Excellent Cryogenic Magnetocaloric Properties in Heavy Rare-earth Based HRENiGa2 (HRE = Dy, Ho, or Er) Compounds

link.springer.com link.springer.com Published online 24 June 2022 | https://doi.org/10.1007/s40843-022-2095-6 Sci China Mater 2023, 66(1): 249–256 Excellent cryogenic magnetocaloric properties in heavy rare-earth based HRENiGa2(HRE = Dy, Ho, or Er) compounds Dan Guo1,2,3, Luis M. Moreno-Ramírez2, Jia-Yan Law2*, Yikun Zhang1,3*and Victorino Franco2 ABSTRACT RENiX2compounds, where RE = rare-earth element and X = p-block element, have been highly regarded for cryogenic magnetocaloric applications. Depending on the elements, they can crystallize in CeNiSi2-type, NdNiGa2-type, or MgCuAl2-type crystal structures, showing different types of magnetic ordering and thus affect their magnetic properties. Regarding the magnetocaloric effect, MgCuAl2-type aluminides show larger values than those of the CeNiSi2-type silicides and the NdNiGa2-type gallides due to the favored ferromagnetic ground state. However, RENiGa2gallides can crystallize in either NdNiGa2- or MgCuAl2-type structures depending on the RE element. In this work, we select heavy RE (HRE) elements for exploring the microstructure, magnetic ordering and magnetocaloric performance of HRENiGa2 (HRE = Dy, Ho or Er) gallides. They all crystallize in the desired MgCuAl2-type crystal structure which undergoes a second-order transition from ferro- to para-magnetic state with increasing temperature. The maximum isothermal entropy change (|ΔSisomax|) values are 6.2, 10.4, and 11.4 J kg−1 K−1 (0–5 T) for DyNiGa2, HoNiGa2, and ErNiGa2, respectively, which are comparable to many recently reported cryogenic magnetocaloric materials. Particularly, the excellent magnetocaloric properties of HoNiGa2and ErNiGa2com- pounds, including their composite, fall in the temperature range that enables them for the in-demand hydrogen liquefaction systems. Keywords: rare-earth-nickel-gallides, magnetocaloric effect, magnetic phase transitions INTRODUCTION Cooling technology has become an indispensable application in our modern society, in which the choice of optimal refrigeration technologies needs to be driven by environmental compatibility and energy efficiency. Regarding these aspects, the magnetic refrigeration (MR) technology based on the magnetocaloric effect (MCE), is expected to become a highly competitive cooling method due to its high-energy efficiency and the absence of hazardous and ozone-depleting gases in comparison to the conventional vapor-compression refrigeration [1–6]. The MCE is an inherent magneto-thermodynamic phenomenon that occurs in magnetic materials upon changing the external magnetic field, which is significant at temperatures close to the thermomagnetic phase transitions. It is characterized by an isothermal magnetic entropy change (ΔSiso) and an adiabatic temperature change (ΔTad). By using a large variety of magnetic materials and different thermodynamic cycles, MR can operate in a very wide temperature range (from extremely low temperatures up to room temperature) [7–17]. In recent years, lowtemperature MR has received extensive attention due to its important applications, such as industrial gas liquefaction and storage, cryogenic space technology, and laboratory cryogenic basic research [18–25]. Liquid H2is a clean fuel that is widely used in the aerospace field and is expected to extend its use towards conventional applications. Moreover, for H2storage and transportation, the liquid state is more advantageous than the gaseous H2. Regarding the liquefaction processes, the efficiency of magnetic systems could reach 60% of the theoretical Carnot cycle, which is 50% higher than the actual ones of gas compression systems [26]. For these reasons, exploring MCE materials with excellent performance in the low-temperature range is of great interest. Specifically, it has been reported that several heavy rare-earth (HRE)-transition metal (TM) intermetallic compounds can be considered potential candidates for cryogenic MR materials for H2liquefaction [27–33]. This is