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Room temperature magnetoresistance in La0.67Sr0.33Mn1−xAlxO3 manganites (x⩽0.25)
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 263 (2003) 249–252
Room temperature magnetoresistance in La0.67Sr0.33Mn1 xAlxO3 manganites (xp0.25) Hongwei Qin*, Jifan Hu, Juan Chen, Hongdong Niu, Luming Zhu Department of Physics, Shandong University, Jinan 250100, China Received 1 November 2002
Abstract An enhancement of the room temperature magnetoresistance induced by the Al substitution in La0.67Sr0.33Mn1 xAlxO3 was observed. A peak of the room temperature magnetoresistance occurs at Al content of xE0.05. This enhancement is correlated with the shift of the Curie temperature and metal–insulator transition temperature to near room temperature through the Al substitution. The room temperature magnetoresistance drops again at a large Al content xX0.10, which may be due to the lower metal–insulator transition temperature far from the room temperature. r 2003 Elsevier Science B.V. All rights reserved. PACS: 72.20.My; 75.30. m; 75.80.+q Keywords: Colossal magnetoresistance; Manganites; Curie temperature
1. Introduction Manganites La1 xAxMnO3 (A=Ca, Ba and Sr) have generated great interest due to the colossal magnetoresistance (CMR) effect [1,2]. The double exchange interaction has been firstly adopted to explain the CMR effect [3]. However, recent calculation showed that the double exchange interaction alone cannot explain this effect and suggested that the electron–phonon coupling also plays an important role in transport processes of manganites [4,5]. It has been found that the colossal magnetoresistance strongly depends on the temperature, whose peak usually occurs near *Corresponding author. Fax: +86-531-856-5167. E-mail address: [email protected] (H. Qin).
the Curie temperature TC. Considering the practical application, it is important to improve the magnetoresistance ratio DR/R0 at room temperature. In the present paper, we report the enhancement of the room temperature magnetoresistance induced by the substitution of Al for Mn in the polycrystalline manganite La0.67Sr0.33MnO3.
2. Experiments Polycrystalline manganites La0.67Sr0.33Mn1 xAlxO3 (x=0, 0.03, 0.05, 0.10, 0.15, 0.20 and 0.25) were prepared by the traditional solid-state reaction method. Mixed powders of La2O3, Al2O3, MnO2 and SrCO3 were ground for 20 min, ball milled for 30 min, and pressed into pellets. The
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(02)01571-8
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pellets were firstly heated at 800 C for 24 h and then subsequently pulverized. After milled for 20 min they were pressed into pellets again and then sintered at 1200 C for 24 h, followed by furnace cooling in atmosphere. Powder X-ray diffraction of manganites were carried out with Cu-Ka radiation at room temperature. Results indicated that the investigated La0.67Sr0.33Mn1 xAlxO3 samples were single phase with perovskite rhombohedral structure. Curie temperatures were determined from thermal magnetic curves measured with a thermal magnetic analyzer (TMA) under a low magnetic field of 200 Oe. The metal– insulator transition temperatures of manganites were determined from the temperature dependence of the resistivity measured with a four terminal method under zero field. The magnetoresistance of manganites were measured at 293 K.
Fig. 2. The Al substitution content dependence of the room temperature resistivity at zero field for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
3. Results and discussion Fig. 1 shows the Al substitution content dependence of the Curie temperature TC for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites. The Curie temperature TC of the undoped sample (x=0) is about 362 K. The substitution of Al for Mn in the La0.67Sr0.33Mn1 xAlxO3 lowers the Curie temperature. The TC is found to be 315 K for x=0.05 and 296 K for x=0.10. It means that one can reduce the TC from high temperature
Fig. 1. The Al substitution content dependence of the Curie temperature TC for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
Fig. 3. The temperature dependence of the resistivity r for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites (x=0.05 and 0.10) with zero field.
to near room temperature by adjusting the appropriate amount of Al in La0.67Sr0.33Mn1 xAlxO3. The decrease of TC with Al substitution is mainly due to the local cutoff of magnetic interaction between the spins of the t2g electrons [6]. As shown in Fig. 2, the room temperature resistivity at zero field for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites increases with the increase of Al content, accompanying the reduction of the ferromagnetism. The temperature dependence of the resistivity r for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites (x=0.05 and 0.10) with zero field is shown in Fig. 3. There is a peak in the r T curve,
ARTICLE IN PRESS H. Qin et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 249–252
Fig. 4. The Al substitution content dependence of the metal– insulator transition temperature for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
showing the metal–insulator (M–I) transition. Such M–I peak can be observed in a polycrystalline La0.7Sr0.3MnO3 sample [7,8]. However, for a single crystal La0.7Sr0.3MnO sample, a resistivity step occurs near TC instead of the M–I peak [6,9]. The metal–insulator transition temperature TMI for the polycrystalline manganites La0.67Sr0.33Mn1 xAlxO3 was derived as 295 K with x=0.05 and 253 K with x=0.10. As shown in Fig. 4, the TMI decreases with the increase of Al content, accompanying with the reduction of the Curie temperature TC. This shows that TMI can be shifted from a high temperature to the near room temperature through Al substitution. Fig. 5 shows the magnetic field dependence of the magnetoresistance ratio DR/R0=(R(H) R(0))/R(0) at T=293 K for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites. Negative magnetoresistance effect can be observed for x=0, 0.05 and 0.10, respectively. The ratio of DR/R0 as a function of Al content for the La0.67Sr0.33Mn1 xAlxO3 at T=293 K with H=12 kOe is shown in Fig. 6. The DR/R0 is–2.39% for the undoped sample x=0. With increasing Al substitution, there is an enhancement of the room temperature magnetoresistance. The DR/R0 is –3.22% for x=0.03 and –3.42% for x=0.05. The enhancement of the room temperature magnetoresistance in the La0.67Sr0.33Mn1 xAlxO3 may be connected with the shift of the TC and TMI from high temperatures to near room temperature. Similar enhancement were also observed in magnanites
251
Fig. 5. The magnetic field dependence of the magnetoresistance ratio DR/R0=(R(H) R(0))/R(0) at T=293 K for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
Fig. 6. The Al substitution content dependence of the magnetoresistance DR/R0 for the polycrystalline manganites La0.67Sr0.33Mn1 xAlxO3 at T=293 K with H=12 kOe.
La0.67Sr0.33Mn1 xTxO3 with T=Ti, Cr and Co [10–12]. The DR/R0 become small again at large Al substitution contents. For example, the DR/R0 is – 1.23% for x=0.10, whose TC is 296 K near room temperature but its TMI is 253 K much lower than room temperature. This shows that the room temperature magnetoresistance DR/R0 for manganites La0.67Sr0.33Mn1 xAlxO3 depend not only on the TC but also on the TMI. In summary, the enhancement of the room temperature magnetoresistance induced by the substitution of Al for Mn in La0.67Sr0.33MnO3 was observed. This enhancement is correlated with the shift of the Curie temperature and
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metal–insulator transition temperature to near room temperature through the Al substitution.
Acknowledgements This work was supported by the National Natural Science Foundation of China. The supports from the Trans-Century Foundation and the Key-Teacher Foundation of the Educational Ministry of China were also acknowledged.
References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331.
[2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] C. Zener, Phys. Rev. 8 (82) (1951) 403. [4] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [5] A.J. Millis, B.I. Shraiman, R. Mueller, Phys. Rev. Lett. 76 (1996) 1356. [6] Y. Sawaki, K. Takenaka, A. Osuka, R. Shiozaki, S. Sugai, Phys. Rev. B61 (2000) 11588. [7] G.H. Jonker, J.H. Van Santen, Physica 16 (1950) 599. [8] H. Qin, J. Hu, J. Chen, Mater. Trans. 44 (2003) 104. [9] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, K. Kido, Y. Tokura, Phys. Rev. B51 (1995) 14103. [10] J. Hu, H. Qin, J. Chen, Z. Wang, Mater. Sci. Eng. B90 (2002) 146. [11] J. Hu, H. Qin, J. Chen, Solid State Commun. 124 (2002) 437. [12] J. Hu, H. Qin, J. Chen, R.K. Zheng, J. Appl. Phys. 91 (2002) 8912.
