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Magnetic properties and magnetocaloric effect in metamagnetic RE2Cu2O5 (RE = Dy and Ho) cuprates
Journal of Alloys and Compounds 658 (2016) 500e504
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Magnetic properties and magnetocaloric effect in metamagnetic RE2Cu2O5 (RE ¼ Dy and Ho) cuprates Lingwei Li a, b, c, *, Jing Wang c, Kunpeng Su c, Dexuan Huo c, Yang Qi b a
Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China Institute of Materials Physics and Chemistry, Colleague of Materials Science and Engineering, Northeastern University, Shenyang 110819, China c Institute of Materials Physics, Hangzhou Dianzi University, Hangzhou 310018, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 29 October 2015 Accepted 30 October 2015 Available online 4 November 2015
The magnetic and magnetocaloric properties in Dy2Cu2O5 and Ho2Cu2O5 cuprates have been investigated. The RE2Cu2O5 compounds show a typical paramagnetic (PM) to antiferromagnetic (AFM) transition under low field around 10.3 and 13.5 K for RE ¼ Dy and Ho, respectively. And a field-induced metamagnetic transition from AFM to ferromagnetic (FM) phase was happened with the application of higher magnetic field for both compounds. An inverse MCE under low magnetic field change and at low temperatures together with a normal reversible MCE under high magnetic field change was observed for the present RE2Cu2O5 compounds, which is related to the fact of the AFM state and the fieldinduced first order metamagnetic transition from AFM to FM state, respectively. The maximum values of magnetic entropy change ðDSM max Þ are 12.0 and 12.4 J/kg K under a field change of 0e7 T for Dy2Cu2O5 and Ho2Cu2O5, with the values of the relative cooling power (RCP) of 273 and 256 J/kg, respectively. © 2015 Elsevier B.V. All rights reserved.
Keywords: RE2Cu2O5 (RE ¼ Dy and Ho) cuprates Magnetocaloric effect Magnetic phase transition Metamagnetism
1. Introduction The magnetocaloric effect (MCE) in various materials has been extensively investigated during the last thirty years, not only due to their potential applications for active magnetic refrigeration but also for further understanding the fundamental properties of these materials [1e13]. MCE is an intrinsic thermal response for all the magnetic materials when applying or removing a magnetic field, which manifests as the magnetic entropy change in an isothermal process, DSM, or/and the temperature change in an adiabatic process, DTad. Magnetic refrigeration technology based on the MCE has many advantages, such as more environmental friendly, higher energy efficiency as compared to the conventional gas compression/expansion refrigeration technology [1e4]. From the viewpoint of applications, search for magnetic materials with large MCE is an important requirement. In recent years, the MCE in some rare earth transition metal oxides have also been carried out, and some of them are found to possess interesting magnetocaloric properties. For examples, a giant MCE has been observed in zircon-type DyCrO4 and HoCrO4 [9]. The HoVO3 orthovanadate undergoes a
large negative and conventional MCE around 4 and 15 K, respectively [10]. Large rotating magnetic entropy change, together with large refrigeration capacity and negligible hysteresis in DyFeO3 single crystal has been observed [11]. Very recently, Mo et al. found a giant low field reversible together with large refrigerant capacity in EuTi1-xCrxO3 compounds, indicating that these compounds are promising candidates for low temperature magnetic refrigeration [12,13]. In the ternary rare earth RE-Cu-O phase diagram, the series with the stoichiometry RE2Cu2O5 arises when RE is smaller than Gd. The crystal structure and some physical properties for RE2Cu2O5 have been reported [14e18]. Magnetic studies reveals that the RE2Cu2O5 cuprates ordered antiferromagnetically below 17.7, 9.5, 12.3, 25, 16.7, 17.3, and 10.5 K for RE ¼ Tb, Dy, Ho, Er, Tm, Lu, and Y, respectively [14e18]. In the present study, we further investigated the magnetic and magnetocaloric properties in the Dy2Cu2O5 and Ho2Cu2O5 cuprates. A field induced metamagnetic transition together with a normal and inverse MCE has been observed in both compounds.
