- Email: [email protected]
Magnetocaloric effect in the La0.8Ce0.2Fe11.4−xCoxSi1.6 compounds
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552 www.elsevier.com/locate/jmmm
Current perspectives
Magnetocaloric effect in the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds G.F. Wanga, L. Songa, F.A. Lia, Z.Q. Oua, O. Tegusa,b, E. Bru¨ckb,, K.H.J. Buschowb a
Key Laboratory for Physics and Chemistry of Function Materials, Inner Mongolia Normal University, Hohhot 010022, China b Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands Received 19 November 2007; received in revised form 8 February 2008 Available online 5 March 2008
Abstract The effects of substitution of Co for Fe on the magnetic and magnetocaloric properties of La0.8Ce0.2Fe11.4xCoxSi1.6 (0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds have been investigated. X-ray diffraction shows that all compounds crystallize in the NaZn13-type structure. Magnetic measurements show that the Curie temperature (TC) can be tuned between 184 and 294 K by changing the Co content from 0 to 1. A field-induced methamagnetic transition occurs in samples with x ¼ 0, 0.2 and 0.4. The magnetic entropy changes of the compounds have been determined from the isothermal magnetization measurements by using the Maxwell relation. r 2008 Elsevier B.V. All rights reserved. Keywords: Magnetization process; Magnetic entropy; First-order transition; Magnetic refrigeration
1. Introduction
2. Experiments
Recently, much attention has been paid to magnetic refrigeration, as it might provide an alternative route towards replacing the conventional vapor-compression/ expansion refrigeration technique in use today [1–5]. For developing a magnetic refrigerator, the materials used must exhibit a large magnetocaloric effect (MCE) in a relatively low magnetic field. The La(Fe,Si)13 compounds with NaZn13-type structure belong to the group of promising materials with large MCE [6,7]. The Curie temperature of these compounds can be tuned by replacing Fe by Co or by insertion of the interstitial elements H, C and B into the lattice [8–10]. More recently, there have been some reports showing that the substitution of Ce for La can also have effect on changing the Curie temperature and the MCE [11,12]. However, the TC of these alloys is far below room temperature. In this paper, we report on the magnetic properties and magnetic entropy changes of the La0.8Ce0.2 Fe11.4xCoxSi1.6 compounds with x=0, 0.2, 0.4, 0.6, 0.8 and 1.0.
La0.8Ce0.2Fe11.4xCoxSi1.6 compounds with 0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by arc-melting the starting materials La(3N), Ce(3N), Fe(3N) and Co(3N) and Si(5N) in an argon gas atmosphere. An excess of 15% of La and Ce was added to compensate for the loss during melting. The ingots were remelted five times to ensure homogeneity. The ingots were then inserted into a quartz tube and annealed at 1373 K for 50 h under the protection of highpurity argon gas, followed by quenching in ice water. The crystal structure and the lattice constant were determined by X-ray diffraction (XRD) measurements with Cu-Ka radiation. The magnetization was measured with a LakeShore 7407 VSM and a quantum design SQUID magnetometer. The magnetic entropy change DSm is derived from the magnetization measurements by using the numerical expression of the Maxwell relation DSm ðT; DBÞ ¼
X M i ðT þ DT=2; Bi Þ M i ðT DT=2; Bi Þ DBi DT i
(1) Corresponding author. Tel.: +31 20 525 5640; fax: +31 20 695 1500.
E-mail addresses: [email protected] (O. Tegus), bruck@science. uva.nl (E. Bru¨ck). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.03.004
where Mi is the experimental value of the magnetization, DT is the temperature step and DBi is the field step.
ARTICLE IN PRESS G.F. Wang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552
3549
La0.8Ce0.2Fe11.4-xCoxSi1.6 x = 1.0
Intensity (arb.unit)
x = 0.8
x = 0.6
x = 0.4
x = 0.2
α-Fe
20
30
40
x=0
50 2 (deg)
60
70
80
Fig. 1. XRD patterns of the La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds taken at room temperature.
