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Large reversible magnetocaloric effect of light rare-earth intermetallic compound Pr5Si3
Journal of Alloys and Compounds 496 (2010) 517–520
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Large reversible magnetocaloric effect of light rare-earth intermetallic compound Pr5 Si3 Guang Tian a , Honglin Du a,∗ , Yan Zhang a , Yuanhua Xia a , Changsheng Wang a , Jingzhi Han a , Shunquan Liu a , Jinbo Yang a,b a b
School of Physics, Peking University, Beijing 100871, PR China State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing, 100871, PR China
a r t i c l e
i n f o
Article history: Received 4 December 2009 Received in revised form 7 February 2010 Accepted 11 February 2010 Available online 18 February 2010 Keywords: Magnetocaloric effect Magnetic refrigerant materials Magnetic entropy change Second-order magnetic transition
a b s t r a c t The structure and magnetic properties of Pr5 Si3 compound have been investigated. A large reversible magnetic entropy change was observed at about 47 K accompanied with a second-order magnetic transition from the ferromagnetic to paramagnetic state. The maximum values of −SM are 11.6 and 6.0 J kg−1 K−1 at 47 K, corresponding to the applied magnetic field changes of 50 and 20 kOe, respectively. The study on the magnetocaloric effect of the light rare-earth based compound, may open a field in searching for new magnetic refrigerant materials. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Compared with the conventional vapour-cycle refrigeration, magnetic refrigeration based on the magnetocaloric effect (MCE) has attracted great attention due to its energy-efficient and environment-friendly features [1–3]. The MCE is an intrinsic thermodynamic property of magnetic materials, which is closely related to the magnetic entropy change caused by the applied magnetic field. A large entropy change usually leads to high refrigeration efficiency during a magnetic-refrigeration cycle. As a result, searching for magnetic materials with giant values of the magnetic entropy change is of great importance for the development of magnetic refrigeration. A giant MCE (GMCE) is often associated with a first order magnetic transition (FOMT). For instance, ferromagnetic (FM) materials such as Gd5 (Ge1−x Six )4 , La(Fe1−x Six )13 , MnFeP0.45 As0.55 , ErCo2 , and MnAs [4–8], have large negative magnetic-entropy changes (SM ) near the FOMT from paramagnetic (PM) to FM states [9,10]. In recent years, many studies devoted to the influence of chemical substitutions on the magnetocaloric properties of these promising systems [11–13]. Besides the FOMT ferromagnetic materials, it is also found that large MCE exists in antiferromagnetic (AFM) systems, such as DyCu2 [14] and DySb [15]. Recently, some researchers start to look for
∗ Corresponding author. E-mail address: [email protected] (H. Du). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.093
magnetic refrigerant materials with a large reversible SM based on the second-order magnetic transition (SOMT) [3] which has a small thermal and magnetic hysteresis, and some magnetic materials such as Tb3 Co [16] and La0.7 Ca0.3−x Srx MnO3 [17] have been reported. A few earlier investigations about binary Pr-silicides focused mainly on the preparation, structure chemistry and basic magnetic properties have been reported [18–20]. Among all the Pr-silicides, it is learned that the magnetization of Pr5 Si3 is relatively high at low temperature, and it changes rapidly near its Curie temperature [18], which indicates that a large MCE may be observed near this phase transition temperature. Therefore, we have performed a study on the structure and magnetic properties of Pr5 Si3 compound in this paper, and especially focused on the MCE in it, which has not been investigated before.
2. Experimental The polycrystalline Pr5 Si3 sample was prepared by arc melting of high purity raw materials, 99.9% or better, in a water-cooled copper crucible under a purified argon atmosphere. Approximately 5 wt% excess Pr was added during melting. The sample was arc-melted 5 times with the button being turned over after each melting to improve the homogeneity of the alloy. The ingot was then sealed in an argon-filled quartz tube and annealed at 1050 ◦ C for 7 days. X-ray diffraction (XRD) measurement was carried out by using X’pert Pro MPD diffractometer (Cu K␣ radiation) at room temperature and the XRD pattern was refined with the Rietveld method. The magnetic measurements were performed using the magnetic property measurement system (MPMS-7, manufactured by Quantum Design Inc.) from 10 to 250 K with applied field up to 50 kOe.
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Fig. 1. The observed (open symbols) and calculated (solid lines) X-ray diffraction pattern for Pr5 Si3 . Positions for the Bragg peaks are marked by vertical bars. Differences between the observed and the calculated intensities are shown at the bottom of the figure.
