuning the magnetic exchange interaction strength [14,15], the so-called F-center exchange model [16–18], and the bound magnetic polaron model [19,20], the first mechanism has been discussed more intensively. SnO2 is a wide-gap paramagnetic semiconductor at room temperature, with a typical gap size of 3.6 eV [21], thus being ideal for serving as the host for DMSs. In fact, when the size of the crystallite is reduced to nano-scale, SnO2 itself can show
n
Corresponding authors. Tel.: þ 86 10 82320255; fax: þ86 10 82322624. E-mail addresses: [email protected], [email protected] (Z.-j. Peng), [email protected] (X.-l. Fu). 0304-8853/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2014.01.043
magnetism with a relatively high saturation magnetization due to the spins polarization of 2p and 2s electrons of the oxygen atoms [13]. Regarding the TM doping in SnO2, a variety of elements, including Cr, Fe, Co, Ni, Mn and V, have been tried and they were confirmed to work effectively to establish the magnetic state [22,23]. However, not only the origin but also the magnetism type remains highly controversial. Fe-doped SnO2, for instance, was reported to exhibit FM in Sn0.95Fe0.05O2 thin film with moments ranging from 1.06 to 4.76 μB/Fe and a very high Curie temperature Tc of 610 K [17], while the moment of Sn0.95Fe0.05O2 bulk ceramic (Tc, 360 K) was only 0.95 μB/Fe, about 15% of the high-spin Fe3 þ (3d5, 5.9 μB/Fe) calculated from g[S(Sþ1)]1/2, where g ( ¼2) is the Landé factor and S (¼2) is the spin number, which indicated a nearly paramagnetic nature since about 85% of spins are magnetically ordered. More interestingly, rigidly depending on the synthesis temperature, the Fe-doped SnO2 powder showed either ferromagnetic (r600 1C) or antiferromagnetic (4600 1C) behavior [19,24]. Very recently, the FM in an analog Zn(TM)O was found to be remarkably enhanced by co-doping of group-I elements, which was ascribed to the hole-related defects favoring the magnetic interaction [25–27]. According to a theoretical calculation, Li-doping is capable of lowering the formation energy of the Zn vacancy to produce FM, and they at the substitutional sites rather than the interstitial ones even can increase the total magnetic moment [28]. Throughout the overview, it is clear that a study on the Li-doped Sn(TM)O2 would be necessary, not only due to the importance for achieving more in-depth insights into the magnetism in Sn(TM)O2, and hence a clarification of the longstanding controversy, but also because of the merit of checking the universality of the FM enhancement behavior induced by group-I elements doping. We are thus motivated to carry out the study.
J.-g. Xi et al. / Journal of Magnetism and Magnetic Materials 358-359 (2014) 128–131
129
The structural and magnetic properties of Li-doped Sn0.95Fe0.05O2 are reported herein.
2. Experimental procedure Polycrystalline Sn0.95 x(Fe0.05,Lix)O2 (0 rx r0.08) was synthesized via the conventional solid state reaction method starting from the powders of stoichiometric SnO2 (99.5 wt%), Fe2O3 (98.0 wt%), and LiNO3 (99.0 wt%). The starting powders were first mixed and ball-milled for 24 h in de-ionized water using highly wear-resistant ZrO2 balls as a grinding medium. The resultant slurries were then dried at 110 1C for 12 h in air. The dried chunks were ground and sieved into fine powders, which were subsequently pressed into pellets and placed in an alumina crucible. Then the crucible was heated at 1000 1C for 24 h in an electrical resistance chamber furnace. The furnace was finally cooled down to room temperature after heating by shutting off the electricity source. Powder X-ray diffraction (XRD) patterns were recorded at room temperature on a 8 kW D/max-2550 diffractometer with Cu Kα radiation (λ¼ 1.5418 Å) in the 2θ range from 201 to 801. The dc magnetic susceptibility was measured between 2 and 300 K with the presence of a 10 kOe magnetic field under both zero-fieldcooling (ZFC) and zero-cooling (FC) conditions, on a magnetic property measurement system (MPMS), Quantum Design. Isothermal magnetizations (Μ(H)) at 2 and 300 K were also measured on the same apparatus with the magnetic field within 750 kOe. To avoid the shielding of magnetization, loose powder of each sample was used for the measurement.
