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Electronic states in polycrystalline and crystalline bismuth-based manganites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 885–887 www.elsevier.com/locate/jmmm
Electronic states in polycrystalline and crystalline bismuth-based manganites M. Tokunaga, S. Hakuta, T. Tamegai Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Available online 27 November 2006
Abstract Magnetic and transport properties in polycrystalline and crystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 were studied for various values of z. Polycrystalline samples show metal–insulator transitions at around z ¼ 0:4, which do not coincide with magnetic transitions. On the contrary, crystalline samples show clear metal–insulator transitions coupled with magnetic transitions. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 75.47.Lx Keywords: Manganite; Metal–insulator transition
1. Introduction Among a variety of manganites with perovskite-type structures, bismuth-based system shows the unique electronic states that can be originated from the lone-pair character of Bi-6s electrons. BiMnO3 is known as a ferroelectric ferromagnet below about 100 K [1]. In holedoped Bi1x Srx MnO3 , a charge-ordered state sets in at temperature significantly higher than the room temperature (600 K for x ¼ 0:25) [2]. This enhancement of the transition temperature cannot be interpreted by a simple mechanism such as the reduction of ionic radius in the A-site ion of the perovskite-type structure, making remarkable contrast with the other rare-earth manganites [3]. On the other hand, La1x Srx MnO3 is known as a canonical CMR material with Curie temperature as high as 370 K for x0:3 [4]. Therefore, the mixed material of ðLa1z Biz Þ1x Srx MnO3 , can be expected to show competition between the two phases even at the room temperature. Although this system has been studied in polycrystalline samples [5], detailed magnetic and transport properties are not clear as yet.
Corresponding author. Tel.: +81 3 58416848.
E-mail address: [email protected] (M. Tokunaga). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.1100
We synthesized polycrystalline and crystalline samples of ðLa1z Biz Þ2=3 Sr1=3 MnO3 with various values of z, and measured the magnetic and transport properties. 2. Experimental Polycrystalline samples were synthesized by the standard solid state reaction method. Crystals of these manganites were grown by using Bi2 O3 as flux in platinum crucibles following the procedure reported in Ref. [6]. Rectangular parallelepiped crystals were obtained with a typical dimension about 1 mm for each side. Electron microprobe analyses indicate that the polycrystalline samples have identical composition with the starting mixed-powders while the crystalline samples contain slightly larger amount of Sr than the nominal values and have scattered values of z from piece to piece. We used the measured values of z to describe the samples used in the present study. 3. Results and discussion Fig. 1 shows temperature dependence of (a) magnetization and (b) resistivity in polycrystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 . The samples with zp0:5 show ferromagnetic moment below about 350 K. Substitution of Bi for La systematically diminishes the ferromagnetic moment, whereas the Curie
ARTICLE IN PRESS 886
M. Tokunaga et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 885–887
Fig. 3. Doping dependence ðLa1z Biz Þ2=3 Sr1=3 MnO3 .
Fig. 1. Temperature dependence of (a) magnetization at m0 H ¼ 0:5 T and (b) resistivity in polycrystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 .
of
spontaneous
magnetization
in
peak structure decoupled with magnetic transition have been explained as the effect of the grain boundaries [7]. The effect of grain boundary can be removed in crystalline samples. Figs. 2(a) and (b) show temperature dependence of magnetization and resistivity in crystals of ðLa1z Biz Þ2=3 Sr1=3 MnO3 . Samples with z ¼ 0:53 and 0.61 shows insulator–metal transitions at the Curie temperature. To discuss the relation between magnetic and metal– insulator transition, we have to take into account of the possibility of chemical phase-separation to La2=3 Sr1=3 MnO3 and Bi2=3 Sr1=3 MnO3 . Fig. 3 shows doping dependence of spontaneous magnetization (M 0 ) determined from the magnetization curves at low temperatures. Steep change in the M 0 at around z ¼ 0:5 deviate from the z-dependence expected for the mixed-phase materials (the dashed-line). Discrepancy between the data for polycrystalline and crystalline samples can be ascribed to the difference in carrier concentration. Finally, our recent experiments for Bi1x Srx MnO3 crystals in high magnetic fields demonstrate insulating transport properties in highly spin-polarized state of bismuth manganites [8]. Detailed studies of electronic states in ðLa1z Biz Þ2=3 Sr1=3 MnO3 crystals with 0:61ozo0:98 will clarify the nature of this spin-polarized state. Acknowledgements
Fig. 2. Temperature dependence of (a) magnetization at m0 H ¼ 0:5 T and (b) resistivity in crystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 .
