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Effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7
Journal of Magnetism and Magnetic Materials 323 (2011) 1925–1928
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Effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7 Gongqi Yu a,n, Bin Xu b, Jinming Xiong a, Xiangfei Liu a, Li Liu c, Songliu Yuan c a b c
The Second Artillery Command College, Wuhan 430012, People’s Republic of China Department of Mathematics and Information Science, North China University of Water Resources and Electric Power, Zhengzhou 450008, People’s Republic of China School of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 7 March 2010 Received in revised form 23 October 2010 Available online 4 February 2011
The effect of Cr doping on magnetic and electrical properties in the bilayer manganites La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0–0.1) has been investigated. When yr 0.025, Cr doping enhances the three-dimensional magnetic transition temperature TC and the insulator–metal transition temperature TIM as well as decreases the peak resistivity at TIM, and the saturated magnetization decreases slightly. When yZ 0.035, TIM decreases gradually accompanied by the increase of peak resistivity, but TC remains nearly constant, and the saturated magnetization decreases heavily. In the whole doping region, the two-dimensional magnetic transition temperature Tn monotonously decreases with an increasing of Cr doping level. These results can be explained by considering different magnetic (including ferromagnetic and antiferromagnetic) interactions between Mn ions and Cr ions. & 2011 Elsevier B.V. All rights reserved.
Keywords: Bilayer manganite Double-exchange Magnetic interaction Phase separation
1. Introduction In the past few years, the bilayer manganites La2 2xSr1 + 2xMn2O7 of Ruddlesden–Popper phases (n¼2) attracted considerable attention due to their novel physical properties such as the colossal magnetoresistance effect, the tunneling magnetoresistance and fascinating magnetic properties [1–3]. In this system, two ferromagnetic metallic MnO2 layers are separated by a rock-salt-type block layer (La,Sr)2O2, keeping the quasi-two-dimensional networks of the MnO6 octahedra. The system presents a very rich phase diagram and displays a rich variety of properties depending strongly on the doping level x [4]. For x¼0.3, namely La1.4Sr1.6Mn2O7, the magnetic coupling is ferromagnetic (FM) within the constituent MnO2 bilayer and weakly antiferromagnetic (AFM) between the adjacent bilayers at low temperature, the three-dimensional (3D) magnetic transition temperature beingTC 100 K. For x¼0.32–0.4, the magnetic couplings mentioned above are FM; the maximum TC 131 K for x0.36, where Mn3 + /Mn4+ ratio is an optimal ratio. For x¼0.5, namely LaSr2Mn2O7, 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 with a bilayer unit, the AFM ordering temperature being TN 220 K. It is well known that the correlation between the magnetic and electrical properties in the mixed-valence manganites is generally understood in terms of the double-exchange (DE) interaction mechanism [5]. Besides the DE mechanism, the Jahn–Teller effect,
n
Corresponding author. Tel.: +86 15972962023; fax: + 86 2787544764. E-mail address: [email protected] (G. Yu).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.01.042
phase separation (PS) [6] and AFM superexchange and chargeorbital ordering interactions also play an important role. The bilayer manganites, La2 2xSr1 + 2xMn2O7, offer another unique opportunity to investigate the generic features of the mixedvalence manganites. Mn-site doping in the bilayer manganites can dramatically change the magnetic and electrical properties. Some such studies have been undertaken during the past few years [7–13]. Cr is a very interesting substitution ion for Mn site, because Cr3 + has the 3 0 eg ) as that of Mn4 + , and closely same electronic configuration (t2g identical ionic radius as that of Mn3 + . There may be FM DE interaction between Cr3 + and Mn3 + just as between Mn4 + and Mn3 + . Some reports on the effect of Cr doping in the cubic perovskite manganites have appeared, and several reports on such studies have also appeared in the case of bilayer manganites [14–18]. But there are a few reports on the effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7. In this paper, we report the effect of Cr doping on magnetic and electrical properties, especially on magnetic properties in the bilayer manganites La1.4Sr1.6(Mn1 yCry)2O7 (y¼0–0.1). With Cr doping, Cr3 + will replace Mn3 + , which directly decreases the content of Mn3 + ions. On the other hand, Cr3 + can act as Mn4 + due to the same electronic configuration if there exists FM DE interaction between Cr3+ and Mn3 + . Thus, the Mn3 + /Mn4 + (including Cr3 + ) ratio will be closer to optimal ratio, and an interesting effect on magnetic and electric properties will be observed in the present Cr-doped system. The present measurements show that the magnetic coupling between Cr ions and Mn ions is related to the Cr doping concentration. When yo0.035 the coupling is FM and when y40.035 it is AFM. The total effect is observed and discussed.
