Second-order Magnetic Transition Research Articles

Magnetic properties and magnetocaloric effect in thiospinel Fe1-xCuxCr2S4 (x = 0.0, 0.5) compounds

In this study, we investigate the magnetic and magnetocaloric properties of Cr-based spinel sulphides Fe1-xCuxCr2S4 (x = 0.0 and x = 0.5). The magnetic transition temperature of FeCr2S4 (176 K) increases to 330 K when 50 % of Fe in FeCr2S4 structure is substituted with Cu. Temperature-dependent Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetic moment measurements show a paramagnetic (PM) to ferrimagnetic (FiM) phase transition in our samples. Magnetocaloric properties were investigated through magnetization isotherm measurements. The maximum magnetic entropy change values are −2.78 J/kg. K and −2.11 J/kg. K under 5 T magnetic field, for x = 0 and x = 0.5 in Fe1-xCuxCr2S4, respectively. Analysis of phenomenological universal curves and Arrott plots indicate a second-order magnetic transition for both compounds. Consequently, Fe0.5Cu0.5Cr2S4 emerges as a promising candidate for magnetic refrigeration applications due to its considerable high magnetic entropy change and its proximity to room temperature transition.

Magnetic properties and large low-field magnetocaloric effect of RFe2Si2 (R = Ho, Tm) compounds

Nowadays, magnetic refrigerant technology has drawn much attention for its environmental friendliness. Those magnetic materials possessing giant low-field magnetocaloric effect (MCE) performance are of great importance for practical applications, especially at low temperatures. Herein, we present a detailed study on polycrystalline magnetocaloric compounds HoFe2Si2 and TmFe2Si2. HoFe2Si2 exhibits a transition from antiferromagnetic to paramagnetic phase at 2.2 K, while no long-range magnetic ordering is observed in TmFe2Si2 even temperature down to 2 K. For both compounds, they showed large low-field magnetic entropy change (−ΔSM) with the peak values of 10.6 and 7.9 J kg−1 K−1 under field change of 0–2 T for HoFe2Si2 and TmFe2Si2, respectively, which is comparable or even larger than some low-temperature magnetocaloric materials. In addition, the characteristic of second-order magnetic transitions of both compounds were confirmed on basis of Arrott plots and mean-field theory criterion. The low magnetic ordering temperatures, large low-field MCE performance along with the feature of second order phase transition for HoFe2Si2 and TmFe2Si2 indicate that both compounds are promising candidate for magnetic refrigerant materials at liquid helium temperature.

Designing gadolinium-transition metals-based perovskite type high entropy oxides with good cryogenic magnetocaloric performances

Cryogenic magnetic cooling based on the principle of the magnetocaloric effects (MCEs) of magnetic solids has been recognized as an alternative cooling technology due to its significant economic and social benefits. Designing novel magnetic materials with good magnetocaloric performance is a prerequisite for practical applications. In this study, three gadolinium-transition metal-based high entropy oxides (HEOs) of Gd(Fe1/4Ni1/4Al1/4Cr1/4)O3, Gd(Fe1/5Ni1/5Al1/5Cr1/5Co1/5)O3, and Gd(Fe1/6Ni1/6Al1/6Cr1/6Co1/6Mn1/6)O3 were designed and systematically characterized regarding their structural and cryogenic magnetic properties. These HEOs were confirmed to crystallize into a single-phase perovskite-type orthorhombic structure with a homogeneous microstructure, reveal a second-order magnetic transition at low temperatures, and exhibit significant cryogenic MCEs. The magnetocaloric performances of the present HEOs, identified by magnetic entropy changes, relative cooling power, and temperature-averaged entropy changes, were comparable with recently reported candidate materials. The present study indicates potential applications for cryogenic magnetic cooling of the present HEOs and provides meaningful clues for designing and exploring HEOs with good cryogenic magnetocaloric performances.

Excellent thermomagnetic power generation for harvesting waste heat via a second-order ferromagnetic transition.

