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Compared with traditional techniques, solid‐state magnetocaloric phase transition materials (MPTMs), based on the giant magnetocaloric effect (GMCE), can achieve a higher energy conversion efficiency for caloric applications. As one of the most promising MPTMs, the hexagonal (Mn,Fe)2(P,Si)‐based compounds host some advantages, but the existing hysteresis and relatively unstable GMCE properties need to be properly tackled. In this study, it is found that substitutions with Ni, Pd, and Pt can maintain and even enhance the GMCE (≈8.7% maximum improvement of |Δsm|). For a magnetic field change of Δμ0H = 2 T, all samples obtain a |Δsm| in the range of 20–25 J kg−1 K−1 with a low thermal hysteresis ΔThys (≤5.6 K). The performance surpasses almost all other (Mn,Fe)2(P,Si)‐based materials with ΔThys (<10 K) reported until now. The occupancy of substitutional Ni/Pd/Pt atoms is determined by X‐ray diffraction, neutron diffraction, and density functional theory calculations. The difference in GMCE properties upon doping is understood from the competition between a weakening of the magnetic exchange interactions and the different degrees of orbital hybridization among 3d‐4d‐5d elements. The studies elaborate on the responsible mechanism and provide a general strategy through d‐block doping to further optimize the GMCE of this materials family. [ABSTRACT FROM AUTHOR]

Solid‐state caloric effects as intrinsic thermal responses to different physical external stimuli (magnetic‐, uniaxial stress‐, pressure‐, and electric‐fields) can achieve a higher energy efficiency compared with traditional gas compression techniques. Among these effects, magnetocaloric energy conversion is regarded as the best available alternative and has been exploited extensively for promising application scenarios in the last decades. This review systematically introduces the magnetocaloric effect and its applications, and summarizes the corresponding representative magnetocaloric materials, as well as important progress in recent years. Specifically, the review focuses on some key understandings of the magnetocaloric effect by utilizing state‐of‐the‐art technical tools such as synchrotron X‐ray, neutron scattering, muon spin spectroscopy, positron annihilation spectroscopy, high magnetic fields, etc., and highlights their importance toward advanced materials design and development. An overview of the basic principles and applications of these advanced techniques on magnetocaloric materials is provided. Finally, the challenges and perspectives on further developments in this field are discussed. Further in‐depth understanding and manufacturing technology advancement combined with fast‐developed artificial intelligence and machine learning are expected to advance the magnetocaloric energy conversion technology closer to real applications. [ABSTRACT FROM AUTHOR]

In this work, a Ni-doped MnCoGe alloy has been synthesized by doping 4 wt% of Ni in place of Ge. The room temperature XRD pattern shows that the alloy has crystallized in two phases: hexagonal-MnCoGe and cubic-Co2MnGe and in addition, there are vacancies at Co and Ge sites. Around room temperature, MnCoGe phase has hexagonal structure and as the temperature is lowered, the structural transition takes place to orthorhombic-MnCoGe phase. When external magnetic field is low, orthorhombic phase is antiferromagnetic, but as field strength is increased, the ferromagnetic ground state is established. This alloy undergoes a second-order ferromagnetic transition followed by a first-order structural transition in a narrow temperature width. These have synergistically resulted in a single, reversible - Δ S (T) curve across a very wide temperature span. The magnitude of entropy change has to be, however, improved by Ni–Ge tuning to enhance the relative cooling power. Regardless, a modest magnetocaloric effect with negligible hysteresis has been attained near room temperature via Ni-doping in MnCoGe alloy. [ABSTRACT FROM AUTHOR]

