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Influence of Tb substitution for La on the structure, magnetic and magneto-transport properties of La1.2Sr1.8Mn2O7
Physica B 407 (2012) 2219–2222
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Influence of Tb substitution for La on the structure, magnetic and magneto-transport properties of La1.2Sr1.8Mn2O7 Min Zhou, Hong-Ye Wu, Hong-jin Wang, Lin Zheng, Jian-Jun Zhao, Ru Xing, Yi Lu n Department of Physics, Baotou Normal University, Baotou 014030, China
a r t i c l e i n f o
abstract
Article history: Received 20 October 2011 Received in revised form 29 February 2012 Accepted 1 March 2012 Available online 6 March 2012
The physical properties of Tb3 þ ions substitution at A-site are investigated in the layered manganite La1.2Sr1.8Mn2O7. A series of La1.2 xTbxSr1.8Mn2O7 (x ¼ 0, 0.05, 0.15, and 0.20) shows that doping with a Tb ion of smaller radius in La1.2Sr1.8Mn2O7 caused diffraction peaks to shift to high angle. Some samples have an impure diffraction at about 301, but all samples form single-phase. Samples can be well indexed on a Sr3Ti2O7-type tetragonal structure with the space group I4/mmm. According to the M-T curves, when x r 0.05, the series of samples shows ferromagnetism at low temperatures. With increasing temperature, they have two magnetic transitions at different temperatures. When x Z 0.15, the magnetizations dramatically decrease. The r–T curves of samples show the metal– insulator transition for x ¼ 0, 0.05, and the maximum MR values in magnetic field 5 T are 74% at about 73 K and 94% at about 86 K. When x Z 0.15, the samples remain in the insulator state in the whole observed temperature range, and the maximum MR values of 86% and 69% appeared at 74 K and 42 K. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Manganite Magnetic property Magneto-resistance
1. Introduction Colossal Magneto-Resistance (CMR) has attracted significant interest due to its prospects as magnetic storage, magnetic recording and magnetic sensors. Kimura et al. [1] have studied layered manganite La1.4Sr1.6Mn2O7 and found extremely large magneto-resistance compared with the same charge carrier manganite La0.7Ca0.3MnO3. Layered manganites with A-site doping has stable structure. Rare-earth doping at A-site has an obvious influence on the electronic and magnetic anistrophies, and a low temperature magneto-resistance effect [2–6]. Hole doping concentration (x¼0.4) in 327-type layered manganites seems to be optimal to exhibit rich properties. In this paper, La1.2Sr1.8Mn2O7 is considered as the parent compound; we report the effect of A site substitution by rare-earth element Tb on physical properties of La1.2Sr1.8Mn2O7.
was kept as 2 1C/min). Previous work was repeated again. The resulting mixtures were ground and pressed into pellets and then sintered at 1300 1C for 24 h in air. The size of polycrystalline samples was 13 mm in diameter and 1 mm in thickness. The structure of samples was characterized by powder X-ray diffraction (XRD). The XRD measurements were carried out using CuKa radiation at 40 kV and 40 mA using continuously scanning method. Data for the Rietveld refinement were collected in the 2y range 20–801, with a step size of 0.081. The magnetic measurements were performed by a superconducting quantum interference device (SQUID, MPMS-7). The measurements were taken in both the zero-field-cooled (ZFC) and field-cooled (FC) conditions under the applied field of 100 Oe. The temperature dependence of resistivity in the zero field and in the magnetic field (H¼5 T) was measured by the standard fourpoint technique.
3. Results and discussion 2. Experiment Polycrystalline samples of La1.2 xTbxSr1.8Mn2O7 (x¼0, 0.05, 0.15, 0.02) were prepared by the conventional solid-state reaction method. Stoichiometric amounts of La2O3, SrCO3, MnCO3 and Tb2O3 with a purity higher than 99.9% were mixed, ground for 2–3 h, and calcined at 1000 1C for 12 h in air (speed rate of warming n
Corresponding author. E-mail address: [email protected] (Y. Lu).