3. Results and discussion The XRD patterns of the synthesized compounds are presented in Fig. 1. All the diffraction peaks acquired from each compound match well with a rutile structure of SnO2 (space group: P42/mnm; JCPDS card: 21-1250), implying a single phase of all studied specimens. The small impurity peaks marked by both asterisks and pound signs were identified to arise from Fe2O3. When xo 0.04, only α-Fe2O3 impurity of a hematite phase (space group: R-3c; JCPDS card: 33-0664) was observed, while not only α-Fe2O3 but also γ-Fe2O3 crystalized in a maghemite-C phase (space group: P4132; JCPDS card: 39-1346) was visible when xZ 0.04. Moreover,
Fig. 1. XRD patterns for Sn0.95 x(Fe0.05,Lix)O2 with each being marked by the nominal Li doping level. The peaks marked by pound signs and asterisks arise from α- and γ-Fe2O3 impurities, respectively.
Fig. 2. Evolution of lattice parameters a and c of Sn0.95 x(Fe0.05,Lix)O2 (0 rx r 0.08) with respect to the nominal Li doping level.
the amount level of γ-Fe2O3 remains at the same over the compositions estimated from the intensity of the peaks, while the intensity of α-Fe2O3 shows slight increase when x 40.02, but its amount remains almost the same level in the samples with x¼ 0.04, 0.06 and 0.08. The reason remains unknown, which is possibly caused by the local structure change with the variation of Li concentration in Sn0.95Fe0.05O2. In addition, due to the low resolution of powder XRD, it is hard to judge whether Li is well incorporated into the SnO2 crystal lattice or not. However, from the evolution of lattice parameters and the sensitivity of the magnetic properties to the nominal Li doping level as presented later, it is reasonably to conclude that Li was successfully doped into Sn0.95Fe0.05O2. The lattice parameters calculated from the XRD patterns are plotted in Fig. 2. For the Sn0.95Fe0.05O2 host, the lattice constants a and c are 4.7351(5) and 3.1866(4) Å, respectively, roughly consistent with the previously reported values in Refs. [18,29]. The evolution of a and c is apparently isotropic when x r0.04 since they both increase by approximately 0.1%, to 4.7400(3) Å for a and to 3.1898(2) Å for c at x ¼0.04. Such an isotropic evolution with well obeying Vegard's law possibly indicates an occupation of Li þ ions at the Fe sites, i.e. the center of the tetragonal unit-cell. On the other side, since Sn0.95Fe0.05O2 has mixed valences of Fe2 þ (ionic radius: 0.77 Å) and Fe3 þ (ionic radius: 0.64 Å) [29,30], once Sn4 þ is replaced by Li þ , the released holes will naturally result in a valence state transition of partial Fe2 þ into Fe3 þ due to the charge reservation. The effect will also play a role in changing the lattice parameters and hence competes with the Li-doping effect. When xr 0.04, the valence state transition induced lattice parameters change is possibly overshadowed by the Li-doping effect due to the low Li concentration. While when x 40.04, the former effect may take superiority over the later one, and moreover, partial Li þ ions possibly go onto the substitutional sites of Sn4 þ (ionic radius: 0.76 Å), thus leading to a somewhat anisotropic evolution of the lattice parameters, where c remains almost constant while a monotonically shrinkages from 4.7400(3) Å for x¼ 0.04 to 4.7366 (10) Å for x ¼0.06, decreasing about 0.07%. When x Z 0.06, both a and c show saturation-like behavior, possibly indicating a solubility limit point around x ¼0.06 for Li in SnO2. If it is true, the value is much smaller than that of Li in ZnO, 30 at% [31]. Fig. 3a and b depicts the isothermal magnetizations for Sn0.95 x(Fe0.05,Lix)O2 (0 rx r0.08) measured at 2 and 300 K, respectively. To show more details, the expanded view of the M (H) data for the Sn0.95Fe0.05O2 host at each temperature is inserted into the corresponding figure. From the profiles of the M(H) curves, well-defined hysteresis loops at both temperatures are visible, indicating ferromagnetic state in all samples. Moreover, the M(H) curves at 300 K seem like copies from those at 2 K despite that the corresponding magnetizations are slightly smaller, indicating that the FM state is very robust even up to room temperature. Only at a
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Fig. 3. Isothermal magnetizations for Sn0.95 x(Fe0.05,Lix)O2 (0 r x r0.08) at (a) 2 K and (b) 300 K, respectively. The inset to each figure is the enlarged view for the Sn0.95Fe0.05O2 host. The dotted line in each figure is that used to guide for the eyes about the saturated magnetization.