2. Experimental * Corresponding author. Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China. E-mail address: [email protected] (L. Li). http://dx.doi.org/10.1016/j.jallcom.2015.10.289 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Polycrystalline samples of Dy2Cu2O5 and Ho2Cu2O5 were prepared by solidestate reaction method from dried high-purity
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Dy2O3, Ho2O3, and Cu2O powders. The powders with the stoichiometric composition were mixed, grinding thoroughly and calcined at 900 C for 24 h in air with intermediate grinding. The products were pressed into pellets and sintered at 915 C for 48 h. The final heat treatment of the sintered precursors was at 930 C for 48 h. Both samples were proved to be a single phase by X-ray powder diffraction (XRD), and the lattice parameters a, b, and c were calculated to be 10.83, 3.512, and 12.45 Å for Dy2Cu2O5; and to be 10.81, 3.493, and 12.46 Å for Ho2Cu2O5, respectively. The magnetization measurements were performed with a commercial vibrating sample magnetometer (VSM) which is an option of physical property measurement system (PPMS-9, Quantum Design).
3. Results and discussion The temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetization (M) under the magnetic fields of 0.2 T for Dy2Cu2O5 and Ho2Cu2O5 are shown in Fig. 1(a) and (b), respectively. No difference can be observed in ZFC and FC MeT curves for both compounds, indicating no thermal hysteresis. The MeT curves of RE2Cu2O5 exhibits a maximum around 10.3 and 13.5 K for RE ¼ Dy and Ho, respectively, which is a typical characteristic of paramagnetic (PM) to antiferromagnetic (AFM) transition, being consistent with previously reported results [14,17]. A series of temperature dependence of magnetization under different external field from 0.5 to 3 T for Dy2Cu2O5 and Ho2Cu2O5 are measured, and the results are shown in Fig. 2(a) and (b), respectively. Both compounds show a similar behaviour, i. e. AFM ordering for H 1.5 T, whereas ferromagnetic (FM) or FM-like magnetic ordering for H 2 T. The temperature dependence of the magnetization M (left scale) and the reciprocal susceptibility 1/c (right
Fig. 1. Temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetization (M) under the magnetic fields of 0.2 T for (a) Dy2Cu2O5 and (b) Ho2Cu2O5.
Fig. 2. Temperature dependence of magnetization (M) under different external field from 0.5 to 3 T for (a) Dy2Cu2O5 and (b) Ho2Cu2O5.
Fig. 3. Temperature dependence of the magnetization M (left scale) and the reciprocal susceptibility 1/c (right scale) under a magnetic field H ¼ 1 T for Dy2Cu2O5 and Ho2Cu2O5.
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scale) under a magnetic field H ¼ 1 T for Dy2Cu2O5 and Ho2Cu2O5 are shown in Fig. 3(a) and (b), respectively. The high temperature reciprocal susceptibility for both compounds shows CurieeWeiss behaviour. The values of the effective magnetic moments meff were determined to be 15.83 and 15.62 mB per formula unit (11.19 and 11.05 mB/RE atom) for Dy2Cu2O5 and Ho2Cu2O5 which are obviously larger than that of free rare earth ion values of Dy3þ (10.64 mB) and Ho3þ (10.61 mB), respectively, which was due to the contribution of the magnetic ordering from the CueCu sublattice [14e16]. The corresponding values of the paramagnetic Curie temperature qp were obtained to be 15.6 and 11.8 K. The negative values of qp indicate antiferromagnetic ground state for both compounds. Fig. 4 displays the magnetization isotherms M(H) for Dy2Cu2O5 and Ho2Cu2O5 with increasing and decreasing field at 3 K. Both compounds show quite similar behaviour, the magnetization increases linearly at low magnetic field, and a steep jump can be observed around 1.5 T indicating a field-induced metamagnetic transition from AFM to FM phase was happened with the application of higher magnetic field, and then increases slightly with further increasing magnetic field and does not saturate up to 7 T. The magnetization at 3 K and 7 T is derived to be 11.02 and 11.1 mB per formula unit (5.51 and 5.55 mB/RE atom) for Dy2Cu2O5 and Ho2Cu2O5 which are obviously smaller than that of the free-ion moment of Dy3þ (10 mB) and Ho3þ (10 mB), respectively. This discrepancy is not only due to crystal-field effect but also probably related to the magnetic ordering from the CueCu sublattice and the magnetic moment of Cu is canted to the RE moment [14e16]. Additionally, the MH curves for increasing and decreasing field are identical, i.e., no field hysteresis together with any thermal hysteresis can be observed for both compounds which are favourable for magnetocaloric materials. To evaluate the magnetocaloric effect in Dy2Cu2O5 and Ho2Cu2O5, sets of magnetic isothermal M(H) curves were measured. Several M(H) curves of RE2Cu2O5 are shown in Fig. 5(a) and (b) for RE ¼ Dy and Ho, respectively. It is well known that the MCE have a strong correlation with the order of the corresponding
Fig. 4. Magnetization isotherms M(H) for (a) Dy2Cu2O5 and (b) Ho2Cu2O5 with increasing and decreasing field at 3 K.
Fig. 5. Magnetic field dependence of the magnetization M(H) curves for (a) Dy2Cu2O5 and (b) Ho2Cu2O5 at some selected temperatures.
magnetic phase transition, thus it is important to understand the nature of magnetic transition in RE2Cu2O5 compounds. According to the Banerjee criterion [19], the magnetic transition is of a first order if some of the H/M versus M2 curves (also named as Arrott plot) show negative slope at some point positive slope. On the other hand, if all the H/M versus M2 curves are positive, the magnetic transition is of the second order. To determine the order of magnetic phase transition of RE2Cu2O5, the measured M-H isotherms were converted in to H/M versus M2 plot, and shown in Fig. 6(a) and (b) for RE ¼ Dy and Ho, respectively. Clearly negative slopes can be observed in the Arrot plot for both compounds, indicating the occurrence of first order magnetic transition for Dy2Cu2O5 and Ho2Cu2O5. Fig. 7 shows the temperature dependence of magnetic entropy change DSM for Dy2Cu2O5 and Ho2Cu2O5 which was calculated from the temperature and field dependence of the magnetization M Z Hmax (H, T) by using an integral version of Maxwell's thermodynamic relation, DSM ðT; DHÞ ¼ ðvMðH; TÞ=vT=ÞH dH. It can be seen 0 that the temperature variation of DSM are almost the same for both compounds except for some differences in the peak value and position. Under low magnetic field changes, the values of DSM are negative at low temperature, and they change their signs and become positive at higher temperatures, giving rise to a distinguishable minimum and a maximum for both compounds. As it is well known, for a conventional ferromagnet, DSM is negative and it is termed as a normal MCE. On the other hand, there are some substances that exhibit inverse MCE, i.e., DSM is positive. Generally, inverse MCE occurs in antiferromagnetic materials, in which the applied magnetic field increases the magnetic entropy. Similar behaviours were also observed in recently reported TbCo2B2 [20],
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Fig. 6. The plots of H/M versus M2 for (a) Dy2Cu2O5 and (b) Ho2Cu2O5 at some selected temperatures.
