- Email: [email protected]
Anisotropy of the magnetocaloric effect in DyNiAl
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 2318–2321
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Anisotropy of the magnetocaloric effect in DyNiAl J. Kasˇtil a,, P. Javorsky´ a, A.V. Andreev b a b
Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16 Prague 2, Czech Republic Institute of Physics ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic
a r t i c l e in f o
a b s t r a c t
Article history: Received 2 September 2008 Received in revised form 2 February 2009 Available online 27 February 2009
We present study of the anisotropic magnetocaloric effect in DyNiAl. This compound crystallizes in the hexagonal ZrNiAl-type structure, orders magnetically below T C ¼ 31 K and undergoes a further magnetic phase transition at T 1 ¼ 15 K. The Dy-moments are aligned ferromagnetically along the hexagonal c-axis below T C , the additional antiferromagnetic component develops within the basal plane below T 1 . The magnetocaloric effect was evaluated from the magnetization measurements with field applied along the c-axis and perpendicular to it. Our data reveal a strong anisotropy of the magnetocaloric effect. The large effect occurs for field applied along the c-axis whereas the entropy change is small for the perpendicular field direction. & 2009 Elsevier B.V. All rights reserved.
PACS: 75.30.Sg 75.30.Gw Keywords: Magnetocaloric effect DyNiAl Magnetism Anisotropy
1. Introduction The search for new magnetic materials with a large magnetocaloric effect (MCE), i.e. materials showing a change in temperature induced by a change in magnetic field under adiabatic conditions, is challenging researchers owing to their potential application in magnetic refrigeration with a large impact on energy savings and environmental concerns. The feasibility of using magnetic refrigeration as an alternative to conventional gas compression methods depends largely upon the design of suitable magnetic refrigerant materials. For optimal efficiency, the materials should posses considerable entropy change jDSmag j over the full temperature span of the cooling cycle. The search for potential magnetic refrigerant materials is frequently focused toward intermetallic compounds with rare-earth and transition-metal elements [1]. These materials often exhibit a large change in magnetic entropy near the magnetic transitions. In this context, materials which undergo multiple magnetic transitions are promising for further studies and in the search of ‘‘table-like’’ MCE. The RNiAl compounds crystallizing in the hexagonal ZrNiAltype structure show complex magnetic order at low temperatures with two magnetic phases when R ¼ Sm; Gd; Tb; Dy; Ho or Tm [2,3]. The second phase transition occurs here at about one half of the ordering temperature, T 1 ’ T C =2, what can be favorable for
Corresponding author.
E-mail address: [email protected] (J. Kasˇtil). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.02.118
MCE. The magnetocaloric properties in the RNiAl compounds has been recently studied and revealed large MCE comparable to those of many potential magnetic refrigerants [4–7]. Some of the RNiAl compounds, like GdNiAl or TbNiAl, show an abrupt transition in the temperature dependence of the lattice constants [8]. The c=a ratio changes strongly at the transition temperature; however, the unit-cell volume remains unchanged. DyNiAl does not show such a transition, but similar effect is observed in the concentration dependence when substituting Ni by Cu in the DyNi1x Cux Al series between x ¼ 0:3 and 0.4 [8,9]. The MCE in the DyNi1x Cux Al series is a subject of a separate paper [10]. In this paper, we focus on a seldom discussed aspect of MCE—the anisotropy of the magnetocaloric properties. The use of intrinsic anisotropic properties is one of the promising ways to increase the refrigerant capacity of a given material and has been theoretically and experimentally studied [11–13]. Among the hexagonal RNiAl compounds we have chosen to investigate DyNiAl, compound with intensively studied magnetic properties and showing significant MCE [6]. DyNiAl orders magnetically below T C ¼ 31 K and undergoes a further magnetic phase transition at T 1 ¼ 15 K [2,14,15]. The Dy-moments are aligned ferromagnetically along the hexagonal c-axis between T C and T 1 . The additional antiferromagnetic basal-plane component develops below T 1 leading to a canted magnetic structure at low temperatures [15]. Magnetization measurements on a DyNiAl single crystal [16] revealed a large uniaxial magnetic anisotropy and field-induced transitions in fields applied along
ARTICLE IN PRESS J. Kasˇtil et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2318–2321
the basal plane. The transition associated with a rotation of the ferromagnetic component toward the field direction is observed between 1 and 2 T in both magnetic phases. The transition corresponding to a spin flop of the antiferromagnetic component occurs only in fields above 10 T [16].
