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The effect of transition element doping on the electronic and magnetic properties of La1.4Sr1.6Mn2O7
Solid State Communications 141 (2007) 136–140 www.elsevier.com/locate/ssc
The effect of transition element doping on the electronic and magnetic properties of La1.4Sr1.6Mn2O7 G.Q. Yu, Y.Q. Wang, L. Liu, S.Y. Yin, G.M. Ren, J.H. Miao, X. Xiao, S.L. Yuan ∗ Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 8 February 2006; received in revised form 4 October 2006; accepted 10 October 2006 by B.-F. Zhu Available online 23 October 2006
Abstract The effect of transition element (TE = Cr, Fe, Co, Ni, Cu, Zn) doping on the electronic transport and magnetic properties in the bilayer manganite La1.4 Sr1.6 Mn2 O7 is studied for the same dopant concentration fixed at 2%. Doping does not cause change in structure but different behavior in magnetic and transport properties. Except for Cr, all the other dopings significantly shift the magnetic transition temperature (TC ) to a lower temperature. Associated with such a decrease, the insulator–metal transition temperature (TIM ) decreases and the peak resistivity (ρ p ) at TIM increases. Cr doping enhances TC and TIM as well as decreases ρ p . Fe doping apparently has a stronger effect than Co and Ni doping. It is also indicated that Cu doping causes an anomalously large increase in ρ p . These behaviors are compared with those observed in other bilayer manganites such as La1.2 Sr1.8 Mn2 O7 as well as in La0.7 Ca0.3 Mn1−x TEx O3 . c 2006 Elsevier Ltd. All rights reserved.
PACS: 75.47.Lx; 74.25.Fy; 75.30.Kz; 61.72.Ww Keywords: A. Bilayer manganites; B. Doping; D. Electronic transport; D. Magnetic properties
Both the bilayer manganites La2−2x Sr1+2x Mn2 O7 (n = 2) and the three-dimensional (3D) manganites La1−x Ax MnO3 (n = ∞) are in the Ruddlesden–Popper series (R, A)n+1 Mnn On+1 , where R and A are trivalent rare-earth and divalent alkaline-earth ions, respectively. Recently, bilayer manganites La2−2x Sr1+2x Mn2 O7 have attracted considerable attention due to their novel properties such as the colossal magnetoresistance effect, tunneling magnetoresistance and fascinating electronic and magnetic properties [1–3]. The system forms natural quasi-two-dimensional tunneling structures consisting of two ferromagnetic (FM) metallic MnO2 layers separated by a rocksalt-type block layer (La, Sr)2 O2 , and shows a rich variety of magnetic structures and properties depending strongly on the doping level x. For example, for x = 0.3, namely, La1.4 Sr1.6 Mn2 O7 , the magnetic coupling is FM within the constituent single MnO2 layer and a bilayer unit and is weakly antiferromagnetic (AFM) between the adjacent bilayers at low temperature [4–6]; for x = 0.32–0.4, all the magnetic couplings
∗ Corresponding author. Tel.: +86 27 87556580; fax: +86 27 87544525.
E-mail address: [email protected] (S.L. Yuan). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.10.007
mentioned above are FM; for x = 0.5, namely, LaSr2 Mn2 O7 , with charge-ordered state, the magnetic coupling is FM within the constituent single MnO2 layer, but shows A-type AFM order between the respective MnO2 layers within a bilayer unit. This is very different from the well-studied 3D manganites La1−x Ax MnO3 (A = Ca, Sr, Ba). Generally, the magnetic and transport properties in the mixed-valence manganites can be understood in terms of the double-exchange (DE) interaction mechanism [7]. Besides the DE mechanism, the Jahn–Teller effect, phase separation, AFM superexchange and charge-orbital ordering interactions also play important roles. In the bilayer manganites, the DE interaction essentially works only within the respective MnO2 layers, and there also exists a strong interplay among spin, charge, orbital and lattice degrees of freedom. The bilayer manganites, La2−2x Sr1+2x Mn2 O7 , offer the opportunity not only to investigate the generic features of the mixedvalence manganites in reduced dimensions, but also to explore phenomena that are not found in the 3D manganites. Mn-site doping in the bilayer manganites is very interesting because it can dramatically change the magnetic and electronic properties due to the crucial role of Mn ions in manganites.
