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Magnetic entropy change in Fe74−xCrxCu1Nb3Si13B9 (x = 14 and 17) amorphous alloys
Journal of Non-Crystalline Solids 351 (2005) 2373–2377 www.elsevier.com/locate/jnoncrysol
Magnetic entropy change in Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) amorphous alloys S. Atalay *, H. Gencer, V.S. Kolat Department of Physics, Inonu University, Science and Art Faculty, 44069 Malatya, Turkey Received 9 February 2005; received in revised form 2 July 2005
Abstract In this study, the magnetic entropy variation in Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) amorphous alloys was investigated. The temperature dependence of the magnetic entropy variation was calculated from the magnetization data. It was found that the Curie temperature of the sample decreases with increasing Cr content. The maximum entropy change corresponding to a magnetic field variation of 3 T is about 0.9 J/kg K for Fe60Cr14Cu1Nb3Si13B9 and 0.6 J/kg K for Fe57Cr17Cu1Nb3Si13B9. Ó 2005 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.30.Sg
1. Introduction The search for materials with a large magnetocaloric effect (MCE) has continued since the discovery of MCE in 1881 by Warburg [1]. When a dc magnetic field is applied to a magnetic material, the magnetic moments align parallel to the magnetic field direction, which lowers the magnetic entropy and leads to heating of the material. On the other hand, when the field is set to zero, the magnetic moments align randomly, which increases the entropy and leads to cooling of the material. This phenomenon is defined as the magnetocaloric effect. It has been reported that several important factors are required for the application of magnetic materials in magnetic refrigeration technology [2–6]. The material should have (i) nearly zero magnetic hysteresis, (ii) large electric resistance to avoid eddy current loss, (iii) sufficiently large enough spontaneous magnetization and (iv) fine molding. Considering these requirements, amorphous *
Corresponding author. Tel.: +90 422 3410010; fax: +90 422 3410037. E-mail address: [email protected] (S. Atalay). 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.07.012
magnetic materials seem to be good candidates for magnetic refrigerants, because amorphous materials are flexible (can be produced in ribbon or bulk form), have high resistivity due to non-crystalline structure and high saturation magnetization [7]. Up to now, there have not been many reports about MCE in amorphous alloys. Only recently, Didukh and Slawska-Waniewska [8] have reported that CoNbCuSiB amorphous alloy shows a large MCE and such a material could be a good candidate for MCE effect applications. In this article, therefore, the magnetocaloric effect was investigated in the as-received and heat-treated Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) alloys in ribbon form.
2. Experimental Amorphous Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) ribbons were prepared by the melt-spinning technique. The samples were annealed at 873 K for 60 min under argon atmosphere to form nanocrystalline structure. X-ray diffractograms were recorded with a power diffractometer at room temperature using CuKa radiation.
S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377
The magnetic measurements were made using a Q-3398 (Cryogenic) magnetometer in a temperature range from 2 to 300 K and 6 T maximum magnetic field was applied. The magnetic entropy, which is associated with the magnetocaloric effect, can be calculated from the isothermal magnetization curves under the influence of a magnetic field. According to classical thermodynamic theory, the magnetic entropy change (DSm) produced by the variation of a magnetic field from 0 to Hmax is given by Z H max oM dH . ð1Þ DS m ðT ; H Þ ¼ oT H 0 In order to evaluate the magnetic entropy change one needs to make a numerical approximation of the integral in Eq. (1). The usual method is to use isothermal magnetization measurements at small discrete field intervals; jDSmj can be approximated from Eq. (1) by X M i M iþ1 DH ; ð2Þ jDS m j ¼ T iþ1 T i i
30
0.0
20 -0.4
dM/dT
where Mi and Mi+1 are the experimental values of the magnetization at Ti and Ti+1 respectively, under an applied magnetic field Hi. Using Eq. (2), by measuring the M–H curve at various temperatures, one can calculate the magnetic entropy change associated with the magnetic field variation.