160
La0.8Ce0.2Fe11.4-xCoxSi1.6
30
140
25
120
La0.8Ce0.2Fe11.4Si1.6
100
B=1T
M (Am2/kg)
M (Am2/kg)
35
20 15
x=0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1.0
10 5
150
60 40 20
0 100
80
200 T (K)
250
300
350
Fig. 2. Temperature dependence of the magnetization of the La0.8Ce0.2 Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds measured in a field of 0.05 T.
0 0
50
100
150
200
250
300
T (K) Fig. 3. Temperature dependence of the magnetization of the La0.8Ce0.2 Fe11.4Si1.6 compounds measured in a field of 1 T in warming and cooling processes.
3. Results and discussion Fig. 1 shows the XRD patterns of La0.8Ce0.2Fe11.4x CoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) taken at room temperature. Evidently, most of the reflections could be identified with the NaZn13-type structure. A refinement analysis of the X-ray data shows that the lattice parameter increases with increasing Co content, the single foreign reflection marked by an arrow being ascribed to a minor amount of a-Fe phase.
Fig. 2 shows the temperature dependence of the magnetization of La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) measured in a low field of 50 mT. The Curie temperatures, TC, were determined to be 184, 208, 224, 252, 272 and 294 K for x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0, respectively. This shows that the Curie temperature is very sensitive to the Co content. The Curie temperature is mainly determined by the exchange interactions between the magnetic atoms. Here, La, Ce and Si are nonmagnetic.
ARTICLE IN PRESS G.F. Wang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552
3550
Thus, for these Co-substituted compounds, the Curie temperature is determined by the Fe–Fe, Fe–Co and Co–Co interactions. The increase of the Curie temperature La0.8Ce0.2Fe11.4Si1.6
140
La0.8Ce0.2Fe11.2Co0.2Si1.6
140
120
120
100
100 M (Am2/kg)
M (Am2/kg)
in the Co-substituted compound is probably associated with the fact that the interaction between Co moments in metal systems is always ferromagnetic, and the Fe–Co and
80 60
80 60 40
40
20
20 170 K-213 K
200 K - 236 K, ΔT = 3 K
ΔT = 3 K
0
0 1
0
2
140 x = 0.4
210K-240K
3 B (T)
4
5
1
0
2
120
ΔT = 2K
x = 0.6
120
3 B (T)
4
5
ΔT=3K
235K-274K
100
100 M (Am2/kg)
M (Am2/kg)
80 80 60
60 40
40
20
20
0
0 0.0
0.2
0.4
0.6
0.8 B (T)
1.0
1.2
1.4
0.0
1.6
0.2
0.4
0.6
0.8 B (T)
1.0
1.2
1.4
1.6
1.0
1.2
1.4
1.6
120 x = 0.8
255K-300K
ΔT = 3K
100
270K-312K
ΔT = 3K
70 60 M (Am2/kg)
80 M (Am2/kg)
x = 1.0
80
60 40
50 40 30 20
20 10 0
0 0.0
0.2
0.4
0.6
0.8 B (T)
1.0
1.2
1.4
1.6
0.0
0.2
0.4
0.6
0.8 B (T)
Fig. 4. Isothermal magnetization of the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds: (a) x ¼ 0, (b) x ¼ 0.2, (c) x ¼ 0.4, (d) x ¼ 0.6, (e) x ¼ 0.8 and (f) x ¼ 1.0 on increasing field in the vicinity of their Curie temperatures, respectively.