6
4K
5
2. Experimental
M (μB /f.u.)
4
The DyNiAl single crystal was grown by Czochralski method in a tri-arc furnace from the stoichiometric melt of the constituent elements as described in detail in Ref. [16]. The nearly cubic piece of 1:6 mm3 cut from the crystal used also in the previous studies [14–16] was used in the present work. Magnetic measurements were performed in the temperature range from 2 to 50 K and in external magnetic field up to 5 T using the MPMS (Quantum Design) instrument. The sample was cooled down in zero magnetic field. Then the one quarter of magnetization curves was measured. The measured data were corrected for the demagnetization field using the demagnetization factor of 0.3 that reflects the nearly cubic sample shape.
40 K
3
50 K 2
1
H // a 0 0
1
In order to determine the anisotropy of the MCE in DyNiAl, we have measured in detail the magnetization curves with magnetic field applied along the hexagonal c-axis and perpendicular to the c-axis. In the second orientation, the field was oriented along the a-axis, but considering the previous studies [14,15], possible anisotropy of MCE within the basal plane is rather small. The magnetization curves are represented in Figs. 1 and 2 for magnetic field oriented along the c- and a-axes, respectively. The measurements were performed between 2 and 40 K with a 2 K step, additional MðHÞ dependence was measured for Hka at 50 K. The results are well in agreement with the previously measured data [16]. We observe a typical ferromagnetic behavior for
12 K 8
6
M (μB /f.u.)
3
4
5
Fig. 2. The magnetization curves of DyNiAl measured with field applied along the a-axis. The curves between 4 and 40 K are shown with the step of 4 K, i.e. only every other curve is shown for better lucidity.
magnetic field parallel to the hexagonal c-axis. When applying magnetic field along the a-axis, we observe a low initial susceptibility reflecting the strong uniaxial anisotropy. Fieldinduced transition is observed between 1.5 and 2 T, both below and above T 1 . We can exclude its metamagnetic nature because above T 1 the compound is a simple collinear uniaxial ferromagnet. Therefore, we can consider the transition as a rotation of the ferromagnetic component of the magnetic moment from the c-axis to the basal plane. This interpretation is corroborated also by the neutron-diffraction results [15]. The resulting magnetic structure is stable in a wide interval of further increasing field, the additional transition occurs only in high fields around 12 T [16]. The magnetic entropy change, DSM , was derived from the MðHÞ dependencies using the following equation:
DSM ðT av ; H1 Þ ¼
Z 0
4 40 k H // c
2
0 1
2
μ0H (T)
3. Results and discussion
0
2319
2
3
4
5
μ0H (T) Fig. 1. The magnetization curves of DyNiAl measured with field applied along the c-axis. The curves between 12 and 40 K are shown with the step of 2 K; the curves measured below 12 K are not displayed because they almost overlap with that obtained at 12 K.
H1
MðT iþ1 ; HÞ MðT i ; HÞ dH. T iþ1 T i
(1)
The results are displayed in Figs. 3 and 4. When the field is applied along the c-axis, the entropy change shows a maximum near the ordering temperature T C and reaches the value of 1 DSM ¼ 14 and 22 J kg K1 for 2 and 5 T, respectively. This behavior clearly reflects the ferromagnetic ordering of Dy moments along the c-axis. For the perpendicular orientation, Hka, the behavior is much more complex. We observe a small negative MCE (positive DSM ) for the applied field of 1 T what reflects the antiferromagnetic character of the magnetic order within the basal plane below T 1 [15]. The negative MCE persists also between T 1 and T C , indicating the antiferromagnetic type of interactions in this temperature region. The entropy change related to the antiferromagnetic component increases with increasing magnetic field, leading to the total value of 1 2 J kg K1 below 10 K in 2 T. The antiferromagnetic order remains intact also in higher fields and certainly contributes to the entropy change, but the total MCE is strongly influenced by the fieldinduced transition in 1:5 T related to the rotation of the ferromagnetic component from the c-axis to the basal plane.
