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The influence of gallium on magnetocaloric effect in Gd60Tb40 alloys
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 307 (2006) 120–123 www.elsevier.com/locate/jmmm
The influence of gallium on magnetocaloric effect in Gd60Tb40 alloys W.Q. Ao, Y.X. Jian, F.S. Liu, X.W. Feng, J.Q. Li Department of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China Received 25 October 2005; received in revised form 16 February 2006 Available online 27 April 2006
Abstract The influences of gallium substitution for terbium in Gd60Tb40 on the phase formation, Curie temperature and magnetic entropy change have been investigated. A series of Gd60Tb40xGax with x ¼ 0, 1, 3 and 5 alloys were prepared by arc-melting method. The X-ray diffraction (XRD) analysis reveals that a small amount of Ga substitution for terbium in Gd60Tb40 can form the solid solution (Gd, Tb). The Curie temperature (Tc) increases from 270 K for Gd60Tb40 to 297 K for Gd60Tb37Ga3, while the maximum magnetic entropy changes DSM, max decreases from 5.15 J/K kg for Gd60Tb40 to 3.32 J/K kg for Gd60Tb37Ga3 with increasing the Ga content. r 2006 Elsevier B.V. All rights reserved. Keywords: Gd–Tb alloy; Gallium substitution; Magnetocaloric effect
1. Introduction Magnetic refrigerators have a number of advantages, such as high efficiency, small volume and ecologically clean, as compared with the conventional vapor-cycle refrigerators [1,2]. In recent years, an increasing attention is paid to find magnetic refrigerants that can be used at higher temperatures, especially near room temperature [3]. A vast number of different compounds were tested so far and comprehensive reviews of magnetocaloric materials are available [4–6]. Gd–Tb alloys are one of the candidates for room-temperature magnetic refrigerant [7]. Alloying with additional elements may be an effective way to increase its Tc and improve its magnetocaloric effect (MCE). The gallium-doped Gd5Si1.985Ge1.985Ga0.03 alloy shows that the magnetic ordering temperature can be selectively modified by Ga without decreasing the MCE as compared with Gd5Si2Ge2 [8]. Moreover, Ga dopant can increase corrosion resistance and allows separation of the magnetic phases in the alloys. In the investigation of the Gd–Tb–Ga ternary system, we found that the maximum solid solubility of Ga in (Gd, Tb) at 500 1C is 5.0 at% [9]. In this work, we Corresponding author. Tel.: +86 755 26538537; fax: +86 755 26536239.
E-mail address: [email protected] (W.Q. Ao). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.03.062
study the influence of the substitution of Ga for Tb on the MCE in Gd60Tb40 alloys. 2. Experimental detail The polycrystalline samples of Gd60Tb40xGax with x ¼ 0, 1, 3 and 5 were prepared by arc melting using a non-consumable tungsten electrode and a water-cooled copper tray in argon atmosphere. Gadolinium (purity of 99.99%), terbium (purity of 99.95%) and gallium (purity of 99.999%) were used as starting materials. Titanium was used as an O getter during the melting process. The alloys were re-melted three to four times in order to ensure complete fusion and homogeneous composition. The melted buttons were sealed in evacuated quartz tubes containing titanium chips as an O getter placed in a resistance furnace for homogenization. The homogenization was performed at 900 1C for 12 days and then furnace cooled to room temperature. The phase structure was confirmed by X-ray diffraction (XRD) with Cu Ka radiation (Rigaku D/max 2500 V). The cell refinement was performed by Rietveld method. The MCEs of the alloys were determined by measuring M as function of T and H by using a VSM (LakeShore 7400).
ARTICLE IN PRESS W.Q. Ao et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 120–123
121
3. Results and discussion
Intensity (a. u.)
