Cryogenics 39 (1999) 915±919
Magnetocaloric properties of a ®rst-order magnetic transition system ErCo2 H. Wada *, S. Tomekawa, M. Shiga Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Received 23 August 1999; accepted 17 September 1999
Abstract The magnetocaloric eect was examined for ErCo2 , which shows a ®rst-order magnetic transition from a ferrimagnetic to paramagnetic state at 32 K. A large magnetic entropy change was caused by a magnetic ®eld. The corresponding entropy change DS and the adiabatic temperature change DTad are 12 J/K mol and 13 K, respectively in a change of magnetic ®eld 0±8 T at 32 K. The substitution of 20% Y for Er leads to a reduction of the magnetocaloric properties by 20±30%. The origin of the large magnetocaloric eect is also discussed. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Magnetocaloric eect; Magnetic entropy; First-order phase transition
1. Introduction Because of high thermodynamic eciency, magnetic refrigeration has been an attractive technology for the low temperature generation. The search for working substances in the temperature range of interest is strongly desired for further improvement of magnetic refrigeration. Recently, several compounds, such as Gd5 (Si2 Ge2 )  and FeRh  have been found to show a large magnetocaloric eect near room temperature. These compounds exhibit a ®rst-order phase transition between two magnetically ordered phases. If the magnetic entropy change associated with a ®rst-order magnetic transition is suciently large and the transition temperature T0 is strongly dependent on the magnetic ®eld, we expect a large magnetocaloric eect. This is the origin of the magnetocaloric eect of the above materials. For example, Fe0:49 Rh0:51 undergoes a ®rst-order phase transition from an antiferromagnetic state to a ferromagnetic one with increasing temperature at T0 313 K . The corresponding magnetic entropy change is 1 J/K mol. This transition temperature shifts to 294 K, when the magnetic ®eld of 1.95 T is applied.
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From this point of view, a ®rst-order magnetic phase transition from a ferromagnetic state to a paramagnetic state is of particular interest, because one would expect a substantial change in magnetic entropy at T0 , giving rise to a large magnetocaloric eect. The Laves phase compounds RCo2 , where R stands for a rare-earth element, are suitable systems. In RCo2 , the Co moment is induced by the exchange ®eld from the R moments. YCo2 and LuCo2 , where Y and Lu are nonmagnetic, are Pauli paramagnets with enhanced magnetic susceptibility . The compounds with R Gd±Er are ferrimagnets. GdCo2 and TbCo2 undergo a second-order magnetic phase transition at Tc , whereas a ®rst-order magnetic phase transition takes place for DyCo2 , HoCo2 and ErCo2 at T0 135, 77 and 32 K, respectively . The induced Co moment is 1 lB , being independent of the species of R. Metamagnetic behavior was reported for HoCo2 in its magnetization curves just above T0 . These results strongly suggest that a ®rst-order phase transition remains even in magnetic ®elds and T0 is sensitive to the magnetic ®eld. Nikitin and Tishin examined the magnetocaloric properties of HoCo2 . They observed the adiabatic temperature change DTad 5.1 K in a change of magnetic ®eld 0±6 T at 82 K. However, the calorimetric studies of HoCo2 indicated that more than 70% of the total magnetic entropy comes out below T0 , suggesting that the available magnetic entropy change at T0 is not large . Recently, we have studied the speci®c heat of ErCo2 . The entropy jump
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H. Wada et al. / Cryogenics 39 (1999) 915±919
of ErCo2 at T0 is 9 J/K mol (shown in Fig. 3), which is about 40% of the total magnetic entropy R ln
2J 1 23.1 J/K mol with J 15/2. Above T0 , the magnetic contribution to speci®c heat still remains and decreases with increasing temperature. This is attributable to the crystal ®eld eects of 4f electrons. The magnetic entropy shows a trend of saturation above 100 K. The saturation value of magnetic entropy is close to R ln
2J 1. This fact suggests that the Co sublattice has little contribution to the magnetic entropy. Aleksandryan et al.  have studied the magnetization curves of single crystal ErCo2 . They observed a metamagnetic transition between 32.6 and 43.0 K in the ®eld range of 0.5±5 T. These results suggest that applying a magnetic ®eld of 5T increases T0 by an amount of 10 K. Therefore, ErCo2 is a good candidate for a large magnetocaloric eect. This paper presents the results of speci®c heat of ErCo2 in magnetic ®elds up to 8 T. We observed a large magnetocaloric eect for ErCo2 , DTad 13 K in 0±8 T at 32 K. The Y subtitution eects on the magnetocaloric properties were also studied. We discuss the origin of the large magnetocaloric eect of ErCo2 . A part of the work was already published .
