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Structural and magnetic properties of La0.6−x□xCa0.4MnO3 (0 ≤ x ≤ 0.2) perovskite manganite
Journal of Alloys and Compounds 485 (2009) 64–68
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural and magnetic properties of La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) perovskite manganite I. Walha a,b , H. Ehrenberg b , H. Fuess b , A. Cheikhrouhou a,∗ a b
Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia Institute for Materials Science, University of Technology, D-64287 Darmstadt, Germany
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 16 June 2009 Accepted 17 June 2009 Available online 25 June 2009 PACS: 71.30.+h 75.50.−y 75.75
a b s t r a c t The lanthanum deficient La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) have been elaborated and investigated by X-ray diffraction (XRD) and magnetic measurements. The samples have been synthesized by the solid-state reaction method at high temperatures. All samples crystallize in the orthorhombic system with Pnma space group. Lanthanum deficiency leads to a decrease of the unit cell volume. Magnetization measurements versus temperature in a magnetic applied field of 50 mT showed that all samples are ferromagnetic at low temperature. With increasing lanthanum deficiency, the Curie temperature Tc decreases from 280 K for x = 0 to 170 K for x = 0.2. © 2009 Elsevier B.V. All rights reserved.
Keywords: Manganites Ferromagnetism Curie temperature
1. Introduction Since the discovery in 1994 of colossal magnetoresistance (CMR) effects in perovskite manganese oxides with general formula Ln1−x Ax MnO3 (Ln is a rare earth element and A is a divalent alkali earth element), these compounds have attracted considerable attention due to their rich physical properties and their high potential for magnetic applications [1–3]. It is well accepted that the substitution of the rare earth element by a divalent alkali metal leads to a mixed valence Mn3+ /Mn4+ state and induces a transition from paramagnetic-insulator to ferromagnetic-metallic phase. The coexistence of the ferromagnetic state and metallicity has been explained on the basis of the double exchange (DE) interaction with a strong Hund coupling between carriers and t2g spin [4–6]. The super-exchange interactions and orbital ordering govern transport and magnetic properties. Many studies have shown that the nature of the magnetic ordering with doped manganites depends on the relative concentration of the Mn3+ and Mn4+ ions, and on the structural properties such as Mn–O bond length
∗ Corresponding author. Tel.: +216 74 676607; fax: +216 74 676607. E-mail addresses: [email protected], [email protected] (A. Cheikhrouhou). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.121
and Mn–O–Mn bond angle. Previous studies showed that lanthanum deficiency in La0.7−x x Ca0.3 MnO3 powder samples leads to an increase of the Curie temperature Tc [7]. In this case, the magnetic and transport properties of manganites can be modified indirectly by changing the carrier density (Mn3+ /Mn4+ ) or structural parameters (Mn–O bond length and Mn–O–Mn bond angle). In this paper, we present a study of structural and magnetic effect of lanthanum vacancies in polycrystalline La0.6−x x Ca0.4 MnO3 with (0 ≤ x ≤ 0.2) manganese oxides. 2. Experimental techniques Powder samples of La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) have been synthesized using the standard solid-state reaction method at high temperature, by mixing La2 O3 , MnO2 and CaCO3 up to 99.9% purity in the desired proportion according to the following reaction at 1400 ◦ C. (0.6 − x)/2La2 O3 + 0.4CaCO3 + MnO2 → La0.6−x x Ca0.4 MnO3 + 0.4CO2 The starting materials were intimately mixed in an agate mortar and then heated in air up to 1000 ◦ C for 60 h. The obtained powders were then pressed into pellets (of about 1 mm thickness) and sintered at 1100 ◦ C in air for 60 h with intermediate regrinding and repelling. Finally, these pellets were rapidly quenched to room temperature in air in order to freeze the structure at the annealed temperature. Phase purity, homogeneity and cell dimensions were determined by powder X-ray diffraction at room temperature (with MoK␣ radiation). The amount of Mn4+ ions has been quantitatively checked by chemical analysis. Structural analysis was carried out using the standard Rietveld technique [8,9]. Magnetization measurements
I. Walha et al. / Journal of Alloys and Compounds 485 (2009) 64–68
65
versus temperature in the range 5–300 K and versus magnetic applied field up to 6 T were carried out using a SQUID magnetometer.
