even though these compounds are most prone to such drawbacks, is the negative role of hysteresis. Since all MCE applications have a cyclic character, one of the main pre-conditions is to ensure a total (or at least partial) reversibility of the effect when either fi eld or temperature oscillations are applied. From a material point of view, this means keeping the fi eld or thermal hysteresis that could occur as small as possible. A second drawback of G-MCE materials is related to their mechanical stability. FOTs bring not only sharp magnetization jumps but also discontinuities of other physical parameters, including the unit cell. This "structural" part can have manifold aspects: symmetry breaking or cell-volume or lattice-parameter changes. The most dramatic for the stability of polycrystalline bulk samples turns out to be the cell-volume change. During thermal or magnetic fi eld cycles, the strains generated by a volume change may cause fractures or even destruction of the bulk piece, which severely hinders the applicability of these materials. Technical solutions can be used to overcome this problem, for instance by embedding the MCE material in a resin or by a porous shaping. [ 18 ] However, in such cases the MCE is "diluted", which is not satisfactory since the gap of the magnet is not effi ciently used and the thermal conductivity governing the heat transfer is decreased. Bulk G-MCE materials with a good mechanical stability should remain the preferred solution. Finally, to allow large-scale applications, a last requirement that should be borne in mind is that the MCE material must consist of elements that are available in large amounts, are not expensive, and are not classifi ed as toxic.In this context, the MnFe(P, x ) system appears to be an ideal playground. This material family is derived from the Fe 2 P compound, a prototypical example known for a long time to exhibit a sharp but weak (the latent heat L is only 0.25 kJ kg −1 ) FOT with a Curie temperature ( T C ) of 217 K. [ 19 ] In this hexagonal system, the Fe atoms occupy two inequivalent atomic positions referred to as 3f (in a tetrahedral environment of non-metallic atoms) and 3g (pyramidal). An intriguing aspect is the disappearance at T C of the magnetic moments of the iron atoms at the 3f sites, whereas there is only a limited decrease of the moments at the 3g site. This theoretical prediction has led to a cooperative description of the FOT that links the loss of longrange magnetic order at T C with the loss of the local moments at the 3f site. [ 20 ] This mechanism has recently been proposed to be the origin of the G-MCE observed in MnFe(P,Si). The disappearance of the magnetic moments is ascribed to a conversion from non-bonding d electrons to a distribution with pronounced hybridization with the surrounding Si/P atoms. [ 11 ] A practical consequence is that the FOT mechanism can be expected to be highly sensitive to substitutions at the nonmetallic site. In the present work, precisely this approach has been used to solve three problems of ...

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MnFeP0.595Si0.33B0.075 has recently been presented as a top class magnetocaloric material combining a large magnetocaloric entropy change, a large temperature change, limited thermal hysteresis, and an enhanced mechanical stability. By providing practical rules to control the transition temperature in the MnFe(P,Si,B) system, we demonstrate that this new material was not a single composition and that a giant magnetocaloric effect (MCE) can be observed over a broad temperature range, a point of great interest for applications. As important prerequisite is the cyclability of the MCE. The thermal hysteresis and the recovery of the MCE during field oscillations have been addressed for MnFe(P,Si,B) materials. It is found that when the thermal hysteresis becomes about as large as the field induced shift of the transition, the MCE becomes partially irreversible and a strong decrease in the cyclic temperature change occurs. For an intermediate field change, typically 1 T, the limit for thermal hysteresis is about δThyst ≈ 4 K in the MnFe(P,Si,B) system. Finally, the interest of this material class for magnetic refrigeration is discussed in terms of the coefficient of refrigerant performance.

Magnetic cooling is a highly efficient refrigeration technique with the potential to replace the traditional vapor compression cycle. It is based on the magnetocaloric effect, which is associated with the temperature change of a material when placed in a magnetic field. We present experimental evidence for the origin of the giant entropy change found in the most promising materials, in the form of an electronic reconstruction caused by the competition between magnetism and bonding. The effect manifests itself as a redistribution of the electron density, which was measured by X-ray absorption and diffraction on MnFe(P,Si,B). The electronic redistribution is consistent with the formation of a covalent bond, resulting in a large drop in the Fe magnetic moments. The simultaneous change in bond length and strength, magnetism, and electron density provides the basis of the giant magnetocaloric effect. This new understanding of the mechanism of first order magneto-elastic phase transitions provides an essential step for new and improved magnetic refrigerants

After almost 20 years of intensive research on magnetocaloric effects near room temperature, magnetic refrigeration with first-order magnetocaloric materials has come close to real-life applications. Many materials have been discussed as potential candidates to be used in multicaloric devices. However, phase transitions in ferroic materials are often hysteretic and a metric is needed to estimate the detrimental effects of this hysteresis. We propose the coefficient of refrigerant performance, which compares the net work in a reversible cycle with the positive work on the refrigerant, as a universal metric for ferroic materials. Here, we concentrate on examples from magnetocaloric materials and only consider one barocaloric experiment. This is mainly due to lack of data on electrocaloric materials. It appears that adjusting the field-induced transitions and the hysteresis effects can minimize the losses in first-order materials.This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

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