Fig. 1. Temperature dependence of speci®c heat of ErCo2 in magnetic ®elds up to 8 T. Solid lines are guide to the eye.
2. Experiments Polycrystalline samples of ErCo2 and Er0:8 Y0:2 Co2 were prepared by melting the pure constituents (3N) in an argon arc furnace. The ingots were annealed in an evacuated quartz tube at 800 C for one week. The cubic Laves phase structure was con®rmed by X-ray diraction measurements. The obtained lattice parameter a is for ErCo2 and 7.168 Afor Er0:8 Y0:2 Co2 . The 7.160 A speci®c heat was measured by a conventional heat pulse technique with an adiabatic shield in a ®eld of 0±8 T between 1.4 and 55 K. A calibrated carbon-glass thermometer was used for temperature measurements in a magnetic ®eld. 3. Results Fig. 1 shows the temperature dependence of speci®c heat of ErCo2 in magnetic ®elds up to 8 T. In zero ®eld, the speci®c heat shows a sharp peak at T0 32.2 K, indicating a ®rst-order magnetic phase transition. The transition temperature is in good agreement with a previous work . As the magnetic ®eld is increased, the peak shifts towards a higher temperature. As shown in Fig. 2, T0 increases in a linear fashion with increasing ®eld at a rate of 2 K/T. The sharp transition is retained up to 8 T, although the peak height falls o. The entropy S was obtained by integrating C=T with respect to T except the temperature range where speci®c heat
Fig. 2. First-order phase transition temperature T0 vs magnetic ®eld of ErCo2 . A Solid line is the result of least square ®tting of the data to a straight line.
anomalies appear. P Near T0 , we evaluated S from the equation, S Q=T , where Q is the energy input. The total entropy of ErCo2 in various magnetic ®elds is de-
H. Wada et al. / Cryogenics 39 (1999) 915±919
picted in Fig. 3 as a function of temperature. We note that the entropy jump at T0 in zero ®eld is 9 J/K mol, which is 40% of the total entropy just above T0 . This implies that the magnetic contribution to the entropy is comparable to the lattice entropy load in this temperature range, being favorable for the application to refrigeration cycle devices. With increasing magnetic ®eld, the entropy jump becomes less sharp and shifts towards a higher temperature. Fig. 4 shows the temperature dependence of the entropy change caused by a magnetic ®eld DS. Because of the nature of a ®rst-order phase transition in zero ®eld, the DS rise is quite precipitous at 32 K. At high ®elds, DS decreases gradually with increasing temperature. The maximum value of DS is 10± 12 J/K mol regardless of magnetic ®eld. The adiabatic temperature change DTad (magnetocaloric eect) vs T of ErCo2 is shown in Fig. 5, which shows a peak, similarly to DS ÿ T . There is no signi®cant dierence in the magnitude of DTad between 6 and 8 T. The substitution of Y for Er lowers T0 . Duc et al.  studied magnetic properties of Er1ÿx Yx Co2 . According to their results, T0 shifts to 24 K by the substitution of 20% of Y with retaining a ®rst-order phase transition. In order to study eects of Y substitution on the magnetocaloric properties, we have measured speci®c heat of Er0:8 Y0:2 Co2 . The speci®c heat curves at H 0, 4 and 8 T are displayed in Fig. 6. A sharp peak is observed for H 0 and 4 T, while the transition is rounded o in case of 8 T. The DS ÿ T and DTad ÿ T curves of Er0:8 Y0:2 Co2
are shown in Figs. 7 and 8, respectively. Overall features of DS ÿ T and DTad ÿ T are similar to those of ErCo2 but the maximum values of DS and DTad are 20±30%
Fig. 3. Temperature dependence of the total entropy of ErCo2 in magnetic ®elds up to 8 T.