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) has been recognized as an environmentally benign and energy-efficient cooling technology. Exploring suitable magnetocaloric materials is a crucial prerequisite for practical MR applications. We have herein provided a systematic investigation of the crystal structure, microstructure, electronic structure, magnetic phase transition, critical behavior, and MCE of the GdCoC compound featuring excellent cryogenic magnetocaloric performance by means of experimental determination and theoretical calculation. The GdCoC compound is crystallized in a simple layered tetragonal crystal structure with a P 4 2 / mmc space group and undergoes two successive ferromagnetic (FM) transitions along with a low-temperature weak antiferromagnetic (AFM) transition under low magnetic fields. Density functional theory calculations confirms the FM coupling of the Gd and Co intra-sublattice interactions, whereas AFM coupling for their inter-sublattice interaction. The magnetic transitions are merged in to one under high magnetic fields which has been confirmed to be second-order type and its critical behavior can be understood in the framework of tri-critical mean-field model, whereas the low-temperature weak AFM transition is belonging to the first-order type. The excellent magnetocaloric performance of the GdCoC compound was identified by the parameters of magnetic entropy change, adiabatic temperature change, temperature-averaged entropy change, relative cooling power, and refrigerant capacity, which are superior to most of the well-known magnetocaloric materials with similar working temperatures, making it attractive for practical cryogenic MR applications. [Display omitted] [ABSTRACT FROM AUTHOR]

The magnetocaloric effect (MCE) is an important phenomenon occurring in cryogenic cooling. Herein, a promising MCE material, GdVO4 nanopowder, was synthesized using a microwave-assisted hydrothermal method. The magnetic properties of the produced GdVO4 nanopowder were measured to investigate its MCE efficiency. The as-prepared GdVO4 nanopowder exhibited an antiferromagnetic–paramagnetic transition at 2.20 K, lower than that of bulk GdVO4 (2.50 K). The nanocrystalline GdVO4 showed weak antiferromagnetism due to size effects, where the broken symmetry of the surfaces resulted in a low crystalline anisotropy. However, the GdVO4 nanopowder after annealing at 900 °C showed an increased TN of 2.50 K due to aggregation and the resulting suppression of the size effects. Based on the Maxwell relation, the magnetic entropy change was obtained and compared with that of bulk GdVO4. Although the change in the maximum magnetic entropy was reduced for the nanocrystalline GdVO4 compared to that of bulk GdVO4, the absence of hysteresis loss in the nanocrystalline GdVO4 is important for high-efficiency magnetocaloric materials to be used in hydrogen liquefaction applications. [ABSTRACT FROM AUTHOR]

• Magnetocaloric effect of single crystal GdTiO 3 was studied. • GdTiO 3 had a 2nd order transition from paramagnetic to ferrimagnetic state at 33 K. • A small anisotropy was found in the magnetization and specific heat. • The magnetization showed no hysteresis. • Entropy-temperature diagram of the single crystal GdTiO 3 was obtained. • GdTiO 3 has the potential as a magnetic refrigerant for hydrogen liquefaction. We studied the magnetocaloric effect in a single crystal GdTiO 3. Single crystal GdTiO 3 had a second order phase transition from a paramagnetic to ferrimagnetic state at 33 K. The maximum entropy change of - Δ S = 0.10 J/cm3 K was observed in a 5 T field at the transition temperature. A small anisotropy was found in the magnetization and specific heat in low magnetic fields. The magnetization showed no hysteresis. Temperature dependencies of entropy, which are important for analyzing the refrigeration cycle, were calculated from the magnetization and specific heat measurements. These results indicate that GdTiO 3 has the potential for applications as a magnetic refrigerant for hydrogen liquefaction. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) are the core of many future-sustainable technologies. One example is magnetocaloric refrigeration, an emerging field essential for the efficient use of energy. Future adoption of this technology will require adequate processing of end-of-life units and production residues. Currently, REEs have very high supply risk, and their recovery rates are below 1%. So far, their recovery from magnetocaloric materials has not been addressed. This work reports on a leaching and solvent extraction process to recover REEs from genuine magnetocaloric materials comprising cerium, iron, lanthanum, manganese and silicon. Leaching was studied using nitric, hydrochloric and sulfuric acid solutions, with optimizations in terms of temperature, acid concentration and solid-to-liquid ratio. Recovery of REEs from nitric, hydrochloric, and sulfuric acid leachates was investigated with three types of solvating extractants: tributyl phosphate (TBP), trioctylphosphine oxides (Cyanex 923) and tetraoctyl digylcol amide (TODGA). Extraction was most effective from nitric acid media. Very good extraction selectivity between REEs and non-REEs was achieved with TODGA. Cyanex 923 showed better extraction efficiency than TBP, and performed best in aliphatic diluents. A separation factor of 3.3 between cerium and lanthanum was achieved with 1 mol/L Cyanex 923 in Isopar L. [ABSTRACT FROM AUTHOR]