due to the large total orbital quantum numbers (J) of HRE elements, which lead to large magnetic moments that are beneficial to the magnetocaloric properties. Recently, ternary intermetallic compounds, HRETM-X (X = p-block elements) with the stoichiometric ratio of 1:1:2, have received extensive attention for their diverse crystal structure and unique magnetic properties [34–47]. Their magnetic phase transition and magnetocaloric properties can be varied depending on the TM and p-block elements. For TM = Ni and X = Al, the samples crystallize in the orthorhombic MgCuAl2-type structure, presenting ferromagnetic (FM) order at low temperatures (Curie temperature (TC) ranges around 2.4–28.0 K) [39–41]. For X = Si, the compounds crystallize in the CeNiSi2-type structure, revealing antiferromagnetic order at low temperatures (Néel temperature ranges around 3.0–25.0 K) [44]. When X = Ga, depending on the RE element, two different types of crystal structures have been reported [46]: NdNiGa2-type for RE = La–Nd, Sm or Gd and MgCuAl2-type for RE = Y or Tb–Lu. 1Key Laboratory of Novel Materials for Sensor of Zhejiang Province & School of Electronics and Information Engineering, Hangzhou Dianzi University, Hangzhou 310012, China 2Departamento de Física de la Materia Condensada, ICMS-CSIC, Universidad de Sevilla, P.O. Box 1065. 41080-Sevilla, Spain 3State Key Laboratory of Advanced Special Steels & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China *Corresponding authors (emails: [email protected] (Zhang Y); [email protected] (Law JY)) SCIENCE CHINA Materials ARTICLES January 2023 | Vol.66 No.1 249 © The Author(s) 2022. This article is published with open access at link.springer.com. For the former structure, CeNiGa2and GdNiGa2were reported to show antiferromagnetic ordering at 4 and 22 K, respectively, and both direct and inverse MCE were observed in these two samples [48,49]. On the other hand, for MgCuAl2-type gallides, although their MCE properties have not been reported, the isostructural aluminides exhibit excellent direct MCE in low temperature range, making it of interest to investigate the magnetocaloric behavior and properties of MgCuAl2-type gallides. Hence, in this work, with the aim of obtaining suitable materials for cryogenic applications, we vary the type of HRE elements in the HRENiGa2system (HRE = Dy, Ho, or Er) and investigate their influence on the structural, thermomagnetic, and magnetocaloric behavior. EXPERIMENTAL SECTION A series of polycrystalline samples were fabricated by arc-melt- ing from high-purity (at least 99.9%) elements in an Ar atmosphere with a nominal composition of HRENiGa2(HRE = Dy, Ho, or Er). An excess of 2 wt% of HRE elements was used to compensate for the losses caused by volatilization during the arc-melting process. The obtained ingots were wrapped in the Ta foil and sealed in a quartz tube for annealing. The annealing process was performed in a muffle furnace at 1073 K for one week, and then quenched in ice water. The phase and crystal structure analysis of the HRENiGa2was performed by X-ray diffraction (XRD) using a Bruker D8 diffractometer (Cu-Kα radiation). The magnetization data of the HRENiGa2were collected using the Vibrating Sample Magnetometer (VSM) option of the physical properties measurement system (PPMS-9). RESULTS AND DISCUSSION Fig. 1a displays the powder XRD as well as the Rietveld refinement results for DyNiGa2, HoNiGa2, and ErNiGa2compounds. Both DyNiGa2and HoNiGa2samples are single-phase crystallizing in the orthorhombic MgCuAl2-type structure (Cmcm space group). While for ErNiGa2, extra diffraction peaks corresponding to the Er2O3phase (~1.0 wt%) are detected. The refined factors (included in Fig. 1a) reveal a good agreement between the structural model and the experimental data, being the weighted profile Rfactor (Rwp) of ~10% and a goodness of fit (GoF) of 1.5–1.9 [50]. The refined lattice parameters aand care shown in Fig. 1b. It is worth noting that the lattice parameters decrease monotonically with the decrease of RE ion radius. A schematic of the crystal