ARTICLE IN PRESS G.F. Wang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552
netic state between measurements of various magnetic isotherms. In the latter case, the thermal hysteresis makes it difficult to keep the starting states in one and the same M(T) curves. In the present materials, the origin of large magnetic entropy changes has to be attributed to the large variation in magnetization near the Curie temperature due to the first-order field- induced IEM transition. For x ¼ 0.4, 0.6, 0.8 and 1.0, the maximum values of the entropy changes are 15.4, 12.5, 14.6 and 5.4 J/kg K when obtained with a field change of 1 T, and 3.8, 5.4, 3.3 and 4.8 J/kg K when obtained with a field change of 1.5 T. The phenomenon of asymmetrical broadening of the DSm peak with increasing field was also observed for x ¼ 0.2 and 0.4, which is a result of the field-induced IEM transition from paramagnetic to ferromagnetic state above TC. As shown in Fig. 5(a) and (b), the isothermal entropy change is reduced by a factor of about 20 times with increasing Co concentration x from 0 to 1.0. This can be explained by the fact that the
La0.8Ce0.2Fe11.4-xCoxSi1.6
0-1 T 0-2 T 0-5 T
- ΔSm (J/kgK)
30
20 x = 0.2
x=0
10
0 170
15
180
190
200
210 T (K)
220
230
240
ΔB = 1.5T ΔB = 1.0T
x = 0.4
12 - ΔSm (J/kgK)
Co–Co interactions are much stronger than those of the Fe–Fe ones. Note that the LaCo13 compound is ferromagnetic with a very high Curie temperature, TC ¼ 1318 K [13]. Therefore, the substitution of Co for Fe in the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds results in an increase of the Curie temperature, as observed in previous work on the Fe–Co-based compounds [14]. In order to check whether the transition involves thermal hysteresis, the temperature dependence of magnetization of the La0.8Ce0.2Fe11.4Si1.6 compound was measured in a field of 1 T during heating and cooling using a SQUID magnetometer. The result is shown in Fig. 3. It can be clearly seen that a thermal hysteresis of about 2 K exists between the two curves, indicating the transition is of first order. The isothermal magnetization curves measured at different temperatures in the vicinity of the Curie temperature are shown in Figs. 4(a–f) for x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0, respectively. In order to avoid the effect of hysteresis on the magnetization measurements, the isothermal magnetization of the samples with x ¼ 0 and 0.2 was measured in the following way: during each of the M(B) measurements, the samples were slowly cooled down from their paramagnetic state to the measurement temperature. The measurements were performed by using a SQUID magnetometer. It can be seen that there is a fieldinduced itinerant-electron metamagnetic (IEM) transition from the paramagnetic state to the ferromagnetic state for x ¼ 0, 0.2 and 0.4, the transition disappearing in the samples with higher Co content. The magnetic entropy change, DSm(T,DB), is numerically derived from values of the isothermal magnetization based on the Maxwell relation using Eq. (1). Results for an external field change up to 5 T are shown for x ¼ 0 and 0.2 in Fig. 5a. For the other compounds, results obtained for field changes up to 1.5 T are shown in Fig. 5b. The maximum value (in a field change of 5 T) of the magnetic entropy changes in the plateau is about 25 J/kg K (peak value is 30 J/kg K) for x ¼ 0. This value is much smaller than that reported in Ref. [11], which is 78.3 J/kg K under an applied field of 3 T. We believe that the origin of this large difference in values is not the sample preparation, but the method by means of which the magnetic measurements were performed. In fact, when we measured the sample in the same way as reported in Ref. [11], we obtained similar-high values. This opens the important question about the correct way to determine the magnetic entropy change from magnetization measurements. It is important that measurements of the various M(B) curves (as in Fig. 3) are not influenced by the thermal history of the sample. This can be reached by heating the sample to above the Curie temperature after measuring the M(B) curve at a given T and then cooling the sample in zero field to the next measuring temperature T+DT. If this measuring scheme is not followed, the hysteresis behavior can cause the two starting states to be quite different, not corresponding to points on the same single-valued M(T) curve. The procedure we followed differs from that of the simple continuous heating without passing the paramag-
3551
9
x = 0.6 x = 0.8
6
x = 1.0
3
0 220
240
260 T (K)
280
300
320
Fig. 5. (a) Magnetic entropy change of the La0.8Ce0.2Fe11.4Si1.6 compounds for field changes up till 5 T, (b) magnetic entropy changes of the La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0.2, 0.4, 0.6, 0.8 and 1.0) compounds for magnetic field changes of 0–1 and 0–1.5 T.
