Metamagnetic transition and magnetocaloric properties in antiferromagnetic Ho2Ni2Ga and Tm2Ni2Ga compounds

Intermetallics 94 (2018) 17–21

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Metamagnetic transition and magnetocaloric properties in antiferromagnetic Ho2Ni2Ga and Tm2Ni2Ga compounds

T

Yikun Zhanga,b,∗, Yang Yanga, Chunjuan Houc, Dan Guoa, Xi Lia, Zhongming Rena, Gerhard Wildeb a

State Key Laboratory of Advanced Special Steels & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China b Institute of Materials Physics, University of Münster, Wilhelm-Klemm-Straße 10, D-48149 Münster, Germany c Beijing Key Laboratory of Biomass Waste Resource Utilization, Biochemical Engineering College, Beijing Union University, Beijing 100023, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Ho2Ni2Ga and Tm2Ni2Ga Structure Magnetocaloric effect Metamagnetic transition Magnetic refrigeration

The structure, magnetic transition and magnetocaloric properties in the ternary gallium intermetallic compounds of Ho2Ni2Ga and Tm2Ni2Ga have been systematically investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and magnetization measurements. As found from powder XRD and Rietveld refinement, Ho2Ni2Ga and Tm2Ni2Ga crystallized in an orthorhombic structure of the W2B2Co type belonging to the Immm space group. Both compounds are ordering antiferromagnetically at low temperatures and under low magnetic fields together with a field-induced metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM). Peculiar and similar magnetocaloric properties are observed for both compounds, i. e., an inverse magnetocaloric effect (positive magnetic entropy change, ΔSM) under low magnetic field changes (ΔH) and at low temperatures together with a large normal reversible magnetocaloric effect under high ΔH. The positive ΔSM below/at the Néel temperatures (TN) is ascribed to first order transition from antiferromagnetic (AFM) to ferromagnetic (FM) states, whereas the negative ΔSM above TN is due to the fact of the magnetic transition from paramagnetic (PM) to ferromagnetic (FM) states. Based on the field-dependent magnetization data, the maximum magnetic entropy change (−ΔSMmax), relative cooling power (RCP) and refrigerant capacity (RC) are established, and the corresponding values are 9.6 J/kg K, 276 J/kg and 206 J/kg for Ho2Ni2Ga, and are 4.3 J/kg K, 60 J/kg and 46 J/kg for Tm2Ni2Ga, respectively, for a magnetic field change of 0–70 kOe.

1. Introduction The ternary intermetallic compounds RE2T2X 2:2:1 (RE = rare earth, T = transition metal, and X = p block element) with different RE, T and X elements crystallize in different crystallize structures have been successfully synthesized [1–17]. Based on the relationship of the size of the atoms and the valence electron concentration, there exist three types of crystallize structures, i.e. the tetragonal Mo2B2Fe-type with the space group P4/mbm, the orthorhombic Mn2B2Al-type with the space group Cmmm, as well as the orthorhombic W2B2Co-type with the space group Immm [18–20]. Depending on the constituent elements, the RE2T2X compounds show a variety of fascinating physical properties, such as ferromagnetism, antiferromagnetism, mixed or intermediate valence, spin reorientation, heavy fermion behaviors, Kondo effect etc. Among of the X atoms, the structure and magnetic properties in the

compounds with X = In, Sn and Cd, have been widely investigated [3–17]. The RE2Cu2In possess tetragonal Mo2B2Fe-typ structure and ferromagnetically order at their respective transition temperatures of 85.5, 81, 45.5, 26.7, 37.0 and 29.5 K for RE = Gd, Tb, Dy, Ho, Er and Tm, respectively [6]. Two types of crystallize structures have been found in RE2Ni2Sn; the orthorhombic W2B2Co-type for RE = Ce-Dy, and Lu, and the tetragonal Mo2B2Fe-type for Ho2Ni2Sn, Er2Ni2Sn and Tm2Ni2Sn, respectively [3,4]. The compounds of Ce2Ni2Sn, Sm2Ni2Sn and Gd2Ni2Sn ferromagnetically order around the transition temperatures of 8, 50 and 75 K, whereas Nd2Ni2Sn and Tb2Ni2Sn antiferromagnetically order around the transition temperatures of 22 and 66 K, respectively [3]. In parallel, the RE2Cu2Cd have tetragonal Mo2B2Fe-typ structure and ferromagnetic ordering at the transition temperatures of 120, 48.5, 30, 36 and 15 K for RE = Gd, Dy, Ho, Er and Tm, respectively [14–16]. The Gd2Ni2Cd follows a Mn2B2Al-type