structure for HRENiGa2is included in Fig. 1c. HRE and Ni atoms occupy the same Wyckoff sites of 4c with the m2mpoint symmetry, whereas Ga occupies the 8fsites with the m.. point symmetry. The temperature dependence of magnetization (M) and reciprocal susceptibility (1/χ) at μ0H= 1 T for HRENiGa2are shown in Fig. 2a. From the M-T curves, all samples undergo a gradual magnetic transition from FM to paramagnetic (PM) states. The TCcan be estimated by the derivative of M-T curves (dM/dT-T curves) under the field of 0.01 T, which are also shown in Fig. 2b. The corresponding temperatures of the inflection points are 46.0, 26.0, and 11.0 K for DyNiGa2, HoNiGa2, and ErNiGa2 compounds, respectively. According to the Ruderman-Kittel- Kasuya-Yosida (RKKY) indirect interaction theory [51], the TC for RE-based compounds will be expected to be proportional to the de Gennes factor (dG). This factor is determined by the Lande factor (g) and the total orbital quantum number (J): ( ) ( ) dG g J J= 1 + 1 . (1) 2 The relationship between TCand dG for the samples is shown in the inset of Fig. 2b. As expected, a positive linear trend is observed, revealing the dominant role of RE-RE long-range interaction, which is consistent with previously reported data [46]. A linear trend is observed from the 1/χ-Tcurves (Fig. 2a), indicating that all the samples obey the Curie-Weiss law in the PM region: ( ) CT1 / = 1, (2) P where θPis the PM Curie temperature and Cis the Curie con- Figure 1 (a) Powder XRD patterns (room temperature) and the Rietveld refinement results for HRENiGa2compounds. (b) The lattice parameters as a function of RE elements. (c) Schematic of the MgCuAl2-type crystal structure in these compounds. ARTICLES SCIENCE CHINA Materials 250 January 2023 | Vol.66 No.1 © The Author(s) 2022. This article is published with open access at link.springer.com. stant which can be expressed by the following formula: CN µ µ k =3, (3) Aeff 2 B 2 B where NAis the Avogadro’s number, kBis the Boltzmann constant, μBis the Bohr magneton, and μeff is the effective magnetic moment. Therefore, according to the 1/χ-Tcurves, μeff and θP values can be estimated from the linear fit. The obtained values are 10.67 μBfor DyNiGa2, 10.62 μBfor HoNiGa2, and 9.61 μBfor ErNiGa2, which are very close to the values calculated by Hund’s rules, i.e., 10.64 μBfor Dy3+, 10.60 μBfor Ho3+, and 9.58 μBfor Er3+ [51]. This shows that the RE elements play a major magnetic contribution in these compounds. In addition, the θPis determined from the intercept of the fittings with the temperature axis, obtaining 29.3, 11.4, and 7.5 K for DyNiGa2, HoNiGa2, and ErNiGa2, respectively. The positive values of θP prove the FM ground state in the studied series. For exploring the magnetocaloric performance of these compounds, magnetization isotherms were measured at different temperatures around TC(displayed in Fig. 3). It is observed that all samples show FM characteristic at low temperatures (T<TC) whereas PM characteristic at high temperatures (T>TC), in agreement with the previous M-T results. In addition, M(H) loops measured for the three samples show negligible magnetic hysteresis (coercivity values of 30 mT for DyNiGa2, 20 mT for HoNiGa2and 10 mT for ErNiGa2). Using these isothermal magnetization curves, the isothermal entropy change (ΔSiso) can be indirectly determined based on thermodynamic Maxwell relations from: S T H µ M T H TH( , ) = ( , ) d . (4) H H iso 00 The calculated |ΔSiso| values versus temperature for the studied compounds are plotted in Fig. 4a–c under several selected magnetic field changes (ΔH). For all the compounds, the values of |ΔSiso| gradually increase as ΔHincreases. For ΔHof 0–5 T, the maximum |ΔSiso| values (|ΔSisomax|) and their corresponding peak temperatures (Tpeak) are found to be 6.2 J kg−1K−1and 41.5 K, 10.4 J kg−1K−1and 24.5 K, as well as 11.4 J kg−1K−1and 12.0 K for Dy-, Ho-, and Er-containing samples, respectively. It is highlighted that in the case of the Ho-containing compound, the magnetocaloric response is within the range of the H2 liquefaction temperature (20.4 K). For the Er-containing sample, its Tpeak shifts to higher temperatures gradually with increasing external field in contrast to DyNiGa2and HoNiGa2whose Tpeak remains unaffected with fields. This can be ascribed to the Figure 2 (a) The M(T) and 1/χ(T) curves under a magnetic field of 1 T for the HRENiGa2compounds. (b) dM/dT vs.Tcurves for the studied compounds. The inset displays the relation between TCand dG factor. Figure 3 Magnetization isotherms measured around the transition temperatures (Tt) for the studied compounds. Arrows indicate the increase of temperature. SCIENCE CHINA Materials ARTICLES January 2023 | Vol.66 No.1 251 © The Author(s) 2022. This article is published with open access at link.springer.com. critical exponents deviating from mean field for ErNiGa2[52]. When ΔHincreases to 0–7 T, the Tpeak value obtained for ErNiGa2is 18 K, which is close to the H2liquefaction temperature. To quantify the cooling efficiency of the compounds, the relative cooling power (RCP) is calculated by the following equation: S TRCP = × , (5) iso max FWHM where δTFWHM is the full temperature width at a half maximum of the |ΔSiso| peak. For 0–5 T, the RCP values are 275 J kg−1for DyNiGa2, 302 J kg−1for HoNiGa2, as well as 238 J kg−1for ErNiGa2, respectively. Additionally, the values of δTFWHM are 44.6, 29.1, and 20.9 K for Dy-, Ho-, and Er-containing samples, respectively. Griffith et al.[53] proposed the temperature-averaged magnetic entropy change (TEC) as another figure of merit to evaluate the magnetocaloric performance of the materials. For a given temperature span (ΔTlift), the TEC(ΔTlift) can be estimated using the following formula: TTS T TTEC( ) = 1max ( ) d , (6) TT T T T lift lift 2 +2iso mid mid lift mid lift where Tmid refers to the central temperature to make sure the TEC(ΔTlift) achieves the maximum value. In this work, we chose ΔTlift of 5 K and the TEC(5) values were determined to be 6.13, 10.07, and 11.03 J kg−1K−1for Dy, Ho and Er, which are 98.9%, 96.8%, and 96.7% of |ΔSisomax| values, respectively, as shown in Fig. 4d. This indicates that the compounds can properly operate as magnetic refrigerant (e.g., in active magnetic regenerator (AMR) cycles) with a working span of 5 K with performance close to their maximums. It can be observed that while the maximum MC response of HoNiGa2compound is slightly above the H2liquefaction temperature, that of ErNiGa2is slightly below. Therefore, it is worth studying the existence of a composite material based on both HoNiGa2and ErNiGa2, which would operate in a broad temperature range around the desired H2liquefaction temperature. Moreover, composites with two or more phases were found to be helpful in obtaining table-like MCE which is required for an ideal Ericsson cycle [27,54–57]. To explore this, we estimate the total values of |ΔSiso| and TEC(ΔTlift = 10, 15, and 20 K) under ΔHof 0–5 T for the xHoNiGa2+ (1−x)ErNiGa2composite (where xis the mass fraction of the HoNiGa2compound in the composite, being 0 ≤x≤1). This magnitude is selected as it is equivalent to the average value of magnetic entropy change over a wide temperature range (i.e., ∆Tlift), which is more representative for MCE performance evaluation. We found an optimal table-like MCE for x= 0.5 composite, having an isothermal entropy change of 8.7 J kg−1K−1with the temperature range of 11–25 K as presented in Fig. 5a, showing the potential of this composite for Ericsson cycle MR. Fig. 5b displays the |ΔSisomax| and TEC values as a function of the composite composition. It can be observed that the TEC values for 10, 15, and 20 K working span for x= 0.5 and 0.6 composites are much closer to Figure 4 (a–c) The isothermal entropy change (ΔSiso)vs. temperature curves for the studied compounds. (d) The field dependence of |ΔSisomax| and TEC(5) for the studied compounds. ARTICLES SCIENCE CHINA Materials 252 January 2023 | Vol.66 No.1 © The Author(s) 2022. This article is published with open access at link.springer.com. Figure 6 The Arrott plots (M2versus H/M) (a–c) and ΔSiso(T)/ΔSisomax vs. the rescaled