3. Results and discussion Our samples have been elaborated in air; they are consequently stoichiometric in oxygen [10]. A vacancy in the A site implies a partial conversion of Mn3+ to Mn4+ leading to an increase in the Mn4+ content above 40%. The Mn4+ ions amount has been quantitatively cheeked by chemical analysis (iodometric titration) (Table 1). The powder samples was placed in quartz crucible and dissolved in H2 SO4 and H2 C2 O4 solution at about 50 ◦ C. The compound reacts with H2 C2 O4 solution, which deoxidizes Mn3+ and Mn4+ to Mn2+ . The excess of H2 C2 O4 solution was titrated using KMnO4 . The experimental results agree with the theoretical data and confirmed the amount of deficiency in our lanthanum deficient samples. The X-ray diffraction (XRD) patterns of all our synthesized La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) samples recorded at room temperature are shown in Fig. 1. The profile refinement is started with scale and background parameters followed by the unit cell parameters. Then, the peak asymmetry and preferred orientation corrections are applied. Finally, the positional parameters and the individual isotropic parameters are refined. All the reflexion lines were successfully indexed according to an orthorhombic perovskite structure with Pnma space group. To show the good quality of the fits, we present in Fig. 2a and b the room temperature Rietveld plot for x = 0.05 and x = 0.20 samples. Detailed results of the structural parameters and 2 values are summarized in Table 2. Lanthanum deficiency leads to reduce of unit cell volume (Fig. 3a). As a lanthanum deficiency induces an increase of the Mn4+ content with average ionic radius (rMn 4+ = 0.53 Å) smaller then Mn3+ (rMn 3+ = 0.58 Å), the reduction of the unit cell volume with increasing lanthanum deficiency content can be explained by the
Table 1 Chemical analysis data of La0.6−x x Ca0.4 MnO3 samples (0 ≤ x ≤ 0.2). Samples
%Mn4+ theoretical
%Mn4+ experimental
% relative error
La0.6 Ca0.4 MnO3 La0.55 0.05 Ca0.4 MnO3 La0.5 0.1 Ca0.4 MnO3 La0.45 0.15 Ca0.4 MnO3 La0.4 0.2 Ca0.4 MnO3
40 55 70 85 100
39.6 54.2 68.8 83.4 97.6
−1.1 −1.4 −1.6 −1.8 −2
Fig. 2. XRD patterns of La0.6−x x Ca0.4 MnO3 samples. (a) x = 0.05 and (b) x = 0.20. Squares indicate the experimental data and the calculated data is the continuous line overlapping them. The lowest curve shows the difference between experimental and calculated patterns. The vertical bars indicate the expected reflection positions.
increase of the Mn4+ content. A similar result had been observed by Cheikhrouhou-Koubaa et al. [11] for Pr0.6−x x Sr0.4 MnO3 . The average Mn–O–Mn bond angle increases with increasing x, whereas the average Mn–O bond length displays the inverse correlation to the variation of the Mn–O–Mn bond angle (Fig. 3b). We plot in Fig. 4a the magnetization evolution versus temperature in an applied magnetic field of 50 mT for all samples. These curves show that all samples exhibit a ferromagnetic to paramagnetic transition when the temperature increases. The magnetization at low temperature decreases with increasing vacancy content. The Curie temperature Tc (defined as the temperature at which dM/dT shows a minimum), decreases from
Table 2 Refined structural parameters of La0.6−x x Ca0.4 MnO3 samples (0 ≤ x ≤ 0.2) at room temperature.