3. Results and discussion The XRD pattern confirms the single-phase nature of the compound, which crystallizes in the tetragonal Cr5 B3 -type of structure with space group I4/mcm (Fig. 1). The rare-earth atoms occupy the crystallographic sites 16l and 4c, while the silicon atoms enter the 4a and 8h sites [19]. The unit cell parameters a and c are determined to be 7.8089(6) and 13.7510(13) Å, respectively, in good agreement with the values reported in the previous works [19,21]. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves in a magnetic field of 50 Oe are depicted in Fig. 2. A FM to PM transition is observed at the Curie temperature of TC = 47 K, corresponding to the minimum in the dM/dT curve (as shown in the inset (i) of Fig. 2), which is close to the value of 44 K reported in Ref. [18]. The two curves in Fig. 2 are reversible around TC , a characteristic of the SOMT, while a large difference can be seen in the low temperature range below TC . In the ZFC curve, the magnetization rises first with increasing temperature and then decreases sharply around 44 K. This fact can be a direct consequence of several possibilities, such as spin glass, mictomagnetic transition, etc.
Fig. 2. Temperature dependence of the ZFC and FC magnetization of Pr5 Si3 under a magnetic field of 50 Oe. Inset (i): the first derivative of the magnetization (dM/dT) as a function of temperature. Inset (ii): inverse mass susceptibility versus temperature.
Fig. 3. Magnetic isothermals for Pr5 Si3 in the temperature range of 32–62 K measured on field increase (solid squares) and field decrease (solid triangles), with temperature step of 3 K.
For the compound Pr5 Si3 , the cusp in the ZFC curve is considered to be induced by the thermal activation of domain wall movement resulting in an increase of the net magnetization, as have been observed in other Pr-silicides [18]. The crystal field interaction of moments containing an orbital contribution in Pr5 Si3 results in the high magneto-crystalline anisotropy energy, which is the origin of the presence of narrow walls. At low temperature, the thermal energy is not strong enough to surmount the energy barrier needed to move the narrow walls. This leads to the absence of an appreciable net magnetization in the low temperature region in the ZFC curve [22]. While the FC curve was measured after cooling to the low temperature in a field of 50 Oe, the moments were aligned to the field direction before the measurement and there was no magnetization decrease at low temperature. The temperature dependence of the inverse susceptibility (1/) for Pr5 Si3 is represented in the inset (ii) of Fig. 2 along with the Curie–Weiss fit [1/ = C/(T-p )]. Above 80 K, obeys the Curie–Weiss law with a positive paramagnetic Curie temperature p = 39 K. While below 80 K, it has a slight deviation from the Curie–Weiss law. The magnetization isotherms at different temperatures were measured and are represented in Fig. 3. The temperature interval between the isotherms is 3 K and the magnetic field varies between zero and 50 kOe. At the temperatures below 47 K, the magnetic behavior of the sample is of a typical ferromagnetic system. However, the saturation magnetization is not fully reached at 50 kOe, which is induced by the magneto-crystalline anisotropy of this compound. It can be seen that the magnetization varies nearly linearly with the magnetic field above 47 K in the paramagnetic state. Thus the isothermal magnetization curves confirm a FM to PM phase transition at about 47 K, which is consistent with the result derived from the M–T curve. It can also be seen from Fig. 3 that the M–H isotherms around TC with increasing and decreasing field are nearly identical. This reversible behavior shown in the isothermal magnetization curves confirms the second order nature of the phase transition near the ordering temperature, and there appears no magnetic hysteresis in the magnetization, which is highly beneficial to the practical magnetic refrigeration application. The calculated saturation magnetic moment of 1.45 B per Pr atom was obtained by extrapolation of the magnetization data at 47 K, which is far below the corresponding value of 2.14 B at 5 K (Ref. [18]) and the value of 3.65 B of free Pr ion. This may be caused by the crystal field effect.
G. Tian et al. / Journal of Alloys and Compounds 496 (2010) 517–520
Fig. 4. Arrott plots (M2 vs. H/M) of Pr5 Si3 with the temperature step of 3 K from 32 to 62 K.