temperature (θ) (d–f) for the studied compounds. Figure 5 (a) |ΔSiso|vs.Tcurves for the xHoNiGa2+ (1−x)ErNiGa2composite for ΔHof 0–5 T. (b) The |ΔSisomax| and TEC (ΔTlift = 10, 15, and 20 K) as a function of x(the mass fraction of HoNiGa2) for the composite. SCIENCE CHINA Materials ARTICLES January 2023 | Vol.66 No.1 253 © The Author(s) 2022. This article is published with open access at link.springer.com. the |ΔSisomax| (highlighted by a dashed box), reinforcing the excellent table-like behavior (i.e., ΔSiso is relatively constant and similar to |ΔSisomax| over the various ∆Tlift range values) and performance of those composites. As MCE response is linked to the nature of magnetic transitions, it is thus important to determine the order of phase transitions for practical applications mainly to take into account the reversibility of the response. To confirm the nature of magnetic transitions, the Arrott plots (i.e., M2versus H/M curves) were constructed by applying the Banerjee’s criterion [58] (Fig. 6a–c). It can be inferred that the three compounds undergo a second-order phase transition (SOPT) since all isotherms show positive slopes, according to the Banerjee’s criterion. In addition, Franco et al.[52] have introduced a criterion that a universal curve could be achieved for magnetocaloric materials undergoing SOPT upon rescaling the ΔSiso vs.Tcurves for different ΔH. For the universal curve construction, the magnetic entropy change axis and temperature axis were changed to ΔSiso(T)/ΔSisomax and θ, respectively, where θwas defined independently below and above TCas follows: T T T T T T T T T T T T =( ) / ( ) for , ( ) / ( ) for > , (7) C r1 C C C r2 C C where Tr1 and Tr2 (Tr1 <TC<Tr2) denote two reference temperatures and their corresponding ΔSiso(Tr1)/ΔSisomax = ΔSiso(Tr2)/ ΔSisomax = 0.6. Fig. 6d–f present the normalized ΔSiso(T)/ΔSisomax vs.θcurves for the DyNiGa2, HoNiGa2, and ErNiGa2com- pounds, respectively. It can be observed that all rescaled curves are unaffected by the magnetic field and collapse onto a universal curve, revealing the SOPT nature for the three studied samples. These results are in excellent agreement to those of the Banerjee’s criterion. Fig. 7 displays the |ΔSisomax| and RCP values of the studied samples and their Ttindicated by the color legend. For comparison, other RETMX2systems and some recently reported cryogenic materials falling in the range of the H2liquefaction temperature have been included. It can be seen that the magnetocaloric parameters of our studied MgCuAl2-type gallides are much larger than those of the NdNiGa2-type ones, which can be attributed to the favored FM ground state. In addition, the present samples also show competitive cooling efficiency compared with the isostructural MgCuAl2-type aluminides, CeNiSi2- type silicides, and some other materials, making them potential candidates for cryogenic applications. CONCLUSIONS HRENiGa2series (with HRE = Dy, Ho, and Er) crystallizing in the MgCuAl2-type crystal structure were successfully synthesized by arc melting. Rietveld refinement results show that all the samples are almost single-phase (99 wt% of the main phase). All three compounds undergo an FM-PM magnetic phase transition at low temperatures, around 46.0, 26.0, and 11.0 K for Dy-, Ho-, and Er-containing samples, respectively. The second order nature of the magnetic phase transitions was verified by the Banerjee’s criterion and magnetocaloric universal curve method. Regarding the magnetocaloric response, the maximum |ΔSiso| values for 0–5 T are 6.2, 10.4, and 11.4 J kg−1K−1for Dy, Ho, and Er, respectively. The temperature span of these responses, quantified by δTFWHM values, are 44.6 K for DyNiGa2, 29.1 K for HoNiGa2, and 20.9 K for ErNiGa2, showing good cooling performance in a broad temperature range. This has also been checked by the TEC(5) parameter, which shows values rather close to their maximum capabilities (around 99%–97%). For HoNiGa2and ErNiGa2compounds, their peak temperatures of maximum isothermal entropy changes are close to the H2 liquefaction temperature, making