G.Q. Yu et al. / Solid State Communications 141 (2007) 136–140
Some of such studies have been undertaken during the past few years [8–17]. Gundakaram et al. [8] studied the effect of Cr doping in La1.2 Sr1.8 Mn2 O7 . With the increase of Cr doping level, the insulator–metal transition (IMT) was suppressed and the ferromagnetic Curie temperature increased slightly and then decreased, and the magnetoresistance ratio was not significantly affected. Zhang et al. [9,10] studied the magnetic and transport properties of the La1.2 Sr1.8 Mn2−x Tx O7 (T = Fe, Co, Cr), and they showed Fe and Co doping had similar effects, resulting in quick weakening of FM ordering and immediate disappearance of the IMT and enhancement of low-temperature MR, but a different effect was observed for Cr doping, although the IMT also disappeared quickly, and FM ordering was maintained even for x = 0.5. They suggested there existed a FM DE interaction between Cr3+ and Mn3+ . Ru doping in La1.2 Sr1.8 Mn2 O7 could increase TC . To interpret this, Weigand et al. [13] proposed a coupling model of Mn3+ /Mn4+ and Ru3+ /Ru4+ ions in the high-spin state, and Onose et al. [14] suggested Ru doping suppressed the charge-orbital correlation resulting in the enhancement of the Curie temperature TC . Very recently, in the LaSr2 Mn2 O7 system, Nair and Banerjee [15] reported the effect of Al doping; Zhang et al. [16] reported that of Cr doping and Ang et al. [17] reported that of Co doping. For the La1.4 Sr1.6 Mn2 O7 system, there are less reports on Mn-site doping effect. Only Zhu et al. [11,12] studied the effect of Cu and Ti doping in this system. In the 3D perovskite manganites, Ghosh et al. [18] studied the TE doping effects in La0.7 Ca0.3 MnO3 . For comparison, we believe there is a need for studies of series TE doping in the bilayer manganite La1.4 Sr1.6 Mn2 O7 . Choosing the La1.4 Sr1.6 Mn2 O7 as parent phase is also due to its layered structure and different magnetic structure compared with the 3D manganites and other bilayer manganites (e.g. La1.2 Sr1.8 Mn2 O7 , LaSr2 Mn2 O7 , etc.). We expect to have new and different effects. Polycrystalline bulk samples of La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn) were synthesized by a standard ceramic process. A well-ground stoichiometric mixture of high purity La2 O3 , SrCO3 , MnO2 and relevant TE metal oxides powders were calcined at 1200 ◦ C for 24 h in air with intermediate grinding. The powders were well ground again, then palletized and sintered at 1450 ◦ C for 40 h with an intermediate grinding. The structural characterization was done through x-ray diffraction (XRD) at room temperature with Cu K α radiation. The temperature dependence of resistivity (without and with a field of 5 T) and magnetization at low field (100 Oe) was measured in a commercial Physical Property Measurement System (Quantum Design PPMS). XRD patterns for all samples of La1.4 Sr1.6 Mn1.96 TE0.04 O7 are presented in Fig. 1. All diffraction peaks can be indexed using a Sr3 Ti2 O7 -type tetragonal structure (I 4/mmm), which indicates that all samples are of a single phase. Fig. 2 shows the temperature dependence of resistivity for all samples. For the undoped sample, the IMT temperature TIM is 112 K. Except for Cr doped samples, the TIM of other doped samples shifts to lower temperature, while the resistivity increases. The minimum value of TIM is 88 K for Zn doping.
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Fig. 1. XRD patterns of samples La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn).
Fig. 2. Temperature dependence of resistivity for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn) at zero field.
Cu doping causes a larger increase in the peak resistivity (ρ p ). A similar observation has also been reported by Zhu et al. in Ref. [11], where the peak resistivity of the Cu doped sample with x = 0.05 was nearly 20 times larger than that of the undoped sample. The case is significantly different from that in the 3D manganites La0.7 Ca0.3 Mn0.95 TE0.05 O3 [18], where the decrease in TIM was small for Cr and Zn doping, and TIM of Cu doped samples almost did not shift while the resistivity obviously reduced. For Fe and Co doping in the present system, the TIM drops more slowly than that in La1.2 Sr1.8 Mn2 O7 [9, 10], where the IMT disappeared quickly at a very low doping level (x = 0.01). Cr doping results in an increase in TIM (120 K) and a decrease in resistivity especially at temperatures near
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Fig. 3. Temperature dependence of magnetization M at 100 Oe for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn). What is represented by the symbols here is the same as that in Fig. 2. Inset: the magnification plot of the circled area.
TIM , which is not observed in other manganites (including the layered and 3D manganites). The temperature dependence of magnetization M measured at a field of 100 Oe is shown in Fig. 3. All M–T curves show two magnetic transitions lying at lower temperature near 100 K and higher temperature near 240 K, respectively. For the undoped sample, Fig. 3 shows a 3D magnetic transition at TC (defined as the temperature at which dM/dT reaches the maximum) is 106 K, and another upper transition at T ∗ (defined as another maximum in the dM/dT –T curve) is 241 K which has been ascribed to short range two-dimensional (2D) magnetic ordering due to quasi-two-dimensional features [19– 21]. The TIM , TC and T ∗ for La1.4 Sr1.6 Mn2 O7 doped with different transition elements are presented in Fig. 4(a). For Cr doped samples, the TC just like its TIM shifts to higher temperature (116 K) and T ∗ slightly shifts to lower temperature (235 K) compared with the undoped sample. For other TE doped samples, both TC and TIM shift to lower temperature. The T ∗ of all doped samples shifts to lower temperature, which indicates that the TE doping weakens the 2D magnetic ordering in the system. The influence of Cr doping on 2D magnetic ordering is the weakest. Fig. 4(a) also shows a direct correlation between magnetic and transport behavior. As shown in Figs. 2–4(a), the magnetic properties are more sensitive to Fe doping than the electronic properties in the present system, which is an opposite effect compared with Fe doping in La1.2 Sr1.8 Mn2 O7 [9,10]. Figs. 3 and 4(a) further indicate there is a different effect on TC for Cr, Cu and Zn doping but a similar effect for Fe, Co and Ni doping compared with those in 3D manganites [18]. Fig. 4(b) shows the ρ p value at the IMT temperature. These data have a direct correlation to the data shown in Fig. 4(a), although there appears to be a departure in the correlation in the case of Cu. The temperature dependence of MR0 (MR0 = ({[ρ(H = 0) − ρ(H = 5 T)]/ρ(H = 0)} × 100%)) ratios at 5 T for La1.4 Sr1.6 Mn1.96 TE0.04 O7 is shown in Fig. 5. All MR0 peaks appear at the temperature near TIM , below which fairly large
Fig. 4. TE dependence of (a) TIM , TC and T ∗ , (b) the peak resistivity (ρ p ) value at TIM for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn).