ARTICLE IN PRESS J. Kasˇtil et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2318–2321
2320
25
10
H // c
DyNiAl
5
2
0 10
20
30
40
- ΔSm (J/kg.K)
6
T (K)
0
10
20
0
10
20
30
30
40
40
50
50
T (K) Fig. 5. The specific heat of DyNiAl, data taken from Ref. [14]. Inset shows the total (full line) and magnetic (dashed line) entropy estimated using the LuNiAl data as a nonmagnetic analogue.
8
Δ B = 4.5 T ΔB=4T ΔB=3T
6
ΔB=2T
Δ B = 2 T // C Δ B = 5 T // C
ΔB=1T
ΔTad (K)
8
magnetic
2 0
T (K) Fig. 3. The magnetocaloric effect in DyNiAl as determined from the magnetization curves with magnetic field applied along the c-axis.
4
LuNiAl
1
0
total
6
3
5
10
8
T1
4
S (J/kg.K)
15
TC
6
Cp (J/kg.K)
- ΔSm (J/kg.K)
20
7 ΔB=5T ΔB=4T ΔB=3T ΔB=2T ΔB=1T
4 2
4
2
0
0 H // a
-2 10
20 T (K)
30
40
Fig. 4. The magnetocaloric effect in DyNiAl as determined from the magnetization curves with magnetic field applied along the a-axis.
The total entropy change gradually becomes negative and we observe two moderate maxima of DSM : around the T C and around 19 K. The existence of a minimum around 24 K is related to the fact that the field-induced transition shifts to higher fields for temperatures above 20 K (see Fig. 2). The size of the entropy change for Hka and Hkc is well comparable below T 1, but the maximum in DSM around T C for Hkc is considerably larger than any values for the perpendicular orientation. The existence of two or even more maxima in the entropy change is often observed in compounds exhibiting multiple phase transitions, e.g. Tb5 Ge4 [17]. The case of DyNiAl and Hka is different, although also DyNiAl undergoes two magnetic phase transitions. The observation of the two maxima in DSM is related here to the nature and temperature development of the fieldinduced transition and occur only if the applied field exceeds the value of this transition (see Fig. 4). Our results reveal a strong anisotropy of the MCE in DyNiAl. We observe a large entropy change for magnetic field parallel to the easy magnetization c-axis whereas DSM is smaller and has a very complex temperature dependence for the perpendicular orientation Hka. The results obtained for Hkc are qualitatively similar to those reported on polycrystalline sample [6], but the size of the MCE is considerably enhanced. The value of DSM ¼ 1 14 J kg K1 obtained in our study for the field of 2 T represents
-2 10
20 T (K)
30
40
Fig. 6. The adiabatic temperature change in DyNiAl calculated form the entropy change and the specific heat data (see Fig. 5) for magnetic field applied along the caxis.
