Fu Bin, Han Jie. Magnetic entropy change and magnetic properties of LaFe11.5Si1.5 after controlling the Curie temperature by partial substitution of Mn and hydrogenation. Chinese Physics B, 2016, 25(2): 027501
Magnetic entropy change and magnetic properties of LaFe11.5Si1.5 after controlling the Curie temperature by partial substitution of Mn and hydrogenation
Fu Bin1, †, , Han Jie2
School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
College of Science, Tianjin University of Technology, Tianjin 300384, China
† Corresponding author. E-mail: email@example.com
Projct supported by the Science and Technology Development Fund of Higher Education of Tianjin, China (Grant No. 20130301) and the Tianjin Research Program of Application Foundation and Advanced Technology, China (Grant No. 14JCQNJC4000).
Magnetic properties and magnetic entropy changes of La(Fe1−xMnx)11.5Si1.5Hy compounds are investigated. Their Curie temperatures are adjusted to room temperature by partial Mn substitution for Fe and hydrogen absorption in 1-atm (1 atm = 1.01325 × 105 Pa) hydrogen gas. Under a field change from 0 T to 2 T, the maximum magnetic entropy change for La(Fe0.99Mn0.01)11.5Si1.5H1.61 is −11.5 J/kg. The suitable Curie temperature and large value of ΔSm make it an attractive potential candidate for the room temperature magnetic refrigeration application.
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(FexSi1−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 TC and an itinerant-electron metamagnetic (IEM) transition just above TC,[5–9] which causes a large magnetocaloric effect (MCE) of the isothermal magnetic entropy change ΔSm and the adiabatic temperature change ΔTad.[6–8,10] However, its lower Curie temperature (TC ∼ 190 K) makes it difficult to use in domestic appliances. To realize room temperature magnetic refrigerators, it is necessary to raise its TC. Now incorporating an interstitial hydrogen atom into La(FexSi1−x)13 compounds is regarded as an effective way to increase TC significantly.[11–15] In La(FexSi1−x)13Hy interstitial compounds, due to the intrusion of hydrogen into the 24d site the FeII–FeII (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 TC increases up to about 280 K–336 K.[11,12,15] However, for the reasons of properties of La(FexSi1−x)13 compounds and the hydrogenation process, it is difficult to control TC freely without weakening MCEs. Unsaturated hydrogen absorption can lead to asymmetrical occupation of H atoms in La(FexSi1−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 TC = 336 K for LaFe11.4Si1.56H1.6.
Previous research confirmed that the addition of a small amount of Mn brings about the decrease of Curie temperature of La(FexSi1−x)13, with keeping the IEM transition.[17–20] For La(Fe1−xMnx)11.7Si1.3, Mn concentration has an influence on TC which decreases almost linearly from 188 K for x = 0 to 127 K for x = 0.03. The rapid decrease of TC 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(Fe0.99Mn0.01)11.5Si1.5Hy 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.
2. Experimental procedure
La(Fe1−xMnx)11.5Si1.5 (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. 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.
3. Results and discussion
X-ray diffraction analyses of powdered samples are shown in Fig. 1. The La(Fe1−xMnx)11.5Si1.5 (x = 0.01, 0.02, 0.03) and La(Fe0.99Mn0.01)11.5Si1.5H1.61 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(Fe0.99Mn0.01)11.5Si1.5H1.61 shift toward low angles compared with those of La(Fe0.99Mn0.01)11.5Si1.5. 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(Fe0.99Mn0.01)11.5Si1.5, 11.58 Å for La(Fe0.98Mn0.02)11.5Si1.5, 11.48 Å for La(Fe0.97Mn0.03)11.5Si1.5, and 11.6248 Å for La(Fe0.99Mn0.01)11.5Si1.5H1.61, respectively. Because the ionic radius of Mn is larger than that of Fe, the lattice parameter increases with increasing Mn concentration.
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(Fe1−xMnx)11.5Si1.5 (x = 0.01, 0.02, 0.03) compounds. As seen from this figure, an endothermic peak is found at each of the Curie temperature TC 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 TC with increasing Mn concentration. Furthermore, the peak heights of phase transition of La(Fe1−xMnx)11.5Si1.5 compounds decrease with increasing x. The rapid decrease of TC 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]
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.
DSC curves of LaFe11.5Si1.5, La(Fe0.99Mn0.01)11.5Si1.5, and La(Fe0.99Mn0.01)11.5Si1.5H1.61 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 LaFe11.5Si1.5, La(Fe0.99Mn0.01)11.5Si1.5, and La(Fe0.99Mn0.01)11.5Si1.5H1.61, respectively, indicating that the partial substitution of Mn for Fe leads to the decrease of the TC, and then, saturate hydrogen absorption significantly lifts TC to room temperature. In LaFe11.5Si1.5, the substitution of smaller Mn atoms for Fe weakens the magnitude of the exchange coupling between Fe–Fe atoms (JFe−Fe), which gives rise to the decrease of TC. On the other hand, hydrogen absorption of La(Fe0.99Mn0.01)11.5Si1.5 leads to the volume expansion, which enhances the exchange coupling between Fe–Fe atoms (JFe−Fe) and Fe–Mn atoms (JFe−Mn), and results in a significant increase of TC. For this reason, La(Fe1−xSix)13 compounds with TC 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. 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(Fe0.99Mn0.01)11.5Si1.5H1.61 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 TC, the magnetization change becomes large. Figure 4(b) diaplays the Arrott curves of La(Fe0.99Mn0.01)11.5Si1.5H1.61 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 ΔSm is evaluated from the variation of magnetization as a function of M and H in connection with the following Maxwell relation:
The magnetic entropy changes are shown in Fig. 5. For the La(Fe0.99Mn0.01)11.5Si1.5H1.61 compound, the maximum values of ΔSm for magnetic field changes from 0 T to 1 T and from 0 T to 2 T are found to be −8.0 J/kg·K and −11.5 J/kg·K, respectively. Comparing with the scenario of LaFe11.5Si1.5, whose ∣ΔSm∣ is 18 J/kg·K for a field change of 0 T to 2 T, the maximum magnetic entropy change decreases by substituting Mn for Fe. However, ∣ΔSm∣ of La(Fe0.99Mn0.01)11.5Si1.5H1.61 compound notably exceeds that of Gd (about 4.5 J/kg·K for a field change of 0 T to 2 T). Therefore, the La(Fe0.99Mn0.01)11.5Si1.5H1.61 compound is an attractive candidate for magnetic refrigerants near room temperature.
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(Fe0.99Mn0.01)11.5Si1.5 and La(Fe0.99Mn0.01)11.5Si1.5H1.61 compounds each crystallize into a single phase with a cubic NaZn13-type structure. The Curie temperature of La(Fe1−xMnx)11.5Si1.5 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(Fe0.99Mn0.01)11.5Si1.5H1.61 compound, ΔSm 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 TC near room temperature and large magnetic entropy change make the La(Fe0.99Mn0.01)11.5Si1.5H1.61 compound an attractive candidate for magnetic refrigerants at room temperature.