1926
G. Yu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1925–1928
2. Experiment
240 y=0.1 180
La1.4Sr1.6(Mn1-yCry)2O7
120 Resistivity (Ω·cm)
The polycrystalline bulk samples of La1.4Sr1.6(Mn1 yCry)2O7 (y¼0–0.1) were synthesized by a standard ceramic process. Well ground stoichiometric mixture of high purity La2O3, SrCO3, MnO2 and Cr2O3 were calcined at 1200 1C for 24 h in air with intermediate grinding. The powders were well ground again, then pelletized and sintered at 1450 1C for 45 h with an intermediate grinding. The structural characterization was done through X-ray diffraction (XRD) with CuKa radiation at room temperature. The temperature dependence of resistivity was measured by a standard four probe method at zero field. Magnetization was measured using a commercial physical property measurement system (Quantum Design PPMS).
y=0.05 y=0.035
60
y=0 0 25 y=0
20 15
y=0.005 y=0.025
10
3. Results and discussion XRD patterns for all the samples of La1.4Sr1.6(Mn1 yCry)2O7 (y¼0–0.1) is presented in Fig. 1. All diffraction peaks are indexed using Sr3Ti2O7-type tetragonal structure (I4/mmm), indicating a single phase of bilayer structure. The lattice parameters obtained from XRD data are listed in Table 1. Fig. 2 shows the temperature (77–300 K) dependence of resistivity for all samples. For undoped sample (y¼ 0), the insulator–metal transition (IMT) temperature TIM is 110 K. At
y=0.01
5
y=0.015 0
50
100
150
200
250
300
T(K) Fig. 2. Temperature dependence of resistivity for samples La1.4Sr1.6(Mn1 yCry)2O7 (0ryr 0.1) at zero field.
0.7 y=0
M (µB /Mn1-yCry site)
0.6
y=0.1 y=0.05 y=0.035
0.5
La1.4Sr1.6(Mn1-yCry)2O7
y=0.05
0.4
y=0.035
0.3
y=0.025
0.2
y=0.1
Intensity (a.u.)
0.1
y=0.025
0.0 0
y=0.005 y=0
30
40
50 60 2θ (degree)
70
100
150
200
250
300
Fig. 3. Temperature dependence of magnetization M at 0.01 T for samples La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0, 0.025, 0.035, 0.05, 0.1).
y=0.01
20
50
T (K)
y=0.015
80
Fig. 1. XRD patterns of all samples La1.4Sr1.6(Mn1 yCry)2O7 (0ry r0.1).
Table 1 Lattice parameters for samples La1.4Sr1.6(Mn1 yCry)2O7 (0r yr0.1). y
˚ a (A)
˚ c (A)
V (A˚ 3)
y¼ 0 y¼ 0.005 y¼ 0.01 y¼ 0.015 y¼ 0.025 y¼ 0.035 y¼ 0.05 y¼ 0.1
3.8751 3.8746 3.8729 3.8732 3.8711 3.8704 3.8688 3.8667
20.171 20.174 20.175 20.181 20.183 20.187 20.194 20.202
302.90 302.86 302.61 302.75 302.45 302.40 302.26 302.05
lower Cr doping level (yr0.025), the IMTs shift gradually to higher temperature with increasing Cr doping level. TIM reaches the maximum value of 121 K for y¼0.025 sample, decreases gradually with increasing Cr doping level and when y¼0.1, IMT disappears. Associated with such TIM change, the resistivity near TIM decreases gradually for yo0.025 samples, and increases for yZ0.025 samples. Such results for Cr doping are not observed in other manganites (including the bilayer and 3D cubic manganites), which are very interesting. In addition, with increasing Cr doping, IMTs become broader and broader; similar results have also been observed for Cr doping in La1.2Sr1.8Mn2O7 [16]. The temperature dependence of magnetization M measured at a field of 0.01 T for samples La1.4Sr1.6(Mn1 yCry)2O7 (y¼0, 0.025, 0.035, 0.05, 0.1) is shown in Fig. 3. All M–T curves show two magnetic transitions lying at lower temperature near 110 K and higher temperature near 200–240 K. For all samples, a visible plateau in the M–T curves extends to higher temperature, which has been ascribed to a short range two-dimensional (2D) FM ordering due to the quasi-two-dimensional feature [19–21]. For the undoped sample (y¼0), Fig. 3 shows a 3D magnetic transition at TC (defined as the temperature at which dM/dT reaches the maximum) is 106 K; another upper transition, namely 2D magnetic transition at Tn (defined as another maximum in dM/dT–T