first glance, the evolution of M(H) upon Li-doping is clearly visible. Though the M(H) curve of the Sn0.95Fe0.05O2 host displays clear hysteresis loop at 300 K, the magnetization remains far from saturation within the measured magnetic field range, indicating that the randomly distributed ferromagnetic domains are difficult to be aligned along the external filed direction. The magnetization of the Sn0.95Fe0.05O2 host at H¼50 kOe, 0.09 μB/Fe, is only about 1.5% of the high-spin Fe3 þ , indicating that the FM is rather weak and the compound is nearly paramagnetic. With the increase of x to 0.02, the magnetization becomes almost saturated at H¼ 50 kOe with a saturated magnetization Ms of about 0.75 μB/Fe, which is about 13% of the high-spin Fe3 þ . The Ms value of x¼0.02 is apparently much larger than the magnetization of x¼0 at 50 kOe, thus ambiguously revealing an enhanced FM by Li doping. For such a behavior, the possibility of an impurity contribution source could be excluded by two facts: the impurity α-Fe2O3 in x¼ 0.02 remains almost the same level as that in the Sn0.95Fe0.05O2 host as we analyzed on the presented XRD data, and α-Fe2O3 has a very small magnetic moment which almost could be neglected [32]. With the further increase of x to 0.06, Ms even reaches 2.59 μB/Fe, being increased more than 26 times than that of the host. Due to the mixed valences of Fe ions in Li-doped Sn0.95Fe0.05O2, the real magnetic moment should be somewhat smaller than that expected for high-spin Fe3 þ , and the large magnetic Ms for x¼0.06 therefore should point to a well established ferromagnetic ground state. This could be demonstrated again by the magnetic susceptibility presented later. When x40.06, Ms changes slightly possibly due to the evolution of the particle size rather than the Li-doping since it already reaches the solubility limit [32]. However, as shown in Fig. 1, since much stronger impurity peaks corresponding to γ-Fe2O3 as compared with those of α-Fe2O3 appear when xZ0.04, the impurity contribution from γ-Fe2O3 to the magnetic moment should be carefully evaluated. The magnetic moment of γ-Fe2O3 is about 1.25 μB/Fe [32], only less than half of that observed for x¼0.06. In addition, the amount level of γ-Fe2O3 changes negligibly from x¼ 0.04 to 0.06 identified by the XRD analysis, and its contribution to the magnetic moment will therefore be almost the same. However, Ms of x¼ 0.04 is only about 1.5 μB/Fe, which is much
Fig. 4. The χ(T) for Sn0.95 x(Fe0.05,Lix)O2 (0 r xr 0.08): (a) x¼ 0, (b) x ¼0.02, (c) x ¼0.04, (d) x¼ 0.06 and (e) x¼ 0.08.