1
ffi 40% increase compared to the value of 10 J kg K1 obtained on polycrystal for the same field. The polycrystalline data represent some average of all the crystallographic directions, so the grains oriented with the c-axis perpendicular to the applied field lessen the total MCE. Another quantity characterizing MCE is the adiabatic temperature change DT ad . Its temperature dependence calculated from the entropy change with the field applied along the c-axis and the specific heat data (see Fig. 5) is shown in Fig. 6. The maximum position in DT ad ðTÞ is shifted to higher temperatures when compared to the DSM ðTÞ dependence. This shift may be due to the fact that the temperature change strongly depends also on the slope of the entropy curve SðTÞ (see inset of Fig. 5). The same entropy change results in a larger temperature change when the slope of the SðTÞ curve is smaller. The maximum values of DT ad are found to be 4.5 and 7.8 K, for field changes of 2 and 5 T, respectively. The value obtained for the 2 T change represents about 30% increase compared to the DT ad ¼ 3:5 K found in polycrystalline sample [6]. The relative cooling power, RCP(S), is also used when comparing different magnetocaloric materials. This quantity is calculated as the product of maximum DSM and the full width at half maximum of the DSM vs. T plot. We can roughly estimate the
ARTICLE IN PRESS J. Kasˇtil et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2318–2321
RCP(S) values of DyNiAl with field applied along c-axis to be 280 1 and 600 J kg for the 2 and 5 T change, respectively. The magnetocaloric characteristics of DyNiAl are comparable with other materials which are considered as promising magnetic refrigerants. For example, the RCP(S) values for 5 T change in Gd, Gd5 Si2 Ge2 or MnFeP0:45 As0:55 lie in the range between 400 and 1 [18–20]. Among materials that exhibit large MCE in 600 Jkg comparable temperature region as DyNiAl, the maximum values 1 of DSM in DyAl2 reaches around 60 K about 11 and 19 J kg K1 for the field change of 2 and 5 T, respectively [21]. The maximum entropy change in ErAl2 occurs around 13 K and reaches about 22 1 and 38 Jkg K1 for 2 and 5 T, respectively [21]. In DyNi2 , the 1 maximum values of DSM around 20 K reaches 11 and 22 Jkg K1 for 2 and 5 T, respectively [21].
4. Conclusions We can conclude that our data reveal a strong anisotropy of the MCE in DyNiAl. The entropy change is large, significantly enhanced compared to polycrystalline data, for the magnetic field applied along the c-axis whereas smaller effect and rather complex behavior is observed for the field applied along the aaxis. The entropy change is mostly due to the ordering of the ferromagnetic component at T c . The entropy change related to the order of the antiferromagnetic component at T 1 is relatively small and appears only when the field is applied perpendicular to the caxis.
Acknowledgments The work was supported by the Grant Agency of the Czech Republic under the Grant no. 202/08/0711 and by the Grant Agency of the Charles University under the Grant no. 21108. This
2321
work is a part of the research program MSM 0021620834 financed by the Ministry of Education of the Czech Republic. References [1] V. Provenzano, A.J. Shapiro, R.D. Shull, Nature 429 (2004) 853. [2] G. Ehlers, H. Maletta, Z. Phys. B 101 (1996) 317. [3] P. Javorsky´, N.C. Tuan, M. Divisˇ, L. Havela, P. Svoboda, V. Sechovsky´, G. Hilscher, J. Magn. Magn. Mater. 140–144 (1995) 1139. [4] B.J. Korte, V.K. Pecharsky, K.A. Gschneidner Jr., J. Appl. Phys. 84 (1998) 5677. [5] N.K. Singh, K.G. Suresh, R. Nirmala, A.K. Nigam, S.K. Malik, J. Magn. Magn. Mater. 