∗ Corresponding author. State Key Laboratory of Advanced Special Steels & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China. E-mail address: [email protected] (Y. Zhang).

https://doi.org/10.1016/j.intermet.2017.12.013 Received 6 August 2017; Received in revised form 27 November 2017; Accepted 9 December 2017 0966-9795/ © 2017 Elsevier Ltd. All rights reserved.

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structure with a ferromagnetic ordering of 65 K [17]. It is worth noting that there are two or multiple magnetic transitions which have been observed in Dy2Cu2In, Ho2Au2In and Ho2Cu2Cd due to the existence of the spin reorientation [9,10,16], and in Sm2Ni2Sn and Tb2Ni2Sn because of the simultaneous existence and completing interaction of the ferromagnetic and antiferromagnetic orders [3]. In recent years, the magnetocaloric effect (MCE) in many rare earth based intermetallic compounds including some compounds of RE2T2X system has been found and the origination has also been investigated in details, and some of them possess large/giant MCE [21–28]. The MCE is an intrinsic property for a magnetic material, and is induced via the coupling of the magnetic sublattice with an external magnetic field. The magnetic refrigeration (MR) based on MCE has a high energy efficiency over the conventional gas techniques [29–33]. Kumar et al. [3] found the RE2Ni2Sn possess considerable MCEs. The maximum magnetic entropy changes are 7.2, 0.1, 4.6 and 2.9 J/kg K for RE = Nd, Sm, Gd and Tb for a magnetic field change of 0–50 kOe, respectively. Moreover, Li et al. [10,16] have investigated the magnetic and magnetocaloric properties in Ho2Au2In, Ho2Cu2In and Ho2Cu2Cd compounds. Very recently, we have synthesized and systematically investigated the magnetic phase transitions, magnetism and magnetocaloric effect in RE2Cu2In (RE = Dy, Er and Tm) and RE2Cu2Cd (RE = Gd, Dy, Er and Tm) compounds [8,9,14,15]. However, up to now, there is no report of the crystalline structure and magnetic properties on the ternary gallium compounds RE2Cu2Ga of the RE2T2X system. In order to learn more about the physical properties of the RE2Ni2Ga compounds, we focus on the crystalline structure, magnetic phase transition and magnetocaloric properties of Ho2Ni2Ga and Tm2Ni2Ga compounds in this paper. It is found that both compounds have a W2B2Co-type orthorhombic structure and a field-induced metamagnetic transition in both compounds is observed. The present study may provide some clues to search for promising candidates of 2:2:1 family in the field of magnetic refrigeration.

Fig. 1. XRD patterns together with Rietveld refinement profiles for (a) Ho2Ni2Ga and (b) Tm2Ni2Ga compounds. Inset of Fig. 1 (b) shows the crystalline structure of Ho2Ni2Ga and Tm2Ni2Ga compounds.

