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Low-temperature large reversible “table-like” magnetocaloric effect in HoNi0.9Cu0.1Al compound
Physica B 457 (2015) 36–39
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Low-temperature large reversible “table-like” magnetocaloric effect in HoNi0.9Cu0.1Al compound R.L. Gao a,n, C.L. Fu a, L. Cui b, Q.Y. Dong b, W. Cai a, Y. Zhang b, X.L. Deng a, Z.Y. Xu b, G. Chen a a b
School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Department of Physics, Capital Normal University, Beijing 100048, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 July 2014 Received in revised form 15 September 2014 Accepted 18 September 2014 Available online 1 October 2014
Magnetic properties and magnetocaloric effect (MCE) of HoNi0.9Cu0.1Al have been investigated. The compound experienced two phase transitions at about 5.7 K and 11.8 K which was assumed as the combination and competence between the ferromagnetic and antiferromagnetic ordering components. “Table-like” magnetic entropy changes were found under low field changes. The maximum values of ΔS were –10.3 and –23.5 J/kg K with corresponding RC as 98 J/kg and 330 J/kg for field changes of 0–20 kOe and 0–50 kOe, respectively. The “table-like” ΔS, large RC, low ordering temperatures and no hysteresis loss enabled it a potential material for commercial utilization in the future. & 2014 Elsevier B.V. All rights reserved.
Keywords: HoNi0.9Cu0.1Al Two phase transitions Magnetocaloric effect “Table-like” magnetic entropy changes
1. Introduction The magnetocaloric effect (MCE) was firstly reported by Warburg and is intrinsic to all materials with magnetic domains [1]. When verified magnetic fields applied to the magnetic working materials, the orders of these materials would change, which could induce entropy changes (ΔS) of the materials. Generally speaking, large ΔS were always found in anti-ferromagnetic (AFM) or ferromagnetic (FM) materials, which were thought to possess long range orderings. The magnetic entropy changes increased with the increasing magnetic field changes. The study of a swath of materials indicated that the maximal valves of ΔS were always found to be around the magnetic transition temperatures experimentally and theoretically [2–6]. In recent years, large MCE found in Gd5(Si,Ge)4 and La(Fe,Si)13 compounds has made the rare-earth (R) based intermetallic compounds a natural choice in the search for materials with excellent MCE performance [5,6]. Although large ΔS have been obtained in these materials with first order phase transition, only narrow working temperature span can be employed. Magnetic materials with large MCE over a considerable working temperature span find its way in potential application as working substances used in magnetic refrigeration by employing the Ericsson-cycle [7]. One remarkable merit of the Ericsson-cycle was that it could utilize as much heat as possible that stemming from the magnetic entropy change in the verified process of applied magnetic field [8]. Therefore, composition of some magnetic materials with magnetic materials with adjacent
n
Corresponding author. E-mail address: [email protected] (R.L. Gao).
http://dx.doi.org/10.1016/j.physb.2014.09.030 0921-4526/& 2014 Elsevier B.V. All rights reserved.
working temperature span or single material with multiple magnetic transitions that can generate a “tablelike” MCE are of importance for searching proper materials suitable for Ericsson-cycle. Until now, most thoughts about composition materials stayed at the level of theoretical calculations, and single materials with multiple magnetic transitions approached closer to the practical application[7–10]. HoNi0.9Cu0.1Al belonged to the RTX compounds (R¼rare-earth metal, T¼ transition metal, X¼p-metal) family with a large members. For these compounds, all the R atoms and one-third of T atoms lied in a basal plane layer and the other nonmagnetic layer made up by all the X atoms and two-thirds of T atoms. These two layers stacked alternately along the c-axis.[11–17] This stack structure may lead to large magnetocrystalline anisotropy. In most cases, AFM ordering found in RNiAl while FM ordering found in RCuAl [13–18]. Different with the other samples, both AFM and FM ordering have been found in HoNiAl and DyNiAl [19–21]. In recent years, many works focused on the dopping at the R and T sites and their magnetic properties have been studied in details [15–18]. But the magnetocaloric properties of RNi1 xCuxAl are seldom reported. For HoNi1 xCuxAl, their magnetic and MCE properties have not been declared. In this paper, we synthesize HoNi0.9Cu0.1Al with two phase transition temperatures and declared a detailed investigation in its magnetic and MCE properties.
