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Low-field magnetic entropy change in Dy(Co1−xSix)2
PERGAMON
Solid State Communications 121 (2002) 199±202
www.elsevier.com/locate/ssc
Low-®eld magnetic entropy change in Dy(Co12xSix)2 D.H. Wang*, H.D. Liu, S.L. Tang, T. Tang, J.F. Wen, Y.W. Du Department of Physics, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People's Republic of China Received 15 September 2001; accepted 15 October 2001 by Z. Gan
Abstract A series of Dy(Co12xSix)2 (x 0; 0.01, 0.03, 0.05 and 0.07) compounds have been prepared by arc-melting method. The X-ray diffraction (XRD) analysis reveals that all samples are in the MgCu2-type structure. The magnetization were measured by a vibrating sample magnetometer (VSM). A ®rst-order transition disappeared by partial substitution of Co by Si (x . 0:05) in the Dy(Co12xSix)2 alloys. The magnetic entropy changes (uDSMu) as a function of the temperature in magnetic ®eld (1 T) for Dy(CoxSi12x)2 have been calculated, while the maximum values of uDSM are 5.8, 4.1, 3.3, 3.3 and 2.6 J/kg K for x 0; 0.01, 0.03, 0.05 and 0.07, respectively. The Curie temperature (TC) can be adjusted between 142 and 168 K by partial substitution of Co for Si with some reduction of the uDSMaxu. The origin of the large magnetocaloric effect (MCE) and the potential application of Dy(Co,Si)2 as a working material for magnetic refrigerants have also been discussed. q 2002 Published by Elsevier Science Ltd. PACS: 75.30.sg Keywords: Dy(Co12xSix)2 compounds; Magnetic entropy changes; Metamagnetic transition
1. Introduction Materials showing large magnetic entropy change (uDSMu) have attracted much attention recently for their potential application as magnetic refrigerants. Compared with gas refrigerators, magnetic refrigerators have a number of advantages, such as low noise, high ef®ciency, small volume, absence of freon. Up to date, the magnetocaloric effects (MCE) have been extensively studied in two kinds of working substance for magnetic refrigeration: paramagnetic salts and ferromagnetic compounds. The former have been conveniently used to obtain low temperatures, T , 15 K, while the latter are useful for magnetic refrigeration at high temperature, T . 20 K. In high temperature range, in order to remove the effect of the lattice entropy, the Ericsson cycle has been utilized [1±4]. The working materials which possess large magnetic entropy change only near the transition temperature are not suitable for use in devices utilizing the Ericsson cycle [5], so the magnetic refrigerants with
* Corresponding author. Fax: 186-25-359-5535. E-mail address: [email protected] (D.H. Wang).
large MCE in a relatively wide temperature range are of great interest and urgency. Usually, intermetallic compounds and alloys of rare earth are selected. Recently, a large MCE was found in ErCo2 and Er(Co12xNix)2 [6,7]. In the study of magnetic properties of DyCo2 and Dy(Co12xSix)2, we discovered a large magnetic entropy change in them.
2. Experiment A series of Dy(Co12xSix)2 compounds (x 0; 0.01, 0.03, 0.05 and 0.07) were prepared by arc-melting raw materials with a purity of 99.9% in argon atmosphere. To avoid the appearance of foreign phases, a 6 wt% excess of Dy over the stoichiometric composition is necessary. The melted buttons were wrapped in Ta foil, sealed under argon in quartz tubes and annealed at 1223 K for 120 h. The structures of the compounds were examined using X-ray diffraction (XRD) with monochromatic Cu Ka radiation and only the expected C15 phase was observed. Magnetic properties were measured by vibrating sample magnetometer (VSM) (Lakeshore 7307).
0038-1098/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00486-0
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D.H. Wang et al. / Solid State Communications 121 (2002) 199±202
Fig. 1. Temperature dependence of magnetization for Dy(Co12xSix)2 in an applied ®eld of 0.1 T.