smaller than that of x¼0.06, 2.59 μB/Fe. This fact also implies a Lidoping induced FM enhancement behavior in Sn0.95 x(Fe0.05,Lix)O2 rather than an impurity effect. Fig. 4a–e presents the χ(T) for Sn0.95 x(Fe0.05,Lix)O2. For the Sn0.95Fe0.05O2 host, the magnetization increases steeply with decreasing temperature below 50 K, indicating almost paramagnetic behavior. However, the weak temperature dependence χ(T) above T ¼50 K, as well as the cusp between the ZFC and FC data, indicates the weak FM. To achieve more insights into the magnetism in Sn0.95Fe0.05O2, the Curie–Weiss (CW) law was used to plot the paramagnetic parts ( 4150 K) of χ(Τ), which is shown by the insets to each corresponding figure in Fig. 4a. The analytical formula was χ(T)¼NAμ2eff/3kB(T ΘW), where NA is the Avogadro constant, kB is the Boltzmann constant, ΘW is the Weiss temperature, and μeff is the effective magnetic moment. The fit revealed an unusual large ΘW of about 3500 K with an unphysical μeff, indicating that the sample is intrinsically paramagnetic which is consistent with the result derived from the isothermal magnetization characterization. With increase of x, the magnitude of magnetization increases remarkably in the whole temperature range. Moreover, no paramagnetic behavior within the measured temperature range was traced for all Li-doped samples, and χ(T) actually displays typical ferromagnetic behavior. Thus an attempt to analyze the data by using the Curie–Weiss plot could not be accomplished. Combining the magnetic characterization results, it is reasonable to conclude that Li-doping in Sn0.95Fe0.05O2 works effectively to enhance the FM, as it does in Mn-doped ZnO [27]. Furthermore, the magnetization of all the presented compounds increases steeply with cooling below 25 K, which is probably related to the defected structure and coupled Fe3 þ ions [32,33]. In Sn0.95 xLixFe0.05O2, the magnetic properties rigidly depend on the interaction among Fe3 þ and Fe2 þ ions bridged by oxygen defects or oxygen ions. The interaction between Fe3 þ should be much stronger than that between Fe2 þ due to the different spin states. As we mentioned above, the Li doping could result in an increase of Fe3 þ concentration. As a consequence, the number of Fe3 þ –O2–Fe3 þ exchange pathways will be increased, which will
J.-g. Xi et al. / Journal of Magnetism and Magnetic Materials 358-359 (2014) 128–131
naturally enhance the FM. However, in addition to this conjecture, other possibilities, such as the variation of the oxygen defects, the particle size effect, and more, should be carefully evaluated based on more solid evidences. 4. Conclusions Li-doped Sn0.95 x(Fe0.05,Lix)O2 (0 r xr 0.08) was synthesized for the first time via the conventional solid-state reaction. Structure analysis based on the powder X-ray diffraction revealed that the Li-doping level could be achieved to x ¼0.06 with well retaining the host structure. Upon Li-doping, the lattice parameters showed intriguing evolution. Magnetic measurements revealed that Sn0.95Fe0.05O2 is nearly paramagnetic with a small fraction of FM. By Li-doping, the weak FM is remarkably enhanced, with the magnetic moment even approaching 2.59 μB/Fe at 300 K when x ¼0.06. The enhanced FM in Li-doped Sn0.95Fe0.05O2 could possibly be interpreted in connection with a carrier mediated mechanism. 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), the Transfer and Industrialization Project of Sci-Tech Achievement (Cooperation Project between University and Factory) from Beijing Municipal Commission of Education, and Excellent Adviser Foundation in China University of Geosciences from The Fundamental Research Funds for the Central Universities. References [1] S.J. Liu, C.Y. Liu, J.Y. Juang, H.W. Fang, J. Appl. Phys. 105 (2009) 013928. [2] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [3] Y.C. Qiu, W. Chen, S. Yang, B. Zhang, X.X. Zhang, Y.C. Zhong, K.S. Wong, Cryst. Growth Des. 10 (2010) 182. [4] M. Naeem, S.K. Hasanain, J. Phys.: Condens. Matter 24 (2012) 245305.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Enhanced ferromagnetism by lithium doping in Sn0.95Fe0.05O2 Jian-guo Xi a,b, Zhi-jian Peng a,n, Xiu-li Fu b,n 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
art ic l e i nf o
a b s t r a c t
Article history: Received 24 March 2013 Received in revised form 31 December 2013 Available online 30 January 2014
Lithium doping effects on Sn0.95Fe0.05O2 were studied in terms of structural and magnetic properties. The nominal lithium doping level in Sn0.95 x(Fe0.05,Lix)O2 was achieved to x ¼0.06 with well retaining the host structure. Although the precise position and real content of lithium were hard to be determined by powder X-ray diffraction, the structure parameters, as well as the magnetic properties, actually show sensitivity to the doping. The most interesting observation herein is the remarkably enhanced ferromagnetism, possibly due to that the doped lithium favors the ferromagnetic exchange interaction in Sn0.95Fe0.05O2 through tuning the concentration of Fe3 þ ions responsible for the magnetic interaction. The observation is very similar with those cases in other group-I elements doped diluted magnetic semiconductors, indicating possibly generic phenomena being significant for spintronics. & 2014 Elsevier B.V. All rights reserved.