about 14 nm using the peak width of a-Fe3Si at half maximum, but the distribution of grain sizes was relatively broad, as reported in the previous studies [9]. Fig. 2 shows magnetization measurements as a function of temperature at zero applied field for Fe60Cr14Cu1Nb3Si13B9. The Curie temperature Tc, which is determined from dM/dT versus temperature curves, is about 226 K for the as-received Fe60Cr14Cu1Nb3Si13B9 sample. Figs. 3 and 4 show magnetization measurements as a function of temperature in various applied magnetic fields for as-received and nanocrystalline Fe57Cr17Cu1Nb3Si13B9 samples, respectively. The Curie temperature is about 156 K for the as-received Fe57Cr17Cu1Nb3Si13B9 sample. The variation of Curie temperature as a function Cr content for the as-received amorphous sample is given in Fig. 5. As can be seen, the Curie temperature decreases linearly with increasing
M (emu/g)
2374
10 -0.8 0
3. Results
Tc -1.2
Fig. 1 shows X-ray diffraction patterns of amorphous and nanocrystalline Fe57Cr17Cu1Nb3Si13B9 alloys. The amorphous structure of the samples was confirmed by the X-ray diffraction measurements. It was found that annealing of the samples up to 823 K does not lead to any significant change in the microstructure of the samples, only stresses induced during the production process were relieved. The XRD studies showed the existence of Fe3Si and Cu crystalline phases in the amorphous matrix of the Fe57Cr17Cu1Nb3Si13B9 sample annealed at 873 K. The grain size of Fe3Si was calculated to be
150
200
250
Temperature (K) Fig. 2. Magnetization as a function of temperature at zero magnetic field for the as-received Fe60Cr14Cu1Nb3Si13B9 ribbon. The inset shows the Curie temperature of the sample.
50
5T 3T 1T 0.1 T 0.005 T 0.002 T
40
M (emu/g)
30
20
10
0 100
150
200
250
T (K) Fig. 1. X-ray diffraction patterns of amorphous and nanocrystalline Fe57Cr17Cu1Nb3Si13B9 ribbons. (1) a-FeSi phase, (2) Fe3Si phase, (3) Cu phase in nanocrystalline sample pattern.
Fig. 3. Magnetization as a function of temperature under different magnetic fields for the as-received Fe57Cr17Cu1Nb3Si13B9 ribbon.
S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377 3T
60
50
1T
M (emu/g)
40
50 mT
30
20
5 mT
10
2 mT 0 0
50
100
150
200
250
300
T (K) Fig. 4. M versus T curves for the nanocrystalline Fe57Cr17Cu1Nb3Si13B9 alloy annealed at 873 K.
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Changes in the magnetic contribution to the entropy under the influence of a magnetic field can be estimated from the isothermal magnetization curves. Therefore, for the amorphous and nanocrystalline samples, a set of magnetization curves was measured for each sample in the temperature range 2–300 K; selected magnetization curves are shown in Fig. 6 for the as-received Fe60Cr14Cu1Nb3Si13B9 sample. The entropy variation was calculated from magnetization curves using Eq. (2). The entropy variation as a function of temperature is given in Figs. 7–9. A very broad peak of jDSmj was observed for the as-received Fe60Cr14Cu1Nb3Si13B9 sample near the Curie temperature. We have found that the maximum entropy change corresponding to a magnetic field variation of 3 T is about 0.9 J/kg K for this sample. A maximum entropy change of about 1.12 J/kg K at 6 T magnetic field was obtained in nanocrystalline
185 K
30 205 K
600
M (emu/g)
Tc (K)
500
400
25
225 K
20
244 K 263 K
15
284 K 295 K
10 300 5 200 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
H (T)
100 2
4
6
8
10
12
14
16
18
20
x (%Cr) Fig. 5. Variation of the Curie temperature as a function of Cr content in as-received amorphous Fe74xCrxCu1Nb3Si13B9 sample. The line is drawn only as a guide eye.