The crystal structure can be obtained by the x-ray powder diffraction. The x-ray diffraction patterns for the samples La1.2 xTbxSr1.8Mn2O7 (x¼0, 0.05, 0.15, and 0.02) are shown in Fig. 1. The series of samples shows that doping with a smaller radius Tb in La1.2Sr1.8Mn2O7 caused diffraction peaks to shift to a higher angle. When x¼0.2, some samples have an impure diffraction at about 301. This may indicate the presence of ABO3-type perovskites, which agrees with the previous report [7]. On the whole, all samples basically form single-phase. Structural parameter data were analyzed using the Rietveld method. The x-ray diffraction
0921-4526/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2012.03.001
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M. Zhou et al. / Physica B 407 (2012) 2219–2222
5 La1.2−xTbxSr1.8Mn2O7
La1.2−xTbxSr1.8Mn2O7
4 Intensity/(a.u.)
x=0.2
x=0.00
x=0.05
M/(emu·g-1)
3 x=0.15
x=0.05 x=0.15
2
x=0.20
1 x=0 0 20
30
40
50 2θ/ (°)
60
70
80 -1 0
Fig. 1. x-ray diffraction pattern for the sample La1.2 xTbxSr1.8Mn2O7 (x¼ 0, 0.05, 0.15, and 0.02).
50
100
150 T/ (K)
200
250
300
Fig. 2. Temperature dependence of magnetization for La1.2 xTbxSr1.8Mn2O7 (x ¼0, 0.05, 0.15 and 0.02) measured with field 0.1 T on the zero-filed-cooled mode. Table 1 Refinement parameter for La1.2 xTbxSr1.8Mn2O7 (x¼ 0, 0.05, 0.15, 0.2).
0.00 0.05 0.15 0.20
˚ a (A) 3.896 3.880 3.874 3.873
˚ c (A) 20.239 20.173 20.080 20.062
V (A˚ 3) 307.135 303.556 301.380 301.001
Rp 20.400 17.706 19.858 15.542
Rwp 27.059 13.291 26.602 22.595
70
w2 6.073 5.006 6.352 5.055
patterns for the samples La1.2 xTbxSr1.8Mn2O7 (x ¼0, 0.05, 0.15, and 0.2) were refined to get the structural parameters. As shown in Table 1, reliability factors Rwp, Rp and w2 show that the refined structure is correct. Samples can be well indexed on a Sr3Ti2O7type tetragonal structure with the space group I4/mmm. Meanwhile, the lattice parameters a and c and volume of unit cell exhibit a simple monotonic decrease with increasing doping concentration x. In the layered manganites LSMO (327) system, the La ion has two kinds of occupation sites, a 12-coordinate site (P-site) in the MnO2 bilayer and a 9-coordinate site (R-site) in the rock-salt layer. According to the previous report [8,9] the smallsized ion preferentially occupies R site. As La3 þ (0.106 nm) is larger than that of Tb3 þ ions (0.092 nm). After substituting smaller Tb3 þ ions for larger La3 þ ions, the average radius of the A-site ion decreases and the tolerance factor t becomes small. Therefore inter-layer lattice distortion increases, causing the lattice parameters a and c to decrease. Temperature dependences of magnetization for La1.2 xTbxSr1.8 Mn2O7 (x ¼0, 0.05, 0.15, and 0.02) measured at magnetic field 0.1 T are shown in Fig. 2. And field dependences of magnetization at 30 K are shown in Fig. 3. According to the M-T and M-H curves, when x r0.05, the samples show typical ferromagnetism at low temperatures, the magnetization decreases with increasing temperature, and the magnetization tends to saturation at high fields. When x Z0.15, the spontaneous magnetization decreases significantly and does not change with temperature increase. Magnetization increases linearly as the filed increases above 0.5 T, and no saturation is reached even at 5 T. The temperature dependences of magnetization for La1.2 xTbxSr1.8 Mn2O7 (x¼0.0, and 0.05) measured at field 0.1 T are shown in Fig. 4. The measurements were taken in both zero-filed-cooled (ZFC) and field-cooled (FC) conditions, and dM/dTT curves are shown in the insets. From Fig. 4(a), it can be seen that the parent compound La1.2Sr1.8Mn2O7 is ferromagnetic at low temperatures. There are
T=30K x=0.00
60
x=0.05
50 M/(emu/g)
X
x=0.15
40
x=0.20
30 20 10 0 0
1
2
3
4
5
H/(T) Fig. 3. Field dependence of magnetization at 30 K for La1.2 xTbxSr1.8Mn2O7 (x ¼0, 0.05, 0.15 and 0.2).