G. Yu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1925–1928
curve) is at 241 K. From Fig. 3 it can be seen that with increasing Cr doping level, the 3D magnetic transition shifts slightly to higher temperature and then keeps the transition temperature nearly constant, which is a picture different from the resistivity data in Fig. 2. Meanwhile, with increasing Cr doping level, the short range 2D magnetic transition shifts monotonously to lower temperature, which is different from the 3D magnetic transition, indicating that there are different mechanisms to dominate the two magnetic transitions. To compare the transition temperatures, TIM, TC and Tn are plotted against the Cr doping concentration y in Fig. 4. For y¼0.025 sample, TIM (121 K) and TC (117 K) reach their maximum values accompanied by the decrease of peak resistivity, indicating that the FM DE interaction is enhanced. For y40.035 samples, the apparent disagreement between TIM and TC indicates that a possible PS occurs. The decrease of Tn indicates that Cr doping suppresses the short range 2D FM ordering. It is worth noticing that Tn against the Cr doping concentration y almost follows linear relation. Fig. 5 shows the field (0–5 T) dependence of magnetization at 10 K for samples La1.4Sr1.6(Mn1 yCry)2O7 (y¼0, 0.025, 0.035, 0.05, 0.1). The magnetization is observed to almost saturate in the high field region ( 5 T) for all samples. For undoped sample (y¼0) the magnetic moment reaches 3.59 mB at 10 K at the field of 5 T, which is close to the theoretical value (3.7 mB). When yr0.035, the saturated magnetization decreases slightly with increasing Cr doping level, and when y40.035, it decreases heavily with increasing Cr doping, indicating that there exist different magnetic interactions between Cr ions and Mn ions with Cr doping. In the inset of Fig. 5,
240
T IM TC * T
116
230 220 210
112
T* (K)
TIM & TC (K)
120
200 108
190 0.000
0.025 0.050 0.075 Cr doping concentration y
Fig. 4. Doping (y) dependence of TIM, TC La1.4Sr1.6(Mn1 yCry)2O7 (y¼ 0, 0.025, 0.035, 0.05, 0.1).
0.100
and
Tn
for
180
samples
M (µB / Mn1-yCry site)
4 3 y=0 y=0.025 y=0.035 y=0.05 y=0.1
M=3.7-y
2 1 Mn
Cr
Mn
M=3.7-7y
1927
the magnetization at 5 T is plotted against the doping concentration y. When yo0.035, the magnetization M decreases with increasing Cr doping almost in parallel with the relation that M¼ 3.7–y, although the observed magnetization is slightly smaller than the relation, indicating that the magnetic coupling between Cr spins and Mn spins is FM. When y40.035, the magnetization M almost follows the relation M¼3.7–7y, indicating that the magnetic coupling between Cr spins and Mn spins is AFM. Generally, the magnetic interaction between Cr3 + and Mn3 + is FM, and the magnetic interaction between Cr3 + and Mn4 + is AFM [18,22]. In the present system, we think that there exists competition of the two kinds of interactions above between Cr ions and Mn ions; when the FM interaction between Cr3 + and Mn3 + preponderates, the magnetic interaction between Cr3 + and Mn3 + as well as the magnetic interaction between Cr3 + and Mn4 + is FM; otherwise when the AFM interaction between Cr3 + and Mn4 + preponderates, the magnetic interaction between Cr3 + and Mn4 + as well as the magnetic interaction between Cr3 + and Mn3 + is AFM. At lower Cr doping level (yr0.025), the FM interaction between Cr3 + and Mn3 + preponderating, with increasing Cr doping level, at higher Cr doping level (y40.035), the AFM interaction between Cr3 + and Mn4 + becomes preponderate because the doped Cr3 + ions directly replace Mn3 + ions resulting in the decrease of Mn3 + ions amount. In La1.4Sr1.6Mn2O7, Mn3 + /Mn4 + ratio is 7/3; thus, as Cr3 + ions replace partially Mn3 + ions, they still have Mn3 + ions besides Mn4 + ions around Cr3 + . When Cr spins