Thermomagnetic generation (TMG), a promising technology to convert low-grade waste heat to electricity, utilizes high performance TMG materials. However, the drawbacks of large hysteresis, poor mechanical properties and inadequate service life hinder the practical applications. For the first time, we evaluated the effect of different phase transitions on the TMG performance by systematically comparing the TMG performance of three typical Heusler alloys with similar composition but different phase transitions. Ni2Mn1.4In0.6 exhibits second-order magnetic transition (SOMT) from the ferromagnetic (FM) to paramagnetic (PM) state around TC = 316 K without thermal hysteresis. It presents highly comprehensive TMG performance, which is not only better than those of other two Heusler alloys with different phase transitions, but also better than those of most typical TMG materials. The maximum power density (1752.3 mW m-3), cost index (2.78 μW per €), and power generation index PGI (8.91 × 10-4) of Ni2Mn1.4In0.6 are 1-5, 1-4, and 1-7 orders of magnitude higher than those of most typical reported materials, respectively. In addition, Ni2Mn1.4In0.6 with SOMT also shows some advantages that first-order magnetic transition (FOMT) materials do not have, such as zero hysteresis and a long-term service life. In contrast to the short lifetime of a few minutes for the materials with FOMT, Ni2Mn1.4In0.6 with SOMT can serve for one month or even longer with excellent cycling stability. Consequently, we conclude that the SOMT Ni2Mn1.4In0.6 Heusler alloy with good TMG performance as well as zero hysteresis and long service life can be a better candidate than FOMT materials for practical applications of TMG.

The effect of the gelation temperature on the structural, magnetic and magnetocaloric properties of perovskite nanoparticles manufactured using the sol-gel method.

This study presents a comprehensive investigation of the structural and magnetic properties of La0.8Sr0.2Mn0.8Co0.2O3 (LS1, LS2 and LS3) compounds synthesized via the sol-gel method at different gelation temperatures through X-ray diffraction and different magnetic measurement techniques. The Rietveld refinement demonstrated that all samples exhibit a rhombohedral perovskite structure with the R3̄C space group. Their magnetic behavior, characterized through magnetization measurements, hysteresis loops, and Arrot plots, demonstrates a ferromagnetic-paramagnetic transition with notable soft ferromagnetic characteristics. The samples also demonstrate second-order magnetic transitions, short-range magnetic order and the presence of both ferromagnetic and antiferromagnetic contributions. AC magnetic susceptibility measurements, allowing the investigation of the magnetic dynamics of the samples, shows that the Vogel-Fulcher and the Conventional Critical Slowing Down models are the most appropriate for describing the dynamic behavior, confirming the spin-glass nature of the compounds and the presence of medium to strong interaction between magnetic nanoparticles. The influence of gelation temperature in the magnetocaloric effect of the compounds was proven and LS1, synthesized at the lowest gelation temperature (70 °C), exhibits the higher magnetic entropy change (|ΔSmax| = 3.25 J kg-1 K-1 and RCP = 209.83 J kg-1 at 5 T). For a better evaluation of the magnetocaloric efficiency, the temperature average entropy change (TEC) parameter was calculated for all three samples and LS1 showed the highest value (TEC (LS1) ∼3.2 J kg-1 K-1 for ΔTH-C = 10 K and ΔH = 5 T).

Magnetic and magnetocaloric properties of Tb1.4Dy0.6In compound

This work summarizes the magnetic, thermal, magnetocaloric, and electrical transport properties of Tb1.4Dy0.6In and its critical behavior. First, we find that 60% Dy at the 2d (1/3, 2/3, 3/4) lattice sites of Tb2In (hexagonal Ni2In -type (space group- P63/mmc (No. 194))) leads to the formation of Tb1.4Dy0.6In, which undergoes a ferromagnetic transition at 155 K (T1) followed by an antiferromagnetic transition around 75 K (T2). Successive second-order magnetic transitions at T1 and T2 demonstrate a table-like hysteresis-free magnetocaloric effect around natural gas's liquefaction temperature (55–196 K). To understand the efficiency of Tb1.4Dy0.6In as a coolant, various cooling figures of merits such as isothermal entropy change, adiabatic temperature change, relative cooling power, the temperature averaged entropy change, and coefficient of refrigerant performance were determined. Along with the studies of magnetic properties, the critical exponent analysis endorses mean–field model that concord with the indirect long-range RKKY interaction between the rare-earth atoms in Tb1.4Dy0.6In. Furthermore, first-principle calculations indicate that the most hybridized dp states lie below the Fermi energy level and support the second-order magnetic nature of the transition. Overall, this study provides insights into the magnetocaloric properties of Dy-doped Tb2In and highlights its potential as an efficient coolant.

Structural, Magnetocaloric, and Magnetic Properties in Heusler Ni50Mn35In10X5 (X = Ga, Fe and Al) Alloys

The structural, magnetocaloric, and magnetic characteristics in Heusler Ni50Mn35In10X5 (X = Ga, Fe, and Al) alloys were examined using X-ray diffraction and field-dependent magnetization measurements. All samples exhibited a mixture structure of cubic L21 and tetragonal L10 and underwent second-order magnetic transitions at TC(Al5) = 220 K, TC(Ga5) = 252 K, and TC(Fe5) = 298 K. The Ga5 alloy exhibited structural change as indicated by a thermal hysteresis that may be seen in the saturation magnetic field in the M(T) dependences. The transition at the TC point from a ferromagnetic to a paramagnetic state caused a drop in magnetization, supported by thermal hysteresis, at a low magnetic field (0.01 T). On the other hand, the Fe5 alloy presented a gradual decrease in magnetization with similar hysteresis behavior, also at a low magnetic field (0.01 T), whereas at 0.1 T of field, no features characteristic of this transition were detected. This could be due to a large difference in the metallic radius of Fe compared to that of In. Otherwise, magnetic investigations demonstrated that the replacement of In with Al may cause the structural transformation temperatures and TC to be shifted to low temperatures. The present results imply that the structural transformation temperatures and the transition itself are highly dependent on chemical composition. Furthermore, under a magnetic field change of 5 T, the maximum magnetic entropy changes of 0.6 J/kg K, 1.4 J/kg K, and 2.71 J/kg K for the Ga5, Fe5, and Al5 alloys, respectively, were determined by their TC. Refrigeration capacity values were found to be 25 J/kg, 74 J/kg, and 98 J/kg at µ0∆H = 5 T. These ribbons are viable candidates for multifunctional applications due to their cheaper cost and their physical characteristics disclosed during the magnetostructural transition, which takes place close to the room temperature.

Investigation of doping effect on structural, magnetic and magnetoresistance properties by Ru doping in the manganite La0.6Ce0.05Ba0.35Mn1-xRuxO3 (0 ≤ x ≤ 0.2)

During this study, we were conducted to analyze the properties of structural, magnetic, magnetocaloric, electrical resistivity and magnetoresistance (by applying magnetic fields of zero, 2 and 5T) of ceramics La0.6Ce0.05Ba0.35Mn1-xRuxO3 (0 ≤ x ≤ 0.2). These samples are prepared by solid-state reaction. The compounds have simple rhombohedral structure R-3c space group. Arrot curves display that all materials exhibit a second-order magnetic transition from ferromagnetic to paramagnetic order. All the manganites synthesized show a maximum variation in magnetic entropy (|ΔSM|) with values between 6.2 J/kg.K for x = 0 and 3.4 J/kg.K for x = 0.2. In addition, the relative cooling power (RCP) values are very important (160 J/kg for x = 0 to 248 J/kg for x = 0.2). Environment-friendly compounds applied in the domestic refrigeration.The samples La0.6Ce0.05Ba0.35Mn1-xRuxO3 (0 ≤ x ≤ 0.1) show a transition from ferromagnetic-metal (FM) to paramagnetic-insulator (MI). Sample La0.6Ce0.05Ba0.35Mn0.8Ru0.2O3 exhibits semiconductor behavior over the entire temperature range. The metallic conduction is due to the diffusion of electrons (by the phonons and the grain boundaries) while the semiconducting behavior is due to the attendance of polarons. The curves of the compounds (0 ≤ x ≤ 0.1) in the metallic region (T < Tp), have been fitted to the equationρ=αT2+βT5+1(γ+δT1/2). For the high temperature, the semiconductor region was fitted by the small polaron hopping model (ASPH) and by the variable hopping range model (VRH). The electrical resistivity behavior indicates that the substitution of Ru3+ ions weakened the double exchange process and improved the Jahn-Teller effect. Studies of magnetoresistance (MR) under 2 and 5 T magnetic field show two kinds of contribution: one is intrinsic MR and the other is extrinsic MR.

