- Email: [email protected]
Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1
RARE METALS, Vol. 27, No. 4, Aug 2008, p. 345
Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1 DENG Jianqiua, CHEN Xiangb, and ZHUANG Yinghongb a b
Guangxi Key Laboratory for the Advance Materials and New Preparation Technology, Nanning 530004, China Key laboratory of the Ministry of Education of China for Nonferrous Metal Materials and New Processing Technology, Guangxi University, Nanning 530004, China
Received 25 September 2007; received in revised form 26 November 2007; accepted 13 December 2007
Abstract The lattice parameter and magnetocaloric properties of three samples of LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03 and 0.05 have been investigated by X-ray powder diffraction and magnetization measurements. The lattice parameter increases slightly and the Curie temperature increases somewhat with increasing gallium content. However, a small amount of Ga doping into the sample decreases the magnetic entropy change of the sample. All the samples remain in the first-order magnetic phase transition. The most striking effect of the Ga doping is that the cooling capacity in the samples increases significantly. The maximum magnetic entropy change, –ǻSM, max, and the cooling capacity of the sample LaFe11.2Co0.7Si1.07Ga0.03 are 11.9 Jkg1K1 and 254.8 Jkg1, respectively. Keywords: rare earth; magnetization; magnetocaloric effect; cooling capacity; relative cooling power
1. Introduction Since the discovery of the giant magnetocaloric effect (MCE) in Gd5Si2Ge2 by Pecharsky and Gschneidner in 1997 [1], giant MCEs have been observed in materials that experience a first-order phase transition, such as Gd5SixGe4x [2], MnAs [3], and MnFeP1xAsx [4]. As the first-order transition can cause a sharp drop in magnetization, the entropy change is usually quite large. Nowadays, increasing attention is paid to La(FexSi1x)13 compounds [5-7], which exhibit large magnetic entropy change because of the itinerant-electron metamagnetic (IEM) transition, that is, the field-induced first-order magnetic transition from the paramagnetic to ferromagnetic state. The maximum ǻSM | 3.2 Jkg1K1 was found at its Curie temperature, ~242 K, upon a 2 T magnetic field change for LaFe10.6Si2.4 compound [5]. LaFe11.4Si1.6 compound exhibits large magnetic entropy change, 19.4 Jkg1K1, near its Curie temperatures TC of ~208 K under a field of 5 T [6]. Although La(Fe1xSix)13 compounds display large magnetic entropy change, their Curie temperatures are far from room temperature. However, the Curie temperature and magnetic entropy change can be increased by Co substitution of Fe and hydrogen absorption into the La(FexSi1x)13 compounds [8-22]. The value of the entropy change is 20.3 Jkg1K1 in LaFe11.2Co0.7Si1.1 compound near the Curie Corresponding author: ZHUANG Yinghong
E-mail: [email protected]
temperature TC of 274 K under a magnetic field of 5 T [8]. It markedly exceeds that of pure Gd [23] at the corresponding temperature range. The great entropy change produced by the sharp change of magnetization is associated with a large negative lattice expansion at TC. The values of magnetic entropy change (ǻSM) and adiabatic temperature change (ǻTad) for the La(Fe0.90Si0.10)13H1.1 compound because of the itinerant-electron metamagnetic (IEM) transition are 31 Jkg1K1 and 15.4 K, respectively, at TC = 287 K in the magnetic field change from 0 to 5 T [16]. Recently, the influence of the partial substitution of rare earth elements RE (RE = Nd, Ce, Er, and Pr) for La on the MCE in the La(Fe1xSix)13 compounds was also reported [24-30]. The isothermal magnetic entropy change ǻSM and the adiabatic temperature change ǻTad because of the IEM transition are enhanced by partial substitution of Ce and Pr for La. However, Nd and Er substitution decreases the MCE in the La(Fe1xSix)13 compounds and increases the Curie temperature. Doping B and C into La(Fe0.9Si0.1)13 also increases the Curie temperature and retains the giant MCE because of the first-order magnetic transition property [31-32]. According to Ref. [33], small amount (~0.33 at.%) of Ga substitution for Si + Ge raises the Curie temperature and preserves the giant MCE of the compound Gd5Si2Ge2. So, there has been an attempt to study the influence of a small amount of Ga displacement in LaFe11.2Co0.7Si1.1 alloy on the
346
Curie temperature and MCE. The results showed that Ga doping increases TC and simultaneously has a positive influence on the cooling capacity.
2. Experimental Three samples of LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03 and 0.05 were prepared in an electric arc furnace under an argon atmosphere using a nonconsumable tungsten electrode and a water-cooled copper tray. Lanthanum, iron (purity of 99.9 wt.%), silicon, and gallium (purity of 99.999 wt.%) were used as starting materials. Titanium was used as an O getter during the melting process. To assure homogeneity, the alloys were melted five times to achieve complete fusion and homogeneous composition. The heat treatment was carried out in a vacuum quartz tube at 1223 K for 50 d, followed by quenching in liquid nitrogen. The X-ray powder diffraction data were collected at room temperature on a Rigaku D/Max 2500 diffractometer with Cu KD radiation and graphite monochromator operated at 40 kV, 200 mA. 2ș was between 20° and 60° with datum collection step 0.02°. The Materials Data Inc. software Jade 5.0 and the Powder Diffraction File were used for the phase analysis. Magnetization measurements were carried out using a vibrating sample magnetometer (VSM, Lake Shore 7410) in an applied field up to 2.0 T. The Curie temperature was identified as the minimum in the first differential of the M-T curve, which was carried out at an applied field of 0.01 T. The MCE was evaluated from the calculated magnetic entropy change, 'SM(T, H), in the vicinity of the Curie temperature according to the thermodynamic Maxwell relation.