ErNi2B2C [21] and HoVO3 [10] compounds. The observed inverse MCE in present RE2Cu2O5 is also attributed to the fact of the AFM state under low field and at low temperatures and the normal MCE under high magnetic field change is related to the field e induced metamagnetic transition from AFM to FM state. For a magnetic field change of 0e2 T, the minimum and maximum values of DSM are evaluated to be 4.5 and 1.3 J/kg K for Dy2Cu2O5, and to be 3.6 and 2.4 J/kg K for Ho2Cu2O5, respectively. For the field changes of 0e5 and 0e7 T, the maximum values of magnetic entropy change ðDSM max Þ are evaluated to be 7.5 and 12.0 J/kg K for Dy2Cu2O5; and to be 9.2 and 12.4 J/kg K for Ho2Cu2O5, respectively. Another important quality factor of the refrigerant materials is the relative cooling power (RCP) which is a measure of the amount of heat transfer between the cold and hot reservoirs in an ideal refrigeration cycle. The RCP is defined as the product of the maximum magnetic entropy change DSM max and full width at half maximum in DSM (T) curve dTFWHM. For the field changes of 0e5 and 0e7 T, the values of RCP are determined to be 154 and 273 J/kg; and to be 149 and 256 J/kg for Dy2Cu2O5 and Ho2Cu2O5, respectively. The present results may provide some clue and new data for searching novel magnetic materials for magnetic refrigeration.
4. Conclusions In summary, two single phased cuprates of Dy2Cu2O5 and Ho2Cu2O5 were synthesized and the magnetic properties and MCE have been investigated. For both compounds, a paramagnetic to antiferromagnetic (PMeAFM) magnetic transition was observed under low magnetic fields, whereas a field-induced metamagnetic transition from antiferromagnetic to ferromagnetic (AFMeFM) has
Fig. 7. Temperature dependence of magnetic entropy change DSM for (a) Dy2Cu2O5 and (b) Ho2Cu2O5.
been observed with the application of higher magnetic fields. Both compounds exhibit an inverse and normal MCE for different magnetic field changes and at different temperature. The observed inverse MCE is attributed to the fact of AFM state under low field and at low temperatures for RE2Cu2O5, the reversible normal MCE under higher magnetic field change is related to the first order metamagnetic transition from AFM to FM state. For a field change of 0e5 T, the maximum values of magnetic entropy change ðDSM max Þ reach 7.5 and 9.2 J/kg K for Dy2Cu2O5 and Ho2Cu2O5, respectively. The corresponding values of relative cooling power (RCP) are evaluated to be 154 and 149 J/kg. Acknowledgements This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 11374081 and 11574066), and the Fundamental Research Funds for the Central Universities (Grant Nos. N140901001 and L1509006). References [1] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [2] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Tsoko, Rep. Prog. Phys. 68 (2005) 1479. [3] N.A. de Oliveira, P.J. von Ranke, Phys. Rep. 489 (2010) 89.