Obs
Cal
Bragg position Obs-Cal 60
400
5.73 3.64
76.5 76.0 75.5
3.63
Cell Volume (Å3 )
a, b c Volume
75.0 74.5 0
1
2
(b)
3 4 5 6 Ga content (x)
7
8
9
74.0 10
Fig. 2. Rietveld fitted XRD patterns for the Gd60Tb40xGax with x ¼ 3 (a) and the lattice parameters for the Gd60Tb40xGax with x ¼ 0, 1, 2, 3, 4, 5, 7, 9 obtained by Rietveld refinement (b).
x=0 x=3 Tc (K)
x=1
25 20
305 300 295 290 285 280 275 270 265 -1 0 1 2 3 4 5 6 x
15 10
(b) x=1
300
5
200 100
77.0
5.74
3.61 -1
M (Am2/kg)
cps
500
120
77.5
30
(c) x=3
110
5.75
x=5
600
100
78.0
35
700
90 2θ (degree)
3.62
(112) (201)
(103)
(101)
(110)
(d) x=5
(102)
800
(100) (002)
900
80
5.76
40
1000
70
(a)
Lattice paramemters (Å)
Fig. 1 shows the XRD patterns for the Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5, which indicate that the patterns for these alloys are coincidence with those of the solid solution (Gd, Tb). It means that the Ga containing alloys Gd60Tb40xGax can form the solid solution of (Gd, Tb) up to x ¼ 5. A full Rietveld analysis was performed using the XRD patterns of the samples with different Ga contents. A respective analysis result is shown in Fig. 2(a), which shows for the sample of x ¼ 3 the reliability factors of Rietveld refinement of Rp ¼ 10%, Rwp ¼ 12% and Rexp ¼ 8%. Fig. 2(b) shows the cell parameters as a function of Ga content obtained by the Rietveld refinement. The Rietveld refinement results cannot tell that whether Ga substituted Gd or Tb directly since Ga and Tb locate in disorder in the same Wyckoff position in the Gd60Tb40 structure. We may discuss the substitution with the combination of the cell refinement results in Fig. 2(b) and the atom size characteristic (the atom size’s order is Gd4Tb4Ga). In the Gd60Tb40xGax alloy with x ¼ 1, the Ga substituted Tb prior to Gd since the size of Ga is closer to Tb than Gd, leading to the lattice compressing and the cell volume smaller than that of the original alloy (x ¼ 0). As x41, Ga may enter the octahedral interstices and substite the Tb as well, thus the two effects resulted in the cell volume enlarging. With the Ga content further increasing (xX5), the larger atom Gd may also be substituted by Ga, so in general the result was that the cell volume trended to decrease slightly. The temperature dependence of the magnetization (M–T curves) measured in an applied field of 0.1 T and temperature range from 200 to 350 K for the Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5 are shown in Fig. 3. The Curie temperature Tc was determined by minimum point of the first differentiated curve of the M–T
0
(a) x=0
200
0 20
30
40
50
60
220
240
260 280 300 320 Temperature (K)
340
360
380
70
2θ Fig. 1. X-ray diffraction patterns for the Ga containing alloys Gd60Tb40xGax with x ¼ 0 (a), 1 (b), 3 (c) and 5 (d).
Fig. 3. Temperature dependence of the magnetization (M–T) curves for the Ga-containing alloys Gd60Tb40xGax with x ¼ 0, 1, 3 and 5 measured in an applied field of 0.1 T and temperature range from 200 to 350 K; the inset shows the Curie temperature Tc with different Ga concentration x.