Cr content. The X-ray data (Fig. 1) and our previous studies [9] indicate that the thermal treatment changes the microstructure of amorphous alloys. We have found that crystalline a-FeSi + Fe3Si phases, crystalline Cu and amorphous FeCrNbB phase, which has a much lower Fe content compared to the as-received sample, exist in the heat-treated Fe57Cr17Cu1Nb3Si13B9 sample. Fig. 4 indicates that the nanocrystalline Fe57Cr17Cu1Nb3Si13B9 material exhibits an unusual magnetic behavior with two phase transitions: (i) the phase transition of the amorphous matrix from ferromagnetic to paramagnetic is at about 60 K, (ii) the phase transition of Fe3Si grains from ferromagnetic to paramagnetic occurs at about 850 K, as previously reported by Franco et al. [10]. Therefore, the magnetization value of the nanocrystalline sample does not go to zero above 60 K due to the ferromagnetic Fe3Si phase.
Fig. 6. Isothermal magnetization versus magnetic field curves at different temperatures for the as-received Fe60Cr14Cu1Nb3Si13B9 sample.
1.0
3T 0.8
2.5 T
∆S M (J/kg.K)
0
2T
0.6
1.5 T 0.4
1T 0.2
0.5 T 0.0 180
200
220
240
260
280
300
T (K) Fig. 7. Temperature dependence of magnetic entropy jDSmj at different magnetic fields for as-received Fe60Cr14Cu1Nb3Si13B9 ribbon.
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S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377
1.0
0.8
∆S M (J/kg.K)
netic coupling exists between Fe and Cr [14,15]. This indicates that the total magnetic moment of the transition metal atoms in the alloy decreases with increasing Cr content. Mott and Jones [14] developed a method to calculate the average magnetic moment of an alloy using the rigid band model. According to this model, the variation in total magnetic moment of Fe-based alloy with Cr concentration can be deduced from the relation [15,16]:
1T 2T 3T 4T 5T 6T
0.6
0.4
0.2
0.0 100
lTM ¼ 150
200
250
300
T (K) Fig. 8. Temperature dependence of magnetic entropy jDSmj at different magnetic fields for as-received Fe57Cr17Cu1Nb3Si13B9 sample.
1.2
0.5T 1T 2T 3T 4T 5T
∆SM (J/kg.K)
1.0
0.8
0.6
0.4
0.2
0.0 50
100
150
T (K) Fig. 9. Temperature dependence of magnetic entropy change under different magnetic fields for Fe57Cr17Cu1Nb3Si13B9 alloy annealed at 873 K for 60 min.
Fe57Cr17Cu1Nb3Si13B9 samples. The maximum entropy change corresponding to a magnetic field variation of 3 T for the as-received Fe57Cr17Cu1Nb3Si13B9 sample is about 0.6 J/kg K.
4. Discussion In many studies, it has been shown that Fe74Cu1Nb3Si13B9 amorphous alloys have ferromagnetic ordering below the Curie temperature [11,12] and show a ferromagnetic paramagnetic phase transition at the Curie temperature. There is a strong short-range ferromagnetic exchange interaction between the magnetic moment of Fe atoms in Fe74Cu1Nb3Si13B9 alloys. Concerning the magnetic moment of Cr involved in these amorphous alloys, lCr has been found to be nearly equal to 2.34lB by Moustaide et al. [13], Mott and Jones [14] and it has been assumed that an antiferromag-
74 x x l þ l ; 100 Fe 100 Cr
ð3Þ
where lFe is the magnetic moment of the Fe atoms, lCr is the magnetic moment of the Cr atoms and x is the Cr concentration. Magnetization data showed that lTM decreases with increasing Cr concentration. The observed decrease in saturation magnetization in our experimental results can be attributed to a decrease in the total magnetic moment of the transition metals in the alloy. Moreover, the antiferromagnetic coupling between Fe and Cr cause to weaken the ferromagnetic coupling. This fact is reflected in a decrease in the Curie temperature with increasing Cr concentration. The experimental variation of Tc as a function of Cr content is given in Fig. 5. There are two main requirements for a magnetic material to possess a large magnetic entropy change. One