3. Results and discussion The X-ray powder diffraction patterns for the samples LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03, and 0.05 are shown in Fig. 1 respectively. X-ray powder diffraction showed that the samples crystallize well in the NaZn13-type structure. A minor phase D-Fe also exists as a secondary phase. A similar phenomenon was reported in the compounds LaFe10.6Si2.4 [5] and LaFe11.2Co0.7Si1.1 [8]. These patterns are similar to those of recent reports for La(Fe1xCox)11.4Si1.6 compounds [15]. From Fig. 1, the small amount of the D-Fe phase in the parent compound LaFe11.2Co0.7Si1.1 is more than that in the Ga-containing LaFe11.2Co0.7Si1.1xGax compounds, but does not obviously change with the Ga content x. It seems that Ga addition forms the cubic NaZn13-type structure phase more easily. The refined lattice parameter for these samples analyzed by Jade 5.0 is given in Table 1. The lattice parameter increases slightly with increasing Ga content, which leads to a volume increase of the compounds. For the sample LaFe11.2Co0.7Si1.1, the lattice parameter is in agreement with the results in Ref. [8]. The results indicate that
RARE METALS, Vol. 27, No. 4, Aug 2008
LaFe11.2Co0.7Si1.1 can dissolve a small amount of Ga. A similar result was also found in the binary silicides NdSi1.8 and NdSi, which dissolve 6 at.% and 7 at.% Ga, respectively [34]. The ion radius of Ga is larger than that of Si. So the X-ray diffraction (XRD) patterns and refinement results cannot ascertain whether Ga substitutes Si sites directly or occupies interstitial sites. According to the results reported by Romano [35], only partial Ga atoms are substitutional in the lattice, the remaining Ga atoms occupy interstitial sites.
Fig. 1. X-ray diffraction patterns of the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03 and 0.05 (the arrows indicate the position of the D-Fe Bragg peaks). Table 1. Lattice parameter a, Curie temperature TC, the maximum absolute entropy change |'SM, max|, and the cooling capacity q under a field change of 2.0 T in the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03, and 0.05 |'SM, max| / q / (Jkg1) (Jkg1K1) 12.4 195.1
x
a / nm
TC / K
0.00
1.1487(2)
275
0.03
1.1491(4)
280
11.9
254.8
0.05
1.1494(1)
285
10.2
230.1
Fig. 2 displays the temperature dependence of magnetization (M-T curves) measured in a field of 0.01 T. The Curie temperature TC was determined as the minimum in the first differential of M-T curve. Table 1 gives the values of TC. By a small amount of Ga doping, the Curie temperature increases with increasing Ga in the samples. For the sample LaFe11.2Co0.7Si1.05Ga0.05, the value of TC can be increased up to about 285 K. It is evident that the Curie temperature of the NaZn13-type structure phase increases with the expansion of its lattice, which should be attributed to the changes in the electronic structure instead of changes in the interatomic distance between Fe ions [36]. From the M-T curves, one can see that all the samples are ferromagnetic and maintain the first-order magnetic phase transition. Fig. 3 shows selected isothermal magnetization curves (M-H curves) for three samples under decreasing fields in a wide
Deng J.Q. et al., Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1
temperature range with different temperature steps. In the vicinity of the Curie temperature, the temperature step of 2 K was chosen and a step of 5 K for the regions far away from TC. The magnetic hysteresis upon altering field is small, which is considered to be a very favorable characteristic for magnetic refrigerant applications.
347
where Tcold and Thot are the temperatures of the cold and hot sinks, respectively [39]. When compared, ǻT (i.e., Thot Tcold) equals 45 K in all the samples. The cooling capacities in these samples are listed in Table 1. The cooling capacities equal 195.1, 254.8, and 230.1 Jkg1 for the samples LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03 and 0.05, respectively. The cooling capacity is evidently increased by Ga doping into the compound LaFe11.2Co0.7Si1.1.
Fig. 2. Temperature dependence of the magnetization (M-T curves) for the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03, and 0.05, measured in an applied field of 0.01 T.
The MCE was evaluated from the calculated magnetic entropy change, 'SM(T, H), according to Eq. (1) [37]. H § wM · 'S M (T , H ) ³ ¨ (1) ¸ dH 0 © wT ¹ H For magnetization measured at discrete field and temperature intervals, the magnetic entropy change defined in Eq. (1) can be approximated by Eq. (2) [38]. H 1 ª H 'S M (T , H ) M (T 'T , H )dH ³ M (T , H )dH º ³ 0 0 « »¼ 'T ¬ (2) The magnetic entropy changes, 'SM(T, H), obtained from Eq. (2) using the isothermal magnetization curves of Fig. 3, as a function of temperature in a magnetic field change of 0-2.0 T for these samples, are present in Fig. 4. The magnetic entropy changes reach maximum near their Curie temperatures. The maximum absolute magnetic entropy changes, |ǻSM, max|ˈ for these samples with different Ga concentrations x are also given in Table 1. The maximum magnetic entropy change of LaFe11.2Co0.7Si1.1 reaches 12.4 Jkg1K1 in the field of 2.0 T in this study, which is slightly smaller than that in Ref. [8] under the same applied field. From Fig. 4 and Table 1, one can see that a small amount of Ga doping decreases the magnetic entropy change of compounds. It is interesting to consider the cooling capacity q based on the magnetic entropy change for applications. The cooling capacity is defined as q
Thot
³T
cold
'SdT
(3)
Fig. 3. Isothermal magnetization curves (M-H curves) for the LaFe11.2Co0.7Si1.1xGax samples with x = 0 (a), 0.03 (b), and 0.05 (c), measured at various temperatures around TC and in the magnetic field 2.0 T.
348
RARE METALS, Vol. 27, No. 4, Aug 2008
[7]
[8]
[9]
[10] Fig. 4. Magnetic entropy change, 'SM, as a function of temperature in a magnetic field change of 0-2.0 T for the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03, and 0.05.
[11]
4. Conclusion Three samples LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03, and 0.05 crystallize in the NaZn13-type structure and remain in the first-order magnetic phase order. The lattice parameter and the Curie temperature increase somewhat with increasing Ga content. The magnetic entropy change is decreased by a small amount of Ga doping into the parent compound. However, the cooling capacity increases significantly. The value of cooling capacity for the sample LaFe11.2Co0.7Si1.07Ga0.03 is 254.8 Jkg-1.
Acknowledgement The study is financially supported by the Opening Foundation of Guangxi Key Laboratory for the Advance Materials and New Preparation Technology.
References [1] Pecharsky V.K. and Gschneidner K.A.Jr, Giant magnetocaloric effect in Gd5Si2Ge2, Phys. Rev. Lett., 1997, 78: 4494. [2] Pecharsky V.K. and Gschneidner K.A.Jr, Tunable magnetic regenerator alloys with a giant magnetocaloric effect for magnetic refrigeration from ~ 20 to ~ 290 K, Appl. Phys. Lett., 1997, 70: 3299. [3] Wada H. and Tanabe Y., Giant magnetocaloric effect of MnAs1xSbx, Appl. Phys. Lett., 2001, 79: 3302. [4] Tegus O., Bruck E., Buschow K.H.J., and de Boer F.R., Transition-metal-based magnetic refrigerants for room-temperature applications, Nature, 2002, 415: 150. [5] Zhang X.X., Wen G.H., Wang F.W., Wang W.H., Yu C.H., and Wu G.H., Magnetic entropy change in Fe-based compound LaFe10.6Si2.4, Appl. Phys. Lett., 2000, 77: 3072. [6] Hu F.X., Shen B.G., Sun J.R., Cheng Z.H, Rao G.H., and Zhang X.X., Influence of negative lattice expansion and
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
metamagnetic transition on magnetic entropy change in the compound LaFe11.4Si1.6, Appl. Phys. Lett., 2001, 78: 3675. Xiao S.F., Chen Y.G., Hao C., Wu J.P., and Tu M.G., Research advance on magnetocaloric effect of La-Fe-M(Al, Si) compounds, J. Rare Earths, 2004, 22: 187. Hu F.X., Shen B.G, Sun J.R, Wang G.J., and Cheng Z.H., Very large magnetic entropy change near room temperature in LaFe11.2Co0.7Si1.1, Appl. Phys. Lett., 2002, 80: 826. Hu F.X., Qian X.L., Sun J.R., Wang G.J., Zhang X.X., Cheng Z.H., and Shen B.G., Magnetic entropy change and its temperature variation in compounds La(Fe1xCox)11.2Si1.8, J. Appl. Phys., 2002, 92 (7): 3620. Liu X.B. and Altounian Z., Effect of Co content on magnetic entropy change and structure of La(Fe1xCox)11.4Si1.6, J. Magn. Magn. Mater., 2003, 264: 209. IIyn Max, Tishin A.M., Hu F.X., Gao J., Sun J.R., and Shen B.G., Magnetocaloric properties of the LaFe11.7Si1.3 and LaFe11.2Co0.7Si1.1 systems, J. Magn. Magn. Mater., 2005, 290-291: 712. Hu F.X., Gao J., Qian X.L., IIyn M., Tishin A.M., Sun J.R., and Shen B.G., Magnetocaloric effect in itinerant electron metamagnetic systems La(Fe1xCox)11.9Si1.1, J. Appl. Phys., 2005, 97: 10M303. Liu X.B., Liu X.D., and Altounian Z., Phase formation and magnetocaloric effect in rapidly quenched La(Fe1xCox)11.4Si1.6, J. Appl. Phys., 2005, 98: 113904. Saito A.T., Kobayashi T., and Tsuji H., Magnetocaloric effect of new spherical magnetic refrigerant particles of La(Fe1xyCoxSiy)13 compounds, J. Magn. Magn. Mater., 2007, 310: 2808. Passamani E.C., Larica C., Proveti J.R., Takeuchi A.Y., Gomes A.M., and Ghivelder L., Magnetic and magnetocaloric properties of La(Fe, Co)11.4SP1.6 compounds (SP = Al or Si), J. Magn. Magn. Mater., 2007, 312: 65. Fujieda S., Fujita A., and Fukamichi K., Large magnetocaloric effects in NaZn13-type La(FexSi1x)13 compounds and their hydrides composed of icosahedral clusters, Sci. Technol. Adv. Mater., 2003, 4: 339. Fujieda S., Hasegawa Y., Fujita A., and Fukamichi K., Direct measurement of magnetocaloric effects in itinerant-electron metamagnets La(FexSi1x)13 compounds and their hydrides, J. Magn. Magn. Mater., 2004, 272-276: 2365. Fujita A. and Fukamichi K., Control of large magnetocaloric effects in metamagnetic La(FexSi1x)13 compounds by hydrogenation, J. Alloys Compd., 2005, 404-406: 554. Mandal K., Gutfleisch O., Yan A., Handstein A., and Müller K.H., Effect of reactive milling in hydrogen on the magnetic and magnetocaloric properties of LaFe11.57Si1.43, J. Magn. Magn. Mater., 2005, 290-291: 673. Podgornykh S.M. and Shcherbakova Y.V., Heat capacity of the La(Fe0.88Si0.12)13 and La(Fe0.88Si0.12)13H1.5 compounds with a large magnetocaloric effect, Phys. Rev. B, 2006, 73: 184421. Fukamichi K., Fujita A., and Fujieda S., Large magnetocaloric effects and thermal transport properties of La(Fe Si)13 and their hydrides, J. Alloys Compd., 2006, 408-412: 307.
Deng J.Q. et al., Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1 [22] de Medeiros L.G.Jr and de Oliveira N.A., Magnetocaloric effect in La(FexSi1x)13 doped with hydrogen and under external pressure, J. Alloys Compd., 2006, 424: 41. [23] Dan’kov S.Y., Tishin A.M., Pecharsky V.K., and Gschneidner K.A.Jr, Magnetic phase transitions and the magnetothermal properties of gadolinium, Phys. Rev. B, 1998, 57: 3478. [24] Kim Anh D.T., Thuy N.P., Duc N.H., Nhien T.T., and Nong N.V., Magnetism and magnetocaloric effect in La1-yNdy(Fe0.88Si0.12)13 compounds, J. Magn. Magn. Mater., 2003, 262: 427. [25] Fujita A., Fujieda S., and Fukamichi K., Influence of partial substitution of Ce on the Curie temperature and magnetic entropy change in itinerant-electron metamagnetic La(FexSi1x)13 compounds, J. Appl. Phys., 2006, 99: 08K910. [26] Xu C., Li G.D., and Wang L.G., Itinerant-electron metamagnetic transition and giant magnetocaloric effect in La0.8Ce0.2Fe11.4Si1.6 compound, J. Appl. Phys., 2006, 99: 123913. [27] Fujieda S., Fujita A., Fukamichi K., Hirano N., and Nagaya S., Large magnetocaloric effects enhanced by partial substitution of Ce for La in La(Fe0.88Si0.12)13 compound, J. Alloys Compd., 2006, 408-412: 1165. [28] Balli M., Fruchart D., Gignoux D., Rosca M., and Miraglia S., Magnetic and magnetocaloric properties of La1xErxFe11.44Si1.56 compounds, J. Magn. Magn. Mater., 2007, 313: 43. [29] Fujita A., Fujieda S., and Fukamichi K., Relative cooling power of La(FexSi1x)13 after controlling the Curie temperature by hydrogenation and partial substitution of Ce, J. Magn. Magn. Mater., 2007, 310: e1006.
349
[30] Fujieda S., Fujita A., and Fukamichi K., Enhancement of magnetocaloric effects in La1zPrz(Fe0.88Si0.12)13 and their hydrides, J. Appl. Phys., 2007, 102: 023907. [31] Xie S.H., Li J.Q., and Zhuang Y.H., Influence of boron on the giant magnetocaloric effect of La(Fe0.9Si0.1)13, J. Magn. Magn. Mater., 2007, 311: 589. [32] Li J.Q., Liu F.S., Ao W.Q., Zhuang Y.H., and Zhou K.W., Influence of carbon on the giant magnetocaloric effect of LaFell.7Sil.3, Rare Met., 2006, 25: 556. [33] Pecharsky V.K. and Gschneidner K.A.Jr, Effect of alloying on the giant magnetocaloric effect of Gd5(Si2Ge2), J. Magn. Magn. Mater., 1997, 167: L179. [34] Tokaychuk Y.O., Fedorchuk A.O., Bodak O.I., and Mokr I.R., Phase relations in the Nd-Ga-Si system at 870 K, J. Alloys Compd., 2004, 367: 64. [35] Romano L., Piro A. M., Napolitani E., Bisognin G., Spada A., Grimaldi M.G., and Rimini E., Electrical activation and lattice location of B and Ga impurities implanted in Si, Nucl. Instrum. Methods Phys. Res. Sect. B, 2004, 219-220: 727. [36] Ekkes B., Developments in magnetocaloric refrigeration, J. Phys. D, 2005, 38: R381. [37] Hashimoto T., Numasawa T., Shino M., and Okada T., Magnetic refrigeration in the temperature range from 10 K to room temperature: the ferromagnetic refrigerants, Cryogenics, 1981, 21: 647. [38] Foldeaki M., Chahine R., and Bose T.K., Magnetic measurements: a powerful tool in magnetic refrigerator design, J. Appl. Phys., 1995, 77: 3528. [39] Pecharsky V.K. and Gschneidner K.A.Jr, Some common misconceptions concerning magnetic refrigerant materials, J. Appl. Phys., 2001, 90: 4614.
Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1 DENG Jianqiua, CHEN Xiangb, and ZHUANG Yinghongb a b
Guangxi Key Laboratory for the Advance Materials and New Preparation Technology, Nanning 530004, China Key laboratory of the Ministry of Education of China for Nonferrous Metal Materials and New Processing Technology, Guangxi University, Nanning 530004, China
Received 25 September 2007; received in revised form 26 November 2007; accepted 13 December 2007
Abstract The lattice parameter and magnetocaloric properties of three samples of LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03 and 0.05 have been investigated by X-ray powder diffraction and magnetization measurements. The lattice parameter increases slightly and the Curie temperature increases somewhat with increasing gallium content. However, a small amount of Ga doping into the sample decreases the magnetic entropy change of the sample. All the samples remain in the first-order magnetic phase transition. The most striking effect of the Ga doping is that the cooling capacity in the samples increases significantly. The maximum magnetic entropy change, –ǻSM, max, and the cooling capacity of the sample LaFe11.2Co0.7Si1.07Ga0.03 are 11.9 Jkg1K1 and 254.8 Jkg1, respectively. Keywords: rare earth; magnetization; magnetocaloric effect; cooling capacity; relative cooling power
1. Introduction Since the discovery of the giant magnetocaloric effect (MCE) in Gd5Si2Ge2 by Pecharsky and Gschneidner in 1997 [1], giant MCEs have been observed in materials that experience a first-order phase transition, such as Gd5SixGe4x [2], MnAs [3], and MnFeP1xAsx [4]. As the first-order transition can cause a sharp drop in magnetization, the entropy change is usually quite large. Nowadays, increasing attention is paid to La(FexSi1x)13 compounds [5-7], which exhibit large magnetic entropy change because of the itinerant-electron metamagnetic (IEM) transition, that is, the field-induced first-order magnetic transition from the paramagnetic to ferromagnetic state. The maximum ǻSM | 3.2 Jkg1K1 was found at its Curie temperature, ~242 K, upon a 2 T magnetic field change for LaFe10.6Si2.4 compound [5]. LaFe11.4Si1.6 compound exhibits large magnetic entropy change, 19.4 Jkg1K1, near its Curie temperatures TC of ~208 K under a field of 5 T [6]. Although La(Fe1xSix)13 compounds display large magnetic entropy change, their Curie temperatures are far from room temperature. However, the Curie temperature and magnetic entropy change can be increased by Co substitution of Fe and hydrogen absorption into the La(FexSi1x)13 compounds [8-22]. The value of the entropy change is 20.3 Jkg1K1 in LaFe11.2Co0.7Si1.1 compound near the Curie Corresponding author: ZHUANG Yinghong
E-mail: [email protected]
temperature TC of 274 K under a magnetic field of 5 T [8]. It markedly exceeds that of pure Gd [23] at the corresponding temperature range. The great entropy change produced by the sharp change of magnetization is associated with a large negative lattice expansion at TC. The values of magnetic entropy change (ǻSM) and adiabatic temperature change (ǻTad) for the La(Fe0.90Si0.10)13H1.1 compound because of the itinerant-electron metamagnetic (IEM) transition are 31 Jkg1K1 and 15.4 K, respectively, at TC = 287 K in the magnetic field change from 0 to 5 T [16]. Recently, the influence of the partial substitution of rare earth elements RE (RE = Nd, Ce, Er, and Pr) for La on the MCE in the La(Fe1xSix)13 compounds was also reported [24-30]. The isothermal magnetic entropy change ǻSM and the adiabatic temperature change ǻTad because of the IEM transition are enhanced by partial substitution of Ce and Pr for La. However, Nd and Er substitution decreases the MCE in the La(Fe1xSix)13 compounds and increases the Curie temperature. Doping B and C into La(Fe0.9Si0.1)13 also increases the Curie temperature and retains the giant MCE because of the first-order magnetic transition property [31-32]. According to Ref. [33], small amount (~0.33 at.%) of Ga substitution for Si + Ge raises the Curie temperature and preserves the giant MCE of the compound Gd5Si2Ge2. So, there has been an attempt to study the influence of a small amount of Ga displacement in LaFe11.2Co0.7Si1.1 alloy on the
346
Curie temperature and MCE. The results showed that Ga doping increases TC and simultaneously has a positive influence on the cooling capacity.
2. Experimental Three samples of LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03 and 0.05 were prepared in an electric arc furnace under an argon atmosphere using a nonconsumable tungsten electrode and a water-cooled copper tray. Lanthanum, iron (purity of 99.9 wt.%), silicon, and gallium (purity of 99.999 wt.%) were used as starting materials. Titanium was used as an O getter during the melting process. To assure homogeneity, the alloys were melted five times to achieve complete fusion and homogeneous composition. The heat treatment was carried out in a vacuum quartz tube at 1223 K for 50 d, followed by quenching in liquid nitrogen. The X-ray powder diffraction data were collected at room temperature on a Rigaku D/Max 2500 diffractometer with Cu KD radiation and graphite monochromator operated at 40 kV, 200 mA. 2ș was between 20° and 60° with datum collection step 0.02°. The Materials Data Inc. software Jade 5.0 and the Powder Diffraction File were used for the phase analysis. Magnetization measurements were carried out using a vibrating sample magnetometer (VSM, Lake Shore 7410) in an applied field up to 2.0 T. The Curie temperature was identified as the minimum in the first differential of the M-T curve, which was carried out at an applied field of 0.01 T. The MCE was evaluated from the calculated magnetic entropy change, 'SM(T, H), in the vicinity of the Curie temperature according to the thermodynamic Maxwell relation.