3. Results The temperature dependence of magnetization for Dy(CoxSi12x)2 (x 0; 0.01, 0.03, 0.05 and 0.07) measured in a DC magnetic ®eld of 0.1 T is presented in Fig. 1. The Curie temperatures (TC) of these compounds which are de®ned as the temperature where udM/dTu is maximum were found to be 142, 152, 162, 167 and 168 K for x 0; 0.01, 0.03, 0.05 and 0.07, respectively. For Dy(Co, Si)2, the ®rst-order transition is characterized by a sharp change of
magnetization at TC [8]. Upon substitution of Co by Si, the sharp transition disappears at x 0:05 in Dy(CoxSi12x)2 series. For the remaining sample (x 0:07; a gradual change of the magnetization is observed around TC, which agrees well with the earlier report [8]. This difference in behavior is ascribed to the change-over from a ®rst-order transition to a second-order one [8]. Magnetization isotherms of Dy(CoxSi12x)2 were measured at different temperature near TC. The maximum applied ®eld is 1 T. Figs. 2(a) and 3(a) shows the magnetization curves of
Fig. 2. Magnetization isotherms of DyCo2 measured at several selected temperatures near TC. (a) M versus B plots, (b) Arrott plots.
D.H. Wang et al. / Solid State Communications 121 (2002) 199±202
201
Fig. 3. Magnetization isotherms of Dy(Co1.93Si0.07)2 measured at several selected temperatures near TC. (a) M versus B plots, (b) Arrott plots.
DyCo2 and Dy(Co1.93Si0.07)2 compounds, whose phase transitions are of ®rst- and second-order type, respectively. From the shape of plots, there is no obvious difference between them. But from the Arrott plots (Figs. 2(b) and 3(b)), we can observe the in¯ection point above TC in the plot of DyCo2, which suggests the occurrence of a metamagnetic transition from the paramagnetic to ferromagnetic order [9]. For the Dy(Co1.93Si0.07)2, the plots display the linear behavior above TC as expected for second-order phase transition.
The magnetic entropy change as a function of temperature and magnetic ®eld for Dy(CoxSi12x)2 (x 0; 0.01, 0.03, 0.05 and 0.07) were calculated from isothermal magnetization curves using Maxwell equation [9,10]. Fig. 4 shows the plot of uDSMu versus temperature for the magnetic ®eld changing from H 0 to 1.0 T. The maximum values of uDSMu occur almost at TC for all the compounds and are 5.8, 4.1, 3.3, 3.3 and 2.6 J/kg K for x 0; 0.01, 0.03, 0.05 and 0.07, respectively.
4. Discussion
Fig. 4. Magnetic entropy change of Dy(Co12xSix)2 as a function of temperature under a ®eld of 1 T.
The magnetic phase transition in RCo2 has been well explained in terms of the Inoue±Shimizu model [11], considering the combination of both effects of the 3ditinerant electrons and localized spins. In this model, the possibility of the appearance of a ®rst-order transition in RCo2 is ®rst of all, determined by the metamagnetic behavior of the 3d subsystem. For the Dy(Co12xSix)2 compounds, the phase transition is of ®rst-order type with x # 0:05; and changes into second-order transition with x . 0:05: The ®rst-order transition in these compounds can thus be understood in terms of the metamagnetic behavior of the 3d-electron subsystem below Tm (Tm is the temperature at which the magnetic susceptibility of the corresponding Y(Lu)(Co12xSix)2 compounds shows a maximum). The second-order transition in the compounds with a higher Si content may be related to the formation of a local Co 3d moment in these compounds already at high temperatures and the obvious absence of band metamagnetism at TC [12]. The origin of the large magnetic entropy in DyCo2 could be attributed to the considerable variation of the
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D.H. Wang et al. / Solid State Communications 121 (2002) 199±202
