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Large magnetic entropy change at room temperature in La0.7Ca0.3−xKxMnO3
Journal of Alloys and Compounds 442 (2007) 136–138
Large magnetic entropy change at room temperature in La0.7Ca0.3−xKxMnO3 M. Bejar a , R. Dhahri a , E. Dhahri a , M. Balli b , E.K. Hlil b,∗ a
b
Laboratoire de Physique Appliqu´ee, Facult´e des Sciences de Sfax, B.P. 802, Sfax 3018, Tunisie Laboratoire de Cristallographie, CNRS, 25 Avenue des Martyrs, B.P. 166, 38042, Grenoble-Cedex 9 France Received 15 May 2006; received in revised form 21 August 2006; accepted 15 October 2006 Available online 30 January 2007
Abstract Polycrystalline perovskite samples of La0.7 Ca0.3−x Kx MnO3 (0.05 ≤ x ≤ 0.10) have been prepared using the solid-state reaction. Detailed measurements of the magnetization as function of temperature and magnetic field for these samples were carried out. Significant magnetic-entropy changes (SMmax ) near the Curie temperature are obtanied from the magnetization data. The increase of the K concentration x is accompanied by a decrease of SMmax from 3.95 to 3.49 J/kg K for x = 0.05 and 0.10, respectively, with μ0 H = 2 T. For all samples, we find quite large values of SMmax , which are very close to that provided by Gd the prototypical magnetocaloric material. In view of theses results, the La0.7 Ca0.3−x Kx MnO3 compounds are strongly recommended to use as an active magnetic refrigerant for magnetic refrigerators. © 2007 Elsevier B.V. All rights reserved. PACS: 75.40.−s; 71.20.LP Keywords: Magnetocaloric; Oxide materials; Entropy
1. Introduction LaMnO3 , in which Mn is present as the high-spin Mn3+ (S = 3/2) is an antiferromagnetic oxide [1]. Substituting this compound with divalent (Ca2+ , Ba2+ ) or monovalent (Na+ , K+ ) ions causes the conversion of a proportional number of Mn3+ to Mn4+ . The Mn3+ /Mn4+ mixed valence state is responsible to ferromagnetism through the fast hopping of electrons between Mn3+ and Mn4+ ions. In a ferromagnetic material near its magnetic ordering temperature (called the Curie temperature TC ) when is placed in a magnetic field, the unpaired spins are aligned and the sample warms. On the other hand, when the field is suppressed, the spins tend to randomise and as the consequence the sample cooled down. This phenomenon is known as the magnetocaloric effect and is used for magnetic refrigeration, which presents an advantage over the conventional gas refrigeration [2], such as amity to the environment and higher efficiency. The principal requirement for such a refrigeration technique is the availability of a spin system whose ∗
Corresponding author. Tel.: +33 476 88 74 22; fax: +33 476 88 10 38. E-mail address: [email protected] (E.K. Hlil).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.170
magnetization strongly depends of both external magnetic field and temperature. Obviously, materials, which undergo a magnetic ordering transition, should present large magnetic entropy changes required for refrigeration. One of such systems is the perovskite manganese oxides La1−x Mx MnO3 (M = divalent ion, such as Ca2+ , Ba2+ , Sr2+ ), which have been largely studied in recent years [3–7]. The aim of this work was to study the effect of monovalent K+ ion substitution for divalent Ca2+ ion on the magnetocaloric properties of La0.7 Ca0.3−x Kx MnO3 compounds. 2. Experimental details Polycrystalline samples of La0.7 Ca0.3−x Kx MnO3 (x = 0.05, 0.075, 0.1) were synthesized by solid-state reaction. Stoichiometric amounts of La2 O3 , K2 CO3 , CaCO3 and MnO2 powders were mixed and then heated at 1173 K for 3 days. Then, the powder was pressed into pellets under 4 tonnes/cm2 and sintered at 1673 K for 1 day in air with several periods of grinding and repelleting. Finally, these pellets were rapidly quenched to room temperature. Identification of the phase and structural analysis were carried out by X-ray diffraction technique with ˚ by step scanning (0.02◦ ). The magnetization M Mo radiation (λ = 0.709362 A) of the samples as a function of the temperature T was measured using a Foner magnetometer (FON) equipped with a superconducting coil between 20 and 300 K and a torsion balance with an applied field μ0 H = 500 Oe for the range 300–420 K.0H
M. Bejar et al. / Journal of Alloys and Compounds 442 (2007) 136–138
Fig. 1. Isothermal magnetization for La0.7 Ca0.25 K0.05 MnO3 sample measured at different temperatures around TC .