is a large saturation magnetization and the other is an abrupt drop in magnetization near the Curie temperature. As mentioned above, the substitution of Fe by Cr causes a decrease in the saturation magnetization of the amorphous alloy. Moreover, the weakening of the ferromagnetic interaction in Cr containing alloy causes a more gradual decrease of the magnetization near the Curie temperature. This causes a decrease in the magnetic entropy change with increasing Cr content. The maximum entropy change observed in Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) alloys is not as large as jDSmj values reported for GdGeSi [17–20], LaCaMnO [21,22] and MnFePAs [20], but we assume that it is sufficiently large to be used in magnetic refrigeration. Also, it has been reported that if a magnetic material with a large jDSmj peaks at the transition temperature, but falls off rapidly on either side, it is therefore not suitable for use in magnetic refrigeration technology [2,6]. The Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) alloys show a broad peak in the jDSmj versus temperature curve. We therefore assume that this property of the as-received sample could be useful in magnetic refrigeration technology. Also, it has been shown that the Curie temperature of Fe74xCrxCu1Nb3Si13B9 alloys decreases nearly linearly with increasing Cr content, for example, for x = 17, the Curie temperature of this alloy is about 156 K and for x = 11 the Curie temperature is about 320 K. From the point of view of potential applications, a layered complex with various Cr contents may be considered as an example of a complex magnetic refrigerant.
S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377
5. Conclusions For the first time, we have investigated the magnetocaloric properties of amorphous Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) ribbons. It was observed that the maximum entropy change corresponding to a magnetic field variation of 3 T is about 0.9 J/kg K for Fe60Cr14Cu1Nb3Si13B9 and 0.6 J/kg K for Fe57Cr17Cu1Nb3Si13B9. A maximum entropy change of about 1.12 J/kg K at 6 T magnetic field was observed in nanocrystalline Fe57Cr17Cu1Nb3Si13B9 samples. Although the maximum entropy change in Fe74xCrxCu1Nb3Si13B9 is not very high compared to the manganites and Gdbased alloys, the results indicate that Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) amorphous materials show a pronounced entropy change. Acknowledgments This work was supported by the Turkish Academy of Sciences, in the frame work of the Young Scientist Award Program (SA/TUBA-GEBIP/2002-1-3) and Inonu University research fund. The authors would also like to thank to Dr P. Sovak for supplying the samples. References [1] E. Warburg, Ann. Phys. 13 (1881) 141. [2] A.M. Tishin, Y.I. Spichkin, The Magnetocaloric Effect and Its Applications, Institute of Physics Publishing Ltd., Bristol, 2003.
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[3] R.D. McMichael, J.J. Ritter, R.D. Shull, J. Appl. Phys. 73 (1993) 6946. [4] A.M. Gomes, M.S. Reis, I.S. Oliveira, A.P. Guimaraes, A.Y. Takeuchi, J. Magn. Magn. Mater. 242–245 (2002) 870. [5] B.F. Yu, Q. Gao, B. Zhang, X.Z. Meng, Z. Chen, Int. J. Refrigeration 26 (2003) 622. [6] D. Wang, K. Peng, et al., J. Alloys Compds. 358 (2003) 312. [7] F.E. Luborsky, Amorphous Metallic Alloys, Butterworths, London, 1983. [8] P. Didukh, A. Slawska-Waniewska, J. Magn. Magn. Mater. 254– 255 (2003) 407. [9] N. Bayri, H.I. Adiguzel, S. Atalay, P. Sovak, Phys. Stat. Solid A 189 (2002) 805. [10] V. Franco, C.F. Conde, A. Conde, B. Varga, A. Lovas, J. Magn. Magn. Mater 215–216 (2000) 404. [11] A. Slawska-Waniewska, P. Didukh, H.K. Lachowich, T. Kulik, J. Magn. Magn. Mater 215–216 (2000) 495. [12] P. Didukh, A. Slawska-Waniewska, O. Nedelko, Mater. Sci. Forum 373–376 (2001) 273. [13] A. Moustaide, R. Berraho, S. Sayouri, A. Hassini, Physica B 270 (1999) 11. [14] N.F. Mott, H. Jones, Metals and Alloys, Oxford, 1936. [15] A. Das, A.K. Majumdar, J. Magn. Magn. Mater. 