ARTICLE IN PRESS W.Q. Ao et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 120–123
122
curve for each alloy. The inset figure shows the Tc value, that obtained from the M–T curves, variation with different Ga concentration x. It can be seen that a small amount of Ga doping can increase the Tc of Gd–Tb alloys. The Tc of the Gd60Tb40xGax alloy increases from 270 to 298 K as x increases from 0 to 3. However, further increasing the Ga substitution leads to decrease of its Tc. The Tc for Gd60Tb35Ga5 alloy decreases to 275 K. Fig. 4 shows the isothermal magnetization curves (M–H curves) measured in the magnetic field range of 0–2.0 T at different temperatures with 3–5 K steps for two typical alloys Gd60Tb40 and Gd60Tb39Ga. The isothermal entropy changes (DSM) was calculated from the M data using the integrated Maxwell relation [10]: Z H qM DS M ðT; HÞ ¼ dH. (1) qT H 0 For M measurements obtained at constant temperature at discrete H intervals, the above Maxwell expression can
120 260 K 265 K 268 K 271 K 274 K 277 K 280K 285 K
x=0 100
60 40 20 0 0
5000
(a)
10000 µ0H (G)
15000
The magnetic entropy change DSM as a function of temperature calculated from the isothermal magnetization curves using Eq. (2) for the Gd60Tb40xGax alloys under the magnetic field changing from 0 to 2.0 T is plotted in Fig. 5. The magnetic entropy changes reach maximum near their Curie temperatures. The inset of Fig. 5 shows the maximum magnetic entropy changes for these samples with different Ga concentration x. The maximum magnetic entropy changes DSM, max for the Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5 are 5.15, 4.43, 3.32 and 4.58 J/K kg, respectively, which indicates that it decreases with Tc increases when Ga content increases up to x ¼ 3, but increases back to 4.58 J/K kg with decrease in its Tc to 275 K when Ga content increases to x ¼ 5. In Ref. [12] the MCE of Gd60Tb40xCox alloys with x ¼ 0, 5, 10 and 15 was investigated up to 1.5 T. With the Co content increasing, the Tc increases from 271 to 286 K and the DSM, max decreases from 3.79 to 2.25 J/K kg, which is worse than our work evidently. The Tc and MCE for Gd1xTbx alloys vary the Gd to Tb ratio, increasing the Curie temperature by increasing the Gd:Tb ratio leads to decrease the MCE. The Ga atom is smaller than Gd or Tb atom. The lattice of the Gd60Tb40xGax would compress when the Ga atoms substitute the Gd or Tb atoms. According to the discussion in Section 3, the Ga atoms in the Gd60Tb40xGax alloys may substitute the Tb atoms and enter the octahedral interstices when the Ga content is small (x less than 3), leading to achieve higher Gd:Tb ratio resulting in higher Tc but lower MCE. However, the Ga atoms may substitute the Gd and Tb
20000 6 260 K
80
270 K 273K 276 K 279 K 282 K 285 K 290 K
60
300 K
100
5 4 ∆SM (J/K.kg)
x=1
120
M (Am2/kg)
5.5
x=0
140
5.0
x=1 x=5
∆S (J/K.kg)
M (Am2/kg)
80
be approximated by the following expression [11]: Z H Z H 1 DS m MðT þ DT; HÞ dH MðT; HÞ dH . DT 0 0 (2)
4.5 4.0 3.5 3.0 -1
3
x=3
0
1
2
3
4
5
6
x
2
40 1 20
(b)
0 0 0 (b)
4000
8000
12000 µ0H (G)
16000
20000
Fig. 4. Isothermal magnetization curves (M–H curves) for two typical alloys Gd60Tb40 (a) and Gd60Tb39Ga (b) measured at various temperatures around Tc and in the magnetic field range of 0–2.0 T.
260 265 270 275 280 285 290 295 300 305 310 315 320 T (K) Fig. 5. The magnetic entropy change DSM as a function of temperature in magnetic field change from 0 to 2.0 T for the alloys Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5; the inset figure shows the maximum magnetic entropy changes for these samples with different Ga concentration x.
ARTICLE IN PRESS W.Q. Ao et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 120–123
atoms, and enter the interstices in the same time when Ga content further increases (x more than 3), so the general result might be lower Gd:Tb ratio resulting in lower Tc but higher MCE. Therefore, the substitution of Ga for Tb in Gd60Tb40 can increase the Curie temperature, but decrease the magnetic entropy change slightly. 4. Conclusion The magnetocaloric properties for Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5 were investigated in this paper. As the Ga content in Gd60Tb40xGax alloys increases from x ¼ 0 to x ¼ 3, the Curie temperature (Tc) increases from 270 to 297 K, while the maximum magnetic entropy changes DSM, max decreases from 5.15 to 3.32 J/K kg. The systems studied here are of easy preparation and present appreciable magnetic entropy change in a low magnetic field change of 2.0 T near room temperature. Acknowledgments We acknowledge the financial support from the National Natural Science Foundation of China (no. 50371058 and 50471108).