Keywords: Diluted magnetic semiconductor SnO2 Li þ ion doping Magnetic property
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted considerable interest owing to the significance for application as a kind of multifunctional material in spintronics [1]. The characteristic virtue of DMSs is that they host both charge and spin degrees of freedom in single material, and the delicate interplay between them thereby gives rise to exotic properties. As proposed by Dietl et al., room-temperature ferromagnetism (FM) 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 [2]. Instructed by this, an array of compounds, for examples, ZnO [3–7], TiO2 [8,9], and SnO2 [10–13], was reported to show FM even above 300 K after being doped. Despite these significant achievements, the physics of FM in such DMSs has not been completely established yet. Regarding the sofar major viewpoints about the origin, such as the carrier mediated mechanism through tuning the magnetic exchange interaction strength [14,15], the so-called F-center exchange model [16–18], and the bound magnetic polaron model [19,20], the first mechanism has been discussed more intensively. SnO2 is a wide-gap paramagnetic semiconductor at room temperature, with a typical gap size of 3.6 eV [21], thus being ideal for serving as the host for DMSs. In fact, when the size of the crystallite is reduced to nano-scale, SnO2 itself can show
n
Corresponding authors. Tel.: þ 86 10 82320255; fax: þ86 10 82322624. E-mail addresses: [email protected], [email protected] (Z.-j. Peng), [email protected] (X.-l. Fu). 0304-8853/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2014.01.043
magnetism with a relatively high saturation magnetization due to the spins polarization of 2p and 2s electrons of the oxygen atoms [13]. Regarding the TM doping in SnO2, a variety of elements, including Cr, Fe, Co, Ni, Mn and V, have been tried and they were confirmed to work effectively to establish the magnetic state [22,23]. However, not only the origin but also the magnetism type remains highly controversial. Fe-doped SnO2, for instance, was reported to exhibit FM in Sn0.95Fe0.05O2 thin film with moments ranging from 1.06 to 4.76 μB/Fe and a very high Curie temperature Tc of 610 K [17], while the moment of Sn0.95Fe0.05O2 bulk ceramic (Tc, 360 K) was only 0.95 μB/Fe, about 15% of the high-spin Fe3 þ (3d5, 5.9 μB/Fe) calculated from g[S(Sþ1)]1/2, where g ( ¼2) is the Landé factor and S (¼2) is the spin number, which indicated a nearly paramagnetic nature since about 85% of spins are magnetically ordered. More interestingly, rigidly depending on the synthesis temperature, the Fe-doped SnO2 powder showed either ferromagnetic (r600 1C) or antiferromagnetic (4600 1C) behavior [19,24]. Very recently, the FM in an analog Zn(TM)O was found to be remarkably enhanced by co-doping of group-I elements, which was ascribed to the hole-related defects favoring the magnetic interaction [25–27]. According to a theoretical calculation, Li-doping is capable of lowering the formation energy of the Zn vacancy to produce FM, and they at the substitutional sites rather than the interstitial ones even can increase the total magnetic moment [28]. Throughout the overview, it is clear that a study on the Li-doped Sn(TM)O2 would be necessary, not only due to the importance for achieving more in-depth insights into the magnetism in Sn(TM)O2, and hence a clarification of the longstanding controversy, but also because of the merit of checking the universality of the FM enhancement behavior induced by group-I elements doping. We are thus motivated to carry out the study.
J.-g. Xi et al. / Journal of Magnetism and Magnetic Materials 358-359 (2014) 128–131
129
The structural and magnetic properties of Li-doped Sn0.95Fe0.05O2 are reported herein.