three-dimensional (3D) to two-dimensional (2D) magnetic transitions at 113 K, and also a weak transition peak at 289 K. The result of the parent compound La1.2Sr1.8Mn2O7 sample agrees with the previous reports [10,11]. As explained in Ref. [10], in the low temperature range, the magnetic moments are orderly aligned, forming 3D longrange ferromagnetic (FM) ordering. With increasing temperature, thermal vibration destroyed out-plane FM ordering, and M–T curves decrease rapidly and exhibit a magnetic transition around the temperature Tc1 113 K (it is determined from dM=dTT curves, see the inset of Fig. 4(a)). Between Tc1 and Tc2, M–T curves exhibit a specific plat which is a ferromagnetic–paramagnetic mixed state (it is similar to superconducting material of Griffiths phase [12]). The magnetic properties reveal that the Mn spin ferromagnetic aligned within the a–b plane within the temperature range, and the crystal forms 2D short-range FM ordering. There exists the possibility of a ferromagnetic cluster state. With further increase in temperature, two-dimensional short range FM is destroyed near Tc2 240 K (see the inset of Fig. 4(a), another minimum in the dM/dT t curves). As Tc1 4Tc2,it indicates that the strength
M. Zhou et al. / Physica B 407 (2012) 2219–2222
of in-plane exchange interaction is stronger than that of the out-of-plane exchange interaction [1,13]. In addition, samples show a weak transition peak at about 289 K and the magnetization further decreases, forming paramagnetic state at high temperature. In Fig. 4(b), Tb3 þ ions doping caused magnetic transition temperatures Tc1 and Tc2 to decrease to 70 K and 221 K, respectively. There is a large difference between FC and ZFC data that occurs at low temperature, indicating the presence of magnetic disorder frozen in a spin-glass-like behavior at low temperature.
The magnetic structure is not a simple ferromagnetic (FM) but a canted anti-ferromagnetic (AFM) phases at low temperature range [14]. The spin-glass-like behavior can be ascribed to the competition between FM and AFM interactions which causes disorder in the system. Fig. 5 shows the temperature dependence of resistivity for La1.2 xTbxSr1.8Mn2O7 at the field of 0 and 5 T. From Fig. 5(a) and (b) it can be concluded that the metal–insulator (MI) transition of the parent compound La1.2Sr1.8Mn2O7 occurs at about TMI ¼96 K. When x¼0.05, the MI transition temperature
5
La1.2Sr1.8Mn2O7
4
La1.15Tb0.05Sr1.8Mn2O7
4
M/(emu·g-1)
ZFC
2
FC
3
FC M/(emu·g-1)
2221
0
ZFC
2 1 0 -1
-2
-2 -3 0
50
100
150 T/(K)
200
250
300
0
50
100
150 T/(K)
200
250
300
Fig. 4. Temperature dependence of magnetization for La1.2 xTbxSr1.8Mn2O7 measured at field 0.1 T for both ZFC and FC curves (a) x ¼0, (b) x¼ 0.05. dM/dT are shown in the insets.
1500
2500
1200 H=0T H=5T
1500
ρ/(Ω·cm)
ρ/(Ω·cm)
2000
1000
600 300
0
0 50
100
150 200 T/(K)
250
300
0
1000
5000
La1.05Tb0.15Sr1.8Mn2O7
800
ρ/(Ω·cm)
400
0
0 100
150 200 T/(K)
250
150 200 T/(K)
300
250
300
La1.0Tb0.2Sr1.8Mn2O7 H=0T H=5T
2000 1000
50
100
3000
200
0
50
4000
H=0T H=5T
600
H=0T H=5T
900
500
0
ρ/(Ω·cm)
La1.15Tb0.05Sr1.8Mn2O7
La1.2Sr1.8Mn2O7
0
50
100
150 200 T(K)
250
300
Fig. 5. Temperature dependence of resistvity for the samples at the field of 0 T and 5 T with (a) x ¼0, (b) x ¼0.05, (c) x¼ 0.15, (d) x ¼0.20.