123
References [1] O. Tegus, E. Bruck, K.H.J. Buschow, F.R. de Boer, Nature 415 (2002) 150. [2] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [3] E. Bruck, O. Tegus, X.W. Li, F.R. de Boer, K.H.J. Buschow, Physica B 327 (2003) 431. [4] A.M. Tishin, Y.I. Spichkin, The Magnetocaloric Effect and its Applications, IOP Publishing, Bristol, Philadelphia, 2003. [5] K.A. Gschneidner Jr., V.K. Pecharsky, Intermetallic compounds for magnetic refrigeration, in: J.H. Westbrook, R.L. Fleischer (Eds.), Intermetallic Compounds—Principles and Practice, vol.3, Wiley, New York, 2001. [6] K.A. Gschneidner Jr., V.K. Pecharsky, Annu. Rev. Mater. Sci. 30 (2000) 387. [7] Y.Z. Shao, J.K.L. Lai, C.H. Shek, J. Magn. Magn. Mater. 163 (1996) 103. [8] K.A. Gschneidner Jr., V.K. Pecharsky, C.B. Zimm, Mater. Technol. 12 (1997) 145. [9] J.Q. Li, Y.X. Jian, W.Q. Ao, Y.H. Zhuang, W. He, The isothermal section at 5001C of the Gd–Tb–Ga ternary system, J. Alloys Compounds, 2005, in press. [10] T. Hashimoto, T. Numasawa, M. Shino, T. Okada, Cryogenics 21 (1981) 647. [11] M. Foldeaki, R. Chahine, T.K. Bose, J. Appl. Phys. 77 (1995) 3528. [12] L.G. Wang, G.D. Li, W. Zhang, Y.J. Jiang, Metall. Funct. Mater. 11 (2004) 8.
Journal of Magnetism and Magnetic Materials 307 (2006) 120–123 www.elsevier.com/locate/jmmm
The influence of gallium on magnetocaloric effect in Gd60Tb40 alloys W.Q. Ao, Y.X. Jian, F.S. Liu, X.W. Feng, J.Q. Li Department of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China Received 25 October 2005; received in revised form 16 February 2006 Available online 27 April 2006
Abstract The influences of gallium substitution for terbium in Gd60Tb40 on the phase formation, Curie temperature and magnetic entropy change have been investigated. A series of Gd60Tb40xGax with x ¼ 0, 1, 3 and 5 alloys were prepared by arc-melting method. The X-ray diffraction (XRD) analysis reveals that a small amount of Ga substitution for terbium in Gd60Tb40 can form the solid solution (Gd, Tb). The Curie temperature (Tc) increases from 270 K for Gd60Tb40 to 297 K for Gd60Tb37Ga3, while the maximum magnetic entropy changes DSM, max decreases from 5.15 J/K kg for Gd60Tb40 to 3.32 J/K kg for Gd60Tb37Ga3 with increasing the Ga content. r 2006 Elsevier B.V. All rights reserved. Keywords: Gd–Tb alloy; Gallium substitution; Magnetocaloric effect
1. Introduction Magnetic refrigerators have a number of advantages, such as high efficiency, small volume and ecologically clean, as compared with the conventional vapor-cycle refrigerators [1,2]. In recent years, an increasing attention is paid to find magnetic refrigerants that can be used at higher temperatures, especially near room temperature [3]. A vast number of different compounds were tested so far and comprehensive reviews of magnetocaloric materials are available [4–6]. Gd–Tb alloys are one of the candidates for room-temperature magnetic refrigerant [7]. Alloying with additional elements may be an effective way to increase its Tc and improve its magnetocaloric effect (MCE). The gallium-doped Gd5Si1.985Ge1.985Ga0.03 alloy shows that the magnetic ordering temperature can be selectively modified by Ga without decreasing the MCE as compared with Gd5Si2Ge2 [8]. Moreover, Ga dopant can increase corrosion resistance and allows separation of the magnetic phases in the alloys. In the investigation of the Gd–Tb–Ga ternary system, we found that the maximum solid solubility of Ga in (Gd, Tb) at 500 1C is 5.0 at% [9]. In this work, we Corresponding author. Tel.: +86 755 26538537; fax: +86 755 26536239.