Fig. 1. The X-ray diffraction (XRD) patterns of all our synthesized samples La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) at room temperature.
a (Å) b (Å) c (Å) V (Å3 ) Mn–O (Å) Mn–O–Mn (◦ ) 2
x = 0.00
x = 0.05
x = 0.10
x = 0.15
x = 0.20
5.4406(6) 7.6817(3) 5.4531(5) 227.90(6) 1.945(9) 160.40(1) 1.3
5.4362(2) 7.6731(4) 5.4471(1) 227.21(4) 1.943(8) 160.83(6) 1.1
5.4285(3) 7.6639(9) 5.4392(3) 226.29(4) 1.942(1) 161.77(5) 1.9
5.4165(7) 7.6351(1) 5.4236(2) 224.29(9) 1.940(2) 163.12(3) 2.5
5.4099(5) 7.6309(2) 5.4163(1) 223.60(1) 1.938(7) 163.66(5) 3.2
66
I. Walha et al. / Journal of Alloys and Compounds 485 (2009) 64–68
Fig. 3. (a) Unit cell volume evolution for La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) and (b) variation of Mn–O–Mn angles and Mn–O lengths as a function of deficient content.
280 K for x = 0.00 to 170 K for x = 0.2 (Fig. 4b). According to the double exchange model the increase of Tc arises from a strong overlap between Mn (3d) and O (2p) orbitals, leading to an enhancement of the carrier bandwidth (W) of the eg band. The bond angle and length are related to W by the relation W ≈ cos /dMn–O 3.5 , where = (Mn–O–Mn)/2 [12]. With the increase of the Mn–O–Mn angle and the decrease of the Mn–O distance we should expect an enhancement in the Curie temperature Tc which is not in agreement with our results. But the increase of W by 0.05% is too small to explain the large reduction of Tc in our compounds. Baby et al. [13] found also that even bond length and bond angle influence the electron band width, the influence is too small to explain the variation of Tc . The reduction of Tc in our samples could be explained by the increase of the Mn4+ content above 40%, which leads to a weakness of the double exchange interactions [4]. The Tc decrease in the lanthanum deficient cannot be governed only by the Mn4+ /Mn3+ ratio and subsequently explained in terms of double exchange interactions. As a consequence, there are surely other factors which lead to such behavior as mismatch effects induced by lanthanum vacancy in the A site. This vacancy must have an average radius rV > # 0 and consequently leads to a change in the average ionic radius rA of the A cation site. In order to confirm the ferromagnetic behaviour at low temperature and to determine with more precision the Curie temperature Tc , we have performed for all samples, magnetization measurements vs. magnetic applied field up to 6 T at several
Fig. 4. (a) Temperature dependence of the magnetization at 0 H = 50 mT and (b) Curie temperature versus deficiencies content for La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) samples.
temperatures in the range 10–300 K. We plot in Fig. 5 the M(H) curves for La0.6−x x Ca0.4 MnO3 . The magnetization at low temperature (T < Tc ), increases sharply with magnetic applied field for H < 1 T and then saturates which shows quite typical ferromagnetic behavior. The saturation magnetization shifts to higher values with decreasing temperature. The values of the spontaneous magnetizations at T = 10 K were calculated by considering the total spins of Mn3+ and Mn4+ ions (SMn 3+ = 2, SMn 4+ = 1.5). The spontaneous magnetizations of La0.6−x x Ca0.4 MnO3 expressed as Msp = 2[2 × (0.6 − 3x) + 1.5 × (0.4 + 3x)] B/Mn where x is the content of lanthanum vacancy and B is the Bohr magneton. The measured spontaneous magnetizations at T = 10 K for x = 0.00, 0.05, 0,10 and 0.15 samples are found to be about 3.58 B/Mn , 3.3 B/Mn , 3.05 B/Mn and 2.7 B/Mn , respectively while the calculated values for full spin alignment are 3.6 B/Mn , 3.45 B/Mn , 3.3 B/Mn and 3.15 B/Mn respectively. The spontaneous magnetization decreases with increasing x and the small difference between the measured and calculated value especially for x = 0.15 could be explained by spin canted state at low temperature (Fig. 6). Fig. 7 shows the Arrott plots (M2 versus H/M) obtained from magnetization isotherms for our synthesized samples. As can be seen, Arrott plots above Tc show a linear behavior which indicates that a second order magnetic transition occurs [14].