2222
M. Zhou et al. / Physica B 407 (2012) 2219–2222
95
MRmax
90
T
90 80
85
70
80
60
75
50
70
T/(K)
MRmax /(%)
100
40
65
Magnetic measurements show that with increasing concentration of Tb3 þ , the magnetization decreases. The maximum MR values of La1.2 xTbxSr1.8Mn2O7 samples are 74%, 94%, 86% and 69%. The MR effect around the ferromagnetic–paramagnetic phase transition temperature is due to the intrinsic magneto-resistance CMR. At low temperatures, the maximum MR can be ascribed to a nonintrinsic intergrain MR effects, which are related to spin-polarized tunneling effect or grain boundary effect.
Acknowledgments
30
60 0.00
0.05
0.10 X
0.15
0.20
Fig. 6. The MRmax value and temperature dependence of doping concentration x for La1.2 xTbxSr1.8Mn2O7.
decreases to 93 K. Fig. 5(c) and (d) shows that doping on La site causes samples to exhibit insulation properties in the whole observed temperature range, and the MI transition disappears. The magneto-resistance (MR) value is defined as MRð%Þ ¼ rð0ÞrðHÞ=rðHÞ 100 %, where r(0) is the resistivity in the zero field and r(H) is the resistivity in the magnetic field H¼5 T. The MRmax value and temperature dependence of doping concentration x for La1.2 xTbxSr1.8Mn2O7 are shown in Fig. 6. When x¼0, the MR peak value reaches up to 74% at about 73 K. When x¼0.05, the maximum MR value increases to 94% at about 86 K. When x¼ 0.15 (0.20), the maximum MR value is 86% (69%), and appears at lower temperature. The MR effect around the ferromagnetic– paramagnetic phase transition temperature should be due to the intrinsic magneto-resistance CMR. At lower temperature, the maximum MR can be ascribed to a non-intrinsic intergrain MR effects, which are related to spin-polarized tunneling effect or grain boundary effect [15,16].
4. Conclusion The influence of rare-earth Tb3 þ ions substitution at A-site on the structure and magnetic properties in the layered manganite La1.2Sr1.8Mn2O7 has been investigated. With Tb3 þ doping, samples can be well indexed on a Sr3Ti2O7-type tetragonal structure.
This work is supported by the National Natural Science Foundation of China under Grant nos. 50862007, 11164019, and 11064008, the Inner Mongolia Natural Science Foundation under Grant nos. 2009MS0101, 2011MS0101 and 2011MS0108, and Inner Mongolia College Research Project under grant no. NJ10163, NJZZ11166, and NJ09141.
