† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 51671022 and 51427806), the Beijing Natural Science Foundation, China (Grant No. 2162022), and the Fundamental Research Funds for the Central Universities, China (Grant No. FRF-TP-15-002A3).
The magnetic properties and magnetocaloric effects (MCE) of HoNiGa compound are investigated systematically. The HoNiGa exhibits a weak antiferromagnetic (AFM) ground state below the Ńeel temperature TN of 10 K, and the AFM ordering could be converted into ferromagnetic (FM) ordering by external magnetic field. Moreover, the field-induced FM phase exhibits a high saturation magnetic moment and a large change of magnetization around the transition temperature, which then result in a large MCE. A large −ΔSM of 22.0 J/kg K and a high RC value of 279 J/kg without magnetic hysteresis are obtained for a magnetic field change of 5 T, which are comparable to or even larger than those of some other magnetic refrigerant materials in the same temperature range. Besides, the μ0H2/3 dependence of
The magnetic refrigeration based on magnetocaloric effect (MCE) is superior to the conventional gas compression-expansion refrigeration technique due to its higher energy transfer efficiency and environmental friendliness.[1–5] In recent years, the environmental degradation and the demand of new energy technique have aroused the passion of scientists to develop this advanced refrigeration technology and search for high-performance magnetocaloric materials, which exhibit large MCE, high refrigerant capacity (RC), and negligible hysteresis loss.[6–9]
In the past few decades, large MCEs have been reported in many materials with magnetic transition from ferromagnetic (FM) to paramagnetic (PM) states, in which a negative magnetic entropy change ΔSM (conventional MCE) is obtained by applying a magnetic field. On the other hand, the antiferromagnetic (AFM) material exhibits positive magnetic entropy change ΔSM (inverse MCE) with applying a magnetic field due to the disordering of AFM sublattices antiparallel to the magnetic field.[10,11] However, recently some AFM materials have also been found to exhibit large conventional MCEs, which are associated with the first-order field-induced metamagnetic transition from AFM to FM states.[12,13] Moreover, the AFM–FM metamagnetic transition shows perfect magnetic reversibility, so it is favorable for the practical application in magnetic refrigeration. These results hint us that large reversible MCE could be found not only in FM materials but also in AFM materials.
As one of the important rare-earth (R) based series, the ternary intermetallic RTX compounds (R = rare earth, T = transitional metal, X = p-block metal) have been investigated systematically in the past decades.[14–16] It is interesting to note that although the X atoms hardly contribute the magnetic moments, the crystallographic structure would vary with the change of X atoms, thus leading to the different magnetic and magnetocaloric properties of RTX compounds.[16] For example, the HoNiSi compound crystallizes in the orthorhombic TiNiSi-type structure and experiences an AFM–PM transition,[13] while the HoNiAl compound crystallizes in the hexagonal ZrNiAl-type structure and undergoes successive AFM–FM and FM–PM transitions.[17] Moreover, the HoNiIn compound also crystallizes into ZrNiAl-type structure as HoNiAl, but it shows successive spin-reorientation-like transition and FM–PM transition.[18] In order to further understand the effects of different p-block metals X on the magnetic and magnetocaloric properties in RTX compounds, in present work, we study the magnetic properties and MCE of HoNiGa compound in detail.
The polycrystalline HoNiGa compound was prepared by arc-melting appropriate proportion of constituent components with the purity better than 99.9 wt%. The ingot was flipped over and melted several times to ensure homogeneity. The as-cast sample was wrapped in tantalum foil and annealed in a high-vacuum quartz tube at 1223 K for 3 weeks, and then quenched in ice water. The crystal structure was investigated by powder x-ray diffraction (XRD) at room temperature through using Cu Kα radiation, and the structure parameters were determined by Rietveld refinement[19] through using GSAS[20]/EXPGUI softwares.[21] The XRD investigation confirmed that HoNiGa compound crystallizes into a single phase with TiNiSi-type orthorhombic structure (space group Pnma). The lattice parameters obtained from Rietveld refinement as shown in Fig.
The temperature (T) dependences of zero-field-cooling (ZFC) and field-cooling (FC) magnetization (M) for HoNiGa compound under a magnetic field of 0.01 T are shown in Fig.
In order to further understand the effect of magnetic field on the phase transition, the temperature dependences of magnetization for HoNiGa compound under various applied fields are measured as shown in Fig.
Figure
The Arrott plots are derived from the M–μ0H curves and are presented in Figs.
The ΔSM values for different magnetic field changes are calculated from magnetization isotherms by using Maxwell relation:
With the further increase of magnetic field, a large positive −ΔSM peak (normal MCE) of −22.0 J/kg K can be found around the transition temperature for a field change of 5 T (Fig.
According to the mean-field approximation, despite the diversities in the shape of −ΔSM–T curve for different materials, the field dependences of −ΔSM for all the materials with second-order magnetic transition can be expressed as
In summary, the HoNiGa compound undergoes an AFM–PM magnetic transition under low magnetic field in the vicinity of TN = 10 K. However, the AFM ground state can be converted into FM state due to its weak stability, and thus the HoNiGa undergoes a second-order FM–PM magnetic transition under high magnetic field. Moreover, perfect thermal and magnetic reversibility are observed in HoNiGa, and it is desirable for the practical applications in magnetic refrigeration. The field-induced FM phase exhibits a high saturation magnetic moment and a large change of magnetization around the transition temperature, thus leading to a large MCE. Besides, the field dependence of
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