302 (2006) 302. [6] N.K. Singh, K.G. Suresh, R. Nirmala, A.K. Nigam, S.K. Malik, J. Appl. Phys. 99 (2006) (Art. no. 08K904). [7] N.K. Singh, K.G. Suresh, R. Nirmala, A.K. Nigam, S.K. Malik, J. Appl. Phys. 101 (2007) 093904. [8] J. Prchal, P. Javorsky´, J. Rusz, F. de Boer, M. Divisˇ, H. Kitazawa, A. Do¨nni, S. Danisˇ, V. Sechovsky´, Phys. Rev. B 77 (2008) 134106 (and references therein). [9] J. Prchal, P. Javorsky´, S. Danisˇ, K. Jurek, J. Dlouhy´, Czechoslovak Journal of Physics 54 (2004) D315. [10] J. Prchal, P. Javorsky´, O. Isnard, B. Detlefs, S. Danisˇ, V. Sechovsky´, to be published. [11] A.L. Lima, A.O. Tsokol, K.A. Gschneidner Jr., V.K. Pecharsky, T.A. Lograsso, D.L. Schlagel, Phys. Rev. B 72 (2005) 024403. [12] M. Zou, Y. Mudryk, V. Pecharsky, K. Gschneidner, D. Schlagel, T. Lograsso, Phys. Rev. B 75 (2007) 024418. [13] A.L. Lima, K.A. Gschneidner Jr., V.K. Pecharsky, J. Appl. Phys. 96 (2004) 2164. [14] J. Prchal, A.V. Andreev, P. Javorsky´, F. Honda, K. Jurek, J. Magn. Magn. Mater. 272–276 (2004) e419. [15] J. Prchal, P. Javorsky´, K. Prokesˇ, B. Ouladdiaf, A.V. Andreev, Physica B 385–386 (2006) 346–348. [16] A.V. Andreev, N.V. Mushnikov, T. Goto, J. Prchal, Physica B 346–347 (2004) 201. [17] L. Morellon, C. Magen, P.A. Algarabel, M.R. Ibarra, C. Ritter, Appl. Phys. Lett. 79 (2001) 1318. [18] O. Tegus, E. Bru¨ck, K.H.J. Buschow, F.R. de Boer, Nature 415 (2002) 150. [19] M.H. Phan, S.-C. Yu, J. Magn. Magn. Mater. 308 (2007) 325. [20] P.W. Egolf (Ed.), Proceedings of the First International Conference on Magnetic Refrigeration near Room Temperature, Montreux, Switzerland, 2005. [21] P.J. von Ranke, V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. B 58 (1998) 12110.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Anisotropy of the magnetocaloric effect in DyNiAl J. Kasˇtil a,, P. Javorsky´ a, A.V. Andreev b a b
Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16 Prague 2, Czech Republic Institute of Physics ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic
a r t i c l e in f o
a b s t r a c t
Article history: Received 2 September 2008 Received in revised form 2 February 2009 Available online 27 February 2009
We present study of the anisotropic magnetocaloric effect in DyNiAl. This compound crystallizes in the hexagonal ZrNiAl-type structure, orders magnetically below T C ¼ 31 K and undergoes a further magnetic phase transition at T 1 ¼ 15 K. The Dy-moments are aligned ferromagnetically along the hexagonal c-axis below T C , the additional antiferromagnetic component develops within the basal plane below T 1 . The magnetocaloric effect was evaluated from the magnetization measurements with field applied along the c-axis and perpendicular to it. Our data reveal a strong anisotropy of the magnetocaloric effect. The large effect occurs for field applied along the c-axis whereas the entropy change is small for the perpendicular field direction. & 2009 Elsevier B.V. All rights reserved.
PACS: 75.30.Sg 75.30.Gw Keywords: Magnetocaloric effect DyNiAl Magnetism Anisotropy
1. Introduction The search for new magnetic materials with a large magnetocaloric effect (MCE), i.e. materials showing a change in temperature induced by a change in magnetic field under adiabatic conditions, is challenging researchers owing to their potential application in magnetic refrigeration with a large impact on energy savings and environmental concerns. The feasibility of using magnetic refrigeration as an alternative to conventional gas compression methods depends largely upon the design of suitable magnetic refrigerant materials. For optimal efficiency, the materials should posses considerable entropy change jDSmag j over the full temperature span of the cooling cycle. The search for potential magnetic refrigerant materials is frequently focused toward intermetallic compounds with rare-earth and transition-metal elements [1]. These materials often exhibit a large change in magnetic entropy near the magnetic transitions. In this context, materials which undergo multiple magnetic transitions are promising for further studies and in the search of ‘‘table-like’’ MCE. The RNiAl compounds crystallizing in the hexagonal ZrNiAltype structure show complex magnetic order at low temperatures with two magnetic phases when R ¼ Sm; Gd; Tb; Dy; Ho or Tm [2,3]. The second phase transition occurs here at about one half of the ordering temperature, T 1 ’ T C =2, what can be favorable for
Corresponding author.
