Adiabatic temperature change at first-order magnetic phase transitions: Ni2.19 Mn0.81 Ga as a case study

A comprehensive study of the adiabatic temperature change Δ Tad in the vicinity of a first-order magnetostructural phase transition has been carried out on a Ni2.19 Mn0.81 Ga Heusler alloy. It was found that the directly measured Δ Tad ∼K is of order of magnitude smaller than that expected from reported in the literature isothermal magnetic entropy change and specific-heat data. A new feature of the adiabatic temperature change in materials with giant magnetocaloric effect, specifically an irreversible character of Δ Tad when the sample is subjected to repeatable action of magnetