2, Università di Parma, Parma, , Italy
3, Università del Sannio, Benevento, , Italy
Room temperature magnetic refrigeration requires materials with large isothermal entropy and adiabatic temperature changes at around 293 K and negligible thermo-magnetic hysteresis, when cycled in magnetic fields below 2T.
Ferromagnetic shape memory Heusler compounds with metamagnetic martensitic transformations are among the most studied materials for magnetocaloric applications thanks to their high adiabatic temperature changes (ΔT_ad)related to their inverse magnetocaloric effect . These materials are rare earth free, easy-to-prepare and offer large tailoring possibilities. Remarkably, thanks to the strong discontinuities of the physical properties at the martensitic transformation (e.g. magnetization, volume), caloric effects can be obtained not only by applying magnetic fields but also stress and pressure, enabling multicaloric applications [2,3]. Although very high values of adiabatic temperature change have been reported, metamagnetic Heuslers show poor reversibility due to hysteresis and spreading of the transition.
By taking advantage of suitable substitutions, in NiMn-based Heusler alloys it is possible to tune the order and the number of transitions that can be exploited for magnetocaloric applications. In the present talk we will report some particular cases in the phase diagram of NiMnGa and NiMnIn compounds and discuss the reversible and irreversible contributions to the magnetocaloric effects, based on in-field calorimetry and direct ΔT_ad measurements. Interesting effects occurs when the first order magnetostructural and second order Curie transition are almost coincident. In In-based compounds, for example, the coexistence of direct and inverse magneto caloric effects can be obtained . The possible exploitation of direct and inverse MCEs in alternative refrigeration cycles will be discussed.
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 X. Moya et al., Nature Mat. 12, 52-58 (2013)
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 C. Bennati et al., submitted.