E-mail address: [email protected] (J. Kasˇtil). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.02.118
MCE. The magnetocaloric properties in the RNiAl compounds has been recently studied and revealed large MCE comparable to those of many potential magnetic refrigerants [4–7]. Some of the RNiAl compounds, like GdNiAl or TbNiAl, show an abrupt transition in the temperature dependence of the lattice constants [8]. The c=a ratio changes strongly at the transition temperature; however, the unit-cell volume remains unchanged. DyNiAl does not show such a transition, but similar effect is observed in the concentration dependence when substituting Ni by Cu in the DyNi1x Cux Al series between x ¼ 0:3 and 0.4 [8,9]. The MCE in the DyNi1x Cux Al series is a subject of a separate paper [10]. In this paper, we focus on a seldom discussed aspect of MCE—the anisotropy of the magnetocaloric properties. The use of intrinsic anisotropic properties is one of the promising ways to increase the refrigerant capacity of a given material and has been theoretically and experimentally studied [11–13]. Among the hexagonal RNiAl compounds we have chosen to investigate DyNiAl, compound with intensively studied magnetic properties and showing significant MCE [6]. DyNiAl orders magnetically below T C ¼ 31 K and undergoes a further magnetic phase transition at T 1 ¼ 15 K [2,14,15]. The Dy-moments are aligned ferromagnetically along the hexagonal c-axis between T C and T 1 . The additional antiferromagnetic basal-plane component develops below T 1 leading to a canted magnetic structure at low temperatures [15]. Magnetization measurements on a DyNiAl single crystal [16] revealed a large uniaxial magnetic anisotropy and field-induced transitions in fields applied along
ARTICLE IN PRESS J. Kasˇtil et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2318–2321
the basal plane. The transition associated with a rotation of the ferromagnetic component toward the field direction is observed between 1 and 2 T in both magnetic phases. The transition corresponding to a spin flop of the antiferromagnetic component occurs only in fields above 10 T [16].
6
4K
5
2. Experimental
M (μB /f.u.)
4
The DyNiAl single crystal was grown by Czochralski method in a tri-arc furnace from the stoichiometric melt of the constituent elements as described in detail in Ref. [16]. The nearly cubic piece of 1:6 mm3 cut from the crystal used also in the previous studies [14–16] was used in the present work. Magnetic measurements were performed in the temperature range from 2 to 50 K and in external magnetic field up to 5 T using the MPMS (Quantum Design) instrument. The sample was cooled down in zero magnetic field. Then the one quarter of magnetization curves was measured. The measured data were corrected for the demagnetization field using the demagnetization factor of 0.3 that reflects the nearly cubic sample shape.
40 K
3
50 K 2
1
H // a 0 0
1
In order to determine the anisotropy of the MCE in DyNiAl, we have measured in detail the magnetization curves with magnetic field applied along the hexagonal c-axis and perpendicular to the c-axis. In the second orientation, the field was oriented along the a-axis, but considering the previous studies [14,15], possible anisotropy of MCE within the basal plane is rather small. The magnetization curves are represented in Figs. 1 and 2 for magnetic field oriented along the c- and a-axes, respectively. The measurements were performed between 2 and 40 K with a 2 K step, additional MðHÞ dependence was measured for Hka at 50 K. The results are well in agreement with the previously measured data [16]. We observe a typical ferromagnetic behavior for
12 K 8
6
M (μB /f.u.)