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[4] V. Franco, J.S. Blazquez, B. Ingale, A. Conde, Annu. Rev. Mater. Res. 42 (2012) 305. [5] Z.H. Huang, X. Luo, L. Hu, S.G. Tan, Y. Liu, B. Yuan, J. Chen, W.H. Song, Y.P. Sun, J. Appl. Phys. 115 (2014) 034903. €ttgen, S. Zhou, Appl. [6] L.W. Li, Y. Yuan, Y. Zhang, T. Namiki, K. Nishimura, R. Po Phys. Lett. 107 (2015) 132401. [7] Z.J. Mo, J. Shen, L.Q. Yan, C.C. Tang, L.C. Wang, J.F. Wu, J.R. Sun, B.G. Shen, Intermetallics 56 (2015) 75. [8] Y.K. Zhang, B. Yang, G. Wilde, J. Alloys Compd. 619 (2015) 12. [9] A. Midya, N. Khan, D. Bhoi, P. Mandal, Appl. Phys. Lett. 103 (2013) 092402. [10] M. Balli, B. Roberge, S. Jandl, P. Fournier, T.T.M. Palstra, A.A. Nugroho, J. Appl. Phys. 118 (2015) 073903. [11] Y.J. Ke, X.Q. Zhang, H. Ge, Y. Ma, Z.H. Cheng, Chin. Phys. B 24 (2015) 037501. [12] Z.J. Mo, Z.H. Hao, J. Shen, L. Li, J.F. Wu, F.X. Hu, J.R. Sun, B.G. Shen, J. Alloys
Compd. 649 (2015) 674. [13] Z.J. Mo, J. Shen, L. Li, Y. Liu, C.C. Tang, F.X. Hu, J.R. Sun, B.G. Shen, Mater. Lett. 158 (2015) 282. [14] I.V. Golosovsky, V.P. Plakhty, V.P. Harchenkov, S.V. Sharigin, J. Schweizer, J. Magn. Magn. Mater. 129 (1994) 233. [15] J.L.G. Munoz, X. Obradors, J.R. Carvajal, Phys. B 194e196 (1994) 277. [16] J.L.G. Munoz, X. Obradors, J.R. Carvajal, Phys. Rev. B 51 (1995) 6594. [17] V.V. Moshchalkov, N.A. Samarin, I.O. Grishchenko, B.V. Mill, Y. Zoubkova, J. Magn. Magn. Mater. 90&91 (1990) 533. [18] I.V. Paukov, M.N. Popova, J. Klamut, Phys. Lett. A 189 (1994) 103. [19] S.K. Banerjee, Phys. Lett. 12 (1964) 16. [20] L. Li, K. Nishimura, G. Usui, D. Huo, Z. Qian, Intermetallics 23 (2012) 101. [21] Y.K. Zhang, B. Yang, J. Alloys Compd. 610 (2014) 540.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Magnetic properties and magnetocaloric effect in metamagnetic RE2Cu2O5 (RE ¼ Dy and Ho) cuprates Lingwei Li a, b, c, *, Jing Wang c, Kunpeng Su c, Dexuan Huo c, Yang Qi b a
Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China Institute of Materials Physics and Chemistry, Colleague of Materials Science and Engineering, Northeastern University, Shenyang 110819, China c Institute of Materials Physics, Hangzhou Dianzi University, Hangzhou 310018, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 29 October 2015 Accepted 30 October 2015 Available online 4 November 2015
The magnetic and magnetocaloric properties in Dy2Cu2O5 and Ho2Cu2O5 cuprates have been investigated. The RE2Cu2O5 compounds show a typical paramagnetic (PM) to antiferromagnetic (AFM) transition under low field around 10.3 and 13.5 K for RE ¼ Dy and Ho, respectively. And a field-induced metamagnetic transition from AFM to ferromagnetic (FM) phase was happened with the application of higher magnetic field for both compounds. An inverse MCE under low magnetic field change and at low temperatures together with a normal reversible MCE under high magnetic field change was observed for the present RE2Cu2O5 compounds, which is related to the fact of the AFM state and the fieldinduced first order metamagnetic transition from AFM to FM state, respectively. The maximum values of magnetic entropy change ðDSM max Þ are 12.0 and 12.4 J/kg K under a field change of 0e7 T for Dy2Cu2O5 and Ho2Cu2O5, with the values of the relative cooling power (RCP) of 273 and 256 J/kg, respectively. © 2015 Elsevier B.V. All rights reserved.