E-mail address: [email protected] (W.Q. Ao). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.03.062
study the influence of the substitution of Ga for Tb on the MCE in Gd60Tb40 alloys. 2. Experimental detail The polycrystalline samples of Gd60Tb40xGax with x ¼ 0, 1, 3 and 5 were prepared by arc melting using a non-consumable tungsten electrode and a water-cooled copper tray in argon atmosphere. Gadolinium (purity of 99.99%), terbium (purity of 99.95%) and gallium (purity of 99.999%) were used as starting materials. Titanium was used as an O getter during the melting process. The alloys were re-melted three to four times in order to ensure complete fusion and homogeneous composition. The melted buttons were sealed in evacuated quartz tubes containing titanium chips as an O getter placed in a resistance furnace for homogenization. The homogenization was performed at 900 1C for 12 days and then furnace cooled to room temperature. The phase structure was confirmed by X-ray diffraction (XRD) with Cu Ka radiation (Rigaku D/max 2500 V). The cell refinement was performed by Rietveld method. The MCEs of the alloys were determined by measuring M as function of T and H by using a VSM (LakeShore 7400).
ARTICLE IN PRESS W.Q. Ao et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 120–123
121
3. Results and discussion
Intensity (a. u.)
Obs
Cal
Bragg position Obs-Cal 60
400
5.73 3.64
76.5 76.0 75.5
3.63
Cell Volume (Å3 )
a, b c Volume
75.0 74.5 0
1
2
(b)
3 4 5 6 Ga content (x)
7
8
9
74.0 10
Fig. 2. Rietveld fitted XRD patterns for the Gd60Tb40xGax with x ¼ 3 (a) and the lattice parameters for the Gd60Tb40xGax with x ¼ 0, 1, 2, 3, 4, 5, 7, 9 obtained by Rietveld refinement (b).
x=0 x=3 Tc (K)
x=1
25 20
305 300 295 290 285 280 275 270 265 -1 0 1 2 3 4 5 6 x
15 10
(b) x=1
300
5
200 100
77.0
5.74
3.61 -1
M (Am2/kg)
cps
500
120
77.5
30
(c) x=3
110
5.75
x=5
600
100
78.0
35
700
90 2θ (degree)
3.62
(112) (201)
(103)
(101)
(110)
(d) x=5
(102)
800
(100) (002)
900
80
5.76
40
1000
70
(a)
Lattice paramemters (Å)
Fig. 1 shows the XRD patterns for the Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5, which indicate that the patterns for these alloys are coincidence with those of the solid solution (Gd, Tb). It means that the Ga containing alloys Gd60Tb40xGax can form the solid solution of (Gd, Tb) up to x ¼ 5. A full Rietveld analysis was performed using the XRD patterns of the samples with different Ga contents. A respective analysis result is shown in Fig. 2(a), which shows for the sample of x ¼ 3 the reliability factors of Rietveld refinement of Rp ¼ 10%, Rwp ¼ 12% and Rexp ¼ 8%. Fig. 2(b) shows the cell parameters as a function of Ga content obtained by the Rietveld refinement. The Rietveld refinement results cannot tell that whether Ga substituted Gd or Tb directly since Ga and Tb locate in disorder in the same Wyckoff position in the Gd60Tb40 structure. We may discuss the substitution with the combination of the cell refinement results in Fig. 2(b) and the atom size characteristic (the atom size’s order is Gd4Tb4Ga). In the Gd60Tb40xGax alloy with x ¼ 1, the Ga substituted Tb prior to Gd since the size of Ga is closer to Tb than Gd, leading to the lattice compressing and the cell volume smaller than that of the original alloy (x ¼ 0). As x41, Ga may enter the octahedral interstices and substite the Tb as well, thus the two effects resulted in the cell volume enlarging. With the Ga content further increasing (xX5), the larger atom Gd may also be substituted by Ga, so in general the result was that the cell volume trended to decrease slightly. The temperature dependence of the magnetization (M–T curves) measured in an applied field of 0.1 T and temperature range from 200 to 350 K for the Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5 are shown in Fig. 3. The Curie temperature Tc was determined by minimum point of the first differentiated curve of the M–T
0
(a) x=0
200
0 20
30
40
50
60
220
240
260 280 300 320 Temperature (K)
340
360
380
70
2θ Fig. 1. X-ray diffraction patterns for the Ga containing alloys Gd60Tb40xGax with x ¼ 0 (a), 1 (b), 3 (c) and 5 (d).