The so-called Arrott plots (H/M versus M2 ) of the sample deduced from the isothermal magnetization curves are shown in Fig. 4. It is widely accepted that the negative slopes or inflection points (S-shaped curve) in Arrott plots express the negative contribution of some higher order terms in the Landau free energy expansion, which is indicative of first-order phase transition. On the contrary, the positive slope and linear behavior near TC often mean that the phase transition is a SOMT [23,24]. Fig. 4 clearly reveals the occurrence of a second-order FM to PM phase transition near TC = 47 K. There is neither S-shaped curve nor negative slope appeared in the curves around this temperature. The isothermal magnetic entropy change, corresponding to a magnetic-field change H starting from zero field, can be calculated from the isothermal magnetization curves by means of the following equation [25]
SM (T, H) = SM (T, H) − SM (T, 0) = 0
H
∂M ∂T
dH H
which can be obtained from the Maxwell relation. With the magnetization data measured at discrete values of the field and temperature, the magnetic entropy change can be calculated approximately using the mathematical formula [26] −SM =
i
1 (Mi+1 − Mi )Hi Ti+1 − Ti
where Mi and Mi+1 are the magnetization values at temperatures Ti and Ti+1 , respectively. The results are shown in Fig. 5. It can be seen that the S curves show a symmetrical broadening with the increase of the applied field, also indicating the second order nature of the magnetic transition to which the large magnetic entropy change is ascribed [27]. The calculated maximum value of the magnetic entropy changes is 11.6 J kg−1 K−1 for the field change from 0
519
Fig. 5. Magnetic entropy change as a function of temperature for different magneticfield change.
to 50 kOe. In particular, even for a field change of 20 kOe, which can be easily provided, the maximum value of −SM is 6.0 J kg−1 K−1 . As predicted by the theory, the possible maximum variation in the magnetic entropy for free ions of total angular momentum, J, is SMAX = −R ln(2J + 1), where R is the universal gas constant. From the saturation magnetic moment of Pr5 Si3 (s = 2.14 B per Pr atom) and gJ = 4/5, the average J value of the Pr ions is estimated to be 2. Because there are five Pr ions per formula unit, we have SMAX = −5R ln(2J + 1) = −64.4 J mol−1 K−1 = −81.6 J kg−1 K−1 , which is much larger than the value obtained from the magnetization measurements. The value (11.6 J kg−1 K−1 ) of the magnetic entropy change in Pr5 Si3 is comparable to some famous refrigerant materials working in the same temperature range. For clarity, we summarize in Table 1 the main parameters of the representative magnetic refrigerant materials around 47 K [28,29]. Compared with the best-known heavy rare-earth based intermetallic compounds used for low temperature magnetic refrigeration, such as (Gd0.5 Dy0.25 Er0.25 )CoAl [29] and (Gd1−x Erx )NiAl [30,31], Pr5 Si3 compound is much cheaper and easier to be fabricated. The light rare-earth compounds based on Pr-silicides, appear to be an attractive candidate for a magnetic refrigerant due to its low cost and extremely large entropy change. Moreover, most of the materials with large MCE, such as the rare-earth intermetallics, transition metal alloys, perovskite oxides, and even some ferromagnetic shape memory alloys, are based on heavy rare-earth or transition metals due to their high magnetic moments. As a result, the magnetic materials based on heavy rare-earth elements or transition metals have been investigated intensely for magnetic refrigeration applications. Therefore, a large magnetic entropy change from Pr5 Si3 without heavy rare-earth and transition metal elements, provides new opportunities in searching for new magnetic refrigerant materials.
Table 1 Comparison of the representative refrigerant materials with working temperature around 47 K. Material
MAX SM (5T ) (J kg−1 K−1 )
MAX SM (2T ) (J kg−1 K−1 )
Transition temperature (K)
Ref.