2. Experimental details The HoNi0.9Cu0.1Al polycrystalline sample was synthesized by arc melting the stoichiometric amounts of starting materials (Ho, Cu, Ni and Al) on a water-cooled copper crucible under protection
R.L. Gao et al. / Physica B 457 (2015) 36–39
of high-purity argon atmosphere. The purity of all the starting metals are better than 99.9 wt%. The weight loss of Ho due to the evaporation during the arc melting was compensated by adding 3 at% excessive Ho. The ingot was turned over and re-melted for four times to ensure the homogeneity. The obtained button was annealed at 973 K for 60 days in a quartz tube filled with high
37
purity argon atmosphere before which the tube was evacuated to 10 3 Pa. Phase composition was determined by powder X-ray diffractometer from Bruker Inc. equipped with Cu Kα radiation. The DC magnetic measurements were done by means of a commercial superconducting quantum interference device magnetometer (SQUID, Quantum Design).
3. Results and discussion
Fig. 1. XRD spectrum for the HoNi0.9Cu0.1Al alloy.
Fig. 1 exhibited the collected standard θ-2θ powder X-ray diffraction patterns for HoNi0.9Cu0.1Al compound. After a careful examination, the refinement of the patterns revealed that the sample crystallized in a clean phase with a hexagonal ZrNiAl-type structure (space group P62m, NO. 189) and the lattice parameters a and c were determined to be 0.689070.0003 nm and 0.382670.0002 nm. Fig. 2 displayed the temperature dependences of magnetization collected under various magnetic fields, which were measured in zero-field cooling (ZFC) and field-cooling (FC) modes. Fig. 2 (a) showed the ZFC and FC magnetization under a field of 0.1 kOe in which two obvious peaks were found both in the ZFC and FC curves at around 5.7 K and 11.8 K. The reversal of the susceptibility (χ 1) in ZFC mode as a function of temperature under 0.1 kOe was plotted in the inset of Fig. 2(a). The χ 1 in the paramagnetic (PM) region obeyed the Curie–Weiss law with a PM Curie temperature
Fig. 2. Temperature dependences of magnetization in ZFC and FC modes for HoNi0.9Cu0.1Al compound measured under various magnetic fields from 0.1 kOe to 10 kOe. The inset of Fig. 2(a) displays the temperature variation of the inverse susceptibility fitted to the Curie–Weiss law.
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R.L. Gao et al. / Physica B 457 (2015) 36–39
θp ¼ 5.9 K and the effective magnetic moment (μeff) derived was equal to 10.8 μB/Ho3 þ . The value of μeff was slightly larger than the free ion value of Ho3 þ (106 μB) that might originate from the polarization of the conduction band as reported in HoNiAl [19]. The positive value of θp indicated the FM ordering existed in the sample. FM ordering has also been reported in HoNiAl polycrystalline sample. Another point we should pay attention was that the θp was lower than the Tord. Such a behavior have been observed in other compounds and was attributed to the presence of AFM correlations in terms of the FM ones [19,20]. It is reasonable to assume the FM ordering existed in HoNi0.9Cu0.1Al that originated from HoNiAl for only a little amount of Ni was substituted by Cu. Suresh et. al. have found two magnetic phase transition temperatures at 5 K and 14 K that were detected in the M–T curves [19]. On the other hand, neutron scattering study of HoNiAl indicated the presence of two magnetic phases with transition temperatures around 5 K and 13 K [21]. The two results agreed well with each other. Considering the similarity between HoNi0.9Cu0.1Al and HoNiAl, we could attribute the transition temperature at 11.8 K to a FM to PM transition. The earlier neutron scattering study pointed out that both the FM and AFM components coexisted in the two phases of HoNiAl. Specifically speaking, the FM component was along the c-axis all the time while the AFM component lied in the ab basal plane for To5 K and paralleled to c-axis when T45 K [21]. Based on this conclusion, the curves from Fig. 2(a) to (f) could be tentatively explained. Considering the similarity between HoNiAl and HoNi0.9Cu0.1Al, we could assume that the FM component always ordered along the c-axis while the AFM component lied in the ab basal plane for To5.7 K and paralleled to c-axis when T45.7 K. In the temperature range of To5.7 K, the AFM component lied in the ab basal plane and dominated the magnetic structure; in the temperature range between the two peaks,