References [1] T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. [2] Y. Yamato, M. Matsukawa, Y. Murano, R. Suryanarayanan, S. Nimori, M. Apostu, A. Revcolevschi, K. Koyama, N. Kobayashi, Appl. Phys. Lett. 94 (2009) 092507. [3] G. Allodi, M. Bimbi, R. De Renzi, C. Baumann, M. Apostu, Phys. Rev. B 78 (2008) 064420. [4] J.J. Zhao, Y. Lu, Haosibayar, R. Xing, R.F. Yang, Q.A. Li, Y. Sun, Z.H. Cheng, Chin. Phys. B 17 (2008) 2717. [5] Myung Whun Kim, H.J. Lee, B. Yang, K.H. Kim, Y. Moritomo, Yu. Jaejun, T.W. Noh, Phys. Rev. Lett. 98 (2007) 187201. [6] R.F. Yang, Y. Sun, W. He, Q.A. Li, Z.H. Cheng,, Appl. Phys. Lett. 90 (2007) 032502. [7] M.A. Green, D.A. Neumann, Chem. Mater. 12 (2000) 90. [8] Y. Moritomo, Aust. J. Phys. 52 (1999) 255. [9] X Ma, Z.Q. Kou, N.L. Di, Z.H. Cheng, Q.A. Li, J. Magn. Magn. Mater. 285 (2005) 439. [10] Y. Moritomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature 380 (1996) 141. [11] H. Asano, J. Hayakawa, M. Matsui, Phys. Rev. B 56 (1997) 5395. [12] R.F. Yang, Y. Sun, W. He, Q.A. Li, Z.H. Cheng, Appl. Phys. Lett. 90 (2007) 032502. [13] T.G. Perring, G. Aeppli, Y. Moritomo, Y. Tokura, Phys. Rev. Lett. 78 (1997) 3197. [14] C.F. Chang, P.H. Chou, H.L. Tsay, S.S. Weng, S Chatterjee, H.D. Yang, R.S. Liu, C.H. Shen, W.H. Li, Phys. Rev. B 58 (1998) 12224. [15] H.-Y. Hwang, S.-W. Cheong, N.P. Ong, B. Batlogg., Phys. Rev. Lett. 77 (1996) 2041. [16] X.W. Li, A. Gupta, G. Xiao, G.Q. Gong, Appl. Phys. Lett. 71 (1997) 1124.
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Physica B journal homepage: www.elsevier.com/locate/physb
Influence of Tb substitution for La on the structure, magnetic and magneto-transport properties of La1.2Sr1.8Mn2O7 Min Zhou, Hong-Ye Wu, Hong-jin Wang, Lin Zheng, Jian-Jun Zhao, Ru Xing, Yi Lu n Department of Physics, Baotou Normal University, Baotou 014030, China
a r t i c l e i n f o
abstract
Article history: Received 20 October 2011 Received in revised form 29 February 2012 Accepted 1 March 2012 Available online 6 March 2012
The physical properties of Tb3 þ ions substitution at A-site are investigated in the layered manganite La1.2Sr1.8Mn2O7. A series of La1.2 xTbxSr1.8Mn2O7 (x ¼ 0, 0.05, 0.15, and 0.20) shows that doping with a Tb ion of smaller radius in La1.2Sr1.8Mn2O7 caused diffraction peaks to shift to high angle. Some samples have an impure diffraction at about 301, but all samples form single-phase. Samples can be well indexed on a Sr3Ti2O7-type tetragonal structure with the space group I4/mmm. According to the M-T curves, when x r 0.05, the series of samples shows ferromagnetism at low temperatures. With increasing temperature, they have two magnetic transitions at different temperatures. When x Z 0.15, the magnetizations dramatically decrease. The r–T curves of samples show the metal– insulator transition for x ¼ 0, 0.05, and the maximum MR values in magnetic field 5 T are 74% at about 73 K and 94% at about 86 K. When x Z 0.15, the samples remain in the insulator state in the whole observed temperature range, and the maximum MR values of 86% and 69% appeared at 74 K and 42 K. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Manganite Magnetic property Magneto-resistance
1. Introduction Colossal Magneto-Resistance (CMR) has attracted significant interest due to its prospects as magnetic storage, magnetic recording and magnetic sensors. Kimura et al. [1] have studied layered manganite La1.4Sr1.6Mn2O7 and found extremely large magneto-resistance compared with the same charge carrier manganite La0.7Ca0.3MnO3. Layered manganites with A-site doping has stable structure. Rare-earth doping at A-site has an obvious influence on the electronic and magnetic anistrophies, and a low temperature magneto-resistance effect [2–6]. Hole doping concentration (x¼0.4) in 327-type layered manganites seems to be optimal to exhibit rich properties. In this paper, La1.2Sr1.8Mn2O7 is considered as the parent compound; we report the effect of A site substitution by rare-earth element Tb on physical properties of La1.2Sr1.8Mn2O7.
was kept as 2 1C/min). Previous work was repeated again. The resulting mixtures were ground and pressed into pellets and then sintered at 1300 1C for 24 h in air. The size of polycrystalline samples was 13 mm in diameter and 1 mm in thickness. The structure of samples was characterized by powder X-ray diffraction (XRD). The XRD measurements were carried out using CuKa radiation at 40 kV and 40 mA using continuously scanning method. Data for the Rietveld refinement were collected in the 2y range 20–801, with a step size of 0.081. The magnetic measurements were performed by a superconducting quantum interference device (SQUID, MPMS-7). The measurements were taken in both the zero-field-cooled (ZFC) and field-cooled (FC) conditions under the applied field of 100 Oe. The temperature dependence of resistivity in the zero field and in the magnetic field (H¼5 T) was measured by the standard fourpoint technique.
3. Results and discussion 2. Experiment Polycrystalline samples of La1.2 xTbxSr1.8Mn2O7 (x¼0, 0.05, 0.15, 0.02) were prepared by the conventional solid-state reaction method. Stoichiometric amounts of La2O3, SrCO3, MnCO3 and Tb2O3 with a purity higher than 99.9% were mixed, ground for 2–3 h, and calcined at 1000 1C for 12 h in air (speed rate of warming n
Corresponding author. E-mail address: [email protected] (Y. Lu).
The crystal structure can be obtained by the x-ray powder diffraction. The x-ray diffraction patterns for the samples La1.2 xTbxSr1.8Mn2O7 (x¼0, 0.05, 0.15, and 0.02) are shown in Fig. 1. The series of samples shows that doping with a smaller radius Tb in La1.2Sr1.8Mn2O7 caused diffraction peaks to shift to a higher angle. When x¼0.2, some samples have an impure diffraction at about 301. This may indicate the presence of ABO3-type perovskites, which agrees with the previous report [7]. On the whole, all samples basically form single-phase. Structural parameter data were analyzed using the Rietveld method. The x-ray diffraction
0921-4526/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2012.03.001
2220
M. Zhou et al. / Physica B 407 (2012) 2219–2222
5 La1.2−xTbxSr1.8Mn2O7
La1.2−xTbxSr1.8Mn2O7
4 Intensity/(a.u.)
x=0.2
x=0.00
x=0.05
M/(emu·g-1)
3 x=0.15
x=0.05 x=0.15
2
x=0.20
1 x=0 0 20
30
40
50 2θ/ (°)
60
70
80 -1 0
Fig. 1. x-ray diffraction pattern for the sample La1.2 xTbxSr1.8Mn2O7 (x¼ 0, 0.05, 0.15, and 0.02).
50
100
150 T/ (K)
200
250
300
Fig. 2. Temperature dependence of magnetization for La1.2 xTbxSr1.8Mn2O7 (x ¼0, 0.05, 0.15 and 0.02) measured with field 0.1 T on the zero-filed-cooled mode. Table 1 Refinement parameter for La1.2 xTbxSr1.8Mn2O7 (x¼ 0, 0.05, 0.15, 0.2).
0.00 0.05 0.15 0.20
˚ a (A) 3.896 3.880 3.874 3.873
˚ c (A) 20.239 20.173 20.080 20.062
V (A˚ 3) 307.135 303.556 301.380 301.001
Rp 20.400 17.706 19.858 15.542
Rwp 27.059 13.291 26.602 22.595
70
w2 6.073 5.006 6.352 5.055
patterns for the samples La1.2 xTbxSr1.8Mn2O7 (x ¼0, 0.05, 0.15, and 0.2) were refined to get the structural parameters. As shown in Table 1, reliability factors Rwp, Rp and w2 show that the refined structure is correct. Samples can be well indexed on a Sr3Ti2O7type tetragonal structure with the space group I4/mmm. Meanwhile, the lattice parameters a and c and volume of unit cell exhibit a simple monotonic decrease with increasing doping concentration x. In the layered manganites LSMO (327) system, the La ion has two kinds of occupation sites, a 12-coordinate site (P-site) in the MnO2 bilayer and a 9-coordinate site (R-site) in the rock-salt layer. According to the previous report [8,9] the smallsized ion preferentially occupies R site. As La3 þ (0.106 nm) is larger than that of Tb3 þ ions (0.092 nm). After substituting smaller Tb3 þ ions for larger La3 þ ions, the average radius of the A-site ion decreases and the tolerance factor t becomes small. Therefore inter-layer lattice distortion increases, causing the lattice parameters a and c to decrease. Temperature dependences of magnetization for La1.2 xTbxSr1.8 Mn2O7 (x ¼0, 0.05, 0.15, and 0.02) measured at magnetic field 0.1 T are shown in Fig. 2. And field dependences of magnetization at 30 K are shown in Fig. 3. According to the M-T and M-H curves, when x r0.05, the samples show typical ferromagnetism at low temperatures, the magnetization decreases with increasing temperature, and the magnetization tends to saturation at high fields. When x Z0.15, the spontaneous magnetization decreases