Fig. 3. Temperature dependence of the magnetization (M–T) curves for the Ga-containing alloys Gd60Tb40xGax with x ¼ 0, 1, 3 and 5 measured in an applied field of 0.1 T and temperature range from 200 to 350 K; the inset shows the Curie temperature Tc with different Ga concentration x.
ARTICLE IN PRESS W.Q. Ao et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 120–123
122
curve for each alloy. The inset figure shows the Tc value, that obtained from the M–T curves, variation with different Ga concentration x. It can be seen that a small amount of Ga doping can increase the Tc of Gd–Tb alloys. The Tc of the Gd60Tb40xGax alloy increases from 270 to 298 K as x increases from 0 to 3. However, further increasing the Ga substitution leads to decrease of its Tc. The Tc for Gd60Tb35Ga5 alloy decreases to 275 K. Fig. 4 shows the isothermal magnetization curves (M–H curves) measured in the magnetic field range of 0–2.0 T at different temperatures with 3–5 K steps for two typical alloys Gd60Tb40 and Gd60Tb39Ga. The isothermal entropy changes (DSM) was calculated from the M data using the integrated Maxwell relation [10]: Z H qM DS M ðT; HÞ ¼ dH. (1) qT H 0 For M measurements obtained at constant temperature at discrete H intervals, the above Maxwell expression can
120 260 K 265 K 268 K 271 K 274 K 277 K 280K 285 K
x=0 100
60 40 20 0 0
5000
(a)
10000 µ0H (G)
15000
The magnetic entropy change DSM as a function of temperature calculated from the isothermal magnetization curves using Eq. (2) for the Gd60Tb40xGax alloys under the magnetic field changing from 0 to 2.0 T is plotted in Fig. 5. The magnetic entropy changes reach maximum near their Curie temperatures. The inset of Fig. 5 shows the maximum magnetic entropy changes for these samples with different Ga concentration x. The maximum magnetic entropy changes DSM, max for the Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5 are 5.15, 4.43, 3.32 and 4.58 J/K kg, respectively, which indicates that it decreases with Tc increases when Ga content increases up to x ¼ 3, but increases back to 4.58 J/K kg with decrease in its Tc to 275 K when Ga content increases to x ¼ 5. In Ref. [12] the MCE of Gd60Tb40xCox alloys with x ¼ 0, 5, 10 and 15 was investigated up to 1.5 T. With the Co content increasing, the Tc increases from 271 to 286 K and the DSM, max decreases from 3.79 to 2.25 J/K kg, which is worse than our work evidently. The Tc and MCE for Gd1xTbx alloys vary the Gd to Tb ratio, increasing the Curie temperature by increasing the Gd:Tb ratio leads to decrease the MCE. The Ga atom is smaller than Gd or Tb atom. The lattice of the Gd60Tb40xGax would compress when the Ga atoms substitute the Gd or Tb atoms. According to the discussion in Section 3, the Ga atoms in the Gd60Tb40xGax alloys may substitute the Tb atoms and enter the octahedral interstices when the Ga content is small (x less than 3), leading to achieve higher Gd:Tb ratio resulting in higher Tc but lower MCE. However, the Ga atoms may substitute the Gd and Tb
20000 6 260 K
80
270 K 273K 276 K 279 K 282 K 285 K 290 K
60
300 K
100
5 4 ∆SM (J/K.kg)
x=1
120
M (Am2/kg)
5.5
x=0
140
5.0
x=1 x=5
∆S (J/K.kg)
M (Am2/kg)
80
be approximated by the following expression [11]: Z H Z H 1 DS m MðT þ DT; HÞ dH MðT; HÞ dH . DT 0 0 (2)
4.5 4.0 3.5 3.0 -1
3
x=3
0
1
2
3
4
5
6
x
2
40 1 20
(b)
0 0 0 (b)
4000
8000
12000 µ0H (G)
16000
20000
Fig. 4. Isothermal magnetization curves (M–H curves) for two typical alloys Gd60Tb40 (a) and Gd60Tb39Ga (b) measured at various temperatures around Tc and in the magnetic field range of 0–2.0 T.