magnetization near the transition temperature: in RCo2 compounds, the Co moment is induced by exchange interactions between the 3d (Co) and 4f (R) spins. In RCo2 compounds with the non-magnetic R ions, Y and Lu are exchange enhanced Pauli paramagnets and undergo a metamagnetic transition, i.e. a ®eld induced magnetic phase transition from the paramagnetic to the ferromagnetic state under an external magnetic ®eld exceeding a certain critical value. This critical ®eld (or induction) was found to be 69 and 74 T for YCo2 and LuCo2, respectively. In the RCo2 compounds with magnetic R ions, in the ordered state the molecular ®eld may exceed the critical ®eld necessary to induce the metamagnetic transition in the d-electron subsystem [13]. The phase transitions in DyCo2, HoCo2 and ErCo2 are of ®rst-order type. Application of magnetic ®eld can assist the process of Co moment formation and induce a metamagnetic transition [12]. Meanwhile, the magnetic transition of rare earth moment itself is of second-order, just between a paramagnetic and a ferromagnetic phase [14]. ErCo2, HoCo2 and DyCo2 are of speci®c interest because their temperature of metamagnetic transition and second-order transition are fairly close to each other. Therefore this character causes a sharp change of the magnetization near TC. On the other hand, E. Gratz has shown that HoCo2, whose phase transition is also of ®rst-order type, undergoes a cubic to tetragonal crystal structure change at TC [15]. Presumably this also occurs in DyCo2 and ErCo2. So there is a crystallographic transition which occurs simultaneously with the paramagnetic/ferromagnetic transition. Meanwhile a large and sudden increase in volume which is characteristic for a ®rst-order transition [16±18] has been found in DyCo2 at the transition from the paramagnetic to the magnetic state. The crystallographic transition and volume change will lead to an additional change in magnetization and ultimately result in a large magnetic entropy change. For the Dy(Co12xSix)2 compounds, we should consider that Si atoms introduce additional types of exchange interactions (3p(Si)±3d(Co)) and the statistical distribution of Si in the Co sublattice may lead to clustering effects and consequent broadening of the critical temperature interval [19]. Thus the magnetization decreases more gradually near TC with the increase of Si content. So the magnetic entropy change in DyCo2 is larger than that of Dy(Co12xSix)2 compounds. For the Dy(Co12xSix)2 (x 0:01; 0.03 and 0.05) compounds, since they all have metamagnetic transitions and sudden volume changes at the ®rst-order magnetic transition, there is a relatively larger magnetic entropy change in them than that of Dy(Co1.93Si0.07)2. Recently, an active magnetic regenerator (AMR) has been developed as new magnetic refrigeration [20]. Since the AMR cycle allows relatively large temperature span, the MCE requirements cannot be achieved by a single material. Consequently, a series of magnetic materials with large MCE at the vicinity of a certain temperature should be combined or blended. From this point of view,
Dy(Co12xSix)2 (x 0; 0.01, 0.03 and 0.05) are advantageous, because they exhibit large magnetic entropy changes in a relatively low magnetic ®eld. 5. Conclusion We have studied the MCE of Dy(Co12xSix)2 (x 0; 0.01, 0.03, 0.05 and 0.07). Large magnetic entropy change has been observed for x 0; 0.01, 0.03 and 0.05, respectively. TC can be tuned between 142 and 168 K by the substitution of Si. These results strongly suggest that Dy(Co12xSix)2 has a potential application as a working substance of magnetic refrigeration. Acknowledgements This work was supported by G1999064508, the National key Project for Basic Research; No.50072007, NSFCÐNational Natural Science Foundation of China; 99-A30-01-03, the National Key Project for Science and Technology. References [1] T. Hashimoto, T. Numasawa, M. Shino, T. Okada, Cryogenics 21 (1981) 647. [2] R.D. McMichael, J. Jritter, R.D. Shull, J. Appl. Phys. 73 (1993) 6946. [3] G.V. Brown, J. Appl. Phys. 47 (1976) 36,731. [4] Y. Hakuraku, J. Appl. Phys. 62 (1987) 1560. [5] B.J. Korte, K. Pecharsky, K.A. Gschneidne Jr, J. Appl. Phys. 84 (1998) 5677. [6] H. Wada, S. Tomekawa, M. Shiga, Cryogenics 39 (1999) 915. [7] H. Wada, Y. Tanabe, M. Shiga, H. Sugawara, H. Sato, J. Alloys Comp. 316 (2001) 245. [8] N.H. Duc, J. Magn. Magn. Mater. 