Magneto-structural transitions driven giant magnetocaloric effect in Ni42Mn43Cr4Sn11 Shape Memory Heusler Alloy near room temperature

In the current work, a Ni–Mn-X (X = Sn, In) type off-stoichiometric, polycrystalline magnetic shape memory alloy (MSMA) having nominal composition Ni42Mn43Cr4Sn11 (at. %) has been thoroughly investigated in terms of its crystal structure, microstructure, magnetic properties, and magnetocaloric effect (MCE). X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), differential scanning calorimetry (DSC), and magnetic and magneto-transport measurements are used to characterize it. By offering a significant value of parameters related to MCE at the first-order martensite transition (FOMT) and the second-order magnetic transition (SOMT), the “Cr” doping at the “Ni” site enables considerable magnetic refrigeration to occur close to room temperature (RT). The alloy crystallises into coexisting cubic L21 austenite and tetragonal L10 martensite phases, among which the first one prevails as the leading phase at room temperature (RT) without exhibiting any geometric frustration. Lattice strain is enumerated using the Williamson-Hall method. A huge working temperature spans of 29.65 K and 26.16 K; the maximum magnetic entropy changes of +8.70 J kg−1.K−1 and −5.04 J kg−1.K−1 and the maximum refrigerant capacities (RC) of 284.24 J. Kg−1 and 234.10 J. Kg−1 across FOMT and SOMT, respectively at an applied magnetic field of 6 T are found to be most suitable for real technological applications. This MSMA offers a comparatively low cost of the constituent elements (Ni, Mn, Cr, and Sn), as well as exhibits giant MCE and RC across both FOMT and SOMT, making it an excellent candidate for magnetic refrigeration near RT. It also opens up new avenues for future research on novel Heusler alloys of this type.

Combinatorial design of partial ordered Al–Cr–Mn–Co medium-entropy alloys for room temperature magnetic refrigeration applications

Multi-component alloys have received increasing interest for functional applications in recent years. Here, we explore the magnetocaloric response for Al–Cr–Mn–Co medium-entropy alloys by integrated theoretical and experimental methods. Under the guidance of thermodynamic and ab initio calculations, a dual-phase system with large magnetic moment, i.e., Al50Cr19Mn19Co12, is synthesized, and the structural and magnetocaloric properties are confirmed via characterization. The obtained results indicate that the selected alloy exhibits a co-continuous mixture of a disordered body-centered cubic and an ordered B2 phase. The ab initio and Monte Carlo calculations indicate that the presence of the ordered B2 phase is responsible for the substantial magnetocaloric effect. The magnetization measurements demonstrated that this alloy undergoes a second-order magnetic transition with the Curie temperature of ∼300 K. The magnetocaloric properties are examined using magnetic entropy change, refrigeration capacity, and adiabatic temperature change. The property-directed strategy explored here is intended to contribute to the study of potential multi-component alloys in magnetocaloric applications.