Journal of Magnetism and Magnetic Materials 263 (2003) 249–252
Room temperature magnetoresistance in La0.67Sr0.33Mn1 xAlxO3 manganites (xp0.25) Hongwei Qin*, Jifan Hu, Juan Chen, Hongdong Niu, Luming Zhu Department of Physics, Shandong University, Jinan 250100, China Received 1 November 2002
Abstract An enhancement of the room temperature magnetoresistance induced by the Al substitution in La0.67Sr0.33Mn1 xAlxO3 was observed. A peak of the room temperature magnetoresistance occurs at Al content of xE0.05. This enhancement is correlated with the shift of the Curie temperature and metal–insulator transition temperature to near room temperature through the Al substitution. The room temperature magnetoresistance drops again at a large Al content xX0.10, which may be due to the lower metal–insulator transition temperature far from the room temperature. r 2003 Elsevier Science B.V. All rights reserved. PACS: 72.20.My; 75.30. m; 75.80.+q Keywords: Colossal magnetoresistance; Manganites; Curie temperature
1. Introduction Manganites La1 xAxMnO3 (A=Ca, Ba and Sr) have generated great interest due to the colossal magnetoresistance (CMR) effect [1,2]. The double exchange interaction has been firstly adopted to explain the CMR effect [3]. However, recent calculation showed that the double exchange interaction alone cannot explain this effect and suggested that the electron–phonon coupling also plays an important role in transport processes of manganites [4,5]. It has been found that the colossal magnetoresistance strongly depends on the temperature, whose peak usually occurs near *Corresponding author. Fax: +86-531-856-5167. E-mail address: [email protected] (H. Qin).
the Curie temperature TC. Considering the practical application, it is important to improve the magnetoresistance ratio DR/R0 at room temperature. In the present paper, we report the enhancement of the room temperature magnetoresistance induced by the substitution of Al for Mn in the polycrystalline manganite La0.67Sr0.33MnO3.
2. Experiments Polycrystalline manganites La0.67Sr0.33Mn1 xAlxO3 (x=0, 0.03, 0.05, 0.10, 0.15, 0.20 and 0.25) were prepared by the traditional solid-state reaction method. Mixed powders of La2O3, Al2O3, MnO2 and SrCO3 were ground for 20 min, ball milled for 30 min, and pressed into pellets. The
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(02)01571-8
ARTICLE IN PRESS 250
H. Qin et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 249–252
pellets were firstly heated at 800 C for 24 h and then subsequently pulverized. After milled for 20 min they were pressed into pellets again and then sintered at 1200 C for 24 h, followed by furnace cooling in atmosphere. Powder X-ray diffraction of manganites were carried out with Cu-Ka radiation at room temperature. Results indicated that the investigated La0.67Sr0.33Mn1 xAlxO3 samples were single phase with perovskite rhombohedral structure. Curie temperatures were determined from thermal magnetic curves measured with a thermal magnetic analyzer (TMA) under a low magnetic field of 200 Oe. The metal– insulator transition temperatures of manganites were determined from the temperature dependence of the resistivity measured with a four terminal method under zero field. The magnetoresistance of manganites were measured at 293 K.
Fig. 2. The Al substitution content dependence of the room temperature resistivity at zero field for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
3. Results and discussion Fig. 1 shows the Al substitution content dependence of the Curie temperature TC for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites. The Curie temperature TC of the undoped sample (x=0) is about 362 K. The substitution of Al for Mn in the La0.67Sr0.33Mn1 xAlxO3 lowers the Curie temperature. The TC is found to be 315 K for x=0.05 and 296 K for x=0.10. It means that one can reduce the TC from high temperature
Fig. 1. The Al substitution content dependence of the Curie temperature TC for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
Fig. 3. The temperature dependence of the resistivity r for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites (x=0.05 and 0.10) with zero field.