5.349(2) and 8.230(3) Å for Ho2Ni2Ga, and to be 4.140(4), 5.316(3) and 8.164(4) Å for Tm2Ni2Ga, respectively. According to the Rietveld refinement, it is found that no impurity phase is detected for Ho2Ni2Ga, whereas one obviously extra peak at around 2θ ∼36.1° for Tm2Ni2Ga is observed which is due to the existence of TmNiGa impurity phase with the content of about 5.9 wt%. Fig. 2 (a) and (b) display the backscattered scanning electron micrographs of the Ho2Ni2Ga and Tm2Ni2Ga and the main grey phases are determined as Ho2Ni2Ga and Tm2Ni2Ga, respectively. EDS analysis is performed on the dark grey phases of Tm2Ni2Ga compound, illustrating compositions of 32.30 at.% Tm, 34.13 at.% Ni, and 33.57 at.% Ga, which further confirms the existence of TmNiGa phase. The magnetization M (left hand scale) as a function of temperature for Ho2Ni2Ga and Tm2Ni2Ga compounds for a magnetic field of 10 kOe are illustrated in Figs. 3 and 4, respectively. The right hand scales of Figs. 3 and 4 show the corresponding reciprocal susceptibility (1/χ) data with high temperature Curie-Weiss linear fit above 80 K. Based on the fitted equation, 1/χ = (T-θp)/C (C is the Curie constant and θp is the paramagnetic Curie temperature), the effective magnetic moment (μeff) and θp for both compounds can be computed. The values of μeff for Ho2Ni2Ga and Tm2Ni2Ga are 10.55 μB/Ho3+and 7.37 μB/Tm3+, respectively. The μeff for Ho2Ni2Ga is consistent with the corresponding free ion value of Ho3+ (10.60 μB), whereas the μeff for Tm2Ni2Ga is obviously smaller than the theoretical value of Tm3+ (7.56 μB) which may be due to the existence of the TmNiGa impurity phase. The corresponding θp values are obtained to be −14.3 and −7.5 K, further indicating the antiferromagnetic interactions are dominant at ground state for both compounds. The temperature dependence of magnetization (M) measured at 2 kOe in field-cooled (FC) and zero-field-cooled (ZFC) modes for Ho2Ni2Ga and Tm2Ni2Ga compounds are presented in the insets (a) of Figs. 3 and 4, respectively. It is clearly seen that both compounds show similar features except for some differences in values and there is no obvious thermal and magnetic hysteresis. The maximum values in ZFC and FC M-T curves are considered to be the magnetic transition temperatures from paramagnetic (PM) to antiferromagnetic (AFM), i.e. Néel temperature (TN), and which come out to be ∼12.5 and ∼5.5 K for Ho2Ni2Ga and Tm2Ni2Ga, respectively. The insets (b) of

2. Experimental The polycrystalline Ho2Ni2Ga and Tm2Ni2Ga samples were produced by arc-melting from elements of Ho, Tm, Ni, and Ga (with the purities all better than 99.9 at.%) under a Ti-gettered argon atmosphere. Both samples were turned over and re-melted for six times to ensure chemical homogeneity. Then, the samples were wrapped in a Tafoil and sealed in an evacuated quartz tube to subsequently anneal at 1083 K for seven days, and followed by quench in the ice-cold water. Powder X-ray diffraction (XRD) measurements at room temperature were carried out with the Bruker D8 Advance XRD using Cu-Kα radiation. The microstructure and composition of Ho2Ni2Ga and Tm2Ni2Ga were characterized by using a scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) attached (Hitachi SU70). The DC magnetization measurements were performed by using a commercial vibrating sample magnetometer (VSM) with a sensitivity range of ∼10−7 emu at room temperature, which is an option of the Cryogen Free Measurement System (CFMS -9T) in the temperature range of 3–300 K and with the magnetic field up to 70 kOe. 3. Results and discussion The observed XRD patterns together with the Rietveld refinement profiles for Ho2Ni2Ga and Tm2Ni2Ga which were analyzed by MAUD software are shown in Fig. 1 (a) and (b), respectively. The vertical bars demonstrate predicted Bragg peak positions for a given phase in both compounds. The factors RB (%) and Rexp (%) of Rietveld refinement are 10.91% and 4.64% for Ho2Ni2Ga; and 10.34% and 4.77% for Tm2Ni2Ga, respectively. The refinement results proved that both samples are crystallized in W2B2Co-type orthorhombic structure belonging to the space group Immm, as illustrated in the inset of Fig. 1(b). The refined lattice parameters a, b and c are evaluated to be 4.166(3), 18

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Fig. 4. Temperature dependence of the magnetization (Μ, left scale) and the reciprocal susceptibility (1/χ, right scale) for Tm2Ni2Ga. Insets show (a) the temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetization (M) for Tm2Ni2Ga under the magnetic field of 2 kOe, (b) the temperature dependence of magnetization (M) for Tm2Ni2Ga under various magnetic fields.