the FM and AFM components ordered in the same direction and was comparable to each other. With the increasing temperature, the AF component took predominant position. It could be seen from Fig. 2 that the two peaks became gently and vanished with the magnetic field increased to 10 kOe. The peak at 11.8 K disappeared at field of about 3–5 kOe as shown in Fig. 2(c) and (d) while the peak at 5.7 K vanished at about 7 kOe as displayed in Fig. 2(e). In addition, as shown in Fig. 2(a), a distinct thermal irreversibility between the ZFC and FC branches was observed. It was generally thought that the thermo-magnetic irreversibility could be observed in narrow-domain wall pinning system and/or frustrated system [16–18]. The crystal anisotropy induced by the layer structure and the magnetic anisotropy ascertained by the neutron scattering study generated strong anisotropy in HiNi0.9Cu0.1Al. Taking into account of the low ordering temperature of the sample, the discrepancy could be attributed to narrow-domain wall pinning effect. The domain walls were pinned in ZFC condition, and the thermal energy was not so strong enough that could break through the energy barriers. However, in FC condition, the magnetic field during the cooling prevented the pinning effect. This was the reason for the magnetization at low temperature in FC condition was higher than that in ZFC condition. Moreover, the difference between ZFC and FC got disappeared with increasing fields, which might induced by the sufficient energy provided by the high magnetic field that can overcome the energy barriers. Fig. 3(a) and (b) presented the isothermal magnetizations verses magnetic field for HoNi0.9Cu0.1Al measured at various temperatures around the transition temperatures. Ordered state could be found in the curvature behavior as shown in Fig. 3(a) and (b). Another phenomenon that caught our attention was that some curves above TC also exhibit curvature behavior. Similar phenomena have also been reported in HoNiAl and the other intermetallic alloys which
Fig. 3. Initial isothermal magnetization curves at typical temperatures from 2 K to 40 K with the inset showing the magnetic hysteresis loop at 2 K (a); initial isothermal magnetization curves from 2 K to 6 K (b); the Arrott plot from 2 K to 30 K (c) and the enlarged parts of the arrott plot from 2 K to 10 K (d).
R.L. Gao et al. / Physica B 457 (2015) 36–39
39
temperatures and no hysteresis loss enabled it a potential material for commerce utilization in the future.
4. Conclusions In summary, magnetic properties and magnetocaloric effects (MCEs) of HoNi0.9Cu0.1Al were studied and the compound was found to experience two phase transitions at about 5.7 K and 11.8 K. An assumption based on the combination of ferromagnetic and antiferromagnetic ordering components was proposed in this paper. A “table-like” magnetic entropy changes derived from two ΔS peaks were found under low field changes. The maximum values of ΔS were equal to 10.3 and 23.5 J/kg K with corresponding RC of 98 J/kg and 330 J/kg for a field change of 0–20 kOe and 0–50 kOe, respectively. The “table-like” ΔS, large RC, low ordering transition temperatures and no hysteresis loss enabled it a potential material for low temperature (o 10 K) commercial utilization in the future. Acknowledgments
Fig. 4. Magnetic entropy changes as a function of temperature for HoNi0.9Cu0.1Al compound for field changes up to 10 kOe (a); for field changes up to 50 kOe (b).