to near room temperature by adjusting the appropriate amount of Al in La0.67Sr0.33Mn1 xAlxO3. The decrease of TC with Al substitution is mainly due to the local cutoff of magnetic interaction between the spins of the t2g electrons [6]. As shown in Fig. 2, the room temperature resistivity at zero field for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites increases with the increase of Al content, accompanying the reduction of the ferromagnetism. The temperature dependence of the resistivity r for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites (x=0.05 and 0.10) with zero field is shown in Fig. 3. There is a peak in the r T curve,
ARTICLE IN PRESS H. Qin et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 249–252
Fig. 4. The Al substitution content dependence of the metal– insulator transition temperature for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
showing the metal–insulator (M–I) transition. Such M–I peak can be observed in a polycrystalline La0.7Sr0.3MnO3 sample [7,8]. However, for a single crystal La0.7Sr0.3MnO sample, a resistivity step occurs near TC instead of the M–I peak [6,9]. The metal–insulator transition temperature TMI for the polycrystalline manganites La0.67Sr0.33Mn1 xAlxO3 was derived as 295 K with x=0.05 and 253 K with x=0.10. As shown in Fig. 4, the TMI decreases with the increase of Al content, accompanying with the reduction of the Curie temperature TC. This shows that TMI can be shifted from a high temperature to the near room temperature through Al substitution. Fig. 5 shows the magnetic field dependence of the magnetoresistance ratio DR/R0=(R(H) R(0))/R(0) at T=293 K for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites. Negative magnetoresistance effect can be observed for x=0, 0.05 and 0.10, respectively. The ratio of DR/R0 as a function of Al content for the La0.67Sr0.33Mn1 xAlxO3 at T=293 K with H=12 kOe is shown in Fig. 6. The DR/R0 is–2.39% for the undoped sample x=0. With increasing Al substitution, there is an enhancement of the room temperature magnetoresistance. The DR/R0 is –3.22% for x=0.03 and –3.42% for x=0.05. The enhancement of the room temperature magnetoresistance in the La0.67Sr0.33Mn1 xAlxO3 may be connected with the shift of the TC and TMI from high temperatures to near room temperature. Similar enhancement were also observed in magnanites
251
Fig. 5. The magnetic field dependence of the magnetoresistance ratio DR/R0=(R(H) R(0))/R(0) at T=293 K for La0.67Sr0.33Mn1 xAlxO3 polycrystalline manganites.
Fig. 6. The Al substitution content dependence of the magnetoresistance DR/R0 for the polycrystalline manganites La0.67Sr0.33Mn1 xAlxO3 at T=293 K with H=12 kOe.
La0.67Sr0.33Mn1 xTxO3 with T=Ti, Cr and Co [10–12]. The DR/R0 become small again at large Al substitution contents. For example, the DR/R0 is – 1.23% for x=0.10, whose TC is 296 K near room temperature but its TMI is 253 K much lower than room temperature. This shows that the room temperature magnetoresistance DR/R0 for manganites La0.67Sr0.33Mn1 xAlxO3 depend not only on the TC but also on the TMI. In summary, the enhancement of the room temperature magnetoresistance induced by the substitution of Al for Mn in La0.67Sr0.33MnO3 was observed. This enhancement is correlated with the shift of the Curie temperature and
ARTICLE IN PRESS 252
H. Qin et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 249–252
metal–insulator transition temperature to near room temperature through the Al substitution.
Acknowledgements This work was supported by the National Natural Science Foundation of China. The supports from the Trans-Century Foundation and the Key-Teacher Foundation of the Educational Ministry of China were also acknowledged.
References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331.
[2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] C. Zener, Phys. Rev. 8 (82) (1951) 403. [4] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [5] A.J. Millis, B.I. Shraiman, R. Mueller, Phys. Rev. Lett. 76 (1996) 1356. [6] Y. Sawaki, K. Takenaka, A. Osuka, R. Shiozaki, S. Sugai, Phys. Rev. B61 (2000) 11588. [7] G.H. Jonker, J.H. Van Santen, Physica 16 (1950) 599. [8] H. Qin, J. Hu, J. Chen, Mater. Trans. 44 (2003) 104. [9] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, K. Kido, Y. Tokura, Phys. Rev. B51 (1995) 14103. [10] J. Hu, H. Qin, J. Chen, Z. Wang, Mater. Sci. Eng. B90 (2002) 146. [11] J. Hu, H. Qin, J. Chen, Solid State Commun. 124 (2002) 437. [12] J. Hu, H. Qin, J. Chen, R.K. Zheng, J. Appl. Phys. 91 (2002) 8912.