Gd33 Er22 Al25 Co20 (Gd0.5 Dy0.25 Er0.25 )CoAl Pr5 Si3
9.47 14.0 11.6
∼4.5 6.3 6.0
52 45 47
[28] [29] this work
520
G. Tian et al. / Journal of Alloys and Compounds 496 (2010) 517–520
4. Conclusions The magnetic and magnetocaloric properties of Pr5 Si3 have been investigated. A maximum values of −SM of 11.6 and 6.0 J kg−1 K−1 are observed at 47 K for field changes of 50 and 20 kOe, respectively. The large reversible entropy change −SM originates from a second-order phase transition from the ferromagnetic to paramagnetic state. The good magnetocaloric features of the compound Pr5 Si3 , in addition to the low material costs and simple component, make it a promising candidate material for magnetic refrigeration. Acknowledgements This work has been supported by the National Natural Science Foundation of China (Grant Nos. 10875007 and 50701003), and the National 973 Project (No. 2010CB833104, MOST of China). References [1] J. Glanz, Science 279 (1998) 2045. [2] C.B. Zimm, A. Jastrab, A. Sternberg, V.K. Pecharsky, K.A. Gschneidner Jr., M. Osborne, I. Anderson, Adv. Cryog. Eng. 43 (1998) 1759. [3] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Tsokol, Rep. Prog. Phys. 68 (2005) 1479. [4] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [5] F.X. Hu, B.G. Shen, J.R. Sun, Z.H. Cheng, G.H. Rao, X.X. Zhang, Appl. Phys. Lett. 78 (2001) 3675. [6] O. Tegus, E. Brück, K.H.J. Buschow, F.R. de Boer, Nature (London) 415 (2002) 150. [7] H. Wada, S. Tomekawa, M. Shiga, Cryogenics 39 (1999) 915. [8] S. Gama, A.A. Coelho, A. de Campos, A.M.G. Carvalho, F.C.G. Gandra, P.J. von Ranke, N.A. de Oliveira, Phys. Rev. Lett. 93 (2004) 237202.
[9] S. Stadler, M. Khan, J. Mitchell, N. Ali, A.M. Gomes, I. Dubenko, A.Y. Takeuchi, A.P. Guimarães, Appl. Phys. Lett. 88 (2006) 192511. [10] T. Krenke, E. Duman, M. Acet, E.F. Wassermann, X. Moya, L. Manosa, A. and, Planes, Nat. Mater. 4 (2005) 450. [11] V. Paul-Boncour, T. Mazet, J. Appl. Phys. 105 (2009) 013914. [12] Zhiping Lin, Shandong Li, Meimei Liu, Jenq-Gong Duh, Ken Peng, Xingyu Mao, J. Alloys Compd. 489 (2010) 1. [13] M. Balli, D. Fruchart, D. Gignoux, C. Dupuis, A. Kedous-Lebouc, R. Zach, J. Appl. Phys. 103 (2008) 103908. [14] Parul Arora, Pragya Tiwari, V.G. Sathe, M.K. Chattopadhyay, J. Magn. Magn. Mater. 321 (2009) 3278. [15] W.J. Hu, J. Du, B. Li, Q. Zhang, Z.D. Zhang, Appl. Phys. Lett. 92 (2008) 192505. [16] B. Li, J. Du, W.J. Ren, W.J. Hu, Q. Zhang, D. Li, Z.D. Zhang, Appl. Phys. Lett. 92 (2008) 242504. [17] M.H. Phan, S.C. Yu, N.H. Hur, Appl. Phys. Lett. 86 (2005) 072504. [18] N. Pinguet, F. Weitzer, K. Hiebl, J.C. Schuster, H. Noël, J. Alloys Compd. 348 (2003) 1. [19] Dmitri Souptel, Andreas Leithe-Jasper, Wolfgang Löser, Walter Schnelle, Horst Borrmann, Günter Behr, J. Cryst. Growth 273 (2004) 311. [20] S. Legl, C. Franz, A. Neubauer, C. Pfleiderer, D. Souptel, G. Behr, Physica B 404 (2009) 2887. [21] V.N. Eremenko, K.A. Meleshevich, Y.I. Buyanov, I.M. Obushenko, Dop. Akad. Nauk Ukr. RSR (A) 11 (1984) 80. [22] K.H.J. Buschow, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol.1, Elsevier, North-Holland, Amsterdam, 1980, pp. 297. [23] N.H. Duc, D.T.K. Anh, P.E. Brommer, Physica B 319 (2002) 1. [24] N.K. Singh, K.G. Suresh, A.K. Nigam, S.K. Malik, A.A. Coelho, S. Gama, J. Magn. Magn. Mater. 317 (2007) 68. [25] M. Foldeaki, R. Chahine, J. Kbose, J. Appl. Phys. 77 (1995) 3528. [26] R.D. McMichael, J.J. Ritter, R.D. Shull, J. Appl. Phys. 73 (1993) 6946. [27] Jun Shen, Fang Wang, Yang-Xian Li, Ji-Rong Sun, Bao-Gen Shen, J. Alloys Compd. 458 (2008) L6. [28] Q. Luo, D.Q. Zhao, M.X. Pan, R.J. Wang, W.H. Wang, Appl. Phys. Lett. 89 (2006) 81914. [29] X.X. Zhang, F.W. Wang, G.H. Wen, J. Phys.: Condens. Matter 13 (2001) L747. [30] H. Takeya, V.K. Pecharsky, K.A. Gschneidner Jr., J.O. Moorman, Appl. Phys. Lett. 64 (1994) 2739. [31] B.J. Korte, V.K. Pecharsky, K.A. Gschneidner Jr., J. Appl. Phys. 84 (1998) 5677.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Large reversible magnetocaloric effect of light rare-earth intermetallic compound Pr5 Si3 Guang Tian a , Honglin Du a,∗ , Yan Zhang a , Yuanhua Xia a , Changsheng Wang a , Jingzhi Han a , Shunquan Liu a , Jinbo Yang a,b a b