152 (1996) 219. [9] F.-x. Hu, B.-g. Shen, J.-r. Sun, Z.-h. Cheng, G.-h. Rao, X.-X. Zhang, Appl. Phys. Lett. 78 (2001) 3675. [10] J.R. Sun, F.X. Hu, B.G. Shen, Phys. Rev. Lett. 85 (2000) 4191. [11] J. Inoune, M. Shimizu, J. Phys. F. 12 (1982) 1811. [12] T.D. Cuong, N.H. Duc, P.E. Brommer, Z. Arnold, J. Kamarad, V. Schovsky, J. Magn. Magn. Mater. 182 (1998) 143. [13] N.H. Duc, P.E. Brommer, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, 1999, p. 12 chapter 3. [14] M. Foldeaki, A. Giguere, R. Chahine, T.K. Bose, Adv. Cryogen. Engng. 43 (1998) 1553. [15] E. Gratz, Solid State Commun. 48 (1983) 825. [16] A. Kowalczyk, A. Szajek, J. Baszynski, J. Kovac, G. Chelkowska, J. Magn. Magn. Mater. 166 (1997) 237. [17] A. Kowalczyk, J. Baszynski, J. Kovac, A. Szlaferek, J. Magn. Magn. Mater. 176 (1997) 241. [18] J. Kamard, Z. Arnold, M.R. Ibarra, J. Magn. Magn. Mater. 140±149 (1995) 837. [19] T.D. Cuong, L. Havela, V. Sechosky, A.V. Andreev, Z. Amold, J. Kamarad, N.H. Duc, J. Appl. Phys. 81 (1997) 4221. [20] A.J. DeGregoria, L.J. Feuling, J.F. Laatsch, J.R. Rowe, J.R. Trucblood, A.A. Wang, Adv. Cryog. Engng. 37B (1992) 875.
Solid State Communications 121 (2002) 199±202
www.elsevier.com/locate/ssc
Low-®eld magnetic entropy change in Dy(Co12xSix)2 D.H. Wang*, H.D. Liu, S.L. Tang, T. Tang, J.F. Wen, Y.W. Du Department of Physics, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People's Republic of China Received 15 September 2001; accepted 15 October 2001 by Z. Gan
Abstract A series of Dy(Co12xSix)2 (x 0; 0.01, 0.03, 0.05 and 0.07) compounds have been prepared by arc-melting method. The X-ray diffraction (XRD) analysis reveals that all samples are in the MgCu2-type structure. The magnetization were measured by a vibrating sample magnetometer (VSM). A ®rst-order transition disappeared by partial substitution of Co by Si (x . 0:05) in the Dy(Co12xSix)2 alloys. The magnetic entropy changes (uDSMu) as a function of the temperature in magnetic ®eld (1 T) for Dy(CoxSi12x)2 have been calculated, while the maximum values of uDSM are 5.8, 4.1, 3.3, 3.3 and 2.6 J/kg K for x 0; 0.01, 0.03, 0.05 and 0.07, respectively. The Curie temperature (TC) can be adjusted between 142 and 168 K by partial substitution of Co for Si with some reduction of the uDSMaxu. The origin of the large magnetocaloric effect (MCE) and the potential application of Dy(Co,Si)2 as a working material for magnetic refrigerants have also been discussed. q 2002 Published by Elsevier Science Ltd. PACS: 75.30.sg Keywords: Dy(Co12xSix)2 compounds; Magnetic entropy changes; Metamagnetic transition
1. Introduction Materials showing large magnetic entropy change (uDSMu) have attracted much attention recently for their potential application as magnetic refrigerants. Compared with gas refrigerators, magnetic refrigerators have a number of advantages, such as low noise, high ef®ciency, small volume, absence of freon. Up to date, the magnetocaloric effects (MCE) have been extensively studied in two kinds of working substance for magnetic refrigeration: paramagnetic salts and ferromagnetic compounds. The former have been conveniently used to obtain low temperatures, T , 15 K, while the latter are useful for magnetic refrigeration at high temperature, T . 20 K. In high temperature range, in order to remove the effect of the lattice entropy, the Ericsson cycle has been utilized [1±4]. The working materials which possess large magnetic entropy change only near the transition temperature are not suitable for use in devices utilizing the Ericsson cycle [5], so the magnetic refrigerants with
* Corresponding author. Fax: 186-25-359-5535. E-mail address: [email protected] (D.H. Wang).