128 (1993) 47. [16] Z. Jin, W. Tana, H. Qin, J. Zhang, Y. Du, J. Magn. Magn. Mater. 177–181 (1998) 1115. [17] K.A. Gschneidner, V.K. Pecharsky, Ann. Rev. Mater. 30 (2000) 387. [18] L. Morellon, J. Blasco, P.A. Algarabel, M.R. Ibarra, Phys. Rev. B 62 (2000) 1022. [19] V.K. Pecharsky, K.A. Gschneidner, Phys. Rev. Lett. 78 (1997) 4494. [20] O. Tegus, E. Bruck, K.H.J. Buschow, et al., Nature 415 (6868) (2002) 150. [21] A. Szewczyk, H. Szymczak, A. Wisniewski, et al., Appl. Phys. Lett. 77 (2000) 1026. [22] Y. Sun, X.J. Xu, Y.H. Zhang, J. Magn. Magn. Mater. 219 (2000) 183.
Magnetic entropy change in Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) amorphous alloys S. Atalay *, H. Gencer, V.S. Kolat Department of Physics, Inonu University, Science and Art Faculty, 44069 Malatya, Turkey Received 9 February 2005; received in revised form 2 July 2005
Abstract In this study, the magnetic entropy variation in Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) amorphous alloys was investigated. The temperature dependence of the magnetic entropy variation was calculated from the magnetization data. It was found that the Curie temperature of the sample decreases with increasing Cr content. The maximum entropy change corresponding to a magnetic field variation of 3 T is about 0.9 J/kg K for Fe60Cr14Cu1Nb3Si13B9 and 0.6 J/kg K for Fe57Cr17Cu1Nb3Si13B9. Ó 2005 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.30.Sg
1. Introduction The search for materials with a large magnetocaloric effect (MCE) has continued since the discovery of MCE in 1881 by Warburg [1]. When a dc magnetic field is applied to a magnetic material, the magnetic moments align parallel to the magnetic field direction, which lowers the magnetic entropy and leads to heating of the material. On the other hand, when the field is set to zero, the magnetic moments align randomly, which increases the entropy and leads to cooling of the material. This phenomenon is defined as the magnetocaloric effect. It has been reported that several important factors are required for the application of magnetic materials in magnetic refrigeration technology [2–6]. The material should have (i) nearly zero magnetic hysteresis, (ii) large electric resistance to avoid eddy current loss, (iii) sufficiently large enough spontaneous magnetization and (iv) fine molding. Considering these requirements, amorphous *
Corresponding author. Tel.: +90 422 3410010; fax: +90 422 3410037. E-mail address: [email protected] (S. Atalay). 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.07.012
magnetic materials seem to be good candidates for magnetic refrigerants, because amorphous materials are flexible (can be produced in ribbon or bulk form), have high resistivity due to non-crystalline structure and high saturation magnetization [7]. Up to now, there have not been many reports about MCE in amorphous alloys. Only recently, Didukh and Slawska-Waniewska [8] have reported that CoNbCuSiB amorphous alloy shows a large MCE and such a material could be a good candidate for MCE effect applications. In this article, therefore, the magnetocaloric effect was investigated in the as-received and heat-treated Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) alloys in ribbon form.
2. Experimental Amorphous Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) ribbons were prepared by the melt-spinning technique. The samples were annealed at 873 K for 60 min under argon atmosphere to form nanocrystalline structure. X-ray diffractograms were recorded with a power diffractometer at room temperature using CuKa radiation.