3. Results and discussion The temperature and field dependence of the magnetization M (T, μ0 H) was measured for all samples. Fig. 1 shows isothermal magnetization curves for the La0.7 Ca0.25 K0.05 MnO3 sample, measured in the magnetic field range of 0–5 T and a temperature range of 240–300 K. The magnetic-entropy change SM induced by the variation of the magnetic field from 0 to HMax , can be numerically calculated using Eq. (1) H T1 + T 2 1 [ M(T2 , μ0 H)μ0 dH = SM 2 T2 − T 1 0 H − M(T1 , μ0 H)μ0 dH] (1) 0
It is clear from Eq. (1) that SM depends on the temperature gradient change of magnetization and grows up to a maximum value when the temperature approaches Curie temperature TC , and the magnetization decreases rapidly. Fig. 2 shows the temperature dependence of the magnetic-entropy change SM for different applied magnetic field change intervals for La0.7 Ca0.25 K0.05 MnO3 compound. It can be seen that both the magnetic-entropy change and the full-width at the halfmaximum of the SM versus T (␦Tfwhm ) depend on the applied magnetic filed, and at high fields it has the behaviour needed for an Ericsson-cycle magnetic refrigerator. In order to study the effect of the partial substitution of Ca by K, we have calculated the magnetic-entropy change SMmax and the relative cooling power RCP given by [8]:
137
Fig. 2. Temperature dependence of the magnetic-entropy change SM for different magnetic field intervals for La0.7 Ca0.25 K0.05 MnO3 sample.
used as magnetic refrigerants and in all samples the maximumentropy change SMmax is observed at Curie temperature TC . We can deduce from theses curves that the increase of x is accompanied by a reduction of SMmax and RCP values. Thus, due to the high SMmax and RCP, sample with x = 0.05 is the most promising compound to be used as magnetocaloric material. The magnetic Mn ions exist in both Mn3+ and Mn4+ valence states. In general, the large magnetic-entropy change in perovskite manganites originates mainly from the variation of the double-exchange interaction between the Mn3+ and Mn4+ ions arising from the change in the Mn3+ /Mn4+ ratio, under the doping process [9]. This interaction is strongest at the Mn3+ /Mn4+ ratio equal 7:3. With an increasing K content, the fraction of Mn4+ increases, which leads to a reduction of the Mn3+ /Mn4+ ratio. Consequently, the double-exchange interaction drops, which explains the reduction of the maximum magnetic-entropy change SMmax with increasing x.
RCP = − SM max δTfwhm where SMmax is the maximum-entropy change at TC . The temperature dependence of SM determined at an applied magnetic field of 2 T, for K doping concentration x = 0.05, 0.075 and 0.1, is plotted in Fig. 3. Theses curves reveal that all samples show large enough magnetocaloric effect to be
Fig. 3. Temperature dependence of the magnetic-entropy change SM, at the magnetic field change μ0 H = 2 T, for La0.7 Ca0.3−x Kx MnO3 samples, with x = 0.05, 0.075 and 0.10.
138
M. Bejar et al. / Journal of Alloys and Compounds 442 (2007) 136–138
Table 1 The reported values of magnetic-entropy change occurring at the Curie temperature under 0 H = 2 T, for several magnetic materials used as active refrigerants Material
TC (K)
μ0 H (T)
SMmax (J/kg K)
Reference
Gd La0.67 Ca0.33 MnO3 La0.67 Ba0.33 MnO3 La0.7 Ca0.25 K0.05 MnO3 La0.7 Ca0.225 K0.075 MnO3 La0.7 Ca0.20 K0.10 MnO3
299 260 350 270 281 272
2 2 2 2 2 2
4.20 1.90 1.72 3.95 3.75 3.49
[10] [11] [10] This work This work This work
For comparison, the values of the SMmax for several magnetic materials used as active refrigerants, determined at the applied magnetic field of 2 T are listed in Table 1. One can see that the SMmax values of our samples are very close to that of gadolinium, the prototypical magnetocaloric material. In comparison with Gd, perovskite-like structured materials are easier to fabricate, possess a higher stability and their Curie temperature can be easily adjusted by tuning the doping concentration. Thus, our compounds can be considered as active magnetic refrigerants for room-temperature applications. 4. Conclusion In summary, we have studied the magnetocaloric properties of La0.7 Ca0.3−x Kx MnO3 compounds, with 0.05 ≤ x ≤ 0.10.