3. Results and discussion The X-ray powder diffraction patterns for the samples LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03, and 0.05 are shown in Fig. 1 respectively. X-ray powder diffraction showed that the samples crystallize well in the NaZn13-type structure. A minor phase D-Fe also exists as a secondary phase. A similar phenomenon was reported in the compounds LaFe10.6Si2.4 [5] and LaFe11.2Co0.7Si1.1 [8]. These patterns are similar to those of recent reports for La(Fe1xCox)11.4Si1.6 compounds [15]. From Fig. 1, the small amount of the D-Fe phase in the parent compound LaFe11.2Co0.7Si1.1 is more than that in the Ga-containing LaFe11.2Co0.7Si1.1xGax compounds, but does not obviously change with the Ga content x. It seems that Ga addition forms the cubic NaZn13-type structure phase more easily. The refined lattice parameter for these samples analyzed by Jade 5.0 is given in Table 1. The lattice parameter increases slightly with increasing Ga content, which leads to a volume increase of the compounds. For the sample LaFe11.2Co0.7Si1.1, the lattice parameter is in agreement with the results in Ref. [8]. The results indicate that
RARE METALS, Vol. 27, No. 4, Aug 2008
LaFe11.2Co0.7Si1.1 can dissolve a small amount of Ga. A similar result was also found in the binary silicides NdSi1.8 and NdSi, which dissolve 6 at.% and 7 at.% Ga, respectively [34]. The ion radius of Ga is larger than that of Si. So the X-ray diffraction (XRD) patterns and refinement results cannot ascertain whether Ga substitutes Si sites directly or occupies interstitial sites. According to the results reported by Romano [35], only partial Ga atoms are substitutional in the lattice, the remaining Ga atoms occupy interstitial sites.
Fig. 1. X-ray diffraction patterns of the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03 and 0.05 (the arrows indicate the position of the D-Fe Bragg peaks). Table 1. Lattice parameter a, Curie temperature TC, the maximum absolute entropy change |'SM, max|, and the cooling capacity q under a field change of 2.0 T in the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03, and 0.05 |'SM, max| / q / (Jkg1) (Jkg1K1) 12.4 195.1
x
a / nm
TC / K
0.00
1.1487(2)
275
0.03
1.1491(4)
280
11.9
254.8
0.05
1.1494(1)
285
10.2
230.1
Fig. 2 displays the temperature dependence of magnetization (M-T curves) measured in a field of 0.01 T. The Curie temperature TC was determined as the minimum in the first differential of M-T curve. Table 1 gives the values of TC. By a small amount of Ga doping, the Curie temperature increases with increasing Ga in the samples. For the sample LaFe11.2Co0.7Si1.05Ga0.05, the value of TC can be increased up to about 285 K. It is evident that the Curie temperature of the NaZn13-type structure phase increases with the expansion of its lattice, which should be attributed to the changes in the electronic structure instead of changes in the interatomic distance between Fe ions [36]. From the M-T curves, one can see that all the samples are ferromagnetic and maintain the first-order magnetic phase transition. Fig. 3 shows selected isothermal magnetization curves (M-H curves) for three samples under decreasing fields in a wide
Deng J.Q. et al., Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1
temperature range with different temperature steps. In the vicinity of the Curie temperature, the temperature step of 2 K was chosen and a step of 5 K for the regions far away from TC. The magnetic hysteresis upon altering field is small, which is considered to be a very favorable characteristic for magnetic refrigerant applications.
347
where Tcold and Thot are the temperatures of the cold and hot sinks, respectively [39]. When compared, ǻT (i.e., Thot Tcold) equals 45 K in all the samples. The cooling capacities in these samples are listed in Table 1. The cooling capacities equal 195.1, 254.8, and 230.1 Jkg1 for the samples LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03 and 0.05, respectively. The cooling capacity is evidently increased by Ga doping into the compound LaFe11.2Co0.7Si1.1.
Fig. 2. Temperature dependence of the magnetization (M-T curves) for the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03, and 0.05, measured in an applied field of 0.01 T.
The MCE was evaluated from the calculated magnetic entropy change, 'SM(T, H), according to Eq. (1) [37]. H § wM · 'S M (T , H ) ³ ¨ (1) ¸ dH 0 © wT ¹ H For magnetization measured at discrete field and temperature intervals, the magnetic entropy change defined in Eq. (1) can be approximated by Eq. (2) [38]. H 1 ª H 'S M (T , H ) M (T 'T , H )dH ³ M (T , H )dH º ³ 0 0 « »¼ 'T ¬ (2) The magnetic entropy changes, 'SM(T, H), obtained from Eq. (2) using the isothermal magnetization curves of Fig. 3, as a function of temperature in a magnetic field change of 0-2.0 T for these samples, are present in Fig. 4. The magnetic entropy changes reach maximum near their Curie temperatures. The maximum absolute magnetic entropy changes, |ǻSM, max|ˈ for these samples with different Ga concentrations x are also given in Table 1. The maximum magnetic entropy change of LaFe11.2Co0.7Si1.1 reaches 12.4 Jkg1K1 in the field of 2.0 T in this study, which is slightly smaller than that in Ref. [8] under the same applied field. From Fig. 4 and Table 1, one can see that a small amount of Ga doping decreases the magnetic entropy change of compounds. It is interesting to consider the cooling capacity q based on the magnetic entropy change for applications. The cooling capacity is defined as q
Thot
³T
cold
'SdT
(3)
Fig. 3. Isothermal magnetization curves (M-H curves) for the LaFe11.2Co0.7Si1.1xGax samples with x = 0 (a), 0.03 (b), and 0.05 (c), measured at various temperatures around TC and in the magnetic field 2.0 T.