260 265 270 275 280 285 290 295 300 305 310 315 320 T (K) Fig. 5. The magnetic entropy change DSM as a function of temperature in magnetic field change from 0 to 2.0 T for the alloys Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5; the inset figure shows the maximum magnetic entropy changes for these samples with different Ga concentration x.
ARTICLE IN PRESS W.Q. Ao et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 120–123
atoms, and enter the interstices in the same time when Ga content further increases (x more than 3), so the general result might be lower Gd:Tb ratio resulting in lower Tc but higher MCE. Therefore, the substitution of Ga for Tb in Gd60Tb40 can increase the Curie temperature, but decrease the magnetic entropy change slightly. 4. Conclusion The magnetocaloric properties for Gd60Tb40xGax alloys with x ¼ 0, 1, 3 and 5 were investigated in this paper. As the Ga content in Gd60Tb40xGax alloys increases from x ¼ 0 to x ¼ 3, the Curie temperature (Tc) increases from 270 to 297 K, while the maximum magnetic entropy changes DSM, max decreases from 5.15 to 3.32 J/K kg. The systems studied here are of easy preparation and present appreciable magnetic entropy change in a low magnetic field change of 2.0 T near room temperature. Acknowledgments We acknowledge the financial support from the National Natural Science Foundation of China (no. 50371058 and 50471108).
123
References [1] O. Tegus, E. Bruck, K.H.J. Buschow, F.R. de Boer, Nature 415 (2002) 150. [2] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [3] E. Bruck, O. Tegus, X.W. Li, F.R. de Boer, K.H.J. Buschow, Physica B 327 (2003) 431. [4] A.M. Tishin, Y.I. Spichkin, The Magnetocaloric Effect and its Applications, IOP Publishing, Bristol, Philadelphia, 2003. [5] K.A. Gschneidner Jr., V.K. Pecharsky, Intermetallic compounds for magnetic refrigeration, in: J.H. Westbrook, R.L. Fleischer (Eds.), Intermetallic Compounds—Principles and Practice, vol.3, Wiley, New York, 2001. [6] K.A. Gschneidner Jr., V.K. Pecharsky, Annu. Rev. Mater. Sci. 30 (2000) 387. [7] Y.Z. Shao, J.K.L. Lai, C.H. Shek, J. Magn. Magn. Mater. 163 (1996) 103. [8] K.A. Gschneidner Jr., V.K. Pecharsky, C.B. Zimm, Mater. Technol. 12 (1997) 145. [9] J.Q. Li, Y.X. Jian, W.Q. Ao, Y.H. Zhuang, W. He, The isothermal section at 5001C of the Gd–Tb–Ga ternary system, J. Alloys Compounds, 2005, in press. [10] T. Hashimoto, T. Numasawa, M. Shino, T. Okada, Cryogenics 21 (1981) 647. [11] M. Foldeaki, R. Chahine, T.K. Bose, J. Appl. Phys. 77 (1995) 3528. [12] L.G. Wang, G.D. Li, W. Zhang, Y.J. Jiang, Metall. Funct. Mater. 11 (2004) 8.