large MCE in a relatively wide temperature range are of great interest and urgency. Usually, intermetallic compounds and alloys of rare earth are selected. Recently, a large MCE was found in ErCo2 and Er(Co12xNix)2 [6,7]. In the study of magnetic properties of DyCo2 and Dy(Co12xSix)2, we discovered a large magnetic entropy change in them.
2. Experiment A series of Dy(Co12xSix)2 compounds (x 0; 0.01, 0.03, 0.05 and 0.07) were prepared by arc-melting raw materials with a purity of 99.9% in argon atmosphere. To avoid the appearance of foreign phases, a 6 wt% excess of Dy over the stoichiometric composition is necessary. The melted buttons were wrapped in Ta foil, sealed under argon in quartz tubes and annealed at 1223 K for 120 h. The structures of the compounds were examined using X-ray diffraction (XRD) with monochromatic Cu Ka radiation and only the expected C15 phase was observed. Magnetic properties were measured by vibrating sample magnetometer (VSM) (Lakeshore 7307).
0038-1098/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00486-0
200
D.H. Wang et al. / Solid State Communications 121 (2002) 199±202
Fig. 1. Temperature dependence of magnetization for Dy(Co12xSix)2 in an applied ®eld of 0.1 T.
3. Results The temperature dependence of magnetization for Dy(CoxSi12x)2 (x 0; 0.01, 0.03, 0.05 and 0.07) measured in a DC magnetic ®eld of 0.1 T is presented in Fig. 1. The Curie temperatures (TC) of these compounds which are de®ned as the temperature where udM/dTu is maximum were found to be 142, 152, 162, 167 and 168 K for x 0; 0.01, 0.03, 0.05 and 0.07, respectively. For Dy(Co, Si)2, the ®rst-order transition is characterized by a sharp change of
magnetization at TC [8]. Upon substitution of Co by Si, the sharp transition disappears at x 0:05 in Dy(CoxSi12x)2 series. For the remaining sample (x 0:07; a gradual change of the magnetization is observed around TC, which agrees well with the earlier report [8]. This difference in behavior is ascribed to the change-over from a ®rst-order transition to a second-order one [8]. Magnetization isotherms of Dy(CoxSi12x)2 were measured at different temperature near TC. The maximum applied ®eld is 1 T. Figs. 2(a) and 3(a) shows the magnetization curves of
Fig. 2. Magnetization isotherms of DyCo2 measured at several selected temperatures near TC. (a) M versus B plots, (b) Arrott plots.
D.H. Wang et al. / Solid State Communications 121 (2002) 199±202
201
Fig. 3. Magnetization isotherms of Dy(Co1.93Si0.07)2 measured at several selected temperatures near TC. (a) M versus B plots, (b) Arrott plots.
DyCo2 and Dy(Co1.93Si0.07)2 compounds, whose phase transitions are of ®rst- and second-order type, respectively. From the shape of plots, there is no obvious difference between them. But from the Arrott plots (Figs. 2(b) and 3(b)), we can observe the in¯ection point above TC in the plot of DyCo2, which suggests the occurrence of a metamagnetic transition from the paramagnetic to ferromagnetic order [9]. For the Dy(Co1.93Si0.07)2, the plots display the linear behavior above TC as expected for second-order phase transition.