Keywords: RE2Cu2O5 (RE ¼ Dy and Ho) cuprates Magnetocaloric effect Magnetic phase transition Metamagnetism
1. Introduction The magnetocaloric effect (MCE) in various materials has been extensively investigated during the last thirty years, not only due to their potential applications for active magnetic refrigeration but also for further understanding the fundamental properties of these materials [1e13]. MCE is an intrinsic thermal response for all the magnetic materials when applying or removing a magnetic field, which manifests as the magnetic entropy change in an isothermal process, DSM, or/and the temperature change in an adiabatic process, DTad. Magnetic refrigeration technology based on the MCE has many advantages, such as more environmental friendly, higher energy efficiency as compared to the conventional gas compression/expansion refrigeration technology [1e4]. From the viewpoint of applications, search for magnetic materials with large MCE is an important requirement. In recent years, the MCE in some rare earth transition metal oxides have also been carried out, and some of them are found to possess interesting magnetocaloric properties. For examples, a giant MCE has been observed in zircon-type DyCrO4 and HoCrO4 [9]. The HoVO3 orthovanadate undergoes a
large negative and conventional MCE around 4 and 15 K, respectively [10]. Large rotating magnetic entropy change, together with large refrigeration capacity and negligible hysteresis in DyFeO3 single crystal has been observed [11]. Very recently, Mo et al. found a giant low field reversible together with large refrigerant capacity in EuTi1-xCrxO3 compounds, indicating that these compounds are promising candidates for low temperature magnetic refrigeration [12,13]. In the ternary rare earth RE-Cu-O phase diagram, the series with the stoichiometry RE2Cu2O5 arises when RE is smaller than Gd. The crystal structure and some physical properties for RE2Cu2O5 have been reported [14e18]. Magnetic studies reveals that the RE2Cu2O5 cuprates ordered antiferromagnetically below 17.7, 9.5, 12.3, 25, 16.7, 17.3, and 10.5 K for RE ¼ Tb, Dy, Ho, Er, Tm, Lu, and Y, respectively [14e18]. In the present study, we further investigated the magnetic and magnetocaloric properties in the Dy2Cu2O5 and Ho2Cu2O5 cuprates. A field induced metamagnetic transition together with a normal and inverse MCE has been observed in both compounds.
2. Experimental * Corresponding author. Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China. E-mail address: [email protected] (L. Li). http://dx.doi.org/10.1016/j.jallcom.2015.10.289 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Polycrystalline samples of Dy2Cu2O5 and Ho2Cu2O5 were prepared by solidestate reaction method from dried high-purity
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501
Dy2O3, Ho2O3, and Cu2O powders. The powders with the stoichiometric composition were mixed, grinding thoroughly and calcined at 900 C for 24 h in air with intermediate grinding. The products were pressed into pellets and sintered at 915 C for 48 h. The final heat treatment of the sintered precursors was at 930 C for 48 h. Both samples were proved to be a single phase by X-ray powder diffraction (XRD), and the lattice parameters a, b, and c were calculated to be 10.83, 3.512, and 12.45 Å for Dy2Cu2O5; and to be 10.81, 3.493, and 12.46 Å for Ho2Cu2O5, respectively. The magnetization measurements were performed with a commercial vibrating sample magnetometer (VSM) which is an option of physical property measurement system (PPMS-9, Quantum Design).
3. Results and discussion The temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetization (M) under the magnetic fields of 0.2 T for Dy2Cu2O5 and Ho2Cu2O5 are shown in Fig. 1(a) and (b), respectively. No difference can be observed in ZFC and FC MeT curves for both compounds, indicating no thermal hysteresis. The MeT curves of RE2Cu2O5 exhibits a maximum around 10.3 and 13.5 K for RE ¼ Dy and Ho, respectively, which is a typical characteristic of paramagnetic (PM) to antiferromagnetic (AFM) transition, being consistent with previously reported results [14,17]. A series of temperature dependence of magnetization under different external field from 0.5 to 3 T for Dy2Cu2O5 and Ho2Cu2O5 are measured, and the results are shown in Fig. 2(a) and (b), respectively. Both compounds show a similar behaviour, i. e. AFM ordering for H 1.5 T, whereas ferromagnetic (FM) or FM-like magnetic ordering for H 2 T. The temperature dependence of the magnetization M (left scale) and the reciprocal susceptibility 1/c (right
Fig. 1. Temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetization (M) under the magnetic fields of 0.2 T for (a) Dy2Cu2O5 and (b) Ho2Cu2O5.
Fig. 2. Temperature dependence of magnetization (M) under different external field from 0.5 to 3 T for (a) Dy2Cu2O5 and (b) Ho2Cu2O5.