2. Experimental procedure Polycrystalline Sn0.95 x(Fe0.05,Lix)O2 (0 rx r0.08) was synthesized via the conventional solid state reaction method starting from the powders of stoichiometric SnO2 (99.5 wt%), Fe2O3 (98.0 wt%), and LiNO3 (99.0 wt%). The starting powders were first mixed and ball-milled for 24 h in de-ionized water using highly wear-resistant ZrO2 balls as a grinding medium. The resultant slurries were then dried at 110 1C for 12 h in air. The dried chunks were ground and sieved into fine powders, which were subsequently pressed into pellets and placed in an alumina crucible. Then the crucible was heated at 1000 1C for 24 h in an electrical resistance chamber furnace. The furnace was finally cooled down to room temperature after heating by shutting off the electricity source. Powder X-ray diffraction (XRD) patterns were recorded at room temperature on a 8 kW D/max-2550 diffractometer with Cu Kα radiation (λ¼ 1.5418 Å) in the 2θ range from 201 to 801. The dc magnetic susceptibility was measured between 2 and 300 K with the presence of a 10 kOe magnetic field under both zero-fieldcooling (ZFC) and zero-cooling (FC) conditions, on a magnetic property measurement system (MPMS), Quantum Design. Isothermal magnetizations (Μ(H)) at 2 and 300 K were also measured on the same apparatus with the magnetic field within 750 kOe. To avoid the shielding of magnetization, loose powder of each sample was used for the measurement.
3. Results and discussion The XRD patterns of the synthesized compounds are presented in Fig. 1. All the diffraction peaks acquired from each compound match well with a rutile structure of SnO2 (space group: P42/mnm; JCPDS card: 21-1250), implying a single phase of all studied specimens. The small impurity peaks marked by both asterisks and pound signs were identified to arise from Fe2O3. When xo 0.04, only α-Fe2O3 impurity of a hematite phase (space group: R-3c; JCPDS card: 33-0664) was observed, while not only α-Fe2O3 but also γ-Fe2O3 crystalized in a maghemite-C phase (space group: P4132; JCPDS card: 39-1346) was visible when xZ 0.04. Moreover,
Fig. 1. XRD patterns for Sn0.95 x(Fe0.05,Lix)O2 with each being marked by the nominal Li doping level. The peaks marked by pound signs and asterisks arise from α- and γ-Fe2O3 impurities, respectively.
Fig. 2. Evolution of lattice parameters a and c of Sn0.95 x(Fe0.05,Lix)O2 (0 rx r 0.08) with respect to the nominal Li doping level.
the amount level of γ-Fe2O3 remains at the same over the compositions estimated from the intensity of the peaks, while the intensity of α-Fe2O3 shows slight increase when x 40.02, but its amount remains almost the same level in the samples with x¼ 0.04, 0.06 and 0.08. The reason remains unknown, which is possibly caused by the local structure change with the variation of Li concentration in Sn0.95Fe0.05O2. In addition, due to the low resolution of powder XRD, it is hard to judge whether Li is well incorporated into the SnO2 crystal lattice or not. However, from the evolution of lattice parameters and the sensitivity of the magnetic properties to the nominal Li doping level as presented later, it is reasonably to conclude that Li was successfully doped into Sn0.95Fe0.05O2. The lattice parameters calculated from the XRD patterns are plotted in Fig. 2. For the Sn0.95Fe0.05O2 host, the lattice constants a and c are 4.7351(5) and 3.1866(4) Å, respectively, roughly consistent with the previously reported values in Refs. [18,29]. The evolution of a and c is apparently isotropic when x r0.04 since they both increase by approximately 0.1%, to 4.7400(3) Å for a and to 3.1898(2) Å for c at x ¼0.04. Such an isotropic evolution with well obeying Vegard's law possibly indicates an occupation of Li þ ions at the Fe sites, i.e. the center of the tetragonal unit-cell. On the other side, since Sn0.95Fe0.05O2 has mixed valences of Fe2 þ (ionic radius: 0.77 Å) and Fe3 þ (ionic radius: 0.64 Å) [29,30], once Sn4 þ is replaced by Li þ , the released holes will naturally result in a valence state transition of partial Fe2 þ into Fe3 þ due to the charge reservation. The effect will also play a role in changing the lattice parameters and hence competes with the Li-doping effect. When xr 0.04, the valence state transition induced lattice parameters