The magnetic entropy change as a function of temperature and magnetic ®eld for Dy(CoxSi12x)2 (x 0; 0.01, 0.03, 0.05 and 0.07) were calculated from isothermal magnetization curves using Maxwell equation [9,10]. Fig. 4 shows the plot of uDSMu versus temperature for the magnetic ®eld changing from H 0 to 1.0 T. The maximum values of uDSMu occur almost at TC for all the compounds and are 5.8, 4.1, 3.3, 3.3 and 2.6 J/kg K for x 0; 0.01, 0.03, 0.05 and 0.07, respectively.
4. Discussion
Fig. 4. Magnetic entropy change of Dy(Co12xSix)2 as a function of temperature under a ®eld of 1 T.
The magnetic phase transition in RCo2 has been well explained in terms of the Inoue±Shimizu model [11], considering the combination of both effects of the 3ditinerant electrons and localized spins. In this model, the possibility of the appearance of a ®rst-order transition in RCo2 is ®rst of all, determined by the metamagnetic behavior of the 3d subsystem. For the Dy(Co12xSix)2 compounds, the phase transition is of ®rst-order type with x # 0:05; and changes into second-order transition with x . 0:05: The ®rst-order transition in these compounds can thus be understood in terms of the metamagnetic behavior of the 3d-electron subsystem below Tm (Tm is the temperature at which the magnetic susceptibility of the corresponding Y(Lu)(Co12xSix)2 compounds shows a maximum). The second-order transition in the compounds with a higher Si content may be related to the formation of a local Co 3d moment in these compounds already at high temperatures and the obvious absence of band metamagnetism at TC [12]. The origin of the large magnetic entropy in DyCo2 could be attributed to the considerable variation of the
202
D.H. Wang et al. / Solid State Communications 121 (2002) 199±202
magnetization near the transition temperature: in RCo2 compounds, the Co moment is induced by exchange interactions between the 3d (Co) and 4f (R) spins. In RCo2 compounds with the non-magnetic R ions, Y and Lu are exchange enhanced Pauli paramagnets and undergo a metamagnetic transition, i.e. a ®eld induced magnetic phase transition from the paramagnetic to the ferromagnetic state under an external magnetic ®eld exceeding a certain critical value. This critical ®eld (or induction) was found to be 69 and 74 T for YCo2 and LuCo2, respectively. In the RCo2 compounds with magnetic R ions, in the ordered state the molecular ®eld may exceed the critical ®eld necessary to induce the metamagnetic transition in the d-electron subsystem [13]. The phase transitions in DyCo2, HoCo2 and ErCo2 are of ®rst-order type. Application of magnetic ®eld can assist the process of Co moment formation and induce a metamagnetic transition [12]. Meanwhile, the magnetic transition of rare earth moment itself is of second-order, just between a paramagnetic and a ferromagnetic phase [14]. ErCo2, HoCo2 and DyCo2 are of speci®c interest because their temperature of metamagnetic transition and second-order transition are fairly close to each other. Therefore this character causes a sharp change of the magnetization near TC. On the other hand, E. Gratz has shown that HoCo2, whose phase transition is also of ®rst-order type, undergoes a cubic to tetragonal crystal structure change at TC [15]. Presumably this also occurs in DyCo2 and ErCo2. So there is a crystallographic transition which occurs simultaneously with the paramagnetic/ferromagnetic transition. Meanwhile a large and sudden increase in volume which is characteristic for a ®rst-order transition [16±18] has been found in DyCo2 at the transition from the paramagnetic to the magnetic state. The crystallographic transition and