were attributed to the polarization of 3d band of Ni [19]. The dependence of magnetization on the magnetic field collected at 2 K up to 50 kOe was plotted in the inset of Fig. 3(a). No magnetic hysterics effect was observed. Obvious crossover among curves as shown in Fig. 3(b) and its inset indicated AFM ground state of this sample and metamagnetic transition occurred in low temperature range under a field of about 7 kOe, agreed well with the graph as shown in Fig. 2(e) in which all peaks disappeared under a field of 7 kOe. Fig. 3(c) displayed the Arrott-plots (M2 vs H/M) of the sample from 2 K to 40 K while Fig. 3(d) specially displayed the Arrott-plots up to 10 K. Negative values of slope as shown in Fig. 3(d) confirmed the transitions occurred at low temperature range were first-order transitions according to the Banerjee criterion, which might due to the coexistence of the AFM and FM ordering [22]. For characterizing the MCE, the isothermal magnetic entropy changes and the refrigerant capacity (RC) were the two parameters that need to be focused on. In this paper, ΔS was determined by RH employing Maxwell's relationship ΔS ¼ 0 ð∂M=∂TÞH dH while RC was calculated by numerically integrating the area under half maximum value of ΔS [23,24]. The temperature variation of ΔS under different magnetic field changes was presented in Fig. 4. Positive values of ΔS and two peaks for different field changes up to 10 kOe are the most distinguish features for the sample as shown in Fig. 4(a). The positive values existed in ΔS coordinated with the AFM ordering in this sample and the position of the two peaks corresponded to the two transition temperatures. The two peaks are so close to each other that can generate a “table-like” effect that can increase the RC value. Fig. 4(b) displayed the ΔS as a function of temperature for different field changes up to 50 kOe. The maximum values of ΔS are 10.3 and 23.5 J/kg K with corresponding RC of 98 J/kg and 330 J/kg for a field change of 0–20 kOe and 0–50 kOe, respectively. This large peak value of ΔS is comparable to the other favorite potential magnetic refrigerant materials in low temperature regime under the same field changes [24,25]. The merits of “table-like” ΔS, large RC, low ordering
This work was supported by the National Natural Science Foundation of China, (51021061, 51271196, 51402031), the Natural Science Foundation Project of CQ (No. CSCT2012jjA50017) and the Cooperative Project of Academician Workstation of Chongqing University of Science & Technology (CKYS2014Z01, CKYS2014Y04). Specially thanks Lichen Wang for supporting samples and his useful discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
E. Warburg, Ann. Phys. 249 (1881) 141. K.A. Gschneidner Jr, V.K. Pecharsky, A.O. Tsokol, Rep. Prog. Phys. 68 (2005) 1479. V. Franco, J.S. Blázquez, A. Conde, Appl. Phys. Lett. 89 (2006) 222512. O. Tegus, E. Bruck, K.H.J. Buschow, F.R. de Boer, Nature 415 (2002) 150. B.G. Shen, F.X. Hu, Q.Y. Dong, J.R. Sun, Chin. Phys. B 22 (2013) 017502. V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 23. Q.Y. Dong, J. Chen, J. Shen, J.R. Sun, B.G. Shen, Appl. Phys. Lett. 99 (2011) 132504. H. Takeya, V.K. Pecharsky, K.A. Gschneidner Jr., J.O. Moorman, Appl. Phys. Lett. 64 (1994) 2739. T. Hashimoto, T. Kuzuhara, M. Sahasi, K. Inomata, A. Tomokiyo, H. Yayama, J. Appl. Phys. 62 (1987) 3873. Z. Yan, J. Chen, J. Appl. Phys. 72 (1992) 1. P. Javorský, L. Havela, V. Sechovský, H. Michor, K. Jurek, J. Alloys Compd. 264 (1998) 38. A.V. Andreev, P. Javorský, A. Lindbaum, J. Alloys Compd. 290 (1999) 10. J. Jarosz, E. Talik, T. Mydlarz, J. Kusz, H. Böhm, A. Winiarski, J. Magn. Magn. Mater. 208 (2000) 169. P. Javorský, P. Burlet, E. Ressouche, V. Sechovský, H. Michor, G. Lapertot, Physica B 225 (1996) 230. B.J. Korte, V.K. Pecharsky, K.A. Gschneidner Jr., J. Appl. Phys. 84 (1998) 5677. Jiří Prchal, Pavel Javorský, Blanka Detlefs, Stanislav Daniš, Olivier Isnard, J. Magn. Magn. Mater. 