and Tm2Ni2Ga compounds were measured in a wide temperature range around their transition temperatures under applied magnetic field up to 70 kOe. Ten typical M-H curves are plotted for the sake of clarity in the temperature range of 3–50 K for both compounds, as displayed in Figs. 5(a) and 6(a), respectively. The M-H curves for Ho2Ni2Ga and Tm2Ni2Ga compounds also show similar features, for the temperature below TN, M shows a linear increase with the increase of H in low magnetic field ranges below 20 kOe, then M has a rapid increase around 20 kOe, but it does not saturate in high H up to 70 kOe. For the temperatures much higher than TN, the M shows a linear dependence of magnetic field. Generally, the slope of the Arrott plot curves by using plotting M2 vs H/M in the critical region has been used to justify the type of the magnetic phase transition of first order or second order [34]. The magnetic transition is first order if several of the M2 vs H/M curves show a negative slope at some point, whereas the magnetic transition is second order if all the curves show a positive slope. The Arrott plot curves for Ho2Ni2Ga and Tm2Ni2Ga compounds in the vicinity of their respective transition temperature ranges are shown in Figs. 5(b) and 6(b). The negative slope observed in the Arrott plot curves below TN, suggesting the occurrence of first order metamagnetic transition in both compounds. The isothermal magnetic entropy change (−ΔSM) is usually taken as an important metric of the MCE, which can be calculated using conventional numerical integration derived from Maxwell's thermo-

Fig. 2. The Back-scattered images for (a) Ho2Ni2Ga and (b) Tm2Ni2Ga compounds.

H max

(∂M (H , T )/ ∂T ) H dH (in which dynamic relation, ΔSM (T , ΔH ) = ∫0 T and H respectively denote absolute temperature and applied magnetic field). Fig. 7 displays the magnetic entropy change −ΔSM as a function of temperature for Ho2Ni2Ga and Tm2Ni2Ga compounds under various magnetic field changes up to ΔH = 70 kOe. It can be found that the maximum value of −ΔSM increase monotonically with increasing magnetic field change for both compounds [insets of Fig. 7 (a) and (b), (left hand scale)]. For Ho2Ni2Ga, we can see that the nature of −ΔSM curve exists distinct differences below the antiferromagnetic transition temperature TN for the magnetic field changes of ΔH = 10–30 kOe compared with the much higher magnetic field changes of ΔH = 40–70 kOe. Under ΔH = 10–30 kOe, the small negative value of −ΔSM (inverse MCE) exists at the temperatures around/below TN, and then it changes to be a positive value (normal MCE) at the temperatures above TN, which corresponds to the field-induced magnetic transition from AFM to FM states. Therefore, a distinguishable minimum and maximum are deduced in Ho2Ni2Ga compound. In contrast, the value of

Fig. 3. Temperature dependence of the magnetization (Μ, left scale) and the reciprocal susceptibility (1/χ, right scale) for Ho2Ni2Ga. Insets show (a) the temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetization (M) for Ho2Ni2Ga under the magnetic field of 2 kOe, (b) the temperature dependence of magnetization (M) for Ho2Ni2Ga under various magnetic fields.

Figs. 3 and 4 illustrate the temperature dependences of magnetization M for Ho2Ni2Ga and Tm2Ni2Ga in various magnetic fields, respectively. We can see that the nature of M-T curves are AFM up to 35 kOe and 20 kOe for Ho2Ni2Ga and Tm2Ni2Ga, respectively, whereas field-induced modification toward FM state occurs with further increasing magnetic field for both compounds. The magnetic field H dependence of magnetization M for Ho2Ni2Ga

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Fig. 6. (a) Magnetic field dependence of the magnetization (increasing field only) for Tm2Ni2Ga, (b) The Arrott plots (H/M versus M2) for Tm2Ni2Ga.

Fig. 5. (a) Magnetic field dependence of the magnetization (increasing field only) for Ho2Ni2Ga, (b) The Arrott plots (H/M versus M2) for Ho2Ni2Ga.