Fig. 5. Temperature dependence of MR0 at 5 T for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn).
low-temperature MR0 ratios remain. The maximum MR0 peak appears in Zn doped samples, and the minimum appears in Cr doped samples. There is an obvious correlation between the maximum MR0 and the ionic radii, which is similar to the observation in 3D manganites in Ref. [18]. However, the lowtemperature (below 40 K) MR0 of all doped samples is less than that of the undoped sample, which is very different from the former observation of TE doping in La1.2 Sr1.8 Mn2 O7 [9,10], where TE doping enhanced the low-temperature MR0 effect, especially for Fe doping. In the bilayer manganites, the layered structure and the decrease of dimensionality results in the DE interaction between Mn ions essentially working only within the respective MnO2 layers, which makes the system more sensitive to the Mn-site doping, and the intrabilayer magnetic interaction is stronger than the interbilayer one. Mn-site doping may influence not only the intrabilayer magnetic interaction but also the interbilayer magnetic interaction. In the present studies, TE doping destroys the Mn–O–Mn network and the DE interaction
G.Q. Yu et al. / Solid State Communications 141 (2007) 136–140
between Mn ions, and dilutes the in-plane FM interaction. Except for Cr doping, TE doping results in a decrease in TIM , TC and T ∗ (with Cr doping) and an increase in ρ p . Cu2+ (r ∼ 0.073 nm) and Zn2+ (r ∼ 0.075 nm) ions partially substituting Mn3+ (r ∼ 0.065 nm) ions causes a bigger local change in the crystal lattice and a serious destruction of Mn–O–Mn network due to the bigger ionic radius, which results in a more obvious decrease in TIM , TC and T ∗ as well as a bigger increase in resistivity. In comparison with all the other dopings, Cu doping, for reasons that are not clear now, causes an anomalously large increase in ρ p . All our experimental results are well repeatable, and accordant results occurred in several repeated experiments from our group. The effect of Cu doping on the peak resistivity is worthy of further study. Now we focus on the case of Fe, Co and Ni doping. From Figs. 2 and 3, it can be seen that Fe doping apparently has a stronger effect than Co and Ni doping. The radii of Fe3+ , Co3+ and Ni3+ (0.065, 0.061 and 0.060 nm, respectively) are close to the radius of Mn3+ (0.065 nm). We notice that the M–T curves for Fe and Zn (with larger ionic radius ∼ 0.075 nm) doping almost overlap at low temperature, and they have the same TC . This implies that except for ionic radius, other factors are important to electronic and magnetic properties in the case of Fe, Co and Ni doping. In the 3D manganites, several reports [22–24] state that the coupling between Mn and doped Fe, Co, Ni ions obeys the AFM superexchange mechanism. We believe that the AFM superexchange interaction between Mn and doped Fe, Co, Ni ions plays an important role in the present system. The electronic configurations for Mn3+ (Mn4+ ), Fe3+ , Co3+ and 3 e1 (t 3 e0 ), t 3 e2 , t 4 e2 and t 5 e2 and hence the Ni3+ are t2g g 2g g 2g g 2g g 2g g numbers of unpaired 3d electrons are 4 (3), 5, 4 and 3, respectively. Due to the AFM coupling between Mn and doped Fe, Co, Ni ions, the more unpaired 3d electrons are, the stronger the AFM coupling is, hence the AFM coupling between Mn and Fe ions is the strongest. The enhancement of the AFM interaction weakens the FM behavior. Meanwhile, in the system, the stronger the AFM interaction is, the weaker the FM DE interaction is. Therefore, Fe doping has the strongest effect among Fe, Co and Ni doping in the present system. For Fe, Co and Ni doping in La1.4 Sr1.6 Mn2 O7 , their major influence is on the intrabilayer magnetic interaction, and so show similar magnetic and transport behaviors with those in 3D manganites [18], while they also influence the interbilayer magnetic coupling resulting in a different MR0 effect from that in La1.2 Sr1.8 Mn2 O7 [9,10]. Cr doping has a different effect. Some researchers [10,25] believe there exists FM DE interactions between Cr3+ and Mn3+ just as between Mn4+ and Mn3+ because Cr3+ has 3 e0 ) as Mn4+ . However, the same electronic configuration (t2g g in the present system, the doped Cr3+ ions directly replace Mn3+ resulting in the appearance of Cr3+ –O–Mn4+ chains which prevent the hopping of carriers (i.e. DE interaction) within the MnO2 layers. Cr doping dilutes the in-plane FM interaction, so a decrease in T ∗ is observed. What causes the increase in TIM and TC and the decrease in resistivity and MR0 ratio for Cr doping in La1.4 Sr1.6 Mn2 O7 ? Besides intrabilayer