Fig. 3. Temperature dependence of the magnetization M (left scale) and the reciprocal susceptibility 1/c (right scale) under a magnetic field H ¼ 1 T for Dy2Cu2O5 and Ho2Cu2O5.
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scale) under a magnetic field H ¼ 1 T for Dy2Cu2O5 and Ho2Cu2O5 are shown in Fig. 3(a) and (b), respectively. The high temperature reciprocal susceptibility for both compounds shows CurieeWeiss behaviour. The values of the effective magnetic moments meff were determined to be 15.83 and 15.62 mB per formula unit (11.19 and 11.05 mB/RE atom) for Dy2Cu2O5 and Ho2Cu2O5 which are obviously larger than that of free rare earth ion values of Dy3þ (10.64 mB) and Ho3þ (10.61 mB), respectively, which was due to the contribution of the magnetic ordering from the CueCu sublattice [14e16]. The corresponding values of the paramagnetic Curie temperature qp were obtained to be 15.6 and 11.8 K. The negative values of qp indicate antiferromagnetic ground state for both compounds. Fig. 4 displays the magnetization isotherms M(H) for Dy2Cu2O5 and Ho2Cu2O5 with increasing and decreasing field at 3 K. Both compounds show quite similar behaviour, the magnetization increases linearly at low magnetic field, and a steep jump can be observed around 1.5 T indicating a field-induced metamagnetic transition from AFM to FM phase was happened with the application of higher magnetic field, and then increases slightly with further increasing magnetic field and does not saturate up to 7 T. The magnetization at 3 K and 7 T is derived to be 11.02 and 11.1 mB per formula unit (5.51 and 5.55 mB/RE atom) for Dy2Cu2O5 and Ho2Cu2O5 which are obviously smaller than that of the free-ion moment of Dy3þ (10 mB) and Ho3þ (10 mB), respectively. This discrepancy is not only due to crystal-field effect but also probably related to the magnetic ordering from the CueCu sublattice and the magnetic moment of Cu is canted to the RE moment [14e16]. Additionally, the MH curves for increasing and decreasing field are identical, i.e., no field hysteresis together with any thermal hysteresis can be observed for both compounds which are favourable for magnetocaloric materials. To evaluate the magnetocaloric effect in Dy2Cu2O5 and Ho2Cu2O5, sets of magnetic isothermal M(H) curves were measured. Several M(H) curves of RE2Cu2O5 are shown in Fig. 5(a) and (b) for RE ¼ Dy and Ho, respectively. It is well known that the MCE have a strong correlation with the order of the corresponding
Fig. 4. Magnetization isotherms M(H) for (a) Dy2Cu2O5 and (b) Ho2Cu2O5 with increasing and decreasing field at 3 K.
Fig. 5. Magnetic field dependence of the magnetization M(H) curves for (a) Dy2Cu2O5 and (b) Ho2Cu2O5 at some selected temperatures.
magnetic phase transition, thus it is important to understand the nature of magnetic transition in RE2Cu2O5 compounds. According to the Banerjee criterion [19], the magnetic transition is of a first order if some of the H/M versus M2 curves (also named as Arrott plot) show negative slope at some point positive slope. On the other hand, if all the H/M versus M2 curves are positive, the magnetic transition is of the second order. To determine the order of magnetic phase transition of RE2Cu2O5, the measured M-H isotherms were converted in to H/M versus M2 plot, and shown in Fig. 6(a) and (b) for RE ¼ Dy and Ho, respectively. Clearly negative slopes can be observed in the Arrot plot for both compounds, indicating the occurrence of first order magnetic transition for Dy2Cu2O5 and Ho2Cu2O5. Fig. 7 shows the temperature dependence of magnetic entropy change DSM for Dy2Cu2O5 and Ho2Cu2O5 which was calculated from the temperature and field dependence of the magnetization M Z Hmax (H, T) by using an integral version of Maxwell's thermodynamic relation, DSM ðT; DHÞ ¼ ðvMðH; TÞ=vT=ÞH dH. It can be seen 0 that the temperature variation of DSM are almost the same for both compounds except for some differences in the peak value and position. Under low magnetic field changes, the values of DSM are negative at low temperature, and they change their signs and become positive at higher temperatures, giving rise to a distinguishable minimum and a maximum for both compounds. As it is well known, for a conventional ferromagnet, DSM is negative and it is termed as a normal MCE. On the other hand, there are some substances that exhibit inverse MCE, i.e., DSM is positive. Generally, inverse MCE occurs in antiferromagnetic materials, in which the applied magnetic field increases the magnetic entropy. Similar behaviours were also observed in recently reported TbCo2B2 [20],
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Fig. 6. The plots of H/M versus M2 for (a) Dy2Cu2O5 and (b) Ho2Cu2O5 at some selected temperatures.
ErNi2B2C [21] and HoVO3 [10] compounds. The observed inverse MCE in present RE2Cu2O5 is also attributed to the fact of the AFM state under low field and at low temperatures and the normal MCE under high magnetic field change is related to the field e induced metamagnetic transition from AFM to FM state. For a magnetic field change of 0e2 T, the minimum and maximum values of DSM are evaluated to be 4.5 and 1.3 J/kg K for Dy2Cu2O5, and to be 3.6 and 2.4 J/kg K for Ho2Cu2O5, respectively. For the field changes of 0e5 and 0e7 T, the maximum values of magnetic entropy change ðDSM max Þ are evaluated to be 7.5 and 12.0 J/kg K for Dy2Cu2O5; and to be 9.2 and 12.4 J/kg K for Ho2Cu2O5, respectively. Another important quality factor of the refrigerant materials is the relative cooling power (RCP) which is a measure of the amount of heat transfer between the cold and hot reservoirs in an ideal refrigeration cycle. The RCP is defined as the product of the maximum magnetic entropy change DSM max and full width at half maximum in DSM (T) curve dTFWHM. For the field changes of 0e5 and 0e7 T, the values of RCP are determined to be 154 and 273 J/kg; and to be 149 and 256 J/kg for Dy2Cu2O5 and Ho2Cu2O5, respectively. The present results may provide some clue and new data for searching novel magnetic materials for magnetic refrigeration.
4. Conclusions In summary, two single phased cuprates of Dy2Cu2O5 and Ho2Cu2O5 were synthesized and the magnetic properties and MCE have been investigated. For both compounds, a paramagnetic to antiferromagnetic (PMeAFM) magnetic transition was observed under low magnetic fields, whereas a field-induced metamagnetic transition from antiferromagnetic to ferromagnetic (AFMeFM) has
Fig. 7. Temperature dependence of magnetic entropy change DSM for (a) Dy2Cu2O5 and (b) Ho2Cu2O5.
been observed with the application of higher magnetic fields. Both compounds exhibit an inverse and normal MCE for different magnetic field changes and at different temperature. The observed inverse MCE is attributed to the fact of AFM state under low field and at low temperatures for RE2Cu2O5, the reversible normal MCE under higher magnetic field change is related to the first order metamagnetic transition from AFM to FM state. For a field change of 0e5 T, the maximum values of magnetic entropy change ðDSM max Þ reach 7.5 and 9.2 J/kg K for Dy2Cu2O5 and Ho2Cu2O5, respectively. The corresponding values of relative cooling power (RCP) are evaluated to be 154 and 149 J/kg. Acknowledgements This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 11374081 and 11574066), and the Fundamental Research Funds for the Central Universities (Grant Nos. N140901001 and L1509006). References [1] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [2] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Tsoko, Rep. Prog. Phys. 68 (2005) 1479. [3] N.A. de Oliveira, P.J. von Ranke, Phys. Rep. 489 (2010) 89.
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