348
RARE METALS, Vol. 27, No. 4, Aug 2008
[7]
[8]
[9]
[10] Fig. 4. Magnetic entropy change, 'SM, as a function of temperature in a magnetic field change of 0-2.0 T for the LaFe11.2Co0.7Si1.1xGax samples with x = 0, 0.03, and 0.05.
[11]
4. Conclusion Three samples LaFe11.2Co0.7Si1.1xGax with x = 0, 0.03, and 0.05 crystallize in the NaZn13-type structure and remain in the first-order magnetic phase order. The lattice parameter and the Curie temperature increase somewhat with increasing Ga content. The magnetic entropy change is decreased by a small amount of Ga doping into the parent compound. However, the cooling capacity increases significantly. The value of cooling capacity for the sample LaFe11.2Co0.7Si1.07Ga0.03 is 254.8 Jkg-1.
Acknowledgement The study is financially supported by the Opening Foundation of Guangxi Key Laboratory for the Advance Materials and New Preparation Technology.
References [1] Pecharsky V.K. and Gschneidner K.A.Jr, Giant magnetocaloric effect in Gd5Si2Ge2, Phys. Rev. Lett., 1997, 78: 4494. [2] Pecharsky V.K. and Gschneidner K.A.Jr, Tunable magnetic regenerator alloys with a giant magnetocaloric effect for magnetic refrigeration from ~ 20 to ~ 290 K, Appl. Phys. Lett., 1997, 70: 3299. [3] Wada H. and Tanabe Y., Giant magnetocaloric effect of MnAs1xSbx, Appl. Phys. Lett., 2001, 79: 3302. [4] Tegus O., Bruck E., Buschow K.H.J., and de Boer F.R., Transition-metal-based magnetic refrigerants for room-temperature applications, Nature, 2002, 415: 150. [5] Zhang X.X., Wen G.H., Wang F.W., Wang W.H., Yu C.H., and Wu G.H., Magnetic entropy change in Fe-based compound LaFe10.6Si2.4, Appl. Phys. Lett., 2000, 77: 3072. [6] Hu F.X., Shen B.G., Sun J.R., Cheng Z.H, Rao G.H., and Zhang X.X., Influence of negative lattice expansion and
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
metamagnetic transition on magnetic entropy change in the compound LaFe11.4Si1.6, Appl. Phys. Lett., 2001, 78: 3675. Xiao S.F., Chen Y.G., Hao C., Wu J.P., and Tu M.G., Research advance on magnetocaloric effect of La-Fe-M(Al, Si) compounds, J. Rare Earths, 2004, 22: 187. Hu F.X., Shen B.G, Sun J.R, Wang G.J., and Cheng Z.H., Very large magnetic entropy change near room temperature in LaFe11.2Co0.7Si1.1, Appl. Phys. Lett., 2002, 80: 826. Hu F.X., Qian X.L., Sun J.R., Wang G.J., Zhang X.X., Cheng Z.H., and Shen B.G., Magnetic entropy change and its temperature variation in compounds La(Fe1xCox)11.2Si1.8, J. Appl. Phys., 2002, 92 (7): 3620. Liu X.B. and Altounian Z., Effect of Co content on magnetic entropy change and structure of La(Fe1xCox)11.4Si1.6, J. Magn. Magn. Mater., 2003, 264: 209. IIyn Max, Tishin A.M., Hu F.X., Gao J., Sun J.R., and Shen B.G., Magnetocaloric properties of the LaFe11.7Si1.3 and LaFe11.2Co0.7Si1.1 systems, J. Magn. Magn. Mater., 2005, 290-291: 712. Hu F.X., Gao J., Qian X.L., IIyn M., Tishin A.M., Sun J.R., and Shen B.G., Magnetocaloric effect in itinerant electron metamagnetic systems La(Fe1xCox)11.9Si1.1, J. Appl. Phys., 2005, 97: 10M303. Liu X.B., Liu X.D., and Altounian Z., Phase formation and magnetocaloric effect in rapidly quenched La(Fe1xCox)11.4Si1.6, J. Appl. Phys., 2005, 98: 113904. Saito A.T., Kobayashi T., and Tsuji H., Magnetocaloric effect of new spherical magnetic refrigerant particles of La(Fe1xyCoxSiy)13 compounds, J. Magn. Magn. Mater., 2007, 310: 2808. Passamani E.C., Larica C., Proveti J.R., Takeuchi A.Y., Gomes A.M., and Ghivelder L., Magnetic and magnetocaloric properties of La(Fe, Co)11.4SP1.6 compounds (SP = Al or Si), J. Magn. Magn. Mater., 2007, 312: 65. Fujieda S., Fujita A., and Fukamichi K., Large magnetocaloric effects in NaZn13-type La(FexSi1x)13 compounds and their hydrides composed of icosahedral clusters, Sci. Technol. Adv. Mater., 2003, 4: 339. Fujieda S., Hasegawa Y., Fujita A., and Fukamichi K., Direct measurement of magnetocaloric effects in itinerant-electron metamagnets La(FexSi1x)13 compounds and their hydrides, J. Magn. Magn. Mater., 2004, 272-276: 2365. Fujita A. and Fukamichi K., Control of large magnetocaloric effects in metamagnetic La(FexSi1x)13 compounds by hydrogenation, J. Alloys Compd., 2005, 404-406: 554. Mandal K., Gutfleisch O., Yan A., Handstein A., and Müller K.H., Effect of reactive milling in hydrogen on the magnetic and magnetocaloric properties of LaFe11.57Si1.43, J. Magn. Magn. Mater., 2005, 290-291: 673. Podgornykh S.M. and Shcherbakova Y.V., Heat capacity of the La(Fe0.88Si0.12)13 and La(Fe0.88Si0.12)13H1.5 compounds with a large magnetocaloric effect, Phys. Rev. B, 2006, 73: 184421. Fukamichi K., Fujita A., and Fujieda S., Large magnetocaloric effects and thermal transport properties of La(Fe Si)13 and their hydrides, J. Alloys Compd., 2006, 408-412: 307.
