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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The study of two aspects of the magnetocaloric effect (MCE), namely, the matching between isothermal entropy change and direct adiabatic temperature change, is not straightforward since huge differences between these two quantities have often been reported. Here we put in relation the direct and indirect measurements on the first order magnetostructural martensitic transformation occurring in Ni-Co-Mn-Ga alloys. In order to complete the characterization of the MCE and to find an explanation of these mismatches, differential scanning calorimeter measurements have been performed at different applied magnetic fields.

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.

In contrast to rare-earth-based materials, cheaper and more environmentally friendly candidates for cooling applications are found within the family of Ni-Mn Heusler alloys. Initial interest in these materials is focused on the first-order magnetostructural transitions. However, large hysteresis makes a magnetocaloric cycle irreversible. Alternatively, here it is shown how the Heusler family can be used to optimize reversible second-order magnetic phase transitions for magnetocaloric applications.

Abstract:In Ni-Mn-Ga ferromagnetic shape memory alloys, Co-doping plays a major role in determining a peculiar phase diagram where, besides a change in the critical temperatures, a change of number, order and nature of phase transitions (e.g., from ferromagnetic to paramagnetic or from paramagnetic to ferromagnetic, on heating) can be obtained, together with a change in the giant magnetocaloric effect from direct to inverse. Here we present a thorough study of the intrinsic magnetic and structural properties, including their dependence on hydrostatic pressure, that are at the basis of the multifunctional behavior of Co and In-doped alloys. We study in depth their magnetocaloric properties, taking advantage of complementary calorimetric and magnetic techniques, and show that if a proper measurement protocol is adopted they all merge to the same values, even in case of first order transitions. A simplified model for the estimation of the adiabatic temperature change that relies only on indirect measurements is OPEN ACCESSEntropy 2014, 16 2205 proposed, allowing for the quick and reliable evaluation of the magnetocaloric potentiality of new materials starting from readily available magnetic measurements.

We compare the magnetocaloric effect of samples prepared with different thermal conductivities to investigate the potential of composite materials. By applying the magnetic field under operating conditions we test the material's response and compare this to heat transfer simulations in order to check the reliability of the adiabatic temperature change probe used. As a result of this study we highlight how the material's thermal conductivity influences τ , the time constant of temperature change. This parameter ultimately limits the maximum frequency of a refrigerant cycle and offers fundamental information about the correlation between thermal conductivity and the magnetocaloric effect.