S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377
The magnetic measurements were made using a Q-3398 (Cryogenic) magnetometer in a temperature range from 2 to 300 K and 6 T maximum magnetic field was applied. The magnetic entropy, which is associated with the magnetocaloric effect, can be calculated from the isothermal magnetization curves under the influence of a magnetic field. According to classical thermodynamic theory, the magnetic entropy change (DSm) produced by the variation of a magnetic field from 0 to Hmax is given by Z H max oM dH . ð1Þ DS m ðT ; H Þ ¼ oT H 0 In order to evaluate the magnetic entropy change one needs to make a numerical approximation of the integral in Eq. (1). The usual method is to use isothermal magnetization measurements at small discrete field intervals; jDSmj can be approximated from Eq. (1) by X M i M iþ1 DH ; ð2Þ jDS m j ¼ T iþ1 T i i
30
0.0
20 -0.4
dM/dT
where Mi and Mi+1 are the experimental values of the magnetization at Ti and Ti+1 respectively, under an applied magnetic field Hi. Using Eq. (2), by measuring the M–H curve at various temperatures, one can calculate the magnetic entropy change associated with the magnetic field variation.
about 14 nm using the peak width of a-Fe3Si at half maximum, but the distribution of grain sizes was relatively broad, as reported in the previous studies [9]. Fig. 2 shows magnetization measurements as a function of temperature at zero applied field for Fe60Cr14Cu1Nb3Si13B9. The Curie temperature Tc, which is determined from dM/dT versus temperature curves, is about 226 K for the as-received Fe60Cr14Cu1Nb3Si13B9 sample. Figs. 3 and 4 show magnetization measurements as a function of temperature in various applied magnetic fields for as-received and nanocrystalline Fe57Cr17Cu1Nb3Si13B9 samples, respectively. The Curie temperature is about 156 K for the as-received Fe57Cr17Cu1Nb3Si13B9 sample. The variation of Curie temperature as a function Cr content for the as-received amorphous sample is given in Fig. 5. As can be seen, the Curie temperature decreases linearly with increasing
M (emu/g)
2374
10 -0.8 0
3. Results
Tc -1.2
Fig. 1 shows X-ray diffraction patterns of amorphous and nanocrystalline Fe57Cr17Cu1Nb3Si13B9 alloys. The amorphous structure of the samples was confirmed by the X-ray diffraction measurements. It was found that annealing of the samples up to 823 K does not lead to any significant change in the microstructure of the samples, only stresses induced during the production process were relieved. The XRD studies showed the existence of Fe3Si and Cu crystalline phases in the amorphous matrix of the Fe57Cr17Cu1Nb3Si13B9 sample annealed at 873 K. The grain size of Fe3Si was calculated to be
150
200
250
Temperature (K) Fig. 2. Magnetization as a function of temperature at zero magnetic field for the as-received Fe60Cr14Cu1Nb3Si13B9 ribbon. The inset shows the Curie temperature of the sample.
50
5T 3T 1T 0.1 T 0.005 T 0.002 T
40
M (emu/g)
30
20
10
0 100
150
200
250
T (K) Fig. 1. X-ray diffraction patterns of amorphous and nanocrystalline Fe57Cr17Cu1Nb3Si13B9 ribbons. (1) a-FeSi phase, (2) Fe3Si phase, (3) Cu phase in nanocrystalline sample pattern.
Fig. 3. Magnetization as a function of temperature under different magnetic fields for the as-received Fe57Cr17Cu1Nb3Si13B9 ribbon.
S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377 3T
60
50
1T
M (emu/g)
40
50 mT
30
20
5 mT
10
2 mT 0 0
50
100
150
200
250
300
T (K) Fig. 4. M versus T curves for the nanocrystalline Fe57Cr17Cu1Nb3Si13B9 alloy annealed at 873 K.