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G.F. Wang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552
replacement of Fe by Co weakens the field-induced magnetic transition near the Curie temperature, as shown by Figs. 4(a–f). Besides, the increase in Co content could completely eliminate the IEM transition and, accordingly, the magnetic transition becomes of second order, as reported in Ref. [15] (Fig. 5). In summary, the magnetic entropy changes in La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds were investigated. The substitution of Co for Fe in the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds allows for tuning of the Curie temperature, but gives rise to decrease of MCE.
Acknowledgment This work was supported by the National Natural Foundation of China (Grant no. 50261003), and was partly performed within the Scientific Exchange Program between the Netherlands and China.
References [1] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [2] H. Wada, Y. Tanade, Appl. Phys. Lett. 79 (2001) 3302. [3] O. Tegus, E. Bru¨ck, K.H.J. Buschow, F.R. de Boer, Nature 415 (2002) 150. [4] A. Fujita, S. Fujieda, Y. Hasegawa, K. Fukamichi, Phys. Rev. B 67 (2003) 104416. [5] Ekkes Bru¨ck, J. Phys. D 38 (2005) R381. [6] F.X. Hu, B.G. Shen, J.R. Sun, Z.H. Cheng, G.H. Rao, X.X. Zhang, Appl. Phys. Lett. 78 (2001) 3675. [7] S. Fujieda, A. Fujita, K. Fukamichi, Appl. Phys. Lett. 81 (2002) 1276. [8] A. Fujita, S. Fujieda, K. Fukamichi, H. Mitamura, T. Goto, Phys. Rev. B 65 (2002) 014410. [9] Y.F. Chen, F. Wang, B.G. Shen, G.J. Wang, J.R. Sun, J. Appl. Phys. 93 (2003) 1323. [10] S.H. Xie, J.Q. Li, Y.H. Zhuang, J. Magn. Magn. Mater. 311 (2007) 589. [11] C. Xu, G.D. Li, L.G. Wang, J. Appl. Phys. 99 (2006) 123913. [12] S. Fujieda, A. Fujita, K. Fukamichi, Mater. Trans. 45 (2004) 3228. [13] W.A.J.J. Velge, K.H.J. Buschow, J. Appl. Phys. 39 (1986) 1717. [14] S. Sinnema, J.J.M. Franse, R.J. Radwanski, A. Menovsky, F.R. de Boer, J. Phys. F: Met. Phys. 17 (1987) 233. [15] F.X. Hu, J. Gao, X.L. Qian, Max. Ilyn, A.M. Tishin, J.R. Sun, B.G. Shen, J. Appl. Phys. 97 (2005) 10M303.
Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552 www.elsevier.com/locate/jmmm
Current perspectives
Magnetocaloric effect in the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds G.F. Wanga, L. Songa, F.A. Lia, Z.Q. Oua, O. Tegusa,b, E. Bru¨ckb,, K.H.J. Buschowb a
Key Laboratory for Physics and Chemistry of Function Materials, Inner Mongolia Normal University, Hohhot 010022, China b Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands Received 19 November 2007; received in revised form 8 February 2008 Available online 5 March 2008
Abstract The effects of substitution of Co for Fe on the magnetic and magnetocaloric properties of La0.8Ce0.2Fe11.4xCoxSi1.6 (0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds have been investigated. X-ray diffraction shows that all compounds crystallize in the NaZn13-type structure. Magnetic measurements show that the Curie temperature (TC) can be tuned between 184 and 294 K by changing the Co content from 0 to 1. A field-induced methamagnetic transition occurs in samples with x ¼ 0, 0.2 and 0.4. The magnetic entropy changes of the compounds have been determined from the isothermal magnetization measurements by using the Maxwell relation. r 2008 Elsevier B.V. All rights reserved. Keywords: Magnetization process; Magnetic entropy; First-order transition; Magnetic refrigeration
1. Introduction
2. Experiments
Recently, much attention has been paid to magnetic refrigeration, as it might provide an alternative route towards replacing the conventional vapor-compression/ expansion refrigeration technique in use today [1–5]. For developing a magnetic refrigerator, the materials used must exhibit a large magnetocaloric effect (MCE) in a relatively low magnetic field. The La(Fe,Si)13 compounds with NaZn13-type structure belong to the group of promising materials with large MCE [6,7]. The Curie temperature of these compounds can be tuned by replacing Fe by Co or by insertion of the interstitial elements H, C and B into the lattice [8–10]. More recently, there have been some reports showing that the substitution of Ce for La can also have effect on changing the Curie temperature and the MCE [11,12]. However, the TC of these alloys is far below room temperature. In this paper, we report on the magnetic properties and magnetic entropy changes of the La0.8Ce0.2 Fe11.4xCoxSi1.6 compounds with x=0, 0.2, 0.4, 0.6, 0.8 and 1.0.
La0.8Ce0.2Fe11.4xCoxSi1.6 compounds with 0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by arc-melting the starting materials La(3N), Ce(3N), Fe(3N) and Co(3N) and Si(5N) in an argon gas atmosphere. An excess of 15% of La and Ce was added to compensate for the loss during melting. The ingots were remelted five times to ensure homogeneity. The ingots were then inserted into a quartz tube and annealed at 1373 K for 50 h under the protection of highpurity argon gas, followed by quenching in ice water. The crystal structure and the lattice constant were determined by X-ray diffraction (XRD) measurements with Cu-Ka radiation. The magnetization was measured with a LakeShore 7407 VSM and a quantum design SQUID magnetometer. The magnetic entropy change DSm is derived from the magnetization measurements by using the numerical expression of the Maxwell relation DSm ðT; DBÞ ¼
X M i ðT þ DT=2; Bi Þ M i ðT DT=2; Bi Þ DBi DT i
(1) Corresponding author. Tel.: +31 20 525 5640; fax: +31 20 695 1500.
E-mail addresses: [email protected] (O. Tegus), bruck@science. uva.nl (E. Bru¨ck). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.03.004
where Mi is the experimental value of the magnetization, DT is the temperature step and DBi is the field step.
ARTICLE IN PRESS G.F. Wang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552
3549
La0.8Ce0.2Fe11.4-xCoxSi1.6 x = 1.0
Intensity (arb.unit)
x = 0.8
x = 0.6
x = 0.4
x = 0.2
α-Fe
20
30
40
x=0
50 2 (deg)
60
70
80
Fig. 1. XRD patterns of the La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds taken at room temperature.
160
La0.8Ce0.2Fe11.4-xCoxSi1.6
30
140
25
120
La0.8Ce0.2Fe11.4Si1.6
100
B=1T
M (Am2/kg)
M (Am2/kg)
35
20 15
x=0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1.0
10 5
150
60 40 20
0 100
80
200 T (K)
250
300
350
Fig. 2. Temperature dependence of the magnetization of the La0.8Ce0.2 Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds measured in a field of 0.05 T.
0 0
50
100
150
200
250
300
T (K) Fig. 3. Temperature dependence of the magnetization of the La0.8Ce0.2 Fe11.4Si1.6 compounds measured in a field of 1 T in warming and cooling processes.
3. Results and discussion Fig. 1 shows the XRD patterns of La0.8Ce0.2Fe11.4x CoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) taken at room temperature. Evidently, most of the reflections could be identified with the NaZn13-type structure. A refinement analysis of the X-ray data shows that the lattice parameter increases with increasing Co content, the single foreign reflection marked by an arrow being ascribed to a minor amount of a-Fe phase.
Fig. 2 shows the temperature dependence of the magnetization of La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) measured in a low field of 50 mT. The Curie temperatures, TC, were determined to be 184, 208, 224, 252, 272 and 294 K for x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0, respectively. This shows that the Curie temperature is very sensitive to the Co content. The Curie temperature is mainly determined by the exchange interactions between the magnetic atoms. Here, La, Ce and Si are nonmagnetic.
ARTICLE IN PRESS G.F. Wang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552
3550
Thus, for these Co-substituted compounds, the Curie temperature is determined by the Fe–Fe, Fe–Co and Co–Co interactions. The increase of the Curie temperature La0.8Ce0.2Fe11.4Si1.6
140
La0.8Ce0.2Fe11.2Co0.2Si1.6
140
120
120
100
100 M (Am2/kg)
M (Am2/kg)
in the Co-substituted compound is probably associated with the fact that the interaction between Co moments in metal systems is always ferromagnetic, and the Fe–Co and
80 60
80 60 40
40
20
20 170 K-213 K
200 K - 236 K, ΔT = 3 K
ΔT = 3 K
0
0 1
0
2
140 x = 0.4
210K-240K
3 B (T)
4
5
1
0
2
120
ΔT = 2K
x = 0.6
120
3 B (T)
4
5
ΔT=3K
235K-274K
100
100 M (Am2/kg)
M (Am2/kg)
80 80 60
60 40
40
20
20
0
0 0.0
0.2
0.4
0.6
0.8 B (T)
1.0
1.2
1.4
0.0
1.6
0.2
0.4
0.6
0.8 B (T)
1.0
1.2
1.4
1.6
1.0
1.2
1.4
1.6
120 x = 0.8
255K-300K
ΔT = 3K
100
270K-312K
ΔT = 3K
70 60 M (Am2/kg)
80 M (Am2/kg)
x = 1.0
80
60 40
50 40 30 20
20 10 0
0 0.0
0.2
0.4
0.6
0.8 B (T)
1.0
1.2
1.4
1.6
0.0
0.2
0.4
0.6
0.8 B (T)
Fig. 4. Isothermal magnetization of the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds: (a) x ¼ 0, (b) x ¼ 0.2, (c) x ¼ 0.4, (d) x ¼ 0.6, (e) x ¼ 0.8 and (f) x ¼ 1.0 on increasing field in the vicinity of their Curie temperatures, respectively.
ARTICLE IN PRESS G.F. Wang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3548–3552
netic state between measurements of various magnetic isotherms. In the latter case, the thermal hysteresis makes it difficult to keep the starting states in one and the same M(T) curves. In the present materials, the origin of large magnetic entropy changes has to be attributed to the large variation in magnetization near the Curie temperature due to the first-order field- induced IEM transition. For x ¼ 0.4, 0.6, 0.8 and 1.0, the maximum values of the entropy changes are 15.4, 12.5, 14.6 and 5.4 J/kg K when obtained with a field change of 1 T, and 3.8, 5.4, 3.3 and 4.8 J/kg K when obtained with a field change of 1.5 T. The phenomenon of asymmetrical broadening of the DSm peak with increasing field was also observed for x ¼ 0.2 and 0.4, which is a result of the field-induced IEM transition from paramagnetic to ferromagnetic state above TC. As shown in Fig. 5(a) and (b), the isothermal entropy change is reduced by a factor of about 20 times with increasing Co concentration x from 0 to 1.0. This can be explained by the fact that the
La0.8Ce0.2Fe11.4-xCoxSi1.6
0-1 T 0-2 T 0-5 T
- ΔSm (J/kgK)
30
20 x = 0.2
x=0
10
0 170
15
180
190
200
210 T (K)
220
230
240
ΔB = 1.5T ΔB = 1.0T
x = 0.4
12 - ΔSm (J/kgK)
Co–Co interactions are much stronger than those of the Fe–Fe ones. Note that the LaCo13 compound is ferromagnetic with a very high Curie temperature, TC ¼ 1318 K [13]. Therefore, the substitution of Co for Fe in the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds results in an increase of the Curie temperature, as observed in previous work on the Fe–Co-based compounds [14]. In order to check whether the transition involves thermal hysteresis, the temperature dependence of magnetization of the La0.8Ce0.2Fe11.4Si1.6 compound was measured in a field of 1 T during heating and cooling using a SQUID magnetometer. The result is shown in Fig. 3. It can be clearly seen that a thermal hysteresis