School of Physics, Peking University, Beijing 100871, PR China State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing, 100871, PR China
a r t i c l e
i n f o
Article history: Received 4 December 2009 Received in revised form 7 February 2010 Accepted 11 February 2010 Available online 18 February 2010 Keywords: Magnetocaloric effect Magnetic refrigerant materials Magnetic entropy change Second-order magnetic transition
a b s t r a c t The structure and magnetic properties of Pr5 Si3 compound have been investigated. A large reversible magnetic entropy change was observed at about 47 K accompanied with a second-order magnetic transition from the ferromagnetic to paramagnetic state. The maximum values of −SM are 11.6 and 6.0 J kg−1 K−1 at 47 K, corresponding to the applied magnetic field changes of 50 and 20 kOe, respectively. The study on the magnetocaloric effect of the light rare-earth based compound, may open a field in searching for new magnetic refrigerant materials. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Compared with the conventional vapour-cycle refrigeration, magnetic refrigeration based on the magnetocaloric effect (MCE) has attracted great attention due to its energy-efficient and environment-friendly features [1–3]. The MCE is an intrinsic thermodynamic property of magnetic materials, which is closely related to the magnetic entropy change caused by the applied magnetic field. A large entropy change usually leads to high refrigeration efficiency during a magnetic-refrigeration cycle. As a result, searching for magnetic materials with giant values of the magnetic entropy change is of great importance for the development of magnetic refrigeration. A giant MCE (GMCE) is often associated with a first order magnetic transition (FOMT). For instance, ferromagnetic (FM) materials such as Gd5 (Ge1−x Six )4 , La(Fe1−x Six )13 , MnFeP0.45 As0.55 , ErCo2 , and MnAs [4–8], have large negative magnetic-entropy changes (SM ) near the FOMT from paramagnetic (PM) to FM states [9,10]. In recent years, many studies devoted to the influence of chemical substitutions on the magnetocaloric properties of these promising systems [11–13]. Besides the FOMT ferromagnetic materials, it is also found that large MCE exists in antiferromagnetic (AFM) systems, such as DyCu2 [14] and DySb [15]. Recently, some researchers start to look for
∗ Corresponding author. E-mail address: [email protected] (H. Du). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.093
magnetic refrigerant materials with a large reversible SM based on the second-order magnetic transition (SOMT) [3] which has a small thermal and magnetic hysteresis, and some magnetic materials such as Tb3 Co [16] and La0.7 Ca0.3−x Srx MnO3 [17] have been reported. A few earlier investigations about binary Pr-silicides focused mainly on the preparation, structure chemistry and basic magnetic properties have been reported [18–20]. Among all the Pr-silicides, it is learned that the magnetization of Pr5 Si3 is relatively high at low temperature, and it changes rapidly near its Curie temperature [18], which indicates that a large MCE may be observed near this phase transition temperature. Therefore, we have performed a study on the structure and magnetic properties of Pr5 Si3 compound in this paper, and especially focused on the MCE in it, which has not been investigated before.
2. Experimental The polycrystalline Pr5 Si3 sample was prepared by arc melting of high purity raw materials, 99.9% or better, in a water-cooled copper crucible under a purified argon atmosphere. Approximately 5 wt% excess Pr was added during melting. The sample was arc-melted 5 times with the button being turned over after each melting to improve the homogeneity of the alloy. The ingot was then sealed in an argon-filled quartz tube and annealed at 1050 ◦ C for 7 days. X-ray diffraction (XRD) measurement was carried out by using X’pert Pro MPD diffractometer (Cu K␣ radiation) at room temperature and the XRD pattern was refined with the Rietveld method. The magnetic measurements were performed using the magnetic property measurement system (MPMS-7, manufactured by Quantum Design Inc.) from 10 to 250 K with applied field up to 50 kOe.
