The LaFe₁₃-based compound with a cubic NaZn₁₃-type structure has attracted a great deal of attention in recent years due to its potential applications as a magnetic refrigerant.^[1–4] The La(Fe_xSi_1−x)₁₃ compounds (space group ) with 0.86 ≤ x ≤ 0.90 each show a thermally-induced first-order magnetic transition between the paramagnetic and the ferromagnetic states at the Curie temperature T_C and an itinerant-electron metamagnetic (IEM) transition just above T_C,^[5–9] which causes a large magnetocaloric effect (MCE) of the isothermal magnetic entropy change ΔS_m and the adiabatic temperature change ΔT_ad.^[6–8,10] However, its lower Curie temperature (T_C ∼ 190 K)^[6] makes it difficult to use in domestic appliances. To realize room temperature magnetic refrigerators, it is necessary to raise its T_C. Now incorporating an interstitial hydrogen atom into La(Fe_xSi_1−x)₁₃ compounds is regarded as an effective way to increase T_C significantly.^[11–15] In La(Fe_xSi_1−x)₁₃H_y interstitial compounds, due to the intrusion of hydrogen into the 24d site the Fe^II–Fe^II (at 96i sites) is expanded, resulting in lattice expansion which yields a considerable increase in Curie temperature.^[16,17] It has been reported that after hydrogen absorption in H₂ gas under 5-to-50 atmospheric pressures, and T_C increases up to about 280 K–336 K.^[11,12,15] However, for the reasons of properties of La(Fe_xSi_1−x)₁₃ compounds and the hydrogenation process, it is difficult to control T_C freely without weakening MCEs. Unsaturated hydrogen absorption can lead to asymmetrical occupation of H atoms in La(Fe_xSi_1−x)₁₃ unit cell, which results in the decrease of magnetic entropy change. Sometimes the Curie temperature is lifted too high to be used at room temperature after saturate hydrogen absorption, such as T_C = 336 K for LaFe_11.4Si_1.56H_1.6.^[11]

Previous research confirmed that the addition of a small amount of Mn brings about the decrease of Curie temperature of La(Fe_xSi_1−x)₁₃, with keeping the IEM transition.^[17–20] For La(Fe_1−xMn_x)_11.7Si_1.3, Mn concentration has an influence on T_C which decreases almost linearly from 188 K for x = 0 to 127 K for x = 0.03.^[17] The rapid decrease of T_C may be attributed to the weakening of the magnitude of the exchange coupling between Fe–Fe atoms, caused by Mn substitution. Therefore, it should be an effective way to adjust Curie temperature while remaining a large magnetic entropy change by jointly utilizing Mn substitution for Fe and saturate hydrogen absorption.

In this paper, we prepare La(Fe_0.99Mn_0.01)_11.5Si_1.5H_y compound in hydrogen gas under 1 atmospheric pressure at 473 K and study the effects of substitution of Mn atom and interstitial H atom on the Curie temperature and magnetic entropy change. The structure and magnetic transition properties of the compounds are also investigated.

La(Fe_1−xMn_x)_11.5Si_1.5 (x = 0, 0.01, 0.02, 0.03) compounds were prepared from pure (≥ 99.9%) elements by using the arc-melting method in argon atmosphere. Each arc-melted ingot was flipped over and remelted at least three times to ensure its homogeneity. The ingots were annealed at 1353 K for 10 days, and after that quenched in ice water. The hydrogen absorption took place at 473 K by annealing in a furnace equipped with a vacuum system after the samples had been crushed into powder with about 0.8 mm in size. The pressure of hydrogen was kept under 1 atmospheric pressure. The time of hydrogen absorption is selected to be 2 h to ensure that the saturation was reached.^[21] The hydrogen concentration was determined by the weighing method. The structures of all parent alloys and corresponding hydrides were identified by powder x-ray diffraction (XRD) and the magnetic measurements were carried out by vibrating sample magnetometer (VSM). To study the phase transitions of the compounds a differential scanning calorimeter (DSC) was used.

X-ray diffraction analyses of powdered samples are shown in Fig. 1. The La(Fe_1−xMn_x)_11.5Si_1.5 (x = 0.01, 0.02, 0.03) and La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 compounds each remain a cubic NaZn₁₃-type structure. Obviously, neither Mn substitution nor hydrogen absorption has an adverse influence on the formation of 1:13 phase. The diffraction peaks of La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 shift toward low angles compared with those of La(Fe_0.99Mn_0.01)_11.5Si_1.5. It is indicated that hydrogen atoms have entered interstices between crystal lattices as interstitial atoms, leading to an expansion of unit cell volume. In Table 1, the lattice parameters, obtained from the XRD patterns, are 11.65 Å for La(Fe_0.99Mn_0.01)_11.5Si_1.5, 11.58 Å for La(Fe_0.98Mn_0.02)_11.5Si_1.5, 11.48 Å for La(Fe_0.97Mn_0.03)_11.5Si_1.5, and 11.6248 Å for La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61, respectively. Because the ionic radius of Mn is larger than that of Fe, the lattice parameter increases with increasing Mn concentration.

Fig. 1.

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	Fig. 1. X-ray diffraction patterns of La(Fe1−xMnx)11.5Si1.5 (x = 0.01, 0.02, 0.03) and La(Fe0.99Mn0.01)11.5Si1.5H1.61 compounds.