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magnetic interactions, we believe that the interbilayer magnetic coupling and magnetic structure play an important role. Cr doping affects the interbilayer magnetic coupling. We will study systematically the Cr doping elsewhere. In conclusion, the effect of transition element doping on the electronic and magnetic properties of La1.4 Sr1.6 Mn2 O7 has been investigated. We find that when various elements are used to partially substitute Mn sites, the TIM , TC , T ∗ , ρ p and MR0 exhibit distinct changes. Different doping effects are also observed compared with those in the 3D manganites and other bilayer manganites. The results suggest that there is a different degree of the influence of different transition element doping on the intrabilayer and interbilayer magnetic interactions. Besides the intrabilayer interaction, the interbilayer magnetic interaction may play an important role on the transition element doping in the bilayer manganites. In order to further clarify this, it is worthwhile to make detailed investigations on the magnetic properties. Acknowledgement This work was supported by the National Science Foundation of China under Grand Nos 10374032 and 10574049. References [1] Y. Moritomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature 380 (1996) 141. [2] T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. [3] See, for a review, T. Kimura, Y. Tokura, Annu. Rev. Mater. Sci. 30 (2000) 451. [4] T.G. Perring, G. Aeppli, T. Kimura, Y. Tokura, M.A. Adams, Phys. Rev. B 58 (1998) R14693. [5] D.N. Argyriou, J.F. Mitchell, P.G. Radaelli, H.N. Bordallo, D.E. Cox, M. Medarde, J.D. Jorgensen, Phys. Rev. B 59 (1999) 8695. [6] M. Konoto, T. Kohashi, K. Koike, T. Arima, Y. Kaneko, T. Kimura, Y. Tokura, Phys. Rev. Lett. 93 (2004) 107201. [7] C. Zener, Phys. Rev. 82 (1951) 403; P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675. [8] R. Gundakaram, J.G. Lin, F.Y. Lee, M.F. Tai, C.H. Shen, C.Y. Huang, J. Phys.: Condens. Matter. 11 (1999) 5187. [9] J. Zhang, F. Wang, P. Zhang, Q. Yan, J. Appl. Phys. 86 (1999) 1604. [10] J. Zhang, Q. Yan, F. Wang, P. Yuan, P. Zhang, J. Phys.: Condens. Matter. 12 (2000) 1981. [11] H. Zhu, X.J. Xu, L. Pi, Y.H. Zhang, Phys. Rev. B 62 (2000) 6754. [12] H. Zhu, X.J. Xu, K.Q. Ruan, Y.H. Zhang, Phys. Rev. B 65 (2002) 104424. [13] F. Weigand, S. Gold, A. Schmid, J. Geissier, E. Goering, Appl. Phys. Lett. 81 (2002) 2035. [14] Y. Onose, J.P. He, Y. Kannko, T. Arima, Y. Tokura, Appl. Phys. Lett. 86 (2005) 242502. [15] Sunil Nair, A. Banerjee, Phys. Rev. B 70 (2004) 104428. [16] R.L. Zhang, B.C. Zhao, W.H. Song, Y.Q. Ma, J. Yang, Z.G. Sheng, J.M. Dai, J. Appl. Phys. 96 (2004) 4965. [17] R. Ang, W.J. Lu, R.L. Zhang, B.C. Zhao, X.B. Zhu, W.H. Song, Y.P. Sun, Phys. Rev. B 72 (2005) 184417. [18] K. Ghosh, S.B. Ogale, R. Ramesh, R.L. Greene, T. Venkatesan, K.M. Gapchup, Ravi Bathe, S.I. Patil, Phys. Rev. B 59 (1999) 533. [19] R. Osborn, S. Rosenkranz, D.N. Argyriou, L. Vasiliu-Doloc, J.W. Lynn, S.K. Sinha, J.F. Mitchell, K.E. Gray, S.D. Bader, Phys. Rev. Lett. 81 (1998) 3964. [20] H. Fujioka, M. Kubota, K. Hirota, H. Yoshizawa, Y. Moritomo, Y. Endoh, J. Phys. Chem. Solids 60 (1999) 1165.
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[21] T. Chatterji, L.P. Regnault, P. Thalmeier, R. Suryanarayanan, G. Dhalenne, A. Revcolevschi, Phys. Rev. B 60 (1999) R6965. [22] A. Pe˜na, J. Guti´errez, J.M. Barandiar´an, J.L. Pizarro, T. Rojo, L. Lezama, M. Insausti, J. Magn. Magn. Mater. 226–230 (2001) 831.
[23] H. Song, W. Kim, S.J. Kwon, J. Kang, J. Appl. Phys. 89 (2001) 3398. [24] S.L. Young, Y.C. Chen, H.Z. Chen, L. Horng, J.F. Hsueh, J. Appl. Phys. 91 (2002) 8915. [25] C.S. Hong, N.H. Hur, Y.N. Choi, Solid State Commun. 133 (2005) 531.