0–70 kOe, the values of RCP are evaluated to be 121 and 276 J/kg for Ho2Ni2Ga, and to be 32 and 60 J/kg for Tm2Ni2Ga; the corresponding values of RC are evaluated to be 93 and 206 J/kg for Ho2Ni2Ga; and to be 25 and 46 J/kg for Tm2Ni2Ga, respectively. The present results may provide some clues and new data for searching novel magnetic materials for magnetic refrigeration.

−ΔSM is positive in the entire temperature range under ΔH = 40–70 kOe in Ho2Ni2Ga since a magnetically more ordered configuration from FM to paramagnetic (PM) exists with applied a much higher magnetic field. Similar behaviours have been reported in EuFe2As2 [35], RE2Cu2O5 (RE = Dy and Ho) [36], and ErRu2Si2 [37] compounds. However, there is no inverse MCE is observed in Tm2Ni2Ga, which is due to the negative −ΔSM should exist in a much lower temperature beyond the limitation of magnetization measurements. For the ΔH of 0–20 kOe, the minimum and maximum values of −ΔSM are −1.5 and 0.9 J/kg K for Ho2Ni2Ga, respectively. For the ΔH of 0–50 and 0–70 kOe, the values of maximum -ΔSM are calculated to be 5.4 and 9.6 J/kg K for Ho2Ni2Ga; and to be 2.5 and 4.3 J/kg K for Tm2Ni2Ga, respectively. The relative cooling power (RCP) and refrigerant capacity (RC) has also been considered as another genuine parameter to evaluate the potentiality in application of a magnetic refrigerant material, which measures the thermal efficiency of a magnetic material of how much heat is transferred between hot and cold reservoirs in a thermodynamic cycle [23]. The RCP is estimated by the product of the value of maximum −ΔSM and full width at half maximum (δTFWHM) in the ΔSM-T max × δT FWHM . In parallel, the RC is the area of the curve, RCP = −ΔSM ΔSM − T curve under the temperatures at δTFWHM, and can be exT pressed as RC = ∫T 2 ΔSM dT in which T1 and T2 are the temperatures 1 of the cold and hot reservoirs of half maximum of the −ΔSM peak. The maximum values of RCP and RC also increase monotonically with increasing magnetic field change for both compounds [insets of Fig. 7 (a) and (b), (right hand scale)]. Consequently, for the ΔH of 0–50 and

4. Conclusions Two polycrystalline Ho2Ni2Ga and Tm2Ni2Ga compounds are prepared, which have Ho2Ni2Ga and Tm2Ni2Ga main phases together with a small amounts minor phases of HoNiGa and TmNiGa for both compounds, respectively. Based on the Rietveld refinements, both compounds crystallize in orthorhombic W2B2Co-type structure with space group Immm. Ho2Ni2Ga and Tm2Ni2Ga are ordering antiferromagnetically at low temperatures and under low magnetic fields together with a field-induced metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM). A positive ΔSM (inverse MCE) under low ΔH and at low temperatures together with a large negative ΔSM (normal MCE) under ΔH was observed in both compounds. The positive ΔSM below/at the Néel temperatures (TN) is originating from the field-induced first order transition from AFM to FM states, whereas the negative ΔSM above TN is because of the fact of the magnetic transition from paramagnetic (PM) to FM states. For the ΔH of 0–50 kOe, the maximum values of magnetic entropy change and relative cooling power are 5.4 J/kg K and 121 J/kg for Ho2Ni2Ga, and are 2.5 J/kg K and 32 J/kg for Tm2Ni2Ga, respectively. 20

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Fig. 7. Temperature dependence of magnetic entropy change -ΔSM for (a) Ho2Ni2Ga and (b) Tm2Ni2Ga. Insets show the maximum values of -ΔSM, RCP and RC vs. H for Ho2Ni2Ga and Tm2Ni2Ga.

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Acknowledgements The present work was supported by the National Natural Science Foundation of China (Nos. 51501036 and 51690160), Independent Research and Development Project of State Key Laboratory of Advanced Special Steel, Shanghai University, and United Innovation Program of Shanghai Commercial Aircraft Engine (Nos. AR910 and AR911).

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