couple ferromagnetically with Mn spins, there may exist the FM DE interaction between Cr3 + and Mn3 + just as between Mn4 + and Mn3 + because Cr3 + has the same 3 0 electronic configuration (t2g eg ) as Mn4 + . For yr0.025 samples, the 3+ substitution of Cr (acts as Mn4 + ) for Mn3 + results in the decrease of Mn3 + /Mn4 + (including Cr3 + ) ratio to being closer to optimal ratio. Therefore, enhancement of TIM and TC, and the decrease of resistivity are observed in Fig. 2 and Fig. 3, respectively. When y40.035, the magnetic coupling between Cr spins and Mn spins becomes AFM, i.e. the magnetic interaction between Cr3 + and Mn4 + as well as the magnetic interaction between Cr3 + and Mn3 + is AFM, which results in the appearance of the AFM second phase in the present system. With increasing Cr doping level, the PS between the FM phase and the AFM second phase probably occurs. The volume fraction of AFM second phase is small due to small Cr doping content. The AFM second phase mostly influences the electrical properties by preventing the hopping of carriers, which results in the decrease of TIM and the increase of resistivity. The 3D magnetic transition is still dominated by the FM phase, and the influence of AFM second phase is little, so TC remains nearly constant. Mn-site doping also influences the short range 2D magnetic ordering. The discussion of this aspect is very little in references. For present studies, Cr doping suppresses the short range 2D magnetic ordering. The 2D magnetic transition accompanied by the insulator behavior of electrical properties occurs in the higher temperature region, where the arrangement of Mn spins tends to long range disorder, but there also exists the short range 2D FM ordering inside the layers. The doped Cr3 + ions directly replace Mn3 + ions, which destroys the short range FM interaction between Mn3 + and Mn4 + , while the doped Cr3 + ions can not FM couple with Mn3 + ions because the doped Cr3 + ions can not only ferromagnetically couple with Mn3 + nearby, but also antiferromagnetically couple with Mn4 + nearby, and the two interactions may counteract in the case of short range effect resulting in the disorder.
0 0
1
2
3
4
5
4. Conclusions
H (T) Fig. 5. Field dependence of magnetization M at 10 K for samples La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0, 0.025, 0.035, 0.05, 0.1). Inset: magnetization at 5 T against the doping concentration y.
We have investigated the Cr doping effect in the bilayer manganites La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0 0.1). The saturated magnetization data suggest that with the increase of Cr doping
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G. Yu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1925–1928
level, the magnetic coupling between Cr spins and Mn spins changes from FM to AFM. The cause of the enhancement of the metallic property and 3D magnetic transition temperature TC is that there exists FM DE interaction between Cr3 + and Mn3 + as well as the Cr doping optimizes Mn3 + /Mn4 + ratio for yo0.035. When y40.035, the AFM second phase suppresses the metallic property; meanwhile, the 3D magnetic transition is still dominated by the FM phase, and TC remains nearly constant. In addition, Cr doping suppresses the short range 2D FM ordering due to no short range effect.
Acknowledgement This work was supported by National 973 Project of China (Grant no. 2006CB921606). References [1] Y. Moritomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature (London) 380 (1996) 141. [2] T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. [3] T. Kimura, Y. Tokura, Annu. Rev. Mater. Sci. 30 (2000) 451 See, for a review. [4] Y. Moritomo, Y. Maruyama, T. Akimoto, A. Nakamura, J. Phys. Soc. Jpn. 67 (1998) 405 and references cited therein.