volume change will lead to an additional change in magnetization and ultimately result in a large magnetic entropy change. For the Dy(Co12xSix)2 compounds, we should consider that Si atoms introduce additional types of exchange interactions (3p(Si)±3d(Co)) and the statistical distribution of Si in the Co sublattice may lead to clustering effects and consequent broadening of the critical temperature interval [19]. Thus the magnetization decreases more gradually near TC with the increase of Si content. So the magnetic entropy change in DyCo2 is larger than that of Dy(Co12xSix)2 compounds. For the Dy(Co12xSix)2 (x 0:01; 0.03 and 0.05) compounds, since they all have metamagnetic transitions and sudden volume changes at the ®rst-order magnetic transition, there is a relatively larger magnetic entropy change in them than that of Dy(Co1.93Si0.07)2. Recently, an active magnetic regenerator (AMR) has been developed as new magnetic refrigeration [20]. Since the AMR cycle allows relatively large temperature span, the MCE requirements cannot be achieved by a single material. Consequently, a series of magnetic materials with large MCE at the vicinity of a certain temperature should be combined or blended. From this point of view,
Dy(Co12xSix)2 (x 0; 0.01, 0.03 and 0.05) are advantageous, because they exhibit large magnetic entropy changes in a relatively low magnetic ®eld. 5. Conclusion We have studied the MCE of Dy(Co12xSix)2 (x 0; 0.01, 0.03, 0.05 and 0.07). Large magnetic entropy change has been observed for x 0; 0.01, 0.03 and 0.05, respectively. TC can be tuned between 142 and 168 K by the substitution of Si. These results strongly suggest that Dy(Co12xSix)2 has a potential application as a working substance of magnetic refrigeration. Acknowledgements This work was supported by G1999064508, the National key Project for Basic Research; No.50072007, NSFCÐNational Natural Science Foundation of China; 99-A30-01-03, the National Key Project for Science and Technology. References [1] T. Hashimoto, T. Numasawa, M. Shino, T. Okada, Cryogenics 21 (1981) 647. [2] R.D. McMichael, J. Jritter, R.D. Shull, J. Appl. Phys. 73 (1993) 6946. [3] G.V. Brown, J. Appl. Phys. 47 (1976) 36,731. [4] Y. Hakuraku, J. Appl. Phys. 62 (1987) 1560. [5] B.J. Korte, K. Pecharsky, K.A. Gschneidne Jr, J. Appl. Phys. 84 (1998) 5677. [6] H. Wada, S. Tomekawa, M. Shiga, Cryogenics 39 (1999) 915. [7] H. Wada, Y. Tanabe, M. Shiga, H. Sugawara, H. Sato, J. Alloys Comp. 316 (2001) 245. [8] N.H. Duc, J. Magn. Magn. Mater. 152 (1996) 219. [9] F.-x. Hu, B.-g. Shen, J.-r. Sun, Z.-h. Cheng, G.-h. Rao, X.-X. Zhang, Appl. Phys. Lett. 78 (2001) 3675. [10] J.R. Sun, F.X. Hu, B.G. Shen, Phys. Rev. Lett. 85 (2000) 4191. [11] J. Inoune, M. Shimizu, J. Phys. F. 12 (1982) 1811. [12] T.D. Cuong, N.H. Duc, P.E. Brommer, Z. Arnold, J. Kamarad, V. Schovsky, J. Magn. Magn. Mater. 182 (1998) 143. [13] N.H. Duc, P.E. Brommer, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, 1999, p. 12 chapter 3. [14] M. Foldeaki, A. Giguere, R. Chahine, T.K. Bose, Adv. Cryogen. Engng. 43 (1998) 1553. [15] E. Gratz, Solid State Commun. 48 (1983) 825. [16] A. Kowalczyk, A. Szajek, J. Baszynski, J. Kovac, G. Chelkowska, J. Magn. Magn. Mater. 166 (1997) 237. [17] A. Kowalczyk, J. Baszynski, J. Kovac, A. Szlaferek, J. Magn. Magn. Mater. 176 (1997) 241. [18] J. Kamard, Z. Arnold, M.R. Ibarra, J. Magn. Magn. Mater. 140±149 (1995) 837. [19] T.D. Cuong, L. Havela, V. Sechosky, A.V. Andreev, Z. Amold, J. Kamarad, N.H. Duc, J. Appl. Phys. 81 (1997) 4221. [20] A.J. DeGregoria, L.J. Feuling, J.F. Laatsch, J.R. Rowe, J.R. Trucblood, A.A. Wang, Adv. Cryog. Engng. 37B (1992) 875.