The effect of transition element doping on the electronic and magnetic properties of La1.4Sr1.6Mn2O7 G.Q. Yu, Y.Q. Wang, L. Liu, S.Y. Yin, G.M. Ren, J.H. Miao, X. Xiao, S.L. Yuan ∗ Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 8 February 2006; received in revised form 4 October 2006; accepted 10 October 2006 by B.-F. Zhu Available online 23 October 2006
Abstract The effect of transition element (TE = Cr, Fe, Co, Ni, Cu, Zn) doping on the electronic transport and magnetic properties in the bilayer manganite La1.4 Sr1.6 Mn2 O7 is studied for the same dopant concentration fixed at 2%. Doping does not cause change in structure but different behavior in magnetic and transport properties. Except for Cr, all the other dopings significantly shift the magnetic transition temperature (TC ) to a lower temperature. Associated with such a decrease, the insulator–metal transition temperature (TIM ) decreases and the peak resistivity (ρ p ) at TIM increases. Cr doping enhances TC and TIM as well as decreases ρ p . Fe doping apparently has a stronger effect than Co and Ni doping. It is also indicated that Cu doping causes an anomalously large increase in ρ p . These behaviors are compared with those observed in other bilayer manganites such as La1.2 Sr1.8 Mn2 O7 as well as in La0.7 Ca0.3 Mn1−x TEx O3 . c 2006 Elsevier Ltd. All rights reserved.
PACS: 75.47.Lx; 74.25.Fy; 75.30.Kz; 61.72.Ww Keywords: A. Bilayer manganites; B. Doping; D. Electronic transport; D. Magnetic properties
Both the bilayer manganites La2−2x Sr1+2x Mn2 O7 (n = 2) and the three-dimensional (3D) manganites La1−x Ax MnO3 (n = ∞) are in the Ruddlesden–Popper series (R, A)n+1 Mnn On+1 , where R and A are trivalent rare-earth and divalent alkaline-earth ions, respectively. Recently, bilayer manganites La2−2x Sr1+2x Mn2 O7 have attracted considerable attention due to their novel properties such as the colossal magnetoresistance effect, tunneling magnetoresistance and fascinating electronic and magnetic properties [1–3]. The system forms natural quasi-two-dimensional tunneling structures consisting of two ferromagnetic (FM) metallic MnO2 layers separated by a rocksalt-type block layer (La, Sr)2 O2 , and shows a rich variety of magnetic structures and properties depending strongly on the doping level x. For example, for x = 0.3, namely, La1.4 Sr1.6 Mn2 O7 , the magnetic coupling is FM within the constituent single MnO2 layer and a bilayer unit and is weakly antiferromagnetic (AFM) between the adjacent bilayers at low temperature [4–6]; for x = 0.32–0.4, all the magnetic couplings
∗ Corresponding author. Tel.: +86 27 87556580; fax: +86 27 87544525.
E-mail address: [email protected] (S.L. Yuan). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.10.007
mentioned above are FM; for x = 0.5, namely, LaSr2 Mn2 O7 , with charge-ordered state, the magnetic coupling is FM within the constituent single MnO2 layer, but shows A-type AFM order between the respective MnO2 layers within a bilayer unit. This is very different from the well-studied 3D manganites La1−x Ax MnO3 (A = Ca, Sr, Ba). Generally, the magnetic and transport properties in the mixed-valence manganites can be understood in terms of the double-exchange (DE) interaction mechanism [7]. Besides the DE mechanism, the Jahn–Teller effect, phase separation, AFM superexchange and charge-orbital ordering interactions also play important roles. In the bilayer manganites, the DE interaction essentially works only within the respective MnO2 layers, and there also exists a strong interplay among spin, charge, orbital and lattice degrees of freedom. The bilayer manganites, La2−2x Sr1+2x Mn2 O7 , offer the opportunity not only to investigate the generic features of the mixedvalence manganites in reduced dimensions, but also to explore phenomena that are not found in the 3D manganites. Mn-site doping in the bilayer manganites is very interesting because it can dramatically change the magnetic and electronic properties due to the crucial role of Mn ions in manganites.