310 (2007) e589. J. Prchal, P. Javorský, V. Sechovský, M. Dopita, O. Isnard, K. Jurek, J. Magn. Magn. Mater. 283 (2004) 34. Jiří Prchal, Takashi Naka, Martin Míšek, Olivier Isnard, Pavel Javorský, J. Magn. Magn. Mater. 316 (2007) e499. N.K. Singh, K.G. Suresh, R. Nirmala, A.K. Nigam, S.K. Malik, J. Appl. Phys. 101 (2007) 093904. R. Mallik, E.V. Sampathkumaran, P.L. Paulose, Solid State Commun. 106 (1998) 169. P. Javorský, P. Burlet, E. Ressouche, V. Sechovský, G. Lapertot, J. Magn. Magn. Mater. 159 (1996) 324. S.K. Banerjee, Phys. Lett 12 (1964) 16–17. K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Pecharsky, C.B. Zimm, Mater. Sci. Forum 69 (1999) 315. L.C. Wang, Q.Y. Dong, Z.Y. Xu, F.X. Hu, J.R. Sun, B.G. Shen, J. Appl. Phys. 113 (2013) 023916. H. Zhang, B.G. Shen, Z.Y. Xu, J. Shen, F.X. Hu, J.R. Sun, Y. Long, Appl. Phys. Lett. 102 (2013) 092401.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Low-temperature large reversible “table-like” magnetocaloric effect in HoNi0.9Cu0.1Al compound R.L. Gao a,n, C.L. Fu a, L. Cui b, Q.Y. Dong b, W. Cai a, Y. Zhang b, X.L. Deng a, Z.Y. Xu b, G. Chen a a b
School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Department of Physics, Capital Normal University, Beijing 100048, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 July 2014 Received in revised form 15 September 2014 Accepted 18 September 2014 Available online 1 October 2014
Magnetic properties and magnetocaloric effect (MCE) of HoNi0.9Cu0.1Al have been investigated. The compound experienced two phase transitions at about 5.7 K and 11.8 K which was assumed as the combination and competence between the ferromagnetic and antiferromagnetic ordering components. “Table-like” magnetic entropy changes were found under low field changes. The maximum values of ΔS were –10.3 and –23.5 J/kg K with corresponding RC as 98 J/kg and 330 J/kg for field changes of 0–20 kOe and 0–50 kOe, respectively. The “table-like” ΔS, large RC, low ordering temperatures and no hysteresis loss enabled it a potential material for commercial utilization in the future. & 2014 Elsevier B.V. All rights reserved.
Keywords: HoNi0.9Cu0.1Al Two phase transitions Magnetocaloric effect “Table-like” magnetic entropy changes
1. Introduction The magnetocaloric effect (MCE) was firstly reported by Warburg and is intrinsic to all materials with magnetic domains [1]. When verified magnetic fields applied to the magnetic working materials, the orders of these materials would change, which could induce entropy changes (ΔS) of the materials. Generally speaking, large ΔS were always found in anti-ferromagnetic (AFM) or ferromagnetic (FM) materials, which were thought to possess long range orderings. The magnetic entropy changes increased with the increasing magnetic field changes. The study of a swath of materials indicated that the maximal valves of ΔS were always found to be around the magnetic transition temperatures experimentally and theoretically [2–6]. In recent years, large MCE found in Gd5(Si,Ge)4 and La(Fe,Si)13 compounds has made the rare-earth (R) based intermetallic compounds a natural choice in the search for materials with excellent MCE performance [5,6]. Although large ΔS have been obtained in these materials with first order phase transition, only narrow working temperature span can be employed. Magnetic materials with large MCE over a considerable working temperature span find its way in potential application as working substances used in magnetic refrigeration by employing the Ericsson-cycle [7]. One remarkable merit of the Ericsson-cycle was that it could utilize as much heat as possible that stemming from the magnetic entropy change in the verified process of applied magnetic field [8]. Therefore, composition of some magnetic materials with magnetic materials with adjacent
n
Corresponding author. E-mail address: [email protected] (R.L. Gao).