change is possibly overshadowed by the Li-doping effect due to the low Li concentration. While when x 40.04, the former effect may take superiority over the later one, and moreover, partial Li þ ions possibly go onto the substitutional sites of Sn4 þ (ionic radius: 0.76 Å), thus leading to a somewhat anisotropic evolution of the lattice parameters, where c remains almost constant while a monotonically shrinkages from 4.7400(3) Å for x¼ 0.04 to 4.7366 (10) Å for x ¼0.06, decreasing about 0.07%. When x Z 0.06, both a and c show saturation-like behavior, possibly indicating a solubility limit point around x ¼0.06 for Li in SnO2. If it is true, the value is much smaller than that of Li in ZnO, 30 at% [31]. Fig. 3a and b depicts the isothermal magnetizations for Sn0.95 x(Fe0.05,Lix)O2 (0 rx r0.08) measured at 2 and 300 K, respectively. To show more details, the expanded view of the M (H) data for the Sn0.95Fe0.05O2 host at each temperature is inserted into the corresponding figure. From the profiles of the M(H) curves, well-defined hysteresis loops at both temperatures are visible, indicating ferromagnetic state in all samples. Moreover, the M(H) curves at 300 K seem like copies from those at 2 K despite that the corresponding magnetizations are slightly smaller, indicating that the FM state is very robust even up to room temperature. Only at a
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Fig. 3. Isothermal magnetizations for Sn0.95 x(Fe0.05,Lix)O2 (0 r x r0.08) at (a) 2 K and (b) 300 K, respectively. The inset to each figure is the enlarged view for the Sn0.95Fe0.05O2 host. The dotted line in each figure is that used to guide for the eyes about the saturated magnetization.
first glance, the evolution of M(H) upon Li-doping is clearly visible. Though the M(H) curve of the Sn0.95Fe0.05O2 host displays clear hysteresis loop at 300 K, the magnetization remains far from saturation within the measured magnetic field range, indicating that the randomly distributed ferromagnetic domains are difficult to be aligned along the external filed direction. The magnetization of the Sn0.95Fe0.05O2 host at H¼50 kOe, 0.09 μB/Fe, is only about 1.5% of the high-spin Fe3 þ , indicating that the FM is rather weak and the compound is nearly paramagnetic. With the increase of x to 0.02, the magnetization becomes almost saturated at H¼ 50 kOe with a saturated magnetization Ms of about 0.75 μB/Fe, which is about 13% of the high-spin Fe3 þ . The Ms value of x¼0.02 is apparently much larger than the magnetization of x¼0 at 50 kOe, thus ambiguously revealing an enhanced FM by Li doping. For such a behavior, the possibility of an impurity contribution source could be excluded by two facts: the impurity α-Fe2O3 in x¼ 0.02 remains almost the same level as that in the Sn0.95Fe0.05O2 host as we analyzed on the presented XRD data, and α-Fe2O3 has a very small magnetic moment which almost could be neglected [32]. With the further increase of x to 0.06, Ms even reaches 2.59 μB/Fe, being increased more than 26 times than that of the host. Due to the mixed valences of Fe ions in Li-doped Sn0.95Fe0.05O2, the real magnetic moment should be somewhat smaller than that expected for high-spin Fe3 þ , and the large magnetic Ms for x¼0.06 therefore should point to a well established ferromagnetic ground state. This could be demonstrated again by the magnetic susceptibility presented later. When x40.06, Ms changes slightly possibly due to the evolution of the particle size rather than the Li-doping since it already reaches the solubility limit [32]. However, as shown in Fig. 1, since much stronger impurity peaks corresponding to γ-Fe2O3 as compared with those of α-Fe2O3 appear when xZ0.04, the impurity contribution from γ-Fe2O3 to the magnetic moment should be carefully evaluated. The magnetic moment of γ-Fe2O3 is about 1.25 μB/Fe [32], only less than half of that observed for x¼0.06. In addition, the amount level of γ-Fe2O3 changes negligibly from x¼ 0.04 to 0.06 identified by the XRD analysis, and its contribution to the magnetic moment will therefore be almost the same. However, Ms of x¼ 0.04 is only about 1.5 μB/Fe, which is much
Fig. 4. The χ(T) for Sn0.95 x(Fe0.05,Lix)O2 (0 r xr 0.08): (a) x¼ 0, (b) x ¼0.02, (c) x ¼0.04, (d) x¼ 0.06 and (e) x¼ 0.08.