I. Walha et al. / Journal of Alloys and Compounds 485 (2009) 64–68
Fig. 5. Magnetization evolution versus magnetic applied field at several temperatures for La0.55 0.05 Ca0.4 MnO3 and La0.45 0.15 Ca0.4 MnO3 samples.
67
Fig. 7. M2 versus H/M isotherms for La0.55 0.05 Ca0.4 MnO3 and La0.45 0.15 Ca0.4 MnO3 samples.
4. Conclusions In this work we have studied the effect of lanthanum deficiency on the structural and magnetic properties of La0.6 Ca0.4 MnO3 perovskite manganese oxide. The structural study shows that all our synthesized samples crystallize in the orthorhombic structure with Pnma space group. Lanthanum deficiency leads to a decrease of the Curie temperature with increasing lanthanum vacancy. The spontaneous magnetization value at T = 10 K is lower than the expected theoretical value and may be ascribed to the presence of a spin canted state. Acknowledgments This study has been supported by the Tunisian Ministry of Higher Education, Scientific Research and Technology. One of us (I. Walha) would like to thank the Deutscher Akademischer Austausch-Dienst (DAAD) for support. References
Fig. 6. The spontaneous magnetization experimental (Msp exp ) and calculated (Msp ca ) as a function of vacancy content for La0.6−x x Ca0.4 MnO3 (0.05 ≤ x ≤ 0.15) at T = 10 K.
[1] J. Heremans, J. Phys. D 26 (1993) 1149. [2] T. Venkatesan, M. Rajeswari, Z.-W. Dong, S.B. Ogale, R. Ramesh, Philos. Trans. R. Soc. Lond. Ser. A 356 (1998) 1661. [3] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [4] C. Zener, Phys. Rev. 82 (1951) 403.
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[5] P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675. [6] P.G. de Genes, Phys. Rev. 118 (1960) 141. [7] I. Kammoun, W. Boujelben, A. Cheikhrouhou, H. Roussel, R. Madar, Phys. Stat. Sol. (C) 1 (2004) 1631. [8] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [9] T. Roisnel, J. Rodriguez-Carvajal, Computer Program FULLPROF, LLB-LCSIM, May 2003.
[10] J.H. Kuo, H.U. Anderson, D.M. Sparlin, J. Solid-State Chem. 83 (1989) 52. [11] W. Cheikhrouhou-Koubaa, M. Koubaa, A. Cheikhrouhou, W. Boujelben, A.-M. Haghiri-Gosnet, J. Alloys Comp. 455 (2008) 67. [12] M. Medarde, J. Mesot, P. Lacorre, S. Rosenkranz, P. Fischer, K. Gobrecht, Phys. Rev. B 52 (1995) 9248. [13] P.D. Baby, A. Das, S.K. Paranjpe, Solid-State Commun. 118 (2001) 91. [14] N.K. Singh, K.G. Suresh, A.K. Nigam, Solid-State Commun. 127 (2003) 373.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural and magnetic properties of La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) perovskite manganite I. Walha a,b , H. Ehrenberg b , H. Fuess b , A. Cheikhrouhou a,∗ a b
Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia Institute for Materials Science, University of Technology, D-64287 Darmstadt, Germany
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 16 June 2009 Accepted 17 June 2009 Available online 25 June 2009 PACS: 71.30.+h 75.50.−y 75.75
a b s t r a c t The lanthanum deficient La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) have been elaborated and investigated by X-ray diffraction (XRD) and magnetic measurements. The samples have been synthesized by the solid-state reaction method at high temperatures. All samples crystallize in the orthorhombic system with Pnma space group. Lanthanum deficiency leads to a decrease of the unit cell volume. Magnetization measurements versus temperature in a magnetic applied field of 50 mT showed that all samples are ferromagnetic at low temperature. With increasing lanthanum deficiency, the Curie temperature Tc decreases from 280 K for x = 0 to 170 K for x = 0.2. © 2009 Elsevier B.V. All rights reserved.