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Changes in the magnetic contribution to the entropy under the influence of a magnetic field can be estimated from the isothermal magnetization curves. Therefore, for the amorphous and nanocrystalline samples, a set of magnetization curves was measured for each sample in the temperature range 2–300 K; selected magnetization curves are shown in Fig. 6 for the as-received Fe60Cr14Cu1Nb3Si13B9 sample. The entropy variation was calculated from magnetization curves using Eq. (2). The entropy variation as a function of temperature is given in Figs. 7–9. A very broad peak of jDSmj was observed for the as-received Fe60Cr14Cu1Nb3Si13B9 sample near the Curie temperature. We have found that the maximum entropy change corresponding to a magnetic field variation of 3 T is about 0.9 J/kg K for this sample. A maximum entropy change of about 1.12 J/kg K at 6 T magnetic field was obtained in nanocrystalline
185 K
30 205 K
600
M (emu/g)
Tc (K)
500
400
25
225 K
20
244 K 263 K
15
284 K 295 K
10 300 5 200 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
H (T)
100 2
4
6
8
10
12
14
16
18
20
x (%Cr) Fig. 5. Variation of the Curie temperature as a function of Cr content in as-received amorphous Fe74xCrxCu1Nb3Si13B9 sample. The line is drawn only as a guide eye.
Cr content. The X-ray data (Fig. 1) and our previous studies [9] indicate that the thermal treatment changes the microstructure of amorphous alloys. We have found that crystalline a-FeSi + Fe3Si phases, crystalline Cu and amorphous FeCrNbB phase, which has a much lower Fe content compared to the as-received sample, exist in the heat-treated Fe57Cr17Cu1Nb3Si13B9 sample. Fig. 4 indicates that the nanocrystalline Fe57Cr17Cu1Nb3Si13B9 material exhibits an unusual magnetic behavior with two phase transitions: (i) the phase transition of the amorphous matrix from ferromagnetic to paramagnetic is at about 60 K, (ii) the phase transition of Fe3Si grains from ferromagnetic to paramagnetic occurs at about 850 K, as previously reported by Franco et al. [10]. Therefore, the magnetization value of the nanocrystalline sample does not go to zero above 60 K due to the ferromagnetic Fe3Si phase.
Fig. 6. Isothermal magnetization versus magnetic field curves at different temperatures for the as-received Fe60Cr14Cu1Nb3Si13B9 sample.
1.0
3T 0.8
2.5 T
∆S M (J/kg.K)
0
2T
0.6
1.5 T 0.4
1T 0.2
0.5 T 0.0 180
200
220
240
260
280
300
T (K) Fig. 7. Temperature dependence of magnetic entropy jDSmj at different magnetic fields for as-received Fe60Cr14Cu1Nb3Si13B9 ribbon.
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S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377
1.0
0.8
∆S M (J/kg.K)
netic coupling exists between Fe and Cr [14,15]. This indicates that the total magnetic moment of the transition metal atoms in the alloy decreases with increasing Cr content. Mott and Jones [14] developed a method to calculate the average magnetic moment of an alloy using the rigid band model. According to this model, the variation in total magnetic moment of Fe-based alloy with Cr concentration can be deduced from the relation [15,16]:
1T 2T 3T 4T 5T 6T
0.6
0.4
0.2
0.0 100
lTM ¼ 150
200
250
300
T (K) Fig. 8. Temperature dependence of magnetic entropy jDSmj at different magnetic fields for as-received Fe57Cr17Cu1Nb3Si13B9 sample.
1.2
0.5T 1T 2T 3T 4T 5T
∆SM (J/kg.K)
1.0
0.8
0.6
0.4
0.2
0.0 50
100
150
T (K) Fig. 9. Temperature dependence of magnetic entropy change under different magnetic fields for Fe57Cr17Cu1Nb3Si13B9 alloy annealed at 873 K for 60 min.
Fe57Cr17Cu1Nb3Si13B9 samples. The maximum entropy change corresponding to a magnetic field variation of 3 T for the as-received Fe57Cr17Cu1Nb3Si13B9 sample is about 0.6 J/kg K.