G.Q. Yu et al. / Solid State Communications 141 (2007) 136–140
Some of such studies have been undertaken during the past few years [8–17]. Gundakaram et al. [8] studied the effect of Cr doping in La1.2 Sr1.8 Mn2 O7 . With the increase of Cr doping level, the insulator–metal transition (IMT) was suppressed and the ferromagnetic Curie temperature increased slightly and then decreased, and the magnetoresistance ratio was not significantly affected. Zhang et al. [9,10] studied the magnetic and transport properties of the La1.2 Sr1.8 Mn2−x Tx O7 (T = Fe, Co, Cr), and they showed Fe and Co doping had similar effects, resulting in quick weakening of FM ordering and immediate disappearance of the IMT and enhancement of low-temperature MR, but a different effect was observed for Cr doping, although the IMT also disappeared quickly, and FM ordering was maintained even for x = 0.5. They suggested there existed a FM DE interaction between Cr3+ and Mn3+ . Ru doping in La1.2 Sr1.8 Mn2 O7 could increase TC . To interpret this, Weigand et al. [13] proposed a coupling model of Mn3+ /Mn4+ and Ru3+ /Ru4+ ions in the high-spin state, and Onose et al. [14] suggested Ru doping suppressed the charge-orbital correlation resulting in the enhancement of the Curie temperature TC . Very recently, in the LaSr2 Mn2 O7 system, Nair and Banerjee [15] reported the effect of Al doping; Zhang et al. [16] reported that of Cr doping and Ang et al. [17] reported that of Co doping. For the La1.4 Sr1.6 Mn2 O7 system, there are less reports on Mn-site doping effect. Only Zhu et al. [11,12] studied the effect of Cu and Ti doping in this system. In the 3D perovskite manganites, Ghosh et al. [18] studied the TE doping effects in La0.7 Ca0.3 MnO3 . For comparison, we believe there is a need for studies of series TE doping in the bilayer manganite La1.4 Sr1.6 Mn2 O7 . Choosing the La1.4 Sr1.6 Mn2 O7 as parent phase is also due to its layered structure and different magnetic structure compared with the 3D manganites and other bilayer manganites (e.g. La1.2 Sr1.8 Mn2 O7 , LaSr2 Mn2 O7 , etc.). We expect to have new and different effects. Polycrystalline bulk samples of La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn) were synthesized by a standard ceramic process. A well-ground stoichiometric mixture of high purity La2 O3 , SrCO3 , MnO2 and relevant TE metal oxides powders were calcined at 1200 ◦ C for 24 h in air with intermediate grinding. The powders were well ground again, then palletized and sintered at 1450 ◦ C for 40 h with an intermediate grinding. The structural characterization was done through x-ray diffraction (XRD) at room temperature with Cu K α radiation. The temperature dependence of resistivity (without and with a field of 5 T) and magnetization at low field (100 Oe) was measured in a commercial Physical Property Measurement System (Quantum Design PPMS). XRD patterns for all samples of La1.4 Sr1.6 Mn1.96 TE0.04 O7 are presented in Fig. 1. All diffraction peaks can be indexed using a Sr3 Ti2 O7 -type tetragonal structure (I 4/mmm), which indicates that all samples are of a single phase. Fig. 2 shows the temperature dependence of resistivity for all samples. For the undoped sample, the IMT temperature TIM is 112 K. Except for Cr doped samples, the TIM of other doped samples shifts to lower temperature, while the resistivity increases. The minimum value of TIM is 88 K for Zn doping.
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Fig. 1. XRD patterns of samples La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn).
Fig. 2. Temperature dependence of resistivity for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn) at zero field.
Cu doping causes a larger increase in the peak resistivity (ρ p ). A similar observation has also been reported by Zhu et al. in Ref. [11], where the peak resistivity of the Cu doped sample with x = 0.05 was nearly 20 times larger than that of the undoped sample. The case is significantly different from that in the 3D manganites La0.7 Ca0.3 Mn0.95 TE0.05 O3 [18], where the decrease in TIM was small for Cr and Zn doping, and TIM of Cu doped samples almost did not shift while the resistivity obviously reduced. For Fe and Co doping in the present system, the TIM drops more slowly than that in La1.2 Sr1.8 Mn2 O7 [9, 10], where the IMT disappeared quickly at a very low doping level (x = 0.01). Cr doping results in an increase in TIM (120 K) and a decrease in resistivity especially at temperatures near
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Fig. 3. Temperature dependence of magnetization M at 100 Oe for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn). What is represented by the symbols here is the same as that in Fig. 2. Inset: the magnification plot of the circled area.
TIM , which is not observed in other manganites (including the layered and 3D manganites). The temperature dependence of magnetization M measured at a field of 100 Oe is shown in Fig. 3. All M–T curves show two magnetic transitions lying at lower temperature near 100 K and higher temperature near 240 K, respectively. For the undoped sample, Fig. 3 shows a 3D magnetic transition at TC (defined as the temperature at which dM/dT reaches the maximum) is 106 K, and another upper transition at T ∗ (defined as another maximum in the dM/dT –T curve) is 241 K which has been ascribed to short range two-dimensional (2D) magnetic ordering due to quasi-two-dimensional features [19– 21]. The TIM , TC and T ∗ for La1.4 Sr1.6 Mn2 O7 doped with different transition elements are presented in Fig. 4(a). For Cr doped samples, the TC just like its TIM shifts to higher temperature (116 K) and T ∗ slightly shifts to lower temperature (235 K) compared with the undoped sample. For other TE doped samples, both TC and TIM shift to lower temperature. The T ∗ of all doped samples shifts to lower temperature, which indicates that the TE doping weakens the 2D magnetic ordering in the system. The influence of Cr doping on 2D magnetic ordering is the weakest. Fig. 4(a) also shows a direct correlation between magnetic and transport behavior. As shown in Figs. 2–4(a), the magnetic properties are more sensitive to Fe doping than the electronic properties in the present system, which is an opposite effect compared with Fe doping in La1.2 Sr1.8 Mn2 O7 [9,10]. Figs. 3 and 4(a) further indicate there is a different effect on TC for Cr, Cu and Zn doping but a similar effect for Fe, Co and Ni doping compared with those in 3D manganites [18]. Fig. 4(b) shows the ρ p value at the IMT temperature. These data have a direct correlation to the data shown in Fig. 4(a), although there appears to be a departure in the correlation in the case of Cu. The temperature dependence of MR0 (MR0 = ({[ρ(H = 0) − ρ(H = 5 T)]/ρ(H = 0)} × 100%)) ratios at 5 T for La1.4 Sr1.6 Mn1.96 TE0.04 O7 is shown in Fig. 5. All MR0 peaks appear at the temperature near TIM , below which fairly large
Fig. 4. TE dependence of (a) TIM , TC and T ∗ , (b) the peak resistivity (ρ p ) value at TIM for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn).