[5] C. Zener, Phys. Rev. 82 (1951) 403; P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955)675 100 (1955). [6] E. Dagotto, T. Hotta, A. Moreo, Phys. Rep. 344 (2001) 1. [7] J. Zhang, F. Wang, P. Zhang, Q. Yan, J. Appl. Phys. 86 (1999) 1604. [8] H. Zhu, X.J. Xu, L. Pi, Y.H. Zhang, Phys. Rev. B 62 (2000) 6754. [9] H. Zhu, X.J. Xu, K.Q. Ruan, Y.H. Zhang, Phys. Rev. B 65 (2002) 104424. [10] F. Weigand, S. Gold, A. Schmid, J. Geissier, E. Goering, Appl. Phys. Lett. 81 (2002) 2035. [11] Y. Onose, J.P. He, Y. Kannko, T. Arima, Y. Tokura, Appl. Phys. Lett. 86 (2005) 242502. [12] A.Banerjee Sunil Nair, Phys. Rev. B 70 (2004) 104428. [13] 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. [14] R. Gundkaram, J.G. Lin, F.Y. Lee, M.F. Tai, C.H. Shen, C.Y. Huang, J. Phys.: Condens. Matter 11 (1999) 5187. [15] J. Zhang, Q. Yan, F. Wang, P. Yuan, P. Zhang, J. Phys.: Condens. Matter 12 (2000) 1981. [16] K.B. Chashka, B. Fisher, J. Genossar, A. Keren, L. Patlagon, G.M. Reisner, E. Shimshoni, J.F. Mitchell, Phys. Rev. B 65 (2002) 134441. [17] 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. [18] M. Matsukawa, M. Chiba, E. Kikuchi, R. Suryanarayanan, M. Apostu, S. Nimori, K. Sugimoto, N. Kobayashi, Phys. Rev. B 72 (2005) 224422. [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. [21] T. Chatterji, L.P. Regnault, P. Thalmeier, R. Suryanarayanan, G. Dhalenne, A. Revcolevschi, Phys. Rev. B 60 (1999) R6965. [22] J.B. Goodenough, Magnetism and the Chemical Bond, Interscience, New York, 1963.
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Effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7 Gongqi Yu a,n, Bin Xu b, Jinming Xiong a, Xiangfei Liu a, Li Liu c, Songliu Yuan c a b c
The Second Artillery Command College, Wuhan 430012, People’s Republic of China Department of Mathematics and Information Science, North China University of Water Resources and Electric Power, Zhengzhou 450008, People’s Republic of China School of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 7 March 2010 Received in revised form 23 October 2010 Available online 4 February 2011
The effect of Cr doping on magnetic and electrical properties in the bilayer manganites La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0–0.1) has been investigated. When yr 0.025, Cr doping enhances the three-dimensional magnetic transition temperature TC and the insulator–metal transition temperature TIM as well as decreases the peak resistivity at TIM, and the saturated magnetization decreases slightly. When yZ 0.035, TIM decreases gradually accompanied by the increase of peak resistivity, but TC remains nearly constant, and the saturated magnetization decreases heavily. In the whole doping region, the two-dimensional magnetic transition temperature Tn monotonously decreases with an increasing of Cr doping level. These results can be explained by considering different magnetic (including ferromagnetic and antiferromagnetic) interactions between Mn ions and Cr ions. & 2011 Elsevier B.V. All rights reserved.
Keywords: Bilayer manganite Double-exchange Magnetic interaction Phase separation
1. Introduction In the past few years, the bilayer manganites La2 2xSr1 + 2xMn2O7 of Ruddlesden–Popper phases (n¼2) attracted considerable attention due to their novel physical properties such as the colossal magnetoresistance effect, the tunneling magnetoresistance and fascinating magnetic properties [1–3]. In this system, two ferromagnetic metallic MnO2 layers are separated by a rock-salt-type block layer (La,Sr)2O2, keeping the quasi-two-dimensional networks of the MnO6 octahedra. The system presents a very rich phase diagram and displays a rich variety of properties depending strongly on the doping level x [4]. For x¼0.3, namely La1.4Sr1.6Mn2O7, the magnetic coupling is ferromagnetic (FM) within the constituent MnO2 bilayer and weakly antiferromagnetic (AFM) between the adjacent bilayers at low temperature, the three-dimensional (3D) magnetic transition temperature beingTC 100 K. For x¼0.32–0.4, the magnetic couplings mentioned above are FM; the maximum TC 131 K for x0.36, where Mn3 + /Mn4+ ratio is an optimal ratio. For x¼0.5, namely LaSr2Mn2O7, 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 with a bilayer unit, the AFM ordering temperature being TN 220 K. It is well known that the correlation between the magnetic and electrical properties in the mixed-valence manganites is generally understood in terms of the double-exchange (DE) interaction mechanism [5]. Besides the DE mechanism, the Jahn–Teller effect,
n
Corresponding author. Tel.: +86 15972962023; fax: + 86 2787544764. E-mail address: [email protected] (G. Yu).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.01.042
phase separation (PS) [6] and AFM superexchange and chargeorbital ordering interactions also play an important role. The bilayer manganites, La2 2xSr1 + 2xMn2O7, offer another unique opportunity to investigate the generic features of the mixedvalence manganites. Mn-site doping in the bilayer manganites can dramatically change the magnetic and electrical properties. Some such studies have been undertaken during the past few years [7–13]. Cr is a very interesting substitution ion for Mn site, because Cr3 + has the 3 0 eg ) as that of Mn4 + , and closely same electronic configuration (t2g identical ionic radius as that of Mn3 + . There may be FM DE interaction between Cr3 + and Mn3 + just as between Mn4 + and Mn3 + . Some reports on the effect of Cr doping in the cubic perovskite manganites have appeared, and several reports on such studies have also appeared in the case of bilayer manganites [14–18]. But there are a few reports on the effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7. In this paper, we report the effect of Cr doping on magnetic and electrical properties, especially on magnetic properties in the bilayer manganites La1.4Sr1.6(Mn1 yCry)2O7 (y¼0–0.1). With Cr doping, Cr3 + will replace Mn3 + , which directly decreases the content of Mn3 + ions. On the other hand, Cr3 + can act as Mn4 + due to the same electronic configuration if there exists FM DE interaction between Cr3+ and Mn3 + . Thus, the Mn3 + /Mn4 + (including Cr3 + ) ratio will be closer to optimal ratio, and an interesting effect on magnetic and electric properties will be observed in the present Cr-doped system. The present measurements show that the magnetic coupling between Cr ions and Mn ions is related to the Cr doping concentration. When yo0.035 the coupling is FM and when y40.035 it is AFM. The total effect is observed and discussed.