Deng J.Q. et al., Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1 [22] de Medeiros L.G.Jr and de Oliveira N.A., Magnetocaloric effect in La(FexSi1x)13 doped with hydrogen and under external pressure, J. Alloys Compd., 2006, 424: 41. [23] Dan’kov S.Y., Tishin A.M., Pecharsky V.K., and Gschneidner K.A.Jr, Magnetic phase transitions and the magnetothermal properties of gadolinium, Phys. Rev. B, 1998, 57: 3478. [24] Kim Anh D.T., Thuy N.P., Duc N.H., Nhien T.T., and Nong N.V., Magnetism and magnetocaloric effect in La1-yNdy(Fe0.88Si0.12)13 compounds, J. Magn. Magn. Mater., 2003, 262: 427. [25] Fujita A., Fujieda S., and Fukamichi K., Influence of partial substitution of Ce on the Curie temperature and magnetic entropy change in itinerant-electron metamagnetic La(FexSi1x)13 compounds, J. Appl. Phys., 2006, 99: 08K910. [26] Xu C., Li G.D., and Wang L.G., Itinerant-electron metamagnetic transition and giant magnetocaloric effect in La0.8Ce0.2Fe11.4Si1.6 compound, J. Appl. Phys., 2006, 99: 123913. [27] Fujieda S., Fujita A., Fukamichi K., Hirano N., and Nagaya S., Large magnetocaloric effects enhanced by partial substitution of Ce for La in La(Fe0.88Si0.12)13 compound, J. Alloys Compd., 2006, 408-412: 1165. [28] Balli M., Fruchart D., Gignoux D., Rosca M., and Miraglia S., Magnetic and magnetocaloric properties of La1xErxFe11.44Si1.56 compounds, J. Magn. Magn. Mater., 2007, 313: 43. [29] Fujita A., Fujieda S., and Fukamichi K., Relative cooling power of La(FexSi1x)13 after controlling the Curie temperature by hydrogenation and partial substitution of Ce, J. Magn. Magn. Mater., 2007, 310: e1006.
349
[30] Fujieda S., Fujita A., and Fukamichi K., Enhancement of magnetocaloric effects in La1zPrz(Fe0.88Si0.12)13 and their hydrides, J. Appl. Phys., 2007, 102: 023907. [31] Xie S.H., Li J.Q., and Zhuang Y.H., Influence of boron on the giant magnetocaloric effect of La(Fe0.9Si0.1)13, J. Magn. Magn. Mater., 2007, 311: 589. [32] Li J.Q., Liu F.S., Ao W.Q., Zhuang Y.H., and Zhou K.W., Influence of carbon on the giant magnetocaloric effect of LaFell.7Sil.3, Rare Met., 2006, 25: 556. [33] Pecharsky V.K. and Gschneidner K.A.Jr, Effect of alloying on the giant magnetocaloric effect of Gd5(Si2Ge2), J. Magn. Magn. Mater., 1997, 167: L179. [34] Tokaychuk Y.O., Fedorchuk A.O., Bodak O.I., and Mokr I.R., Phase relations in the Nd-Ga-Si system at 870 K, J. Alloys Compd., 2004, 367: 64. [35] Romano L., Piro A. M., Napolitani E., Bisognin G., Spada A., Grimaldi M.G., and Rimini E., Electrical activation and lattice location of B and Ga impurities implanted in Si, Nucl. Instrum. Methods Phys. Res. Sect. B, 2004, 219-220: 727. [36] Ekkes B., Developments in magnetocaloric refrigeration, J. Phys. D, 2005, 38: R381. [37] Hashimoto T., Numasawa T., Shino M., and Okada T., Magnetic refrigeration in the temperature range from 10 K to room temperature: the ferromagnetic refrigerants, Cryogenics, 1981, 21: 647. [38] Foldeaki M., Chahine R., and Bose T.K., Magnetic measurements: a powerful tool in magnetic refrigerator design, J. Appl. Phys., 1995, 77: 3528. [39] Pecharsky V.K. and Gschneidner K.A.Jr, Some common misconceptions concerning magnetic refrigerant materials, J. Appl. Phys., 2001, 90: 4614.