The intention of this review is therefore, to provide an overview of the scientific and technological aspects related to these coupling mechanisms based on Heusler compounds and La(Fe,Si) 13 as two of the most competitive magnetocaloric materials. The challenge of many MCE materials is, however, that these couplings occur simultaneously and influence each other. Even when occurring individually, many of these mechanisms are not yet fully understood. During operation, MCE materials will experience substantial external magnetic fields and magnetic exchange fields, which will experience magnetostatic coupling across different regions of the sample. Stress coupling of different variants or classes of MCE materials will occur at interfaces, which are in principle present in any realistic material system. Atomic coupling of magnetic and lattice degrees of freedom determines the adiabatic temperature change of the material in first-order magnetostructural transformations.
The types of coupling covered in this Review with the corresponding section numbers are shown in Figure 1. 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).