1926
G. Yu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1925–1928
2. Experiment
240 y=0.1 180
La1.4Sr1.6(Mn1-yCry)2O7
120 Resistivity (Ω·cm)
The polycrystalline bulk samples of La1.4Sr1.6(Mn1 yCry)2O7 (y¼0–0.1) were synthesized by a standard ceramic process. Well ground stoichiometric mixture of high purity La2O3, SrCO3, MnO2 and Cr2O3 were calcined at 1200 1C for 24 h in air with intermediate grinding. The powders were well ground again, then pelletized and sintered at 1450 1C for 45 h with an intermediate grinding. The structural characterization was done through X-ray diffraction (XRD) with CuKa radiation at room temperature. The temperature dependence of resistivity was measured by a standard four probe method at zero field. Magnetization was measured using a commercial physical property measurement system (Quantum Design PPMS).
y=0.05 y=0.035
60
y=0 0 25 y=0
20 15
y=0.005 y=0.025
10
3. Results and discussion XRD patterns for all the samples of La1.4Sr1.6(Mn1 yCry)2O7 (y¼0–0.1) is presented in Fig. 1. All diffraction peaks are indexed using Sr3Ti2O7-type tetragonal structure (I4/mmm), indicating a single phase of bilayer structure. The lattice parameters obtained from XRD data are listed in Table 1. Fig. 2 shows the temperature (77–300 K) dependence of resistivity for all samples. For undoped sample (y¼ 0), the insulator–metal transition (IMT) temperature TIM is 110 K. At
y=0.01
5
y=0.015 0
50
100
150
200
250
300
T(K) Fig. 2. Temperature dependence of resistivity for samples La1.4Sr1.6(Mn1 yCry)2O7 (0ryr 0.1) at zero field.
0.7 y=0
M (µB /Mn1-yCry site)
0.6
y=0.1 y=0.05 y=0.035
0.5
La1.4Sr1.6(Mn1-yCry)2O7
y=0.05
0.4
y=0.035
0.3
y=0.025
0.2
y=0.1
Intensity (a.u.)
0.1
y=0.025
0.0 0
y=0.005 y=0
30
40
50 60 2θ (degree)
70
100
150
200
250
300
Fig. 3. Temperature dependence of magnetization M at 0.01 T for samples La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0, 0.025, 0.035, 0.05, 0.1).
y=0.01
20
50
T (K)
y=0.015
80
Fig. 1. XRD patterns of all samples La1.4Sr1.6(Mn1 yCry)2O7 (0ry r0.1).
Table 1 Lattice parameters for samples La1.4Sr1.6(Mn1 yCry)2O7 (0r yr0.1). y
˚ a (A)
˚ c (A)
V (A˚ 3)
y¼ 0 y¼ 0.005 y¼ 0.01 y¼ 0.015 y¼ 0.025 y¼ 0.035 y¼ 0.05 y¼ 0.1
3.8751 3.8746 3.8729 3.8732 3.8711 3.8704 3.8688 3.8667
20.171 20.174 20.175 20.181 20.183 20.187 20.194 20.202
302.90 302.86 302.61 302.75 302.45 302.40 302.26 302.05
lower Cr doping level (yr0.025), the IMTs shift gradually to higher temperature with increasing Cr doping level. TIM reaches the maximum value of 121 K for y¼0.025 sample, decreases gradually with increasing Cr doping level and when y¼0.1, IMT disappears. Associated with such TIM change, the resistivity near TIM decreases gradually for yo0.025 samples, and increases for yZ0.025 samples. Such results for Cr doping are not observed in other manganites (including the bilayer and 3D cubic manganites), which are very interesting. In addition, with increasing Cr doping, IMTs become broader and broader; similar results have also been observed for Cr doping in La1.2Sr1.8Mn2O7 [16]. The temperature dependence of magnetization M measured at a field of 0.01 T for samples La1.4Sr1.6(Mn1 yCry)2O7 (y¼0, 0.025, 0.035, 0.05, 0.1) is shown in Fig. 3. All M–T curves show two magnetic transitions lying at lower temperature near 110 K and higher temperature near 200–240 K. For all samples, a visible plateau in the M–T curves extends to higher temperature, which has been ascribed to a short range two-dimensional (2D) FM ordering due to the quasi-two-dimensional feature [19–21]. For the undoped sample (y¼0), Fig. 3 shows a 3D magnetic transition at TC (defined as the temperature at which dM/dT reaches the maximum) is 106 K; another upper transition, namely 2D magnetic transition at Tn (defined as another maximum in dM/dT–T
G. Yu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1925–1928
curve) is at 241 K. From Fig. 3 it can be seen that with increasing Cr doping level, the 3D magnetic transition shifts slightly to higher temperature and then keeps the transition temperature nearly constant, which is a picture different from the resistivity data in Fig. 2. Meanwhile, with increasing Cr doping level, the short range 2D magnetic transition shifts monotonously to lower temperature, which is different from the 3D magnetic transition, indicating that there are different mechanisms to dominate the two magnetic transitions. To compare the transition temperatures, TIM, TC and Tn are plotted against the Cr doping concentration y in Fig. 4. For y¼0.025 sample, TIM (121 K) and TC (117 K) reach their maximum values accompanied by the decrease of peak resistivity, indicating that the FM DE interaction is enhanced. For y40.035 samples, the apparent disagreement between TIM and TC indicates that a possible PS occurs. The decrease of Tn indicates that Cr doping suppresses the short range 2D FM ordering. It is worth noticing that Tn against the Cr doping concentration y almost follows linear relation. Fig. 5 shows the field (0–5 T) dependence of magnetization at 10 K for samples La1.4Sr1.6(Mn1 yCry)2O7 (y¼0, 0.025, 0.035, 0.05, 0.1). The magnetization is observed to almost saturate in the high field region ( 5 T) for all samples. For undoped sample (y¼0) the magnetic moment reaches 3.59 mB at 10 K at the field of 5 T, which is close to the theoretical value (3.7 mB). When yr0.035, the saturated magnetization decreases slightly with increasing Cr doping level, and when y40.035, it decreases heavily with increasing Cr doping, indicating that there exist different magnetic interactions between Cr ions and Mn ions with Cr doping. In the inset of Fig. 5,
240
T IM TC * T
116
230 220 210
112
T* (K)
TIM & TC (K)
120
200 108
190 0.000
0.025 0.050 0.075 Cr doping concentration y
Fig. 4. Doping (y) dependence of TIM, TC La1.4Sr1.6(Mn1 yCry)2O7 (y¼ 0, 0.025, 0.035, 0.05, 0.1).
0.100
and
Tn
for
180
samples
M (µB / Mn1-yCry site)
4 3 y=0 y=0.025 y=0.035 y=0.05 y=0.1
M=3.7-y
2 1 Mn
Cr
Mn
M=3.7-7y
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the magnetization at 5 T is plotted against the doping concentration y. When yo0.035, the magnetization M decreases with increasing Cr doping almost in parallel with the relation that M¼ 3.7–y, although the observed magnetization is slightly smaller than the relation, indicating that the magnetic coupling between Cr spins and Mn spins is FM. When y40.035, the magnetization M almost follows the relation M¼3.7–7y, indicating that the magnetic coupling between Cr spins and Mn spins is AFM. Generally, the magnetic interaction between Cr3 + and Mn3 + is FM, and the magnetic interaction between Cr3 + and Mn4 + is AFM [18,22]. In the present system, we think that there exists competition of the two kinds of interactions above between Cr ions and Mn ions; when the FM interaction between Cr3 + and Mn3 + preponderates, the magnetic interaction between Cr3 + and Mn3 + as well as the magnetic interaction between Cr3 + and Mn4 + is FM; otherwise when the AFM interaction between Cr3 + and Mn4 + preponderates, the magnetic interaction between Cr3 + and Mn4 + as well as the magnetic interaction between Cr3 + and Mn3 + is AFM. At lower Cr doping level (yr0.025), the FM interaction between Cr3 + and Mn3 + preponderating, with increasing Cr doping level, at higher Cr doping level (y40.035), the AFM interaction between Cr3 + and Mn4 + becomes preponderate because the doped Cr3 + ions directly replace Mn3 + ions resulting in the decrease of Mn3 + ions amount. In La1.4Sr1.6Mn2O7, Mn3 + /Mn4 + ratio is 7/3; thus, as Cr3 + ions replace partially Mn3 + ions, they still have Mn3 + ions besides Mn4 + ions around Cr3 + . When Cr spins couple ferromagnetically with Mn spins, there may exist the FM DE interaction between Cr3 + and Mn3 + just as between Mn4 + and Mn3 + because Cr3 + has the same 3 0 electronic configuration (t2g eg ) as Mn4 + . For yr0.025 samples, the 3+ substitution of Cr (acts as Mn4 + ) for Mn3 + results in the decrease of Mn3 + /Mn4 + (including Cr3 + ) ratio to being closer to optimal ratio. Therefore, enhancement of TIM and TC, and the decrease of resistivity are observed in Fig. 2 and Fig. 3, respectively. When y40.035, the magnetic coupling between Cr spins and Mn spins becomes AFM, i.e. the magnetic interaction between Cr3 + and Mn4 + as well as the magnetic interaction between Cr3 + and Mn3 + is AFM, which results in the appearance of the AFM second phase in the present system. With increasing Cr doping level, the PS between the FM phase and the AFM second phase probably occurs. The volume fraction of AFM second phase is small due to small Cr doping content. The AFM second phase mostly influences the electrical properties by preventing the hopping of carriers, which results in the decrease of TIM and the increase of resistivity. The 3D magnetic transition is still dominated by the FM phase, and the influence of AFM second phase is little, so TC remains nearly constant. Mn-site doping also influences the short range 2D magnetic ordering. The discussion of this aspect is very little in references. For present studies, Cr doping suppresses the short range 2D magnetic ordering. The 2D magnetic transition accompanied by the insulator behavior of electrical properties occurs in the higher temperature region, where the arrangement of Mn spins tends to long range disorder, but there also exists the short range 2D FM ordering inside the layers. The doped Cr3 + ions directly replace Mn3 + ions, which destroys the short range FM interaction between Mn3 + and Mn4 + , while the doped Cr3 + ions can not FM couple with Mn3 + ions because the doped Cr3 + ions can not only ferromagnetically couple with Mn3 + nearby, but also antiferromagnetically couple with Mn4 + nearby, and the two interactions may counteract in the case of short range effect resulting in the disorder.
0 0
1
2
3
4
5
4. Conclusions
H (T) Fig. 5. Field dependence of magnetization M at 10 K for samples La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0, 0.025, 0.035, 0.05, 0.1). Inset: magnetization at 5 T against the doping concentration y.
We have investigated the Cr doping effect in the bilayer manganites La1.4Sr1.6(Mn1 yCry)2O7 (y ¼0 0.1). The saturated magnetization data suggest that with the increase of Cr doping
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level, the magnetic coupling between Cr spins and Mn spins changes from FM to AFM. The cause of the enhancement of the metallic property and 3D magnetic transition temperature TC is that there exists FM DE interaction between Cr3 + and Mn3 + as well as the Cr doping optimizes Mn3 + /Mn4 + ratio for yo0.035. When y40.035, the AFM second phase suppresses the metallic property; meanwhile, the 3D magnetic transition is still dominated by the FM phase, and TC remains nearly constant. In addition, Cr doping suppresses the short range 2D FM ordering due to no short range effect.
Acknowledgement This work was supported by National 973 Project of China (Grant no. 2006CB921606). References [1] Y. Moritomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature (London) 380 (1996) 141. [2] T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. [3] T. Kimura, Y. Tokura, Annu. Rev. Mater. Sci. 30 (2000) 451 See, for a review. [4] Y. Moritomo, Y. Maruyama, T. Akimoto, A. Nakamura, J. Phys. Soc. Jpn. 67 (1998) 405 and references cited therein.
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