The phase‐down scenario of conventional refrigerants used in gas–vapor compressors and the demand for environmentally friendly and efficient cooling make the search for alternative technologies more important than ever. Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials could be that alternative. However, there are still several challenges to be overcome before having devices that are competitive with those based on the conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant examples of 14 different magnetocaloric material families is presented and those are compared in terms of their adiabatic temperature and isothermal entropy change under cycling in magnetic‐field changes of 1 and 2 T, criticality aspects, and the amount of heat that they can transfer per cycle. The work is based on magnetic, direct thermometric, and calorimetric measurements made under similar conditions and in the same devices. Such a wide‐ranging study has not been carried out before. This data sets the basis for more advanced modeling and machine learning approaches in the near future.

The giant magnetocaloric effect, in which large thermal changes are induced in a material on the application of a magnetic field, can be used for refrigeration applications, such as the cooling of systems from a small to a relatively large scale. However, commercial uptake is limited. We propose an approach to magnetic cooling that rejects the conventional idea that the hysteresis inherent in magnetostructural phase-change materials must be minimized to maximize the reversible magnetocaloric effect. Instead, we introduce a second stimulus, uniaxial stress, so that we can exploit the hysteresis. This allows us to lock-in the ferromagnetic phase as the magnetizing field is removed, which drastically removes the volume of the magnetic field source and so reduces the amount of expensive Nd-Fe-B permanent magnets needed for a magnetic refrigerator. In addition, the mass ratio between the magnetocaloric material and the permanent magnet can be increased, which allows scaling of the cooling power of a device simply by increasing the refrigerant body. The technical feasibility of this hysteresis-positive approach is demonstrated using Ni-Mn-In Heusler alloys. Our study could lead to an enhanced usage of the giant magnetocaloric effect in commercial applications.

Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first‐order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First‐order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first‐order magnetocaloric materials such as Ni–Mn‐based Heusler alloys, FeRh, La(FeSi)13‐based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size‐ and time‐dependence of the MCE at magnetostructural transitions.

Solid-state magnetic refrigeration is a high-potential, resource-efficient cooling technology. However, many challenges involving materials science and engineering need to be overcome to achieve an industry-ready technology. Caloric materials with a first-order transition-associated with a large volume expansion or contraction-appear to be the most promising because of their large adiabatic temperature and isothermal entropy changes. In this study, using experiment and simulation, it is demonstrated with the most promising magnetocaloric candidate materials, La-Fe-Si, Mn-Fe-P-Si, and Ni-Mn-In-Co, that the characteristics of the first-order transition are fundamentally determined by the evolution of mechanical stresses. This phenomenon is referred to as the stress-coupling mechanism. Furthermore, its applicability goes beyond magnetocaloric materials, since it describes the first-order transitions in multicaloric materials as well.under the application of an external field (i.e., electric, magnetic, mechanical stress, or pressure), as illustrated in Figure 1.Recently, the idea of utilizing not just one of these external fields, but a combination of multiple stimuli to transform the caloric material, was proposed, the so-called multicaloric effect. [15][16][17][18][19] In this study, we demonstrate that the caloric effect in materials with a first-order transition is always a consequence of multiple stimuli. Using the example of the three most promising magnetocaloric materials, La-Fe-Si, [20][21][22] Fe 2 P-type, [23][24][25][26] and Heusler alloys, [27][28][29] we develop a model from which the crucial role of mechanical stress is clear. This stress-coupling model does not involve any magnetism and is therefore immediately applicable to all the other caloric effects presented in Figure 1, helping us to understand first-order transitions in general. Cooling Technology

We present a comprehensive study on three selected Heusler alloy systems. Ni-Mn-X(-Co) systems with X ¼ Al, In, Sn are compared with respect to the relevant magnetocaloric properties of their magnetostructural phase transition, namely martensitic transition temperature as well as its field dependence, magnetization change, and width of the thermal hysteresis. The latter one is strongly determining the reversibility of the magnetocaloric effect. Therefore the understanding of how to tailor it by extrinsic and intrinsic factors is of great importance. Our study of the magnetocaloric properties leads to the conclusion that the width of thermal hysteresis can be correlated to the magnetization change of the phase transition. Consequently, the adiabatic temperature change under cycling can largely vary despite similar values of isothermal entropy change for Ni-Mn-In-Co and Ni-Mn-Sn-Co. This result therefore shows the importance of tailoring sharpness, thermal hysteresis, and field dependence of the phase transition to achieve high values for the isothermal entropy change as well as a large magnetocaloric cooling effect in the different Heusler alloys.