significantly and does not change with temperature increase. Magnetization increases linearly as the filed increases above 0.5 T, and no saturation is reached even at 5 T. The temperature dependences of magnetization for La1.2 xTbxSr1.8 Mn2O7 (x¼0.0, and 0.05) measured at field 0.1 T are shown in Fig. 4. The measurements were taken in both zero-filed-cooled (ZFC) and field-cooled (FC) conditions, and dM/dTT curves are shown in the insets. From Fig. 4(a), it can be seen that the parent compound La1.2Sr1.8Mn2O7 is ferromagnetic at low temperatures. There are
T=30K x=0.00
60
x=0.05
50 M/(emu/g)
X
x=0.15
40
x=0.20
30 20 10 0 0
1
2
3
4
5
H/(T) Fig. 3. Field dependence of magnetization at 30 K for La1.2 xTbxSr1.8Mn2O7 (x ¼0, 0.05, 0.15 and 0.2).
three-dimensional (3D) to two-dimensional (2D) magnetic transitions at 113 K, and also a weak transition peak at 289 K. The result of the parent compound La1.2Sr1.8Mn2O7 sample agrees with the previous reports [10,11]. As explained in Ref. [10], in the low temperature range, the magnetic moments are orderly aligned, forming 3D longrange ferromagnetic (FM) ordering. With increasing temperature, thermal vibration destroyed out-plane FM ordering, and M–T curves decrease rapidly and exhibit a magnetic transition around the temperature Tc1 113 K (it is determined from dM=dTT curves, see the inset of Fig. 4(a)). Between Tc1 and Tc2, M–T curves exhibit a specific plat which is a ferromagnetic–paramagnetic mixed state (it is similar to superconducting material of Griffiths phase [12]). The magnetic properties reveal that the Mn spin ferromagnetic aligned within the a–b plane within the temperature range, and the crystal forms 2D short-range FM ordering. There exists the possibility of a ferromagnetic cluster state. With further increase in temperature, two-dimensional short range FM is destroyed near Tc2 240 K (see the inset of Fig. 4(a), another minimum in the dM/dT t curves). As Tc1 4Tc2,it indicates that the strength
M. Zhou et al. / Physica B 407 (2012) 2219–2222
of in-plane exchange interaction is stronger than that of the out-of-plane exchange interaction [1,13]. In addition, samples show a weak transition peak at about 289 K and the magnetization further decreases, forming paramagnetic state at high temperature. In Fig. 4(b), Tb3 þ ions doping caused magnetic transition temperatures Tc1 and Tc2 to decrease to 70 K and 221 K, respectively. There is a large difference between FC and ZFC data that occurs at low temperature, indicating the presence of magnetic disorder frozen in a spin-glass-like behavior at low temperature.
The magnetic structure is not a simple ferromagnetic (FM) but a canted anti-ferromagnetic (AFM) phases at low temperature range [14]. The spin-glass-like behavior can be ascribed to the competition between FM and AFM interactions which causes disorder in the system. Fig. 5 shows the temperature dependence of resistivity for La1.2 xTbxSr1.8Mn2O7 at the field of 0 and 5 T. From Fig. 5(a) and (b) it can be concluded that the metal–insulator (MI) transition of the parent compound La1.2Sr1.8Mn2O7 occurs at about TMI ¼96 K. When x¼0.05, the MI transition temperature
5
La1.2Sr1.8Mn2O7
4
La1.15Tb0.05Sr1.8Mn2O7
4
M/(emu·g-1)
ZFC
2
FC
3
FC M/(emu·g-1)
2221
0
ZFC
2 1 0 -1
-2
-2 -3 0
50
100
150 T/(K)
200
250
300
0
50
100
150 T/(K)
200
250
300
Fig. 4. Temperature dependence of magnetization for La1.2 xTbxSr1.8Mn2O7 measured at field 0.1 T for both ZFC and FC curves (a) x ¼0, (b) x¼ 0.05. dM/dT are shown in the insets.