temperature is less sensitive to z for zp0:5. Transport measurements show the presence of the metal–insulator transition at around z ¼ 0:40. However, the temperature dependence of resistivity in the metallic samples (z ¼ 0:25 and 0.33) shows broad maxima at temperatures considerably lower than the Curie temperature, which is consistent with the early report on polycrystals [5]. Such a broad resistive
This work is supported by Grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. Chemical analyses were performed using facilities of the Institute for Solid State Physics, the University of Tokyo. References [1] T. Kimura, S. Kawamoto, I. Yamada, M. Azuma, M. Takano, Y. Tokura, Phys. Rev. B 67 (2003) 180401(R).
ARTICLE IN PRESS M. Tokunaga et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 885–887 [2] C. Frontera, J.L. Garcia-Munoz, M.A.G. Aranda, M. Hervieu, C. Ritter, L. Manosa, X.G. Capdevila, A. Calleja, Phys. Rev. B 68 (2003) 134408. [3] J.L. Garcia-Munoz, C. Frontera, M.A.G. Aranda, M. Hervieu, C. Ritter, Phys. Rev. B 63 (2001) 064415. [4] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [5] Z.X. Cheng, T.M. Silver, A.H. Li, X.L. Wang, H. Kimura, J. Magn. Magn. Mater. 283 (2004) 143.
887
[6] M. Nevriva, P. Beran, D. Sedmidubsky, A. Strejc, J. Hejtmanek, C. Martin, O. Perez, S. Malo, B. Raveau, Solid State Phenomena 90–91 (2003) 371. [7] R. Gross, L. Alff, B. Buchner, B.H. Freitag, C. Hofener, J. Klein, Y. Lu, W. Mader, J.B. Philipp, M.S.R. Rao, P. Reutler, S. Ritter, S. Thienhaus, S. Uhlenbruck, B. Wiedenhorst, J. Magn. Magn. Mater. 211 (2000) 150. [8] M. Tokunaga, S. Hakuta, T. Tamegai, E. Ohmichi, T. Osada, J. Phys.: Conference Series (2007), to appear.
Journal of Magnetism and Magnetic Materials 310 (2007) 885–887 www.elsevier.com/locate/jmmm
Electronic states in polycrystalline and crystalline bismuth-based manganites M. Tokunaga, S. Hakuta, T. Tamegai Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Available online 27 November 2006
Abstract Magnetic and transport properties in polycrystalline and crystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 were studied for various values of z. Polycrystalline samples show metal–insulator transitions at around z ¼ 0:4, which do not coincide with magnetic transitions. On the contrary, crystalline samples show clear metal–insulator transitions coupled with magnetic transitions. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 75.47.Lx Keywords: Manganite; Metal–insulator transition
1. Introduction Among a variety of manganites with perovskite-type structures, bismuth-based system shows the unique electronic states that can be originated from the lone-pair character of Bi-6s electrons. BiMnO3 is known as a ferroelectric ferromagnet below about 100 K [1]. In holedoped Bi1x Srx MnO3 , a charge-ordered state sets in at temperature significantly higher than the room temperature (600 K for x ¼ 0:25) [2]. This enhancement of the transition temperature cannot be interpreted by a simple mechanism such as the reduction of ionic radius in the A-site ion of the perovskite-type structure, making remarkable contrast with the other rare-earth manganites [3]. On the other hand, La1x Srx MnO3 is known as a canonical CMR material with Curie temperature as high as 370 K for x0:3 [4]. Therefore, the mixed material of ðLa1z Biz Þ1x Srx MnO3 , can be expected to show competition between the two phases even at the room temperature. Although this system has been studied in polycrystalline samples [5], detailed magnetic and transport properties are not clear as yet.
Corresponding author. Tel.: +81 3 58416848.