4. Discussion In many studies, it has been shown that Fe74Cu1Nb3Si13B9 amorphous alloys have ferromagnetic ordering below the Curie temperature [11,12] and show a ferromagnetic paramagnetic phase transition at the Curie temperature. There is a strong short-range ferromagnetic exchange interaction between the magnetic moment of Fe atoms in Fe74Cu1Nb3Si13B9 alloys. Concerning the magnetic moment of Cr involved in these amorphous alloys, lCr has been found to be nearly equal to 2.34lB by Moustaide et al. [13], Mott and Jones [14] and it has been assumed that an antiferromag-
74 x x l þ l ; 100 Fe 100 Cr
ð3Þ
where lFe is the magnetic moment of the Fe atoms, lCr is the magnetic moment of the Cr atoms and x is the Cr concentration. Magnetization data showed that lTM decreases with increasing Cr concentration. The observed decrease in saturation magnetization in our experimental results can be attributed to a decrease in the total magnetic moment of the transition metals in the alloy. Moreover, the antiferromagnetic coupling between Fe and Cr cause to weaken the ferromagnetic coupling. This fact is reflected in a decrease in the Curie temperature with increasing Cr concentration. The experimental variation of Tc as a function of Cr content is given in Fig. 5. There are two main requirements for a magnetic material to possess a large magnetic entropy change. One is a large saturation magnetization and the other is an abrupt drop in magnetization near the Curie temperature. As mentioned above, the substitution of Fe by Cr causes a decrease in the saturation magnetization of the amorphous alloy. Moreover, the weakening of the ferromagnetic interaction in Cr containing alloy causes a more gradual decrease of the magnetization near the Curie temperature. This causes a decrease in the magnetic entropy change with increasing Cr content. The maximum entropy change observed in Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) alloys is not as large as jDSmj values reported for GdGeSi [17–20], LaCaMnO [21,22] and MnFePAs [20], but we assume that it is sufficiently large to be used in magnetic refrigeration. Also, it has been reported that if a magnetic material with a large jDSmj peaks at the transition temperature, but falls off rapidly on either side, it is therefore not suitable for use in magnetic refrigeration technology [2,6]. The Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) alloys show a broad peak in the jDSmj versus temperature curve. We therefore assume that this property of the as-received sample could be useful in magnetic refrigeration technology. Also, it has been shown that the Curie temperature of Fe74xCrxCu1Nb3Si13B9 alloys decreases nearly linearly with increasing Cr content, for example, for x = 17, the Curie temperature of this alloy is about 156 K and for x = 11 the Curie temperature is about 320 K. From the point of view of potential applications, a layered complex with various Cr contents may be considered as an example of a complex magnetic refrigerant.
S. Atalay et al. / Journal of Non-Crystalline Solids 351 (2005) 2373–2377
5. Conclusions For the first time, we have investigated the magnetocaloric properties of amorphous Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) ribbons. It was observed that the maximum entropy change corresponding to a magnetic field variation of 3 T is about 0.9 J/kg K for Fe60Cr14Cu1Nb3Si13B9 and 0.6 J/kg K for Fe57Cr17Cu1Nb3Si13B9. A maximum entropy change of about 1.12 J/kg K at 6 T magnetic field was observed in nanocrystalline Fe57Cr17Cu1Nb3Si13B9 samples. Although the maximum entropy change in Fe74xCrxCu1Nb3Si13B9 is not very high compared to the manganites and Gdbased alloys, the results indicate that Fe74xCrxCu1Nb3Si13B9 (x = 14 and 17) amorphous materials show a pronounced entropy change. Acknowledgments This work was supported by the Turkish Academy of Sciences, in the frame work of the Young Scientist Award Program (SA/TUBA-GEBIP/2002-1-3) and Inonu University research fund. The authors would also like to thank to Dr P. Sovak for supplying the samples. References [1] E. Warburg, Ann. Phys. 13 (1881) 141. [2] A.M. Tishin, Y.I. Spichkin, The Magnetocaloric Effect and Its Applications, Institute of Physics Publishing Ltd., Bristol, 2003.
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