Keywords: Manganites Ferromagnetism Curie temperature
1. Introduction Since the discovery in 1994 of colossal magnetoresistance (CMR) effects in perovskite manganese oxides with general formula Ln1−x Ax MnO3 (Ln is a rare earth element and A is a divalent alkali earth element), these compounds have attracted considerable attention due to their rich physical properties and their high potential for magnetic applications [1–3]. It is well accepted that the substitution of the rare earth element by a divalent alkali metal leads to a mixed valence Mn3+ /Mn4+ state and induces a transition from paramagnetic-insulator to ferromagnetic-metallic phase. The coexistence of the ferromagnetic state and metallicity has been explained on the basis of the double exchange (DE) interaction with a strong Hund coupling between carriers and t2g spin [4–6]. The super-exchange interactions and orbital ordering govern transport and magnetic properties. Many studies have shown that the nature of the magnetic ordering with doped manganites depends on the relative concentration of the Mn3+ and Mn4+ ions, and on the structural properties such as Mn–O bond length
∗ Corresponding author. Tel.: +216 74 676607; fax: +216 74 676607. E-mail addresses: [email protected], [email protected] (A. Cheikhrouhou). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.121
and Mn–O–Mn bond angle. Previous studies showed that lanthanum deficiency in La0.7−x x Ca0.3 MnO3 powder samples leads to an increase of the Curie temperature Tc [7]. In this case, the magnetic and transport properties of manganites can be modified indirectly by changing the carrier density (Mn3+ /Mn4+ ) or structural parameters (Mn–O bond length and Mn–O–Mn bond angle). In this paper, we present a study of structural and magnetic effect of lanthanum vacancies in polycrystalline La0.6−x x Ca0.4 MnO3 with (0 ≤ x ≤ 0.2) manganese oxides. 2. Experimental techniques Powder samples of La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) have been synthesized using the standard solid-state reaction method at high temperature, by mixing La2 O3 , MnO2 and CaCO3 up to 99.9% purity in the desired proportion according to the following reaction at 1400 ◦ C. (0.6 − x)/2La2 O3 + 0.4CaCO3 + MnO2 → La0.6−x x Ca0.4 MnO3 + 0.4CO2 The starting materials were intimately mixed in an agate mortar and then heated in air up to 1000 ◦ C for 60 h. The obtained powders were then pressed into pellets (of about 1 mm thickness) and sintered at 1100 ◦ C in air for 60 h with intermediate regrinding and repelling. Finally, these pellets were rapidly quenched to room temperature in air in order to freeze the structure at the annealed temperature. Phase purity, homogeneity and cell dimensions were determined by powder X-ray diffraction at room temperature (with MoK␣ radiation). The amount of Mn4+ ions has been quantitatively checked by chemical analysis. Structural analysis was carried out using the standard Rietveld technique [8,9]. Magnetization measurements
I. Walha et al. / Journal of Alloys and Compounds 485 (2009) 64–68
65
versus temperature in the range 5–300 K and versus magnetic applied field up to 6 T were carried out using a SQUID magnetometer.