Fig. 5. The adiabatic temperature change, DTad vs temperature curves of ErCo2 .
Fig. 4. Temperature dependence of the entropy change caused by a magnetic ®eld, DS of ErCo2 .
H. Wada et al. / Cryogenics 39 (1999) 915±919
Fig. 6. Temperature dependence of speci®c heat of Er0:8 Y0:2 Co2 in magnetic ®elds up to 8 T. Solid lines are guide to the eye.
Fig. 8. The adiabatic temperature change, DTad vs temperature curves of Er0:8 Y0:2 Co2 .
Fig. 7. Temperature dependence of the entropy change caused by a magnetic ®eld, DS of Er0:8 Y0:2 Co2 .
smaller than those of ErCo2 . These results indicate that the Y substitution is less eective to improve magnetocaloric properties of ErCo2 .
The present results have shown a large magnetocaloric eect of ErCo2 at around 30 K. In a magnetic ®eld of 4 T, DS reaches 11 J/K mol at 32 K, which exceeds those of other Laves phase compounds RAl2 and RNi2 , 7±8 J/K mol at 5 T , although large DS extends over a narrow temperature range. The maximum value of DTad at 8T equals to 13 K, which is twice as large as that of HoCo2 . This value is comparable to that of Dy0:5 Er0:5 Al2 , which was recently proposed as an active magnetic regenerative material at around 40 K . These results suggest a high potential of ErCo2 for a working substance of magnetic refrigeration at 30±50 K. The reasons for the large magnetocaloric eect of ErCo2 are (i) the entropy jump at T0 is large; (ii) T0 increases considerably with increasing magnetic ®eld and (iii) a sharp transition is retained up to 8 T. The large magnetic entropy change at T0 is attributable to a multiplet of Er in a paramagnetic ground state. In a previous paper, we estimated the ground state in the paramagnetic region of ErCo2 as a
3 C8 quadruplet . Below T0 , the exchange ®eld suddenly lifts the degeneracy of the ground state multiplet, which yields the entropy change of R ln 4 11:5 J/K mol. This is the origin of the large entropy change of ErCo2 at T0 . Strong magnetic ®eld dependence of T0 is a unique feature of RCo2 . The ®rst-order magnetic transition of RCo2 with R Dy, Ho and Er originates from an itinerant electron metamagnetic character of the Co sub-
H. Wada et al. / Cryogenics 39 (1999) 915±919
lattice. The Co moment is induced by the exchange ®eld from the R site, Hex , when it is larger than the critical ®eld, Hcr 70 T . In case of ErCo2 , Hex barely exceeds Hcr in the ground state. With increasing temperature, Hex decreases due to thermal ¯uctuations of Er moments and the Co atoms suddenly lose a magnetic moment when Hex < Hcr at T0 . This is a scenario of a ®rst-order magnetic transition in RCo2 . In this context, it is natural that T0 is sensitive to magnetic ®eld. A sharp transition in magnetic ®elds up to 8 T is surprising, because the measurements were performed on a polycrystalline sample. Generally, the rare earth ions, except Gd, show strong magnetic anisotropy. The magnetization easy axis of ErCo2 is a h1 1 1i direction and the anisotropy constant K1 is ÿ5 J/kg at 5 K . This value is 1.4 times as large as that of ErFe2 . The corresponding anisotropy ®eld is 60 T. This implies that the Er moment remains in a nearly h1 1 1i direction by applying a magnetic ®eld of 8 T. In case of a polycrystalline sample, each grain has an angle between its easy axis and the magnetic ®eld direction. Random distribution of the angle broadens the transition. Fortunately, ErCo2 has a cubic structure. The largest angle between h1 1 1i and H is realized in a grain which has a h1 0 0i axis along the magnetic ®eld direction. The Er moment in this grain feels the least p magnetic ®eld of H = 3. Presumably, the narrow distribution of the magnetic ®eld inside the sample is responsible for a sharp transition of ErCo2 in magnetic ®elds up to 8 T. The transition temperature T0 is lowered by the substitution of Y for Er. We observed a 20±30% reduction in DS and DTad compared with ErCo2 . This is because the number of magnetic rare earth metals is decreased by 20%. Therefore, the substitution for Co site is more preferable to improve the magnetocaloric properties at lower temperatures. Along this line, the measurements on Er(Co1ÿx Nix )2 are under progress.