The results show that La0.7 Ca0.3−x Kx MnO3 compounds can be strongly suggested to use as an active magnetic refrigerant near room temperature, namely: (i) a large enough magnetic-entropy change SMmax upon a low applied magnetic field, (ii) a well defined Curie temperature and (iii) a high relative cooling power RCP. References [1] E.O. Wollan, W.C. Koehler, Phys. Rev. 100 (1955) 545. [2] A.F. Lacaze, R. Beranger, G.B. Mardion, G. Claudet, A.A. Lacaze, Cryogenics 23 (1983) 427. [3] G.C. Lin, Q. Wie, J.X. Zhang, J. Magn. Magn. Mater. 300 (2006) 392. [4] M.H. Phan, S.B. Tian, D.Q. Hoang, S.C. Yu, C. Nguyen, A.N. Ulyanos, J. Magn. Magn. Mater. 258–259 (2005) 309. [5] Z.M. Wang, G. Ni, Q.Y. Xu, H. Sang, Y.W. Du, J. Appl. Phys. 90 (2001) 5689–5691. [6] M. Bejar, R. Dhahri, F. El Halouani, E. Dhahri, J. Alloys Compd. 414 (2006) 31–35. [7] M. Bejar, N. Sdiri, M. Hussein, S. Mazen, E. Dhahri, J. Magn. Magn. Mater., in press. [8] K.A. Gschneidner, V.K. Pecharsky, Annu. Rev. Mater. Sci. 30 (2000) 387. [9] Z.B. Guo, Y.M. Du, J.S. Zhu, H. Huang, W.P. Ding, D. Feng, Phys. Rev. Lett. 78 (1997) 1142. [10] Y. Xu., M. Meier, P. Das, M.R. Koblischka, U. Hartmann, Crystal. Eng. 5 (2002) 383–389. [11] X.X. Zhang, J. Tejada, Y. Xin, G.F. Sun, K.W. Wong, X. Bohigas, Appl. Phys. Lett. 69 (1996) 3596.
Large magnetic entropy change at room temperature in La0.7Ca0.3−xKxMnO3 M. Bejar a , R. Dhahri a , E. Dhahri a , M. Balli b , E.K. Hlil b,∗ a
b
Laboratoire de Physique Appliqu´ee, Facult´e des Sciences de Sfax, B.P. 802, Sfax 3018, Tunisie Laboratoire de Cristallographie, CNRS, 25 Avenue des Martyrs, B.P. 166, 38042, Grenoble-Cedex 9 France Received 15 May 2006; received in revised form 21 August 2006; accepted 15 October 2006 Available online 30 January 2007
Abstract Polycrystalline perovskite samples of La0.7 Ca0.3−x Kx MnO3 (0.05 ≤ x ≤ 0.10) have been prepared using the solid-state reaction. Detailed measurements of the magnetization as function of temperature and magnetic field for these samples were carried out. Significant magnetic-entropy changes (SMmax ) near the Curie temperature are obtanied from the magnetization data. The increase of the K concentration x is accompanied by a decrease of SMmax from 3.95 to 3.49 J/kg K for x = 0.05 and 0.10, respectively, with μ0 H = 2 T. For all samples, we find quite large values of SMmax , which are very close to that provided by Gd the prototypical magnetocaloric material. In view of theses results, the La0.7 Ca0.3−x Kx MnO3 compounds are strongly recommended to use as an active magnetic refrigerant for magnetic refrigerators. © 2007 Elsevier B.V. All rights reserved. PACS: 75.40.−s; 71.20.LP Keywords: Magnetocaloric; Oxide materials; Entropy
1. Introduction LaMnO3 , in which Mn is present as the high-spin Mn3+ (S = 3/2) is an antiferromagnetic oxide [1]. Substituting this compound with divalent (Ca2+ , Ba2+ ) or monovalent (Na+ , K+ ) ions causes the conversion of a proportional number of Mn3+ to Mn4+ . The Mn3+ /Mn4+ mixed valence state is responsible to ferromagnetism through the fast hopping of electrons between Mn3+ and Mn4+ ions. In a ferromagnetic material near its magnetic ordering temperature (called the Curie temperature TC ) when is placed in a magnetic field, the unpaired spins are aligned and the sample warms. On the other hand, when the field is suppressed, the spins tend to randomise and as the consequence the sample cooled down. This phenomenon is known as the magnetocaloric effect and is used for magnetic refrigeration, which presents an advantage over the conventional gas refrigeration [2], such as amity to the environment and higher efficiency. The principal requirement for such a refrigeration technique is the availability of a spin system whose ∗
Corresponding author. Tel.: +33 476 88 74 22; fax: +33 476 88 10 38. E-mail address: [email protected] (E.K. Hlil).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.170