3
4
5
Fig. 2. The magnetization curves of DyNiAl measured with field applied along the a-axis. The curves between 4 and 40 K are shown with the step of 4 K, i.e. only every other curve is shown for better lucidity.
magnetic field parallel to the hexagonal c-axis. When applying magnetic field along the a-axis, we observe a low initial susceptibility reflecting the strong uniaxial anisotropy. Fieldinduced transition is observed between 1.5 and 2 T, both below and above T 1 . We can exclude its metamagnetic nature because above T 1 the compound is a simple collinear uniaxial ferromagnet. Therefore, we can consider the transition as a rotation of the ferromagnetic component of the magnetic moment from the c-axis to the basal plane. This interpretation is corroborated also by the neutron-diffraction results [15]. The resulting magnetic structure is stable in a wide interval of further increasing field, the additional transition occurs only in high fields around 12 T [16]. The magnetic entropy change, DSM , was derived from the MðHÞ dependencies using the following equation:
DSM ðT av ; H1 Þ ¼
Z 0
4 40 k H // c
2
0 1
2
μ0H (T)
3. Results and discussion
0
2319
2
3
4
5
μ0H (T) Fig. 1. The magnetization curves of DyNiAl measured with field applied along the c-axis. The curves between 12 and 40 K are shown with the step of 2 K; the curves measured below 12 K are not displayed because they almost overlap with that obtained at 12 K.
H1
MðT iþ1 ; HÞ MðT i ; HÞ dH. T iþ1 T i
(1)
The results are displayed in Figs. 3 and 4. When the field is applied along the c-axis, the entropy change shows a maximum near the ordering temperature T C and reaches the value of 1 DSM ¼ 14 and 22 J kg K1 for 2 and 5 T, respectively. This behavior clearly reflects the ferromagnetic ordering of Dy moments along the c-axis. For the perpendicular orientation, Hka, the behavior is much more complex. We observe a small negative MCE (positive DSM ) for the applied field of 1 T what reflects the antiferromagnetic character of the magnetic order within the basal plane below T 1 [15]. The negative MCE persists also between T 1 and T C , indicating the antiferromagnetic type of interactions in this temperature region. The entropy change related to the antiferromagnetic component increases with increasing magnetic field, leading to the total value of 1 2 J kg K1 below 10 K in 2 T. The antiferromagnetic order remains intact also in higher fields and certainly contributes to the entropy change, but the total MCE is strongly influenced by the fieldinduced transition in 1:5 T related to the rotation of the ferromagnetic component from the c-axis to the basal plane.
ARTICLE IN PRESS J. Kasˇtil et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2318–2321
2320
25
10
H // c
DyNiAl
5
2
0 10
20
30
40
- ΔSm (J/kg.K)
6
T (K)
0
10
20
0
10
20
30
30
40
40
50
50
T (K) Fig. 5. The specific heat of DyNiAl, data taken from Ref. [14]. Inset shows the total (full line) and magnetic (dashed line) entropy estimated using the LuNiAl data as a nonmagnetic analogue.
8
Δ B = 4.5 T ΔB=4T ΔB=3T
6
ΔB=2T
Δ B = 2 T // C Δ B = 5 T // C
ΔB=1T
ΔTad (K)
8
magnetic
2 0
T (K) Fig. 3. The magnetocaloric effect in DyNiAl as determined from the magnetization curves with magnetic field applied along the c-axis.
4
LuNiAl
1
0
total
6
3
5
10
8
T1
4
S (J/kg.K)
15
TC
6
Cp (J/kg.K)
- ΔSm (J/kg.K)
20
7 ΔB=5T ΔB=4T ΔB=3T ΔB=2T ΔB=1T
4 2
4
2
0
0 H // a
-2 10
20 T (K)
30
40
Fig. 4. The magnetocaloric effect in DyNiAl as determined from the magnetization curves with magnetic field applied along the a-axis.