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G. Tian et al. / Journal of Alloys and Compounds 496 (2010) 517–520
Fig. 1. The observed (open symbols) and calculated (solid lines) X-ray diffraction pattern for Pr5 Si3 . Positions for the Bragg peaks are marked by vertical bars. Differences between the observed and the calculated intensities are shown at the bottom of the figure.
3. Results and discussion The XRD pattern confirms the single-phase nature of the compound, which crystallizes in the tetragonal Cr5 B3 -type of structure with space group I4/mcm (Fig. 1). The rare-earth atoms occupy the crystallographic sites 16l and 4c, while the silicon atoms enter the 4a and 8h sites [19]. The unit cell parameters a and c are determined to be 7.8089(6) and 13.7510(13) Å, respectively, in good agreement with the values reported in the previous works [19,21]. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves in a magnetic field of 50 Oe are depicted in Fig. 2. A FM to PM transition is observed at the Curie temperature of TC = 47 K, corresponding to the minimum in the dM/dT curve (as shown in the inset (i) of Fig. 2), which is close to the value of 44 K reported in Ref. [18]. The two curves in Fig. 2 are reversible around TC , a characteristic of the SOMT, while a large difference can be seen in the low temperature range below TC . In the ZFC curve, the magnetization rises first with increasing temperature and then decreases sharply around 44 K. This fact can be a direct consequence of several possibilities, such as spin glass, mictomagnetic transition, etc.
Fig. 2. Temperature dependence of the ZFC and FC magnetization of Pr5 Si3 under a magnetic field of 50 Oe. Inset (i): the first derivative of the magnetization (dM/dT) as a function of temperature. Inset (ii): inverse mass susceptibility versus temperature.
Fig. 3. Magnetic isothermals for Pr5 Si3 in the temperature range of 32–62 K measured on field increase (solid squares) and field decrease (solid triangles), with temperature step of 3 K.
For the compound Pr5 Si3 , the cusp in the ZFC curve is considered to be induced by the thermal activation of domain wall movement resulting in an increase of the net magnetization, as have been observed in other Pr-silicides [18]. The crystal field interaction of moments containing an orbital contribution in Pr5 Si3 results in the high magneto-crystalline anisotropy energy, which is the origin of the presence of narrow walls. At low temperature, the thermal energy is not strong enough to surmount the energy barrier needed to move the narrow walls. This leads to the absence of an appreciable net magnetization in the low temperature region in the ZFC curve [22]. While the FC curve was measured after cooling to the low temperature in a field of 50 Oe, the moments were aligned to the field direction before the measurement and there was no magnetization decrease at low temperature. The temperature dependence of the inverse susceptibility (1/) for Pr5 Si3 is represented in the inset (ii) of Fig. 2 along with the Curie–Weiss fit [1/ = C/(T-p )]. Above 80 K, obeys the Curie–Weiss law with a positive paramagnetic Curie temperature p = 39 K. While below 80 K, it has a slight deviation from the Curie–Weiss law. The magnetization isotherms at different temperatures were measured and are represented in Fig. 3. The temperature interval between the isotherms is 3 K and the magnetic field varies between zero and 50 kOe. At the temperatures below 47 K, the magnetic behavior of the sample is of a typical ferromagnetic system. However, the saturation magnetization is not fully reached at 50 kOe, which is induced by the magneto-crystalline anisotropy of this compound. It can be seen that the magnetization varies nearly linearly with the magnetic field above 47 K in the paramagnetic state. Thus the isothermal magnetization curves confirm a FM to PM phase transition at about 47 K, which is consistent with the result derived from the M–T curve. It can also be seen from Fig. 3 that the M–H isotherms around TC with increasing and decreasing field are nearly identical. This reversible behavior shown in the isothermal magnetization curves confirms the second order nature of the phase transition near the ordering temperature, and there appears no magnetic hysteresis in the magnetization, which is highly beneficial to the practical magnetic refrigeration application. The calculated saturation magnetic moment of 1.45 B per Pr atom was obtained by extrapolation of the magnetization data at 47 K, which is far below the corresponding value of 2.14 B at 5 K (Ref. [18]) and the value of 3.65 B of free Pr ion. This may be caused by the crystal field effect.