Magnetic Refrigeration: An Environment-friendly Cooling Technology

Magnetic Refrigeration: An Environment-friendly Cooling Technology

The emergence of considerable room temperature magnetocaloric performances in the transition metal high-entropy alloys

We herein investigated the structural, magnetic, and magnetocaloric properties of Mn20Al20Co14Fe20+xCr26-xhigh entropy (HE) alloys by experimental determination and first principle calculations. The results indicated that these alloys were crystallized in a disordered body-centered cubic structure.The Fe substitution of Cr enhanced the energy and net magnetic moment of these alloys, resulting in the improvement of magnetic properties. All the Mn20Al20Co14Fe20+xCr26-x HE alloys underwent a second-order magnetic transition with theCurie temperature ranges of 268.2–310.6 K. The magnetocaloric performances of Mn20Al20Co14Fe20+xCr26-x HE alloys were examined by using the magnetic entropy change, relative cooling power, refrigerant capacity, temperature average entropy change, and mechanical properties, which are superior to the reported transition metal-based magnetocaloric materials with second order magnetic phase transition. By considering the realized considerablemagnetocaloricperformances and the benefits of transition metal HE alloys, the present Mn20Al20Co14Fe20+xCr26-x HE alloys are also of potential for room temperature MR application. The present work would provide a large family of transition metal HE alloys with significant magnetocaloric performances.

Critical Behavior of the Competing Magnetic Nature in the Antiperovskite Compound

Antiperovskite materials are deemed as a promising candidate for data storage and efficient energy conversion, exemplified by giant magnetoresistance (GMR) and magnetocaloric effect (MCE). In particular, the abundant magnetic configurations of antiperovskites endow them with a high degree of diversity in probing novel phenomena. Herein, we report the critical behavior of the antiperovskite Sn0.6NFe3.4 with an obvious second-order magnetic transition around Curie temperature (TC = 230 K). The obtained critical exponents (β, γ, and δ) are well consistent for different theoretical methods, which are also verified by the Widom scaling law and scaling equation. It is found that the critical exponent of β is much in accordance with the value of the mean-field model, indicating a long-range FM coupling in the system, whereas the other γ and δ are accessible to the three-dimensional (3D) Heisenberg model. Our work promotes the understanding of antiperovskite magnetism, providing insight for further fundamental research and resulting applications.

Relating microstructure to magnetocaloric properties in RE36Tb20Co20Al24 (RE = Gd, Dy or Ho) high-entropy metallic-glass microwires designed by binary eutectic clusters method

The new high-entropy metallic-glasses (HE-MGs) are designed by using Dy and Ho to replace Gd in Gd36Tb20Co20Al24 alloy based on the binary eutectic clusters method. Compared with the equiatomic Gd25Tb25Co25Al25 HE-MG, the non-equiatomic RE36Tb20Co20Al24 (RE = Gd, Dy, or Ho) alloys show better glass-forming ability, which is attributed to the deep binary eutectic compositions used for alloy design. All RE36Tb20Co20Al24 alloys undergo second-order magnetic transition. An extreme peak value of magnetic entropy change is obtained as 10.3 J kg–1 K–1 (5 T) for the Ho36Tb20Co20Al24 alloy. In-situ high-energy synchrotron X-ray diffraction was conducted to observe the microstructural difference among non-equiatomic samples at cryogenic temperatures. The results indicate that Gd36Tb20Co20Al24 alloy possesses a relatively large average value of the dispersion of local clusters at a low-temperature range. This, combined with the critical exponent β close to 0.5 of Gd36Tb20Co20Al24 alloy, leads to its widest working temperature span among non-equiatomic samples. This work successfully establishes the connection between microstructure and magnetocaloric properties of HE-MGs, which is beneficial for understanding the physical mechanism of the magnetocaloric behaviors of HE-MGs.