http://dx.doi.org/10.1016/j.physb.2014.09.030 0921-4526/& 2014 Elsevier B.V. All rights reserved.
working temperature span or single material with multiple magnetic transitions that can generate a “tablelike” MCE are of importance for searching proper materials suitable for Ericsson-cycle. Until now, most thoughts about composition materials stayed at the level of theoretical calculations, and single materials with multiple magnetic transitions approached closer to the practical application[7–10]. HoNi0.9Cu0.1Al belonged to the RTX compounds (R¼rare-earth metal, T¼ transition metal, X¼p-metal) family with a large members. For these compounds, all the R atoms and one-third of T atoms lied in a basal plane layer and the other nonmagnetic layer made up by all the X atoms and two-thirds of T atoms. These two layers stacked alternately along the c-axis.[11–17] This stack structure may lead to large magnetocrystalline anisotropy. In most cases, AFM ordering found in RNiAl while FM ordering found in RCuAl [13–18]. Different with the other samples, both AFM and FM ordering have been found in HoNiAl and DyNiAl [19–21]. In recent years, many works focused on the dopping at the R and T sites and their magnetic properties have been studied in details [15–18]. But the magnetocaloric properties of RNi1 xCuxAl are seldom reported. For HoNi1 xCuxAl, their magnetic and MCE properties have not been declared. In this paper, we synthesize HoNi0.9Cu0.1Al with two phase transition temperatures and declared a detailed investigation in its magnetic and MCE properties.
2. Experimental details The HoNi0.9Cu0.1Al polycrystalline sample was synthesized by arc melting the stoichiometric amounts of starting materials (Ho, Cu, Ni and Al) on a water-cooled copper crucible under protection
R.L. Gao et al. / Physica B 457 (2015) 36–39
of high-purity argon atmosphere. The purity of all the starting metals are better than 99.9 wt%. The weight loss of Ho due to the evaporation during the arc melting was compensated by adding 3 at% excessive Ho. The ingot was turned over and re-melted for four times to ensure the homogeneity. The obtained button was annealed at 973 K for 60 days in a quartz tube filled with high
37
purity argon atmosphere before which the tube was evacuated to 10 3 Pa. Phase composition was determined by powder X-ray diffractometer from Bruker Inc. equipped with Cu Kα radiation. The DC magnetic measurements were done by means of a commercial superconducting quantum interference device magnetometer (SQUID, Quantum Design).
3. Results and discussion
Fig. 1. XRD spectrum for the HoNi0.9Cu0.1Al alloy.
Fig. 1 exhibited the collected standard θ-2θ powder X-ray diffraction patterns for HoNi0.9Cu0.1Al compound. After a careful examination, the refinement of the patterns revealed that the sample crystallized in a clean phase with a hexagonal ZrNiAl-type structure (space group P62m, NO. 189) and the lattice parameters a and c were determined to be 0.689070.0003 nm and 0.382670.0002 nm. Fig. 2 displayed the temperature dependences of magnetization collected under various magnetic fields, which were measured in zero-field cooling (ZFC) and field-cooling (FC) modes. Fig. 2 (a) showed the ZFC and FC magnetization under a field of 0.1 kOe in which two obvious peaks were found both in the ZFC and FC curves at around 5.7 K and 11.8 K. The reversal of the susceptibility (χ 1) in ZFC mode as a function of temperature under 0.1 kOe was plotted in the inset of Fig. 2(a). The χ 1 in the paramagnetic (PM) region obeyed the Curie–Weiss law with a PM Curie temperature
Fig. 2. Temperature dependences of magnetization in ZFC and FC modes for HoNi0.9Cu0.1Al compound measured under various magnetic fields from 0.1 kOe to 10 kOe. The inset of Fig. 2(a) displays the temperature variation of the inverse susceptibility fitted to the Curie–Weiss law.