3. Results and discussion Our samples have been elaborated in air; they are consequently stoichiometric in oxygen [10]. A vacancy in the A site implies a partial conversion of Mn3+ to Mn4+ leading to an increase in the Mn4+ content above 40%. The Mn4+ ions amount has been quantitatively cheeked by chemical analysis (iodometric titration) (Table 1). The powder samples was placed in quartz crucible and dissolved in H2 SO4 and H2 C2 O4 solution at about 50 ◦ C. The compound reacts with H2 C2 O4 solution, which deoxidizes Mn3+ and Mn4+ to Mn2+ . The excess of H2 C2 O4 solution was titrated using KMnO4 . The experimental results agree with the theoretical data and confirmed the amount of deficiency in our lanthanum deficient samples. The X-ray diffraction (XRD) patterns of all our synthesized La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) samples recorded at room temperature are shown in Fig. 1. The profile refinement is started with scale and background parameters followed by the unit cell parameters. Then, the peak asymmetry and preferred orientation corrections are applied. Finally, the positional parameters and the individual isotropic parameters are refined. All the reflexion lines were successfully indexed according to an orthorhombic perovskite structure with Pnma space group. To show the good quality of the fits, we present in Fig. 2a and b the room temperature Rietveld plot for x = 0.05 and x = 0.20 samples. Detailed results of the structural parameters and 2 values are summarized in Table 2. Lanthanum deficiency leads to reduce of unit cell volume (Fig. 3a). As a lanthanum deficiency induces an increase of the Mn4+ content with average ionic radius (rMn 4+ = 0.53 Å) smaller then Mn3+ (rMn 3+ = 0.58 Å), the reduction of the unit cell volume with increasing lanthanum deficiency content can be explained by the
Table 1 Chemical analysis data of La0.6−x x Ca0.4 MnO3 samples (0 ≤ x ≤ 0.2). Samples
%Mn4+ theoretical
%Mn4+ experimental
% relative error
La0.6 Ca0.4 MnO3 La0.55 0.05 Ca0.4 MnO3 La0.5 0.1 Ca0.4 MnO3 La0.45 0.15 Ca0.4 MnO3 La0.4 0.2 Ca0.4 MnO3
40 55 70 85 100
39.6 54.2 68.8 83.4 97.6
−1.1 −1.4 −1.6 −1.8 −2
Fig. 2. XRD patterns of La0.6−x x Ca0.4 MnO3 samples. (a) x = 0.05 and (b) x = 0.20. Squares indicate the experimental data and the calculated data is the continuous line overlapping them. The lowest curve shows the difference between experimental and calculated patterns. The vertical bars indicate the expected reflection positions.
increase of the Mn4+ content. A similar result had been observed by Cheikhrouhou-Koubaa et al. [11] for Pr0.6−x x Sr0.4 MnO3 . The average Mn–O–Mn bond angle increases with increasing x, whereas the average Mn–O bond length displays the inverse correlation to the variation of the Mn–O–Mn bond angle (Fig. 3b). We plot in Fig. 4a the magnetization evolution versus temperature in an applied magnetic field of 50 mT for all samples. These curves show that all samples exhibit a ferromagnetic to paramagnetic transition when the temperature increases. The magnetization at low temperature decreases with increasing vacancy content. The Curie temperature Tc (defined as the temperature at which dM/dT shows a minimum), decreases from
Table 2 Refined structural parameters of La0.6−x x Ca0.4 MnO3 samples (0 ≤ x ≤ 0.2) at room temperature.
Fig. 1. The X-ray diffraction (XRD) patterns of all our synthesized samples La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) at room temperature.
a (Å) b (Å) c (Å) V (Å3 ) Mn–O (Å) Mn–O–Mn (◦ ) 2
x = 0.00
x = 0.05
x = 0.10
x = 0.15
x = 0.20
5.4406(6) 7.6817(3) 5.4531(5) 227.90(6) 1.945(9) 160.40(1) 1.3
5.4362(2) 7.6731(4) 5.4471(1) 227.21(4) 1.943(8) 160.83(6) 1.1
5.4285(3) 7.6639(9) 5.4392(3) 226.29(4) 1.942(1) 161.77(5) 1.9
5.4165(7) 7.6351(1) 5.4236(2) 224.29(9) 1.940(2) 163.12(3) 2.5
5.4099(5) 7.6309(2) 5.4163(1) 223.60(1) 1.938(7) 163.66(5) 3.2
66
I. Walha et al. / Journal of Alloys and Compounds 485 (2009) 64–68
Fig. 3. (a) Unit cell volume evolution for La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) and (b) variation of Mn–O–Mn angles and Mn–O lengths as a function of deficient content.