E-mail address: [email protected] (M. Tokunaga). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.1100
We synthesized polycrystalline and crystalline samples of ðLa1z Biz Þ2=3 Sr1=3 MnO3 with various values of z, and measured the magnetic and transport properties. 2. Experimental Polycrystalline samples were synthesized by the standard solid state reaction method. Crystals of these manganites were grown by using Bi2 O3 as flux in platinum crucibles following the procedure reported in Ref. [6]. Rectangular parallelepiped crystals were obtained with a typical dimension about 1 mm for each side. Electron microprobe analyses indicate that the polycrystalline samples have identical composition with the starting mixed-powders while the crystalline samples contain slightly larger amount of Sr than the nominal values and have scattered values of z from piece to piece. We used the measured values of z to describe the samples used in the present study. 3. Results and discussion Fig. 1 shows temperature dependence of (a) magnetization and (b) resistivity in polycrystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 . The samples with zp0:5 show ferromagnetic moment below about 350 K. Substitution of Bi for La systematically diminishes the ferromagnetic moment, whereas the Curie
ARTICLE IN PRESS 886
M. Tokunaga et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 885–887
Fig. 3. Doping dependence ðLa1z Biz Þ2=3 Sr1=3 MnO3 .
Fig. 1. Temperature dependence of (a) magnetization at m0 H ¼ 0:5 T and (b) resistivity in polycrystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 .
of
spontaneous
magnetization
in
peak structure decoupled with magnetic transition have been explained as the effect of the grain boundaries [7]. The effect of grain boundary can be removed in crystalline samples. Figs. 2(a) and (b) show temperature dependence of magnetization and resistivity in crystals of ðLa1z Biz Þ2=3 Sr1=3 MnO3 . Samples with z ¼ 0:53 and 0.61 shows insulator–metal transitions at the Curie temperature. To discuss the relation between magnetic and metal– insulator transition, we have to take into account of the possibility of chemical phase-separation to La2=3 Sr1=3 MnO3 and Bi2=3 Sr1=3 MnO3 . Fig. 3 shows doping dependence of spontaneous magnetization (M 0 ) determined from the magnetization curves at low temperatures. Steep change in the M 0 at around z ¼ 0:5 deviate from the z-dependence expected for the mixed-phase materials (the dashed-line). Discrepancy between the data for polycrystalline and crystalline samples can be ascribed to the difference in carrier concentration. Finally, our recent experiments for Bi1x Srx MnO3 crystals in high magnetic fields demonstrate insulating transport properties in highly spin-polarized state of bismuth manganites [8]. Detailed studies of electronic states in ðLa1z Biz Þ2=3 Sr1=3 MnO3 crystals with 0:61ozo0:98 will clarify the nature of this spin-polarized state. Acknowledgements
Fig. 2. Temperature dependence of (a) magnetization at m0 H ¼ 0:5 T and (b) resistivity in crystalline ðLa1z Biz Þ2=3 Sr1=3 MnO3 .
temperature is less sensitive to z for zp0:5. Transport measurements show the presence of the metal–insulator transition at around z ¼ 0:40. However, the temperature dependence of resistivity in the metallic samples (z ¼ 0:25 and 0.33) shows broad maxima at temperatures considerably lower than the Curie temperature, which is consistent with the early report on polycrystals [5]. Such a broad resistive
This work is supported by Grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. Chemical analyses were performed using facilities of the Institute for Solid State Physics, the University of Tokyo. References [1] T. Kimura, S. Kawamoto, I. Yamada, M. Azuma, M. Takano, Y. Tokura, Phys. Rev. B 67 (2003) 180401(R).
ARTICLE IN PRESS M. Tokunaga et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 885–887 [2] C. Frontera, J.L. Garcia-Munoz, M.A.G. Aranda, M. Hervieu, C. Ritter, L. Manosa, X.G. Capdevila, A. Calleja, Phys. Rev. B 68 (2003) 134408. [3] J.L. Garcia-Munoz, C. Frontera, M.A.G. Aranda, M. Hervieu, C. Ritter, Phys. Rev. B 63 (2001) 064415. [4] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [5] Z.X. Cheng, T.M. Silver, A.H. Li, X.L. Wang, H. Kimura, J. Magn. Magn. Mater. 283 (2004) 143.
887
[6] M. Nevriva, P. Beran, D. Sedmidubsky, A. Strejc, J. Hejtmanek, C. Martin, O. Perez, S. Malo, B. Raveau, Solid State Phenomena 90–91 (2003) 371. [7] R. Gross, L. Alff, B. Buchner, B.H. Freitag, C. Hofener, J. Klein, Y. Lu, W. Mader, J.B. Philipp, M.S.R. Rao, P. Reutler, S. Ritter, S. Thienhaus, S. Uhlenbruck, B. Wiedenhorst, J. Magn. Magn. Mater. 211 (2000) 150. [8] M. Tokunaga, S. Hakuta, T. Tamegai, E. Ohmichi, T. Osada, J. Phys.: Conference Series (2007), to appear.