Figure 2 shows DSC curves of La(Fe_1−xMn_x)_11.5Si_1.5 (x = 0.01, 0.02, 0.03) compounds. As seen from this figure, an endothermic peak is found at each of the Curie temperature T_C values of 194 K, 184 K, 173 K, and 151 K for x = 0, 0.01, 0.02, and 0.03, respectively, showing an obvious decease of T_C with increasing Mn concentration. Furthermore, the peak heights of phase transition of La(Fe_1−xMn_x)_11.5Si_1.5 compounds decrease with increasing x. The rapid decrease of T_C and changes of phase transition properties may be attributed to the weakening of the magnitude of the exchange coupling between Fe–Fe atoms caused by the antiferromagnetic coupling between Fe and Mn atoms, as previously reported.^[17,19]

Table 1.

Table 1.

Table 1. Lattice constants of La(Fe1−xMnx)11.5Si1.5 (x = 0.01, 0.02, 0.03) and La(Fe0.99Mn0.01)11.5Si1.5H1.61. . Materials Lattice constant a/Å LaFe11.5Si1.5[21] 11.4867 La(Fe0.99Mn0.01)11.5Si1.5 11.5076 La(Fe0.98Mn0.02)11.5Si1.5 11.4957 La(Fe0.97Mn0.03)11.5Si1.5 11.4837 La(Fe0.99Mn0.01)11.5Si1.5H1.61 11.6248

Table 1. Lattice constants of La(Fe1−xMnx)11.5Si1.5 (x = 0.01, 0.02, 0.03) and La(Fe0.99Mn0.01)11.5Si1.5H1.61. .

Fig. 2.

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	Fig. 2. DSC measurements for La(Fe1−xMnx)11.5Si1.5 (x = 0, 0.01, 0.02, 0.03) compounds.

DSC curves of LaFe_11.5Si_1.5, La(Fe_0.99Mn_0.01)_11.5Si_1.5, and La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 compounds are shown in Fig. 3. In this figure, an endothermic peak is found at each 194 K, 184 K, and 313 K temperatures for LaFe_11.5Si_1.5, La(Fe_0.99Mn_0.01)_11.5Si_1.5, and La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61, respectively, indicating that the partial substitution of Mn for Fe leads to the decrease of the T_C, and then, saturate hydrogen absorption significantly lifts T_C to room temperature. In LaFe_11.5Si_1.5, the substitution of smaller Mn atoms for Fe weakens the magnitude of the exchange coupling between Fe–Fe atoms (J_Fe−Fe), which gives rise to the decrease of T_C. On the other hand, hydrogen absorption of La(Fe_0.99Mn_0.01)_11.5Si_1.5 leads to the volume expansion, which enhances the exchange coupling between Fe–Fe atoms (J_Fe−Fe) and Fe–Mn atoms (J_Fe−Mn), and results in a significant increase of T_C. For this reason, La(Fe_1−xSi_x)₁₃ compounds with T_C around room temperature can be obtained by jointly utilizing the proper proportion of substitution of Mn for Fe and hydrogen absorption. Also, a comparison of endothermic peaks between before and after hydrogen absorption indicates that the temperature range of the phase transition does not become wider.

Fig. 3.

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	Fig. 3. The differential scanning calorimeter (DSC) measurements for LaFe11.5Si1.5, La(Fe0.99Mn0.01)11.5Si1.5, and La(Fe0.99Mn0.01)11.5Si1.5Hy compounds.

Figure 4(a) shows the magnetization isotherms of La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 compound, measured in a wide temperature range around the Curie temperature. It can be seen that the magnetization of the compound decreases with temperature increasing. When the temperature approaches to T_C, the magnetization change becomes large. Figure 4(b) diaplays the Arrott curves of La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 compound. The negative slopes suggest that a first-order field-induced IEM transition from paramagnetic to ferromagnetic order is preserved^[6,9,22,23] after hydrogen absorption, which is of great importance for the large magnetic entropy changes.

The ΔS_m is evaluated from the variation of magnetization as a function of M and H in connection with the following Maxwell relation:

Fig. 4.

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	Fig. 4. Magnetization curves (a) and Arrott curves (b) for La(Fe0.99Mn0.01)11.5Si1.5H1.61 compound.

Fig. 5.

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	Fig. 5. The temperature-dependent magnetic entropy of La(Fe0.99Mn0.01)11.5Si1.5H1.61 compound as static magnetic field varies from 0 T to 1 T and 0 T to 2 T.

In this study, the room-temperature powder XRD patterns show that the La(Fe_0.99Mn_0.01)_11.5Si_1.5 and La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 compounds each crystallize into a single phase with a cubic NaZn₁₃-type structure. The Curie temperature of La(Fe_1−xMn_x)_11.5Si_1.5 is adjusted to about 313 K after saturate hydrogen absorption in H₂ gas under 1 atmospheric pressure and partial substitution of Mn for Fe with x = 0.01, in the remaining first-order magnetic transition. For La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 compound, ΔS_m for a magnetic field change from 0 T to 2 T is found to be −11.5 J/kg·K, which is larger than that of Gd. The T_C near room temperature and large magnetic entropy change make the La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 compound an attractive candidate for magnetic refrigerants at room temperature.