them of great value for magnetocaloric liquefaction systems. Moreover, we illustrate the excellent magnetocaloric performance of a hypothetical HoNi- Ga2and ErNiGa2composite which would exhibit a notable tablelike MCE in a broad temperature range around the desired H2 liquefaction temperature (8.7 J kg−1K−1around 11–25 K). Received 14 March 2022; accepted 27 April 2022; published online 24 June 2022 1 Shen BG, Sun JR, Hu FX, et al. Recent progress in exploring magnetocaloric materials. Adv Mater, 2009, 21: 4545–4564 2 Franco V, Blázquez JS, Ipus JJ, et al. Magnetocaloric effect: From materials research to refrigeration devices. Prog Mater Sci, 2018, 93: 112– 232 3 Tegus O, Brück E, Buschow KHJ, et al. 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J Magn Magn Mater, 2016, 401: 1088–1092 Acknowledgements This work was supported by the National Natural Science Foundation of China (52071197), the Science and Technology Commission of Shanghai Municipality (19ZR1418300 and 19DZ2270200), the Independent Research and Development Project of State Key Laboratory of Advanced Special Steel, Shanghai Key Laboratory of Advanced Ferrometallurgy, Shanghai University (SKLASS 2021-Z05), Grant PID2019- 105720RB-I00 funded by MCIN/AEI/10.13039/501100011033, US/JUNTA/ FEDER-UE (US-1260179), and Consejería de Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía (P18-RT-746). Guo D would like to acknowledge the support provided by China Scholarship Council (CSC) of the Ministry of Education, China (202006890050). Moreno-Ramírez LM acknowledges a postdoctoral fellowship from Junta de Andalucía and European Social Fund (ESF). Author contributions Zhang Y and Law JY designed the idea of the research. Guo D and Zhang Y performed the experiments. Guo D, Moreno- Ramírez LM, and Law JY performed the data analysis and prepared the manuscript. All the authors contributed to the general discussion and review and editing. Franco V and Zhang Y contributed to the conceptualization and supervision. Conflict of interest The authors declare that they have no conflict of interest. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/ by/4.0/. Dan Guo is a PhD student at the School of Materials Science and Engineering, Shanghai University (SHU), China. She is now a visiting scholar at the University of Seville, Spain, sponsored by the China Scholarship Council. Her research interests focus on magnetic properties and magnetocaloric effect of heavy rare-earth based compounds. Jia-Yan Law obtained her PhD degree from the School of Materials Science and Engineering, Nanyang Technological University, Singapore. Currently, she is a postdoctoral researcher at the University of Seville (Spain), leading the research line “Functional High-Entropy Alloys”. Her research interests include the development of magnetic and magnetocaloric materials, device and novel evaluation techniques as well as additive manufacturing. Yikun Zhang received her PhD degree from Northeastern University (China); afterward, she was granted by the Alexander von Humboldt (AvH) scholarship for postdoctoral research. Currently, she is a professor at Hangzhou Dianzi University (China). Her research mainly focuses on designing and exploring magnetic functional materials, especially rare-earth-based magnetic refrigeration materials. 重稀土HRENiGa2(HRE = Dy, Ho或Er)化合物的优异 低温磁热性能 郭丹1,2,3, Luis M. Moreno-Ramírez2, Jia-Yan Law2*,张义坤1,3*, Victorino Franco2 摘要 RENiX2化合物(其中RE为稀土元素, X为p区元素)在低温磁制冷 应用中受到高度关注.它们根据元素的不同可以结晶成CeNiSi2型、 NdNiGa2型或MgCuAl2型晶体结构,并表现出不同类型的磁有序性从 而影响其磁性. MgCuAl2型铝化物由于具有有利的铁磁基态从而表现 出比CeNiSi2型硅化物和NdNiGa2型镓化物更大的磁热性能.此外, RE- NiGa2镓化物根据RE元素的不同可以结晶成NdNiGa2或MgCuAl2型结 构.本文中,我们选择重稀土(HRE)元素来探索HRENiGa2(HRE = Dy, Ho或Er)镓化物的微观结构、磁有序和磁热性能.三种化合物均以 MgCuAl2型晶体结构结晶,并且随着温度的升高经历了从铁磁到顺磁 的二级磁相转变. DyNiGa2, HoNiGa2和ErNiGa2化合物的最大等温磁 熵变(|ΔSisomax|)值分别为6.2, 10.4和11.4 J kg−1K−1(0–5 T), 这与许多最 近报道的低温磁制冷材料性能相当.特别地, HoNiGa2和ErNiGa2化合 物(包括它们的复合材料)在氢气液化的温度范围内表现出优异的磁热 性能. ARTICLES SCIENCE CHINA Materials 256 January 2023 | Vol.66 No.1 © The Author(s) 2022. This article is published with open access at link.springer.com.