smaller than that of x¼0.06, 2.59 μB/Fe. This fact also implies a Lidoping induced FM enhancement behavior in Sn0.95 x(Fe0.05,Lix)O2 rather than an impurity effect. Fig. 4a–e presents the χ(T) for Sn0.95 x(Fe0.05,Lix)O2. For the Sn0.95Fe0.05O2 host, the magnetization increases steeply with decreasing temperature below 50 K, indicating almost paramagnetic behavior. However, the weak temperature dependence χ(T) above T ¼50 K, as well as the cusp between the ZFC and FC data, indicates the weak FM. To achieve more insights into the magnetism in Sn0.95Fe0.05O2, the Curie–Weiss (CW) law was used to plot the paramagnetic parts ( 4150 K) of χ(Τ), which is shown by the insets to each corresponding figure in Fig. 4a. The analytical formula was χ(T)¼NAμ2eff/3kB(T ΘW), where NA is the Avogadro constant, kB is the Boltzmann constant, ΘW is the Weiss temperature, and μeff is the effective magnetic moment. The fit revealed an unusual large ΘW of about 3500 K with an unphysical μeff, indicating that the sample is intrinsically paramagnetic which is consistent with the result derived from the isothermal magnetization characterization. With increase of x, the magnitude of magnetization increases remarkably in the whole temperature range. Moreover, no paramagnetic behavior within the measured temperature range was traced for all Li-doped samples, and χ(T) actually displays typical ferromagnetic behavior. Thus an attempt to analyze the data by using the Curie–Weiss plot could not be accomplished. Combining the magnetic characterization results, it is reasonable to conclude that Li-doping in Sn0.95Fe0.05O2 works effectively to enhance the FM, as it does in Mn-doped ZnO [27]. Furthermore, the magnetization of all the presented compounds increases steeply with cooling below 25 K, which is probably related to the defected structure and coupled Fe3 þ ions [32,33]. In Sn0.95 xLixFe0.05O2, the magnetic properties rigidly depend on the interaction among Fe3 þ and Fe2 þ ions bridged by oxygen defects or oxygen ions. The interaction between Fe3 þ should be much stronger than that between Fe2 þ due to the different spin states. As we mentioned above, the Li doping could result in an increase of Fe3 þ concentration. As a consequence, the number of Fe3 þ –O2–Fe3 þ exchange pathways will be increased, which will
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naturally enhance the FM. However, in addition to this conjecture, other possibilities, such as the variation of the oxygen defects, the particle size effect, and more, should be carefully evaluated based on more solid evidences. 4. Conclusions Li-doped Sn0.95 x(Fe0.05,Lix)O2 (0 r xr 0.08) was synthesized for the first time via the conventional solid-state reaction. Structure analysis based on the powder X-ray diffraction revealed that the Li-doping level could be achieved to x ¼0.06 with well retaining the host structure. Upon Li-doping, the lattice parameters showed intriguing evolution. Magnetic measurements revealed that Sn0.95Fe0.05O2 is nearly paramagnetic with a small fraction of FM. By Li-doping, the weak FM is remarkably enhanced, with the magnetic moment even approaching 2.59 μB/Fe at 300 K when x ¼0.06. The enhanced FM in Li-doped Sn0.95Fe0.05O2 could possibly be interpreted in connection with a carrier mediated mechanism. 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), the Transfer and Industrialization Project of Sci-Tech Achievement (Cooperation Project between University and Factory) from Beijing Municipal Commission of Education, and Excellent Adviser Foundation in China University of Geosciences from The Fundamental Research Funds for the Central Universities. References [1] S.J. Liu, C.Y. Liu, J.Y. Juang, H.W. Fang, J. Appl. Phys. 105 (2009) 013928. [2] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [3] Y.C. Qiu, W. Chen, S. Yang, B. Zhang, X.X. Zhang, Y.C. Zhong, K.S. Wong, Cryst. Growth Des. 10 (2010) 182. [4] M. Naeem, S.K. Hasanain, J. Phys.: Condens. Matter 24 (2012) 245305.
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