G. Tian et al. / Journal of Alloys and Compounds 496 (2010) 517–520
Fig. 4. Arrott plots (M2 vs. H/M) of Pr5 Si3 with the temperature step of 3 K from 32 to 62 K.
The so-called Arrott plots (H/M versus M2 ) of the sample deduced from the isothermal magnetization curves are shown in Fig. 4. It is widely accepted that the negative slopes or inflection points (S-shaped curve) in Arrott plots express the negative contribution of some higher order terms in the Landau free energy expansion, which is indicative of first-order phase transition. On the contrary, the positive slope and linear behavior near TC often mean that the phase transition is a SOMT [23,24]. Fig. 4 clearly reveals the occurrence of a second-order FM to PM phase transition near TC = 47 K. There is neither S-shaped curve nor negative slope appeared in the curves around this temperature. The isothermal magnetic entropy change, corresponding to a magnetic-field change H starting from zero field, can be calculated from the isothermal magnetization curves by means of the following equation [25]
SM (T, H) = SM (T, H) − SM (T, 0) = 0
H
∂M ∂T
dH H
which can be obtained from the Maxwell relation. With the magnetization data measured at discrete values of the field and temperature, the magnetic entropy change can be calculated approximately using the mathematical formula [26] −SM =
i
1 (Mi+1 − Mi )Hi Ti+1 − Ti
where Mi and Mi+1 are the magnetization values at temperatures Ti and Ti+1 , respectively. The results are shown in Fig. 5. It can be seen that the S curves show a symmetrical broadening with the increase of the applied field, also indicating the second order nature of the magnetic transition to which the large magnetic entropy change is ascribed [27]. The calculated maximum value of the magnetic entropy changes is 11.6 J kg−1 K−1 for the field change from 0
519
Fig. 5. Magnetic entropy change as a function of temperature for different magneticfield change.
to 50 kOe. In particular, even for a field change of 20 kOe, which can be easily provided, the maximum value of −SM is 6.0 J kg−1 K−1 . As predicted by the theory, the possible maximum variation in the magnetic entropy for free ions of total angular momentum, J, is SMAX = −R ln(2J + 1), where R is the universal gas constant. From the saturation magnetic moment of Pr5 Si3 (s = 2.14 B per Pr atom) and gJ = 4/5, the average J value of the Pr ions is estimated to be 2. Because there are five Pr ions per formula unit, we have SMAX = −5R ln(2J + 1) = −64.4 J mol−1 K−1 = −81.6 J kg−1 K−1 , which is much larger than the value obtained from the magnetization measurements. The value (11.6 J kg−1 K−1 ) of the magnetic entropy change in Pr5 Si3 is comparable to some famous refrigerant materials working in the same temperature range. For clarity, we summarize in Table 1 the main parameters of the representative magnetic refrigerant materials around 47 K [28,29]. Compared with the best-known heavy rare-earth based intermetallic compounds used for low temperature magnetic refrigeration, such as (Gd0.5 Dy0.25 Er0.25 )CoAl [29] and (Gd1−x Erx )NiAl [30,31], Pr5 Si3 compound is much cheaper and easier to be fabricated. The light rare-earth compounds based on Pr-silicides, appear to be an attractive candidate for a magnetic refrigerant due to its low cost and extremely large entropy change. Moreover, most of the materials with large MCE, such as the rare-earth intermetallics, transition metal alloys, perovskite oxides, and even some ferromagnetic shape memory alloys, are based on heavy rare-earth or transition metals due to their high magnetic moments. As a result, the magnetic materials based on heavy rare-earth elements or transition metals have been investigated intensely for magnetic refrigeration applications. Therefore, a large magnetic entropy change from Pr5 Si3 without heavy rare-earth and transition metal elements, provides new opportunities in searching for new magnetic refrigerant materials.
Table 1 Comparison of the representative refrigerant materials with working temperature around 47 K. Material
MAX SM (5T ) (J kg−1 K−1 )
MAX SM (2T ) (J kg−1 K−1 )
Transition temperature (K)
Ref.
Gd33 Er22 Al25 Co20 (Gd0.5 Dy0.25 Er0.25 )CoAl Pr5 Si3
9.47 14.0 11.6
∼4.5 6.3 6.0
52 45 47
[28] [29] this work
520
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