The total entropy change gradually becomes negative and we observe two moderate maxima of DSM : around the T C and around 19 K. The existence of a minimum around 24 K is related to the fact that the field-induced transition shifts to higher fields for temperatures above 20 K (see Fig. 2). The size of the entropy change for Hka and Hkc is well comparable below T 1, but the maximum in DSM around T C for Hkc is considerably larger than any values for the perpendicular orientation. The existence of two or even more maxima in the entropy change is often observed in compounds exhibiting multiple phase transitions, e.g. Tb5 Ge4 [17]. The case of DyNiAl and Hka is different, although also DyNiAl undergoes two magnetic phase transitions. The observation of the two maxima in DSM is related here to the nature and temperature development of the fieldinduced transition and occur only if the applied field exceeds the value of this transition (see Fig. 4). Our results reveal a strong anisotropy of the MCE in DyNiAl. We observe a large entropy change for magnetic field parallel to the easy magnetization c-axis whereas DSM is smaller and has a very complex temperature dependence for the perpendicular orientation Hka. The results obtained for Hkc are qualitatively similar to those reported on polycrystalline sample [6], but the size of the MCE is considerably enhanced. The value of DSM ¼ 1 14 J kg K1 obtained in our study for the field of 2 T represents
-2 10
20 T (K)
30
40
Fig. 6. The adiabatic temperature change in DyNiAl calculated form the entropy change and the specific heat data (see Fig. 5) for magnetic field applied along the caxis.
1
ffi 40% increase compared to the value of 10 J kg K1 obtained on polycrystal for the same field. The polycrystalline data represent some average of all the crystallographic directions, so the grains oriented with the c-axis perpendicular to the applied field lessen the total MCE. Another quantity characterizing MCE is the adiabatic temperature change DT ad . Its temperature dependence calculated from the entropy change with the field applied along the c-axis and the specific heat data (see Fig. 5) is shown in Fig. 6. The maximum position in DT ad ðTÞ is shifted to higher temperatures when compared to the DSM ðTÞ dependence. This shift may be due to the fact that the temperature change strongly depends also on the slope of the entropy curve SðTÞ (see inset of Fig. 5). The same entropy change results in a larger temperature change when the slope of the SðTÞ curve is smaller. The maximum values of DT ad are found to be 4.5 and 7.8 K, for field changes of 2 and 5 T, respectively. The value obtained for the 2 T change represents about 30% increase compared to the DT ad ¼ 3:5 K found in polycrystalline sample [6]. The relative cooling power, RCP(S), is also used when comparing different magnetocaloric materials. This quantity is calculated as the product of maximum DSM and the full width at half maximum of the DSM vs. T plot. We can roughly estimate the
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RCP(S) values of DyNiAl with field applied along c-axis to be 280 1 and 600 J kg for the 2 and 5 T change, respectively. The magnetocaloric characteristics of DyNiAl are comparable with other materials which are considered as promising magnetic refrigerants. For example, the RCP(S) values for 5 T change in Gd, Gd5 Si2 Ge2 or MnFeP0:45 As0:55 lie in the range between 400 and 1 [18–20]. Among materials that exhibit large MCE in 600 Jkg comparable temperature region as DyNiAl, the maximum values 1 of DSM in DyAl2 reaches around 60 K about 11 and 19 J kg K1 for the field change of 2 and 5 T, respectively [21]. The maximum entropy change in ErAl2 occurs around 13 K and reaches about 22 1 and 38 Jkg K1 for 2 and 5 T, respectively [21]. In DyNi2 , the 1 maximum values of DSM around 20 K reaches 11 and 22 Jkg K1 for 2 and 5 T, respectively [21].
4. Conclusions We can conclude that our data reveal a strong anisotropy of the MCE in DyNiAl. The entropy change is large, significantly enhanced compared to polycrystalline data, for the magnetic field applied along the c-axis whereas smaller effect and rather complex behavior is observed for the field applied along the aaxis. The entropy change is mostly due to the ordering of the ferromagnetic component at T c . The entropy change related to the order of the antiferromagnetic component at T 1 is relatively small and appears only when the field is applied perpendicular to the caxis.
Acknowledgments The work was supported by the Grant Agency of the Czech Republic under the Grant no. 202/08/0711 and by the Grant Agency of the Charles University under the Grant no. 21108. This
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