of about 2 K exists between the two curves, indicating the transition is of first order. The isothermal magnetization curves measured at different temperatures in the vicinity of the Curie temperature are shown in Figs. 4(a–f) for x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0, respectively. In order to avoid the effect of hysteresis on the magnetization measurements, the isothermal magnetization of the samples with x ¼ 0 and 0.2 was measured in the following way: during each of the M(B) measurements, the samples were slowly cooled down from their paramagnetic state to the measurement temperature. The measurements were performed by using a SQUID magnetometer. It can be seen that there is a fieldinduced itinerant-electron metamagnetic (IEM) transition from the paramagnetic state to the ferromagnetic state for x ¼ 0, 0.2 and 0.4, the transition disappearing in the samples with higher Co content. The magnetic entropy change, DSm(T,DB), is numerically derived from values of the isothermal magnetization based on the Maxwell relation using Eq. (1). Results for an external field change up to 5 T are shown for x ¼ 0 and 0.2 in Fig. 5a. For the other compounds, results obtained for field changes up to 1.5 T are shown in Fig. 5b. The maximum value (in a field change of 5 T) of the magnetic entropy changes in the plateau is about 25 J/kg K (peak value is 30 J/kg K) for x ¼ 0. This value is much smaller than that reported in Ref. [11], which is 78.3 J/kg K under an applied field of 3 T. We believe that the origin of this large difference in values is not the sample preparation, but the method by means of which the magnetic measurements were performed. In fact, when we measured the sample in the same way as reported in Ref. [11], we obtained similar-high values. This opens the important question about the correct way to determine the magnetic entropy change from magnetization measurements. It is important that measurements of the various M(B) curves (as in Fig. 3) are not influenced by the thermal history of the sample. This can be reached by heating the sample to above the Curie temperature after measuring the M(B) curve at a given T and then cooling the sample in zero field to the next measuring temperature T+DT. If this measuring scheme is not followed, the hysteresis behavior can cause the two starting states to be quite different, not corresponding to points on the same single-valued M(T) curve. The procedure we followed differs from that of the simple continuous heating without passing the paramag-
3551
9
x = 0.6 x = 0.8
6
x = 1.0
3
0 220
240
260 T (K)
280
300
320
Fig. 5. (a) Magnetic entropy change of the La0.8Ce0.2Fe11.4Si1.6 compounds for field changes up till 5 T, (b) magnetic entropy changes of the La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0.2, 0.4, 0.6, 0.8 and 1.0) compounds for magnetic field changes of 0–1 and 0–1.5 T.
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replacement of Fe by Co weakens the field-induced magnetic transition near the Curie temperature, as shown by Figs. 4(a–f). Besides, the increase in Co content could completely eliminate the IEM transition and, accordingly, the magnetic transition becomes of second order, as reported in Ref. [15] (Fig. 5). In summary, the magnetic entropy changes in La0.8Ce0.2Fe11.4xCoxSi1.6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds were investigated. The substitution of Co for Fe in the La0.8Ce0.2Fe11.4xCoxSi1.6 compounds allows for tuning of the Curie temperature, but gives rise to decrease of MCE.
Acknowledgment This work was supported by the National Natural Foundation of China (Grant no. 50261003), and was partly performed within the Scientific Exchange Program between the Netherlands and China.
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