Fig. 5. Temperature dependence of MR0 at 5 T for La1.4 Sr1.6 Mn1.96 TE0.04 O7 (TE = Cr, Mn, Fe, Co, Ni, Cu, Zn).
low-temperature MR0 ratios remain. The maximum MR0 peak appears in Zn doped samples, and the minimum appears in Cr doped samples. There is an obvious correlation between the maximum MR0 and the ionic radii, which is similar to the observation in 3D manganites in Ref. [18]. However, the lowtemperature (below 40 K) MR0 of all doped samples is less than that of the undoped sample, which is very different from the former observation of TE doping in La1.2 Sr1.8 Mn2 O7 [9,10], where TE doping enhanced the low-temperature MR0 effect, especially for Fe doping. In the bilayer manganites, the layered structure and the decrease of dimensionality results in the DE interaction between Mn ions essentially working only within the respective MnO2 layers, which makes the system more sensitive to the Mn-site doping, and the intrabilayer magnetic interaction is stronger than the interbilayer one. Mn-site doping may influence not only the intrabilayer magnetic interaction but also the interbilayer magnetic interaction. In the present studies, TE doping destroys the Mn–O–Mn network and the DE interaction
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between Mn ions, and dilutes the in-plane FM interaction. Except for Cr doping, TE doping results in a decrease in TIM , TC and T ∗ (with Cr doping) and an increase in ρ p . Cu2+ (r ∼ 0.073 nm) and Zn2+ (r ∼ 0.075 nm) ions partially substituting Mn3+ (r ∼ 0.065 nm) ions causes a bigger local change in the crystal lattice and a serious destruction of Mn–O–Mn network due to the bigger ionic radius, which results in a more obvious decrease in TIM , TC and T ∗ as well as a bigger increase in resistivity. In comparison with all the other dopings, Cu doping, for reasons that are not clear now, causes an anomalously large increase in ρ p . All our experimental results are well repeatable, and accordant results occurred in several repeated experiments from our group. The effect of Cu doping on the peak resistivity is worthy of further study. Now we focus on the case of Fe, Co and Ni doping. From Figs. 2 and 3, it can be seen that Fe doping apparently has a stronger effect than Co and Ni doping. The radii of Fe3+ , Co3+ and Ni3+ (0.065, 0.061 and 0.060 nm, respectively) are close to the radius of Mn3+ (0.065 nm). We notice that the M–T curves for Fe and Zn (with larger ionic radius ∼ 0.075 nm) doping almost overlap at low temperature, and they have the same TC . This implies that except for ionic radius, other factors are important to electronic and magnetic properties in the case of Fe, Co and Ni doping. In the 3D manganites, several reports [22–24] state that the coupling between Mn and doped Fe, Co, Ni ions obeys the AFM superexchange mechanism. We believe that the AFM superexchange interaction between Mn and doped Fe, Co, Ni ions plays an important role in the present system. The electronic configurations for Mn3+ (Mn4+ ), Fe3+ , Co3+ and 3 e1 (t 3 e0 ), t 3 e2 , t 4 e2 and t 5 e2 and hence the Ni3+ are t2g g 2g g 2g g 2g g 2g g numbers of unpaired 3d electrons are 4 (3), 5, 4 and 3, respectively. Due to the AFM coupling between Mn and doped Fe, Co, Ni ions, the more unpaired 3d electrons are, the stronger the AFM coupling is, hence the AFM coupling between Mn and Fe ions is the strongest. The enhancement of the AFM interaction weakens the FM behavior. Meanwhile, in the system, the stronger the AFM interaction is, the weaker the FM DE interaction is. Therefore, Fe doping has the strongest effect among Fe, Co and Ni doping in the present system. For Fe, Co and Ni doping in La1.4 Sr1.6 Mn2 O7 , their major influence is on the intrabilayer magnetic interaction, and so show similar magnetic and transport behaviors with those in 3D manganites [18], while they also influence the interbilayer magnetic coupling resulting in a different MR0 effect from that in La1.2 Sr1.8 Mn2 O7 [9,10]. Cr doping has a different effect. Some researchers [10,25] believe there exists FM DE interactions between Cr3+ and Mn3+ just as between Mn4+ and Mn3+ because Cr3+ has 3 e0 ) as Mn4+ . However, the same electronic configuration (t2g g in the present system, the doped Cr3+ ions directly replace Mn3+ resulting in the appearance of Cr3+ –O–Mn4+ chains which prevent the hopping of carriers (i.e. DE interaction) within the MnO2 layers. Cr doping dilutes the in-plane FM interaction, so a decrease in T ∗ is observed. What causes the increase in TIM and TC and the decrease in resistivity and MR0 ratio for Cr doping in La1.4 Sr1.6 Mn2 O7 ? Besides intrabilayer