magnetization strongly depends of both external magnetic field and temperature. Obviously, materials, which undergo a magnetic ordering transition, should present large magnetic entropy changes required for refrigeration. One of such systems is the perovskite manganese oxides La1−x Mx MnO3 (M = divalent ion, such as Ca2+ , Ba2+ , Sr2+ ), which have been largely studied in recent years [3–7]. The aim of this work was to study the effect of monovalent K+ ion substitution for divalent Ca2+ ion on the magnetocaloric properties of La0.7 Ca0.3−x Kx MnO3 compounds. 2. Experimental details Polycrystalline samples of La0.7 Ca0.3−x Kx MnO3 (x = 0.05, 0.075, 0.1) were synthesized by solid-state reaction. Stoichiometric amounts of La2 O3 , K2 CO3 , CaCO3 and MnO2 powders were mixed and then heated at 1173 K for 3 days. Then, the powder was pressed into pellets under 4 tonnes/cm2 and sintered at 1673 K for 1 day in air with several periods of grinding and repelleting. Finally, these pellets were rapidly quenched to room temperature. Identification of the phase and structural analysis were carried out by X-ray diffraction technique with ˚ by step scanning (0.02◦ ). The magnetization M Mo radiation (λ = 0.709362 A) of the samples as a function of the temperature T was measured using a Foner magnetometer (FON) equipped with a superconducting coil between 20 and 300 K and a torsion balance with an applied field μ0 H = 500 Oe for the range 300–420 K.0H
M. Bejar et al. / Journal of Alloys and Compounds 442 (2007) 136–138
Fig. 1. Isothermal magnetization for La0.7 Ca0.25 K0.05 MnO3 sample measured at different temperatures around TC .
3. Results and discussion The temperature and field dependence of the magnetization M (T, μ0 H) was measured for all samples. Fig. 1 shows isothermal magnetization curves for the La0.7 Ca0.25 K0.05 MnO3 sample, measured in the magnetic field range of 0–5 T and a temperature range of 240–300 K. The magnetic-entropy change SM induced by the variation of the magnetic field from 0 to HMax , can be numerically calculated using Eq. (1) H T1 + T 2 1 [ M(T2 , μ0 H)μ0 dH = SM 2 T2 − T 1 0 H − M(T1 , μ0 H)μ0 dH] (1) 0
It is clear from Eq. (1) that SM depends on the temperature gradient change of magnetization and grows up to a maximum value when the temperature approaches Curie temperature TC , and the magnetization decreases rapidly. Fig. 2 shows the temperature dependence of the magnetic-entropy change SM for different applied magnetic field change intervals for La0.7 Ca0.25 K0.05 MnO3 compound. It can be seen that both the magnetic-entropy change and the full-width at the halfmaximum of the SM versus T (␦Tfwhm ) depend on the applied magnetic filed, and at high fields it has the behaviour needed for an Ericsson-cycle magnetic refrigerator. In order to study the effect of the partial substitution of Ca by K, we have calculated the magnetic-entropy change SMmax and the relative cooling power RCP given by [8]:
137
Fig. 2. Temperature dependence of the magnetic-entropy change SM for different magnetic field intervals for La0.7 Ca0.25 K0.05 MnO3 sample.
used as magnetic refrigerants and in all samples the maximumentropy change SMmax is observed at Curie temperature TC . We can deduce from theses curves that the increase of x is accompanied by a reduction of SMmax and RCP values. Thus, due to the high SMmax and RCP, sample with x = 0.05 is the most promising compound to be used as magnetocaloric material. The magnetic Mn ions exist in both Mn3+ and Mn4+ valence states. In general, the large magnetic-entropy change in perovskite manganites originates mainly from the variation of the double-exchange interaction between the Mn3+ and Mn4+ ions arising from the change in the Mn3+ /Mn4+ ratio, under the doping process [9]. This interaction is strongest at the Mn3+ /Mn4+ ratio equal 7:3. With an increasing K content, the fraction of Mn4+ increases, which leads to a reduction of the Mn3+ /Mn4+ ratio. Consequently, the double-exchange interaction drops, which explains the reduction of the maximum magnetic-entropy change SMmax with increasing x.