1500
2500
1200 H=0T H=5T
1500
ρ/(Ω·cm)
ρ/(Ω·cm)
2000
1000
600 300
0
0 50
100
150 200 T/(K)
250
300
0
1000
5000
La1.05Tb0.15Sr1.8Mn2O7
800
ρ/(Ω·cm)
400
0
0 100
150 200 T/(K)
250
150 200 T/(K)
300
250
300
La1.0Tb0.2Sr1.8Mn2O7 H=0T H=5T
2000 1000
50
100
3000
200
0
50
4000
H=0T H=5T
600
H=0T H=5T
900
500
0
ρ/(Ω·cm)
La1.15Tb0.05Sr1.8Mn2O7
La1.2Sr1.8Mn2O7
0
50
100
150 200 T(K)
250
300
Fig. 5. Temperature dependence of resistvity for the samples at the field of 0 T and 5 T with (a) x ¼0, (b) x ¼0.05, (c) x¼ 0.15, (d) x ¼0.20.
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M. Zhou et al. / Physica B 407 (2012) 2219–2222
95
MRmax
90
T
90 80
85
70
80
60
75
50
70
T/(K)
MRmax /(%)
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40
65
Magnetic measurements show that with increasing concentration of Tb3 þ , the magnetization decreases. The maximum MR values of La1.2 xTbxSr1.8Mn2O7 samples are 74%, 94%, 86% and 69%. The MR effect around the ferromagnetic–paramagnetic phase transition temperature is due to the intrinsic magneto-resistance CMR. At low temperatures, the maximum MR can be ascribed to a nonintrinsic intergrain MR effects, which are related to spin-polarized tunneling effect or grain boundary effect.
Acknowledgments
30
60 0.00
0.05
0.10 X
0.15
0.20
Fig. 6. The MRmax value and temperature dependence of doping concentration x for La1.2 xTbxSr1.8Mn2O7.
decreases to 93 K. Fig. 5(c) and (d) shows that doping on La site causes samples to exhibit insulation properties in the whole observed temperature range, and the MI transition disappears. The magneto-resistance (MR) value is defined as MRð%Þ ¼ rð0ÞrðHÞ=rðHÞ 100 %, where r(0) is the resistivity in the zero field and r(H) is the resistivity in the magnetic field H¼5 T. The MRmax value and temperature dependence of doping concentration x for La1.2 xTbxSr1.8Mn2O7 are shown in Fig. 6. When x¼0, the MR peak value reaches up to 74% at about 73 K. When x¼0.05, the maximum MR value increases to 94% at about 86 K. When x¼ 0.15 (0.20), the maximum MR value is 86% (69%), and appears at lower temperature. The MR effect around the ferromagnetic– paramagnetic phase transition temperature should be due to the intrinsic magneto-resistance CMR. At lower temperature, the maximum MR can be ascribed to a non-intrinsic intergrain MR effects, which are related to spin-polarized tunneling effect or grain boundary effect [15,16].
4. Conclusion The influence of rare-earth Tb3 þ ions substitution at A-site on the structure and magnetic properties in the layered manganite La1.2Sr1.8Mn2O7 has been investigated. With Tb3 þ doping, samples can be well indexed on a Sr3Ti2O7-type tetragonal structure.
This work is supported by the National Natural Science Foundation of China under Grant nos. 50862007, 11164019, and 11064008, the Inner Mongolia Natural Science Foundation under Grant nos. 2009MS0101, 2011MS0101 and 2011MS0108, and Inner Mongolia College Research Project under grant no. NJ10163, NJZZ11166, and NJ09141.
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