Note added in proof Recently, Giguere et al. have reported magnetocaloric eect of ErCo2 , similar to ours.
Acknowledgements This work was supported by Grain-in-Aid for Developmental Scienti®c Research (No. 09555185) from the Ministry of Education, Science and Culture. References  Pecharsky VK, Gschneidner jr. KA. Giant magnetocaloric eect in Gd5 (Si2 Ge2 ). Phys Rev Lett 1997;78:4494.  Annaorazov MP, Asatryan KA, Myalikgulyev G, Nikitin SA, Tishin AM, Tyurin AL. Alloys of the Fe±Rh system as a new class of working material for magnetic refrigerators. Cryogenics 1992;32:867.  Buschow KHJ. Rare earth compounds. In: Wohlfarth EP, editor. Ferromagnetic Materials, vol. 1. Amsterdam: North-Holland, 1980. p. 297.  Lemaire R. Magnetic properties of the intermetallic compounds of Cobalt with the rare earth metals and Yttrium. Cobalt 1966;33:201.  Nikitin SA, Tishin AM. Magnetocaloric eect in HoCo2 compound. Cryogenics 1991;31:167.  Voiron J. Metamagnetisme en champ interne: application aux composes TCo2 . Thesis, University of Grenoble, 1973.  Imai H, Wada H, Shiga M. Calorimetric study on magnetism ErCo2 of. J Magn Magn Mater 1995;140±144:835.  Wada H, Tomekawa S, Shiga M. Magnetocaloric eect of ErCo2 . J Magn Magn Mater 1999;196±197:689.  Aleksandryan VV, Baranov NV, Kozlov AI, Markosyan AS. Band metamagnetism of the d-subsystem of the ErCo2 single crystal: investigation of magnetic and electrical properties. Phys Met Metall 1988;66:50.  Duc NH, Hien TD, Brommer PE, Franse JJM. Electronic and magnetic properties of Er1ÿx Yx Co2 compounds. J Phys F 1988;18:275.  Hashimoto T. Recent investigations on refrigerants for magnetic refrigerators. Adv Cryog Eng 1986;32:261.  Gschneidner KA, Takeya H, Moorman JO, Pecharsky VK. (Dy0:5 Er0:5 )Al2 : a large magnetocaloric eect material for lowtemperature magnetic refrigeration. Appl Phys Lett 1994:25364.  Goto T, Fukamichi K, Sakakibara T, Komatsu H. Itinerant electron metamagnetism in YCo2 . Solid State Commun 1989;72:945.  Andreyev AV, Deryagin AV, Zadvorkin SM, Moskalev VN, Sinitsin YV. In¯uence of the 3d-metal on the magnetic properties of quasibinary rare-earth intermetallics Er(Fe1ÿx Cox )2 . Phys Met Metall 1985;59:57.  Giguere A, Foldeaki M, Schnelle W, Gmelin E. Metamagnetic transition and magnetocaloric eect in ErCo2 . J Phys Condens Matter 1999;11:6969.