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R.L. Gao et al. / Physica B 457 (2015) 36–39
θp ¼ 5.9 K and the effective magnetic moment (μeff) derived was equal to 10.8 μB/Ho3 þ . The value of μeff was slightly larger than the free ion value of Ho3 þ (106 μB) that might originate from the polarization of the conduction band as reported in HoNiAl [19]. The positive value of θp indicated the FM ordering existed in the sample. FM ordering has also been reported in HoNiAl polycrystalline sample. Another point we should pay attention was that the θp was lower than the Tord. Such a behavior have been observed in other compounds and was attributed to the presence of AFM correlations in terms of the FM ones [19,20]. It is reasonable to assume the FM ordering existed in HoNi0.9Cu0.1Al that originated from HoNiAl for only a little amount of Ni was substituted by Cu. Suresh et. al. have found two magnetic phase transition temperatures at 5 K and 14 K that were detected in the M–T curves [19]. On the other hand, neutron scattering study of HoNiAl indicated the presence of two magnetic phases with transition temperatures around 5 K and 13 K [21]. The two results agreed well with each other. Considering the similarity between HoNi0.9Cu0.1Al and HoNiAl, we could attribute the transition temperature at 11.8 K to a FM to PM transition. The earlier neutron scattering study pointed out that both the FM and AFM components coexisted in the two phases of HoNiAl. Specifically speaking, the FM component was along the c-axis all the time while the AFM component lied in the ab basal plane for To5 K and paralleled to c-axis when T45 K [21]. Based on this conclusion, the curves from Fig. 2(a) to (f) could be tentatively explained. Considering the similarity between HoNiAl and HoNi0.9Cu0.1Al, we could assume that the FM component always ordered along the c-axis while the AFM component lied in the ab basal plane for To5.7 K and paralleled to c-axis when T45.7 K. In the temperature range of To5.7 K, the AFM component lied in the ab basal plane and dominated the magnetic structure; in the temperature range between the two peaks,
the FM and AFM components ordered in the same direction and was comparable to each other. With the increasing temperature, the AF component took predominant position. It could be seen from Fig. 2 that the two peaks became gently and vanished with the magnetic field increased to 10 kOe. The peak at 11.8 K disappeared at field of about 3–5 kOe as shown in Fig. 2(c) and (d) while the peak at 5.7 K vanished at about 7 kOe as displayed in Fig. 2(e). In addition, as shown in Fig. 2(a), a distinct thermal irreversibility between the ZFC and FC branches was observed. It was generally thought that the thermo-magnetic irreversibility could be observed in narrow-domain wall pinning system and/or frustrated system [16–18]. The crystal anisotropy induced by the layer structure and the magnetic anisotropy ascertained by the neutron scattering study generated strong anisotropy in HiNi0.9Cu0.1Al. Taking into account of the low ordering temperature of the sample, the discrepancy could be attributed to narrow-domain wall pinning effect. The domain walls were pinned in ZFC condition, and the thermal energy was not so strong enough that could break through the energy barriers. However, in FC condition, the magnetic field during the cooling prevented the pinning effect. This was the reason for the magnetization at low temperature in FC condition was higher than that in ZFC condition. Moreover, the difference between ZFC and FC got disappeared with increasing fields, which might induced by the sufficient energy provided by the high magnetic field that can overcome the energy barriers. Fig. 3(a) and (b) presented the isothermal magnetizations verses magnetic field for HoNi0.9Cu0.1Al measured at various temperatures around the transition temperatures. Ordered state could be found in the curvature behavior as shown in Fig. 3(a) and (b). Another phenomenon that caught our attention was that some curves above TC also exhibit curvature behavior. Similar phenomena have also been reported in HoNiAl and the other intermetallic alloys which
Fig. 3. Initial isothermal magnetization curves at typical temperatures from 2 K to 40 K with the inset showing the magnetic hysteresis loop at 2 K (a); initial isothermal magnetization curves from 2 K to 6 K (b); the Arrott plot from 2 K to 30 K (c) and the enlarged parts of the arrott plot from 2 K to 10 K (d).