Giant low-field magnetocaloric effect in ferromagnetically ordered Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds

Magnetocaloric material is the key working substance for magnetic refrigerant technology, for which the low-field and low-temperature magnetocaloric effect (MCE) performance is of great importance for practical applications at low temperatures. Here, a giant low-field magnetocaloric effect in ferromagnetically ordered Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds was reported, and the magnetic structure was characterized based on low-temperature neutron powder diffraction. With increasing Tm content from 0 to 1, the Curie temperature of Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds decreases from 16.0 K to 3.6 K. For Er0.7Tm0.3Al2 compound, it showed the largest low-field magnetic entropy change (–ΔSM) with the peak value of 17.2 and 25.7 J/(kg K) for 0–1 T and 0–2 T, respectively. The (–ΔSM)max up to 17.2 J/(kg K) of Er0.7Tm0.3Al2 compound for 0–1 T is the largest among the intermetallic magnetocaloric materials ever reported at temperatures below 20 K. The peak value of adiabatic temperature change (ΔTad)max was determined as 4.13 K and 6.87 K for 0–1 T and 0–2 T, respectively. The characteristic of second-order magnetic transitions was confirmed on basis of Arrott plots, the quantitative criterion of exponent n, rescaled universal curves, and the mean-field theory criterion. The outstanding low-field MCE performance with low working temperatures indicates that Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds are promising candidates for magnetic cooling materials at liquid hydrogen and liquid helium temperatures.

Observation of outstanding magnetocaloric effect near room temperature in a Fe88Ce5Pr2B5 amorphous alloy

A Fe88Ce5Pr2B5 amorphous ribbon with outstanding magnetocaloric effect near ambient temperature is reported in this paper. The Fe88Ce5Pr2B5 alloy was successfully quenched into an amorphous ribbon shape. The magnetic properties of the Fe88Ce5Pr2B5 glassy ribbon were measured and the magnetocaloric behavior of the ribbon was studied. It was found that the amorphous ribbon showed the typical magnetocaloric behavior of second-order magnetic transition with a very large maximum magnetic entropy change (−ΔSmpeak, ∼3.97 J/(kg × K) under 5 T) and adiabatic temperature rise (ΔTad, ∼2.71 K under 5 T) near its Curie temperature (∼309 K). The mechanism for the almost highest −ΔSmpeak near 310 K of the Fe88Ce5Pr2B5 metallic glass reported in literature shows a promising perspective for achieving higher −ΔSmpeak in Fe-RE-B amorphous alloys.

Evolution of microstructure, magneto-structural transformation, and magnetocaloric effect in (Fe72-0.9xNi8-0.1xCo8)Zr7B4Cu1Gax (0 ≤ x ≤ 6 at.%) alloys

We report the evolution of microstructure, magnetic properties, and magnetocaloric effects in a series of (Fe72-0.9xNi8-0.1xCo8)Zr7B4Cu1Gax (0 ≤ x ≤ 6 at.%) alloys. The high temperature X-ray diffraction, calorimetric, and thermomagnetic studies revealed that the α-Fe → γ-Fe transformation occurred in the range of 691–733 K in arc-melted ingots (AMIs), whereas the same occurred at 660–723 K in as-spun ribbons (ASRs). The Curie temperature of γ phase lies in the range of 1126–1140 K and 1106–1126 K for AMIs and ASRs, respectively. Addition of Ga reduces the saturation magnetization and induce nanocrystallization in ASRs. A second-order magnetic transition has been identified in x = 0 ASR using Arrott plot exhibiting magnetic entropy change (|ΔSM|) of 0.973 J/kg.K at TCγ under magnetic field of 0.976 T. The refrigeration capacity of x = 0 and x = 6 ASRs has been estimated to be 12.78 J/kg and 22.98 J/kg, respectively, at 0.976 T.