RCP = − SM max δTfwhm where SMmax is the maximum-entropy change at TC . The temperature dependence of SM determined at an applied magnetic field of 2 T, for K doping concentration x = 0.05, 0.075 and 0.1, is plotted in Fig. 3. Theses curves reveal that all samples show large enough magnetocaloric effect to be
Fig. 3. Temperature dependence of the magnetic-entropy change SM, at the magnetic field change μ0 H = 2 T, for La0.7 Ca0.3−x Kx MnO3 samples, with x = 0.05, 0.075 and 0.10.
138
M. Bejar et al. / Journal of Alloys and Compounds 442 (2007) 136–138
Table 1 The reported values of magnetic-entropy change occurring at the Curie temperature under 0 H = 2 T, for several magnetic materials used as active refrigerants Material
TC (K)
μ0 H (T)
SMmax (J/kg K)
Reference
Gd La0.67 Ca0.33 MnO3 La0.67 Ba0.33 MnO3 La0.7 Ca0.25 K0.05 MnO3 La0.7 Ca0.225 K0.075 MnO3 La0.7 Ca0.20 K0.10 MnO3
299 260 350 270 281 272
2 2 2 2 2 2
4.20 1.90 1.72 3.95 3.75 3.49
[10] [11] [10] This work This work This work
For comparison, the values of the SMmax for several magnetic materials used as active refrigerants, determined at the applied magnetic field of 2 T are listed in Table 1. One can see that the SMmax values of our samples are very close to that of gadolinium, the prototypical magnetocaloric material. In comparison with Gd, perovskite-like structured materials are easier to fabricate, possess a higher stability and their Curie temperature can be easily adjusted by tuning the doping concentration. Thus, our compounds can be considered as active magnetic refrigerants for room-temperature applications. 4. Conclusion In summary, we have studied the magnetocaloric properties of La0.7 Ca0.3−x Kx MnO3 compounds, with 0.05 ≤ x ≤ 0.10.
The results show that La0.7 Ca0.3−x Kx MnO3 compounds can be strongly suggested to use as an active magnetic refrigerant near room temperature, namely: (i) a large enough magnetic-entropy change SMmax upon a low applied magnetic field, (ii) a well defined Curie temperature and (iii) a high relative cooling power RCP. References [1] E.O. Wollan, W.C. Koehler, Phys. Rev. 100 (1955) 545. [2] A.F. Lacaze, R. Beranger, G.B. Mardion, G. Claudet, A.A. Lacaze, Cryogenics 23 (1983) 427. [3] G.C. Lin, Q. Wie, J.X. Zhang, J. Magn. Magn. Mater. 300 (2006) 392. [4] M.H. Phan, S.B. Tian, D.Q. Hoang, S.C. Yu, C. Nguyen, A.N. Ulyanos, J. Magn. Magn. Mater. 258–259 (2005) 309. [5] Z.M. Wang, G. Ni, Q.Y. Xu, H. Sang, Y.W. Du, J. Appl. Phys. 90 (2001) 5689–5691. [6] M. Bejar, R. Dhahri, F. El Halouani, E. Dhahri, J. Alloys Compd. 414 (2006) 31–35. [7] M. Bejar, N. Sdiri, M. Hussein, S. Mazen, E. Dhahri, J. Magn. Magn. Mater., in press. [8] K.A. Gschneidner, V.K. Pecharsky, Annu. Rev. Mater. Sci. 30 (2000) 387. [9] Z.B. Guo, Y.M. Du, J.S. Zhu, H. Huang, W.P. Ding, D. Feng, Phys. Rev. Lett. 78 (1997) 1142. [10] Y. Xu., M. Meier, P. Das, M.R. Koblischka, U. Hartmann, Crystal. Eng. 5 (2002) 383–389. [11] X.X. Zhang, J. Tejada, Y. Xin, G.F. Sun, K.W. Wong, X. Bohigas, Appl. Phys. Lett. 69 (1996) 3596.