R.L. Gao et al. / Physica B 457 (2015) 36–39
39
temperatures and no hysteresis loss enabled it a potential material for commerce utilization in the future.
4. Conclusions In summary, magnetic properties and magnetocaloric effects (MCEs) of HoNi0.9Cu0.1Al were studied and the compound was found to experience two phase transitions at about 5.7 K and 11.8 K. An assumption based on the combination of ferromagnetic and antiferromagnetic ordering components was proposed in this paper. A “table-like” magnetic entropy changes derived from two ΔS peaks were found under low field changes. The maximum values of ΔS were equal to 10.3 and 23.5 J/kg K with corresponding RC of 98 J/kg and 330 J/kg for a field change of 0–20 kOe and 0–50 kOe, respectively. The “table-like” ΔS, large RC, low ordering transition temperatures and no hysteresis loss enabled it a potential material for low temperature (o 10 K) commercial utilization in the future. Acknowledgments
Fig. 4. Magnetic entropy changes as a function of temperature for HoNi0.9Cu0.1Al compound for field changes up to 10 kOe (a); for field changes up to 50 kOe (b).
were attributed to the polarization of 3d band of Ni [19]. The dependence of magnetization on the magnetic field collected at 2 K up to 50 kOe was plotted in the inset of Fig. 3(a). No magnetic hysterics effect was observed. Obvious crossover among curves as shown in Fig. 3(b) and its inset indicated AFM ground state of this sample and metamagnetic transition occurred in low temperature range under a field of about 7 kOe, agreed well with the graph as shown in Fig. 2(e) in which all peaks disappeared under a field of 7 kOe. Fig. 3(c) displayed the Arrott-plots (M2 vs H/M) of the sample from 2 K to 40 K while Fig. 3(d) specially displayed the Arrott-plots up to 10 K. Negative values of slope as shown in Fig. 3(d) confirmed the transitions occurred at low temperature range were first-order transitions according to the Banerjee criterion, which might due to the coexistence of the AFM and FM ordering [22]. For characterizing the MCE, the isothermal magnetic entropy changes and the refrigerant capacity (RC) were the two parameters that need to be focused on. In this paper, ΔS was determined by RH employing Maxwell's relationship ΔS ¼ 0 ð∂M=∂TÞH dH while RC was calculated by numerically integrating the area under half maximum value of ΔS [23,24]. The temperature variation of ΔS under different magnetic field changes was presented in Fig. 4. Positive values of ΔS and two peaks for different field changes up to 10 kOe are the most distinguish features for the sample as shown in Fig. 4(a). The positive values existed in ΔS coordinated with the AFM ordering in this sample and the position of the two peaks corresponded to the two transition temperatures. The two peaks are so close to each other that can generate a “table-like” effect that can increase the RC value. Fig. 4(b) displayed the ΔS as a function of temperature for different field changes up to 50 kOe. The maximum values of ΔS are 10.3 and 23.5 J/kg K with corresponding RC of 98 J/kg and 330 J/kg for a field change of 0–20 kOe and 0–50 kOe, respectively. This large peak value of ΔS is comparable to the other favorite potential magnetic refrigerant materials in low temperature regime under the same field changes [24,25]. The merits of “table-like” ΔS, large RC, low ordering
This work was supported by the National Natural Science Foundation of China, (51021061, 51271196, 51402031), the Natural Science Foundation Project of CQ (No. CSCT2012jjA50017) and the Cooperative Project of Academician Workstation of Chongqing University of Science & Technology (CKYS2014Z01, CKYS2014Y04). Specially thanks Lichen Wang for supporting samples and his useful discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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