Achievement of reversible table-like magnetocaloric effect in MnCo1-Gd Ge alloys around room temperature

• The table-like magnetocaloric effect in MnCo 1- x Gd x Ge alloys are achieved. • Two successive magnetic transitions appear in the MnCo 1- x Gd x Ge ( x = 0.04, 0.06). • The refrigerant capacity reach 282 J/kg in MnCo 0.94 Gd 0.06 Ge with Δ H = 50 kOe. • The relative cooling power reach 335 J/kg in MnCo 0.94 Gd 0.06 Ge with Δ H = 50 kOe. • The maximum temperature span are 128 K in MnCo 0.94 Gd 0.06 Ge with Δ H = 50 kOe. The structural, magnetic and magnetocaloric properties in the Gd doped MnCo 1- x Gd x Ge alloys were investigated. With the increase of Gd content, the coexistence of Ni 2 In-type hexagonal structure and TiNiSi-type orthorhombic structure appear from the pure Ni 2 In-type hexagonal structure of MnCoGe. All samples undergo the second-order magnetic transition (SOMT). MnCo 1- x Gd x Ge ( x = 0.04, 0.06) have two successive magnetic transitions and a large reversible table-like MCE has been achieved around room temperature (RT). Moreover, the relative cooling power ( RCP ) and effective refrigeration temperature span (δ T FWHM ) with the magnetic field change of 50 kOe reach 335 J/kg and 128 K, respectively.

Effect of heat treatment on microstructure evolution and magnetocaloric properties of droplet melted Mn–Fe–P–Si alloys

Practical implementation of Mn–Fe–P–Si alloys requires not only excellent magnetocaloric but also mechanical properties. Although these properties can be improved by droplet melting process, the effect of heat treatment on droplets has not been clarified. An investigation has been carried out on the microstructure and magnetocaloric properties of Mn1.20Fe0.75P0.45Si0.55 droplets annealed at different conditions. Experimental results show that the as-cast sample is composed of Fe2P and (Mn,Fe)3Si phases. Tuning the annealing condition leads to fluctuations in Fe2P phase fraction and compositions, thereby affecting the magnetocaloric properties. Increasing the annealing temperature from 1273 to 1373 K, Curie temperature (TC) increases from 204.0 to 277.0 K, thermal hysteresis (ΔThys) reduces from 2.5 to 0.5 K, and isothermal entropy change (ΔSm) doubles to 18.5 J/(kg·K) under a 5 T field. The increase in annealing time leads to the 34.0% increase in TC to 301.5 K, the 66.7% decrease in ΔThys to 1.0 K, but the slight decrease in ΔSm to 11.9 J/(kg·K) at an annealing time of 8 h. The larger relative cooling power (RCP) at higher temperatures and longer times are related to the increase in the Fe2P fraction. Moreover, optimizing the heat treatment condition to 1373 K and 4 h makes the alloy lay at the ideal border of the first-order and second-order magnetic transition, thus obtaining high ΔSm (18.5 J/(kg·K)), large RCP (508.8 J/kg), and extremely low ΔThys (0.5 K) around room temperature. Consequently, this optimal processing produces a competitive candidate for near room temperature magnetic refrigeration.

Large magnetocaloric effect of Tm1 − xYxGa (0 ≤ x ≤0.8) compounds with second-order magnetic transition around liquid helium temperature

A systematic investigation about Tm substitution by rare earth Y atoms in Tm1 − xYxGa (0 ≤ x ≤0.8) compounds on the magnetic properties and magnetocaloric effect (MCE) is presented. Among Tm1 − xYxGa compounds, Tm0.4Y0.6Ga exhibits the optimized working temperatures around the boiling point of liquid helium and large MCE. It undergoes a ferromagnetic (FM) to antiferromagnetic (AFM) transition at TFA = 2.8 K and an AFM to paramagnetic transition at TN = 5.4 K with increasing temperature, respectively. Moreover, the characteristic of second-order magnetic transition was confirmed on the basis of Arrott plots, rescaled universal curves, mean-field theory criterion, and the quantitative criterion of exponent n. Large MCE with the maximum magnetic entropy change of 6.4, 10.1, and 15.6 J/kg K and the maximum adiabatic temperature change of 2.4, 4.2, and 8.4 K for the field changes of 0–1, 0–2, and 0–5 T was observed, respectively. Consequently, the properties of low working temperatures, the characteristic of second-order magnetic transition, and good performance of MCE indicate that Tm0.4Y0.6Ga compounds could be a promising candidate of magnetocaloric materials in the application of helium liquefaction.