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magnetic interactions, we believe that the interbilayer magnetic coupling and magnetic structure play an important role. Cr doping affects the interbilayer magnetic coupling. We will study systematically the Cr doping elsewhere. In conclusion, the effect of transition element doping on the electronic and magnetic properties of La1.4 Sr1.6 Mn2 O7 has been investigated. We find that when various elements are used to partially substitute Mn sites, the TIM , TC , T ∗ , ρ p and MR0 exhibit distinct changes. Different doping effects are also observed compared with those in the 3D manganites and other bilayer manganites. The results suggest that there is a different degree of the influence of different transition element doping on the intrabilayer and interbilayer magnetic interactions. Besides the intrabilayer interaction, the interbilayer magnetic interaction may play an important role on the transition element doping in the bilayer manganites. In order to further clarify this, it is worthwhile to make detailed investigations on the magnetic properties. Acknowledgement This work was supported by the National Science Foundation of China under Grand Nos 10374032 and 10574049. References [1] Y. Moritomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature 380 (1996) 141. [2] T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. [3] See, for a review, T. Kimura, Y. Tokura, Annu. Rev. Mater. Sci. 30 (2000) 451. [4] T.G. Perring, G. Aeppli, T. Kimura, Y. Tokura, M.A. Adams, Phys. Rev. B 58 (1998) R14693. [5] D.N. Argyriou, J.F. Mitchell, P.G. Radaelli, H.N. Bordallo, D.E. Cox, M. Medarde, J.D. Jorgensen, Phys. Rev. B 59 (1999) 8695. [6] M. Konoto, T. Kohashi, K. Koike, T. Arima, Y. Kaneko, T. Kimura, Y. Tokura, Phys. Rev. Lett. 93 (2004) 107201. [7] C. Zener, Phys. Rev. 82 (1951) 403; P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675. [8] R. Gundakaram, J.G. Lin, F.Y. Lee, M.F. Tai, C.H. Shen, C.Y. Huang, J. Phys.: Condens. Matter. 11 (1999) 5187. [9] J. Zhang, F. Wang, P. Zhang, Q. Yan, J. Appl. Phys. 86 (1999) 1604. [10] J. Zhang, Q. Yan, F. Wang, P. Yuan, P. Zhang, J. Phys.: Condens. Matter. 12 (2000) 1981. [11] H. Zhu, X.J. Xu, L. Pi, Y.H. Zhang, Phys. Rev. B 62 (2000) 6754. [12] H. Zhu, X.J. Xu, K.Q. Ruan, Y.H. Zhang, Phys. Rev. B 65 (2002) 104424. [13] F. Weigand, S. Gold, A. Schmid, J. Geissier, E. Goering, Appl. Phys. Lett. 81 (2002) 2035. [14] Y. Onose, J.P. He, Y. Kannko, T. Arima, Y. Tokura, Appl. Phys. Lett. 86 (2005) 242502. [15] Sunil Nair, A. Banerjee, Phys. Rev. B 70 (2004) 104428. [16] R.L. Zhang, B.C. Zhao, W.H. Song, Y.Q. Ma, J. Yang, Z.G. Sheng, J.M. Dai, J. Appl. Phys. 96 (2004) 4965. [17] R. Ang, W.J. Lu, R.L. Zhang, B.C. Zhao, X.B. Zhu, W.H. Song, Y.P. Sun, Phys. Rev. B 72 (2005) 184417. [18] K. Ghosh, S.B. Ogale, R. Ramesh, R.L. Greene, T. Venkatesan, K.M. Gapchup, Ravi Bathe, S.I. Patil, Phys. Rev. B 59 (1999) 533. [19] R. Osborn, S. Rosenkranz, D.N. Argyriou, L. Vasiliu-Doloc, J.W. Lynn, S.K. Sinha, J.F. Mitchell, K.E. Gray, S.D. Bader, Phys. Rev. Lett. 81 (1998) 3964. [20] H. Fujioka, M. Kubota, K. Hirota, H. Yoshizawa, Y. Moritomo, Y. Endoh, J. Phys. Chem. Solids 60 (1999) 1165.
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[21] T. Chatterji, L.P. Regnault, P. Thalmeier, R. Suryanarayanan, G. Dhalenne, A. Revcolevschi, Phys. Rev. B 60 (1999) R6965. [22] A. Pe˜na, J. Guti´errez, J.M. Barandiar´an, J.L. Pizarro, T. Rojo, L. Lezama, M. Insausti, J. Magn. Magn. Mater. 226–230 (2001) 831.
[23] H. Song, W. Kim, S.J. Kwon, J. Kang, J. Appl. Phys. 89 (2001) 3398. [24] S.L. Young, Y.C. Chen, H.Z. Chen, L. Horng, J.F. Hsueh, J. Appl. Phys. 91 (2002) 8915. [25] C.S. Hong, N.H. Hur, Y.N. Choi, Solid State Commun. 133 (2005) 531.