First-order phase transition magnetocaloric materials in multi-stage regenerators are attractive for magnetocaloric refrigeration because of their relatively high available power and the potential for the use of low-cost elements such as iron in their production. In this work a model is used to represent the thermodynamic properties of magnetocaloric materials allowing the magnetocaloric materials to be fully parameterized for the first time. A numerical model is used to carry out a large study including the material parameters and the parameters that describe magnetocaloric machines and their operation. A unique method of evaluating material refrigeration capacity is introduced and used to normalize material inputs. The results are analyzed both in terms of the direct impact of the material parameters and the interactions between machine and material parameters on performance. It was found that machine cooling power was maximized for the cases with the largest adiabatic temperature changes. It was also found that many machine parameters and material parameters interact when determining the performance of a machine. This suggests that materials and machine research would benefit from a closer collaboration. [ABSTRACT FROM AUTHOR]

Hypothermia is an effective neuroprotective strategy for acute stroke. However, in clinical practice, the induction of hypothermia is achieved through the systemic reduction of body temperature (using thermal covers or endovascular cooling devices) which results in a complex system associated in many cases to side effects. Therefore, the aim of this study was to test the magnetocaloric effect as a potential new therapeutic strategy for stroke by means of an adiabatic magnetic refrigerator device. As a first approach, we have developed a simple device to evaluate in vitro the thermodynamic behavior of different concentrations of commercial gadolinium powder as a reference magnetocaloric material. The samples, properly thermally insulated, were cyclically magnetized and demagnetized at room temperature by 1 T permanent magnets in order to induce an adiabatic magnetic effect. Under the experimental conditions tested, results showed a maximun non-accumulative temperature variation of 0.2 °C, insufficient to carry out an effective hypothermia. This study allowed us to discuss about the use of new materials and strategies for further in vivo experiments. [ABSTRACT FROM AUTHOR]

Composites consisted of MnCoGe magnetocaloric alloy and BiSbTe thermoelectric alloy were prepared to explore their potential use in all-solid-state cooling. It was found that in the composites the phase transition of MnCoGe from orthorhombic to hexagonal phase causes a large drop of magnetostructural transition temperature from room temperature to 270 K. The diffusion of Mn into BiSbTe leads to the formation of interfacial phase Mn(Bi,Sb) 2 Te 4 and doping effect that depresses the thermoelectric performance. By adopting a low-temperature sintering strategy, the phase transition was restricted and the doping effect due to Mn diffusion was alleviated. As a result, the magnetostructural transition temperature around 300 K is maintained and the room-temperature magnetocaloric and thermoelectric properties are simultaneously optimized. The maximum magnetic entropy change of 4.3 × 10−3 J kg−1 K−1 at 2.0 T and maximum ZT value of 0.81 at 384 K have been obtained on the composite incorporated with 1 wt % MnCoGe. This work demonstrates that the control of phase transition and the doping effect of MnCoGe is crucial in optimizing the thermo-electro-magnetic properties of MnCoGe/BiSbTe composites. • The diffusion of Mn and formation of interfacial phase Mn(Bi,Sb) 2 Te 4 in MnCoGe/BiSbTe thermo-electro-magnetic composites were identified by the atomic-resolution STEM, which is crucial in affecting the thermoelectric and magnetocaloric performance. By regulating the magnetostructural transition and element diffusion, enhanced thermoelectric and magnetocaloric properties were realized in the composites. [ABSTRACT FROM AUTHOR]

We present magnetocaloric effect measurements of the ferromagnetic semiconductor EuS in the vicinity of its ordering temperature. Single phase EuS powder was synthesized by CS 2 gas sulfurization of Eu 2 O 3 . A sintered compact with relative density over 95% was prepared by pulsed electric current sintering of the powder. Temperature and magnetic field dependence of the magnetization and specific heat were characteristic of a paramagnetic to ferromagnetic second order phase transition. The entropy change induced by an external magnetic field and the specific heat were both close to those of a single crystal. We obtained an entropy-temperature ( S – T ) diagram of the EuS sintered compact. Carnot cycle liquefaction of hydrogen using EuS was compared with several other materials, with results indicating that sintered EuS is an excellent magnetic refrigerant for hydrogen liquefaction. [ABSTRACT FROM AUTHOR]

A hybrid numerical model of the magnetic refrigerator with multi-material microchannel regenerator has been developed. The magnetocaloric effect was implemented using instantaneous temperature rise/drop (discrete method). Two pipe-in-pipe heat exchangers at two ends of the regenerator were treated using ε-NTU method. The commercially available compounds of LaFe 13-x-y Co x Si y as well as hypothetical compounds of Gadolinium were considered as the magnetocaloric materials (MCMs) with different Curie temperatures. The predicted results of the present work for parallel-plate regenerators employing different compounds of LaFe 13-x-y Co x Si y were broadly in good agreement with the available experimental data. The cooling capacity increases as the number of MCMs increase. However, for a given length of regenerator, an optimum number of MCMs was seen yielding the maximum performance of the refrigerator. For a given number of MCMs, a smaller Curie temperature difference Δ T Cu between the MCMs was found to give higher performance. [ABSTRACT FROM AUTHOR]

Mössbauer spectroscopy in the ferromagnetic AlFe B reveals T=299 K and shows good agreement with magnetic measurements. The crystals are plate-shaped. The flakes are found from X-ray diffraction to be in the crystallographic ac-plane in the orthorhombic system. The axes of the principle electric field gradient tensor are, by symmetry, colinear with the crystal a-, b- and c-axes. By using information about the quadrupole splitting and line asymmetry in the paramagnetic regime together with the quadrupole shift of the resonance lines in the ferromagnetic regime the magnetic hyperfine field direction is found to be in the ab-plane having an angle =40 to the b-axis. [ABSTRACT FROM AUTHOR]

This review outlines recent advances in the design of 3d-, 4f-, and 3d–4f type magnetic molecules for use as excellent cryogenic magnetic coolants based on the magnetocaloric effect (MCE), and the structure-magnetocaloric correlations of reported molecular coolants. Further improvements in the MCE values of molecular magnetic materials are also proposed based on assembly strategies from molecular chemistry and crystal engineering. [ABSTRACT FROM AUTHOR]

The magnetocaloric effect is the thermal response of a material to an external magnetic field. This manuscript focuses on the physics and the properties of materials which are commonly used for magnetic refrigeration at cryogenic temperatures. After a brief overview of the magnetocaloric effect and associated thermodynamics, typical requirements on refrigerants are discussed from a standpoint of cooling power density optimization. Finally, a compilation of the most important properties of several common magnetocaloric materials is presented. [ABSTRACT FROM AUTHOR]

Abstract: We report on the magnetic and structural properties of Mn0.66Fe1.29P1−x Si x melt-spun ribbons with 0.34≤x≤0.42 that are promising candidates for high-temperature magnetocaloric applications. A magnetic moment of up to 4.57μB/f.u. for x=0.34 indicates high magnetic density in the system, which is certainly advantageous for the magnetocaloric effects. Introducing site disorder at the 3g site by replacing 1/3 of Fe with Mn appears to enhance the magnetic interaction, while the strong magnetoelastic coupling is maintained. This site disorder also shows a stabilizing effect on the hexagonal crystal structure, which is maintained to a high Si content. The moment alignment within the crystallographic unit cell is also affected when the Si content is increased from x=0.34 to 0.42 in the Mn0.66Fe1.29P1−x Si x compounds as the canting angle with respect to the c-direction increases. [Copyright &y& Elsevier]

• DyVO 4 nanoparticles were synthesized by a microwave-assisted hydrothermal method. • Weak antiferromagnetic DyVO 4 exhibits low temperature magnetocaloric properties. • Low-temperature MCE effects can be applied for hydrogen liquefaction. This study verified the possibility of synthesizing DyVO 4 nanoparticles by a microwave-assisted hydrothermal method as a cryogenic magnetic cooling material. The synthesized DyVO 4 nanoparticles were approximately 10 nm in size with a typical tetragonal zircon-type phase. No antiferromagnetic-to-paramagnetic transition was observed in the temperature-dependent magnetization measurements within our measurement range due to the small particle size. The Curie–Weiss temperature (−2.65 K) obtained from Curie–Weiss fitting indicated weak antiferromagnetism, and an effective moment was calculated as 7.71 μ B. Based on isothermal magnetization measurements with an applied magnetic field, the DyVO 4 nanoparticles showed a different paramagnetic behavior with bulk antiferromagnetic DyVO 4 due to particle size effects, and a positive slope was found in the Arrott plots, indicating a second-order magnetic phase transition. Furthermore, a structural transition due to the Jahn–Teller effect was weakly observed at approximately 10 K. At Δ H = 50 k O e , the maximum change in magnetic entropy and relative cooling power were 10.32 J kg−1 K−1 and 175.44 J kg−1, respectively. [ABSTRACT FROM AUTHOR]

An overview of the magnetocaloric properties of the MnAs 1− x Sb x is presented. The temperature dependence of the isothermal magnetic entropy, Δ S mag , and the refrigerant capacity, RC, have been investigated theoretically and experimentally in a composite based on second order MnAs 1− x Sb x phases. This work demonstrates the outstanding agreement between the experimental results and the continuous curves predicted by numerical calculations, indicating that this approach can be used to design magnetic refrigerant materials with enhanced magnetocaloric response in magnetic refrigerator performing an Ericsson cycle near room temperature. [ABSTRACT FROM AUTHOR]

• Recently, there are surging demand of studying magnetocaloric materials in order to achieve environmentally friendly refrigeration techniques. • So far, a theoretical estimation of the magnetocaloric effect (MCE) has been based on the analysis of specific heat capacity and magnetization. • In the meantime, the direct observation of qausi-adiabatic MCE cannot be overlooked for deeper understanding. • In this manuscript, we introduce quasi-adiabatic and non-equilibrium perturbation terms to equilibrium equation of the MCE. The model well explains the reversible MCE from a paramagnetic material YbPt 2 Sn as well as the irreversible MCE from a complex-phase material Ce 1 - x La x B 6 . Magnetocaloric effect (MCE) has drawn much attention because its magnetic cooling property enables refrigeration without producing noxious gas or using rapidly depleting resources. However, applications for everyday life are yet distant. In addition, we need to understand more about the practical aspect of the MCE. Here, we introduce a phenomenological model to explain the quasi-adiabatic MCE. Correction factors to the equilibrium thermodynamic feature implied by the entropy landscape are devised in analytic forms. To demonstrate the validity of the model, the MCE from two different materials is investigated. The recently discovered metallic paramagnet, YbPt 2 Sn, shows a linear and reversible MCE which is typical of a paramagnetic system and suitable for cryogenics without 3He. On the other hand, a complex-phase material, Ce 0.5 La 0.5 B 6 , exhibits a pronounced irreversible MCE especially across a magnetic phase boundary. A term that describes the field induced heating near a phase transition turns out to be essential in resolving the irreversible, non-equilibrium MCE. [ABSTRACT FROM AUTHOR]