280 K for x = 0.00 to 170 K for x = 0.2 (Fig. 4b). According to the double exchange model the increase of Tc arises from a strong overlap between Mn (3d) and O (2p) orbitals, leading to an enhancement of the carrier bandwidth (W) of the eg band. The bond angle and length are related to W by the relation W ≈ cos /dMn–O 3.5 , where = (Mn–O–Mn)/2 [12]. With the increase of the Mn–O–Mn angle and the decrease of the Mn–O distance we should expect an enhancement in the Curie temperature Tc which is not in agreement with our results. But the increase of W by 0.05% is too small to explain the large reduction of Tc in our compounds. Baby et al. [13] found also that even bond length and bond angle influence the electron band width, the influence is too small to explain the variation of Tc . The reduction of Tc in our samples could be explained by the increase of the Mn4+ content above 40%, which leads to a weakness of the double exchange interactions [4]. The Tc decrease in the lanthanum deficient cannot be governed only by the Mn4+ /Mn3+ ratio and subsequently explained in terms of double exchange interactions. As a consequence, there are surely other factors which lead to such behavior as mismatch effects induced by lanthanum vacancy in the A site. This vacancy must have an average radius rV > # 0 and consequently leads to a change in the average ionic radius rA of the A cation site. In order to confirm the ferromagnetic behaviour at low temperature and to determine with more precision the Curie temperature Tc , we have performed for all samples, magnetization measurements vs. magnetic applied field up to 6 T at several
Fig. 4. (a) Temperature dependence of the magnetization at 0 H = 50 mT and (b) Curie temperature versus deficiencies content for La0.6−x x Ca0.4 MnO3 (0 ≤ x ≤ 0.2) samples.
temperatures in the range 10–300 K. We plot in Fig. 5 the M(H) curves for La0.6−x x Ca0.4 MnO3 . The magnetization at low temperature (T < Tc ), increases sharply with magnetic applied field for H < 1 T and then saturates which shows quite typical ferromagnetic behavior. The saturation magnetization shifts to higher values with decreasing temperature. The values of the spontaneous magnetizations at T = 10 K were calculated by considering the total spins of Mn3+ and Mn4+ ions (SMn 3+ = 2, SMn 4+ = 1.5). The spontaneous magnetizations of La0.6−x x Ca0.4 MnO3 expressed as Msp = 2[2 × (0.6 − 3x) + 1.5 × (0.4 + 3x)] B/Mn where x is the content of lanthanum vacancy and B is the Bohr magneton. The measured spontaneous magnetizations at T = 10 K for x = 0.00, 0.05, 0,10 and 0.15 samples are found to be about 3.58 B/Mn , 3.3 B/Mn , 3.05 B/Mn and 2.7 B/Mn , respectively while the calculated values for full spin alignment are 3.6 B/Mn , 3.45 B/Mn , 3.3 B/Mn and 3.15 B/Mn respectively. The spontaneous magnetization decreases with increasing x and the small difference between the measured and calculated value especially for x = 0.15 could be explained by spin canted state at low temperature (Fig. 6). Fig. 7 shows the Arrott plots (M2 versus H/M) obtained from magnetization isotherms for our synthesized samples. As can be seen, Arrott plots above Tc show a linear behavior which indicates that a second order magnetic transition occurs [14].
I. Walha et al. / Journal of Alloys and Compounds 485 (2009) 64–68
Fig. 5. Magnetization evolution versus magnetic applied field at several temperatures for La0.55 0.05 Ca0.4 MnO3 and La0.45 0.15 Ca0.4 MnO3 samples.
67
Fig. 7. M2 versus H/M isotherms for La0.55 0.05 Ca0.4 MnO3 and La0.45 0.15 Ca0.4 MnO3 samples.
4. Conclusions In this work we have studied the effect of lanthanum deficiency on the structural and magnetic properties of La0.6 Ca0.4 MnO3 perovskite manganese oxide. The structural study shows that all our synthesized samples crystallize in the orthorhombic structure with Pnma space group. Lanthanum deficiency leads to a decrease of the Curie temperature with increasing lanthanum vacancy. The spontaneous magnetization value at T = 10 K is lower than the expected theoretical value and may be ascribed to the presence of a spin canted state. Acknowledgments This study has been supported by the Tunisian Ministry of Higher Education, Scientific Research and Technology. One of us (I. Walha) would like to thank the Deutscher Akademischer Austausch-Dienst (DAAD) for support. References
Fig. 6. The spontaneous magnetization experimental (Msp exp ) and calculated (Msp ca ) as a function of vacancy content for La0.6−x x Ca0.4 MnO3 (0.05 ≤ x ≤ 0.15) at T = 10 K.
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