Magnetocaloric materials for refrigeration near room temperature
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Introduction The magnetocaloric effect (MCE) is the reversible temperature change of a magnetic material upon application and removal of a magnetic field, as illustrated in Figure 1. The phenomenon was discovered in 1917 in nickel by Weiss and Picard.1 Its potential for room-temperature refrigeration utilizing a regenerative cycle2 was demonstrated with elemental gadolinium in 1976. Today, industrial prototypes for different cooling applications exist,3–5 some of them utilizing materials with a “giant” MCE. In these materials, a first-order magnetic phase transition occurs jointly with a change in the structure, which was first observed in Gd5Si2Ge2.6 This finding triggered research that soon led to the discovery of other giant magnetocaloric materials, such as La-Fe-Si-based7 and Mn-Fe-P-based alloys,8 and NiMn-based Heusler alloys.9
Contributions of magnetism and structure to the MCE Magnetocaloric materials release heat when an externally applied magnetic field induces a transition from a disordered to an ordered phase. This is expressed by an (isothermal) entropy change ∆ST. In an adiabatic process, a corresponding temperature change, ∆Tad, is observed. In a first approximation, the total entropy change, ∆ST, may be decomposed into contributions arising from the relevant degrees of freedom—occupation
of electronic bands, magnetic order, and lattice vibrations.10 Apart from their high sensitivity to the magnetic field at the first-order transition, giant magnetocaloric materials may also benefit from the concomitant changes in the entropy contributions not directly related to magnetism. The electronic entropy, Sel, usually provides the smallest contribution, and its change, ∆Sel, is also small in absolute number. Sel is essentially proportional to the electronic density of states (DOS) at the Fermi energy, EF. The highest occupied electronic energy levels at EF are subject to thermal disorder. Sel can be determined by low-temperature calorimetry or photoemission spectroscopy, or alternatively, from first-principles calculations in the framework of density functional theory (DFT).11,12 In magnetic materials, the electronic bands cease to be spin-degenerate, which leads to a spin-dependent occupation of the electronic orbitals and thus the appearance of magnetic moments. The sizes and orientations of the moments may differ from one atomic site to another and form ordered or disordered magnetic configurations in the crystal lattice. These determine the magnetic contribution to the entropy change. In giant magnetocaloric materials, the coupling between magnetism and structure usually inhibits the direct determination of ∆Smag from calorimetry, which yields ∆ST instead.13 However, ∆Smag can be assessed indirectly by subtracting the electronic and lattice contributions from ∆ST, or from theory,
Anja Waske, Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Germany; [email protected] Markus E. Gruner, Department of Physics, Universität Duisburg-Essen, Germany; [email protected] Tino
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