Hereby, a scale-bridging understanding of the involved coupling phenomena and how they determine the functional properties of magnetocaloric materials is provided. In this Review, a wide definition of the term “coupling” is used, that refers to any kind of interaction between neighboring atoms, grains, and even particles. In fact, many of the characteristics of these materials can be understood by an investigation of their coupling effects. 5 The magnetostructural transitions governing first-order magnetocaloric materials show that strong coupling phenomena occur in this material class. 2 In first-order magnetocaloric materials, the magnetic phase transition occurs jointly with a change in the structure of the material, leading to “giant” magnetocaloric effects as observed in, for example, Gd 5Si 2Ge 2, 1 La–Fe–Si-based 3 and Mn–Fe–P-based alloys, 4 and NiMn-based Heusler compounds. Materials exhibiting an MCE can be divided into two classes: second-order magnetocaloric materials show a conventional magnetic transition such as, for example, the ferro- to paramagnetic transition in elemental gadolinium. Firstly, potentially hazardous refrigerants are obsolete and secondly the potential system efficiency improves. 1 Compared to conventional vapor-cycle refrigeration, solid-state cooling shows many advantages. The prospect of efficient solid-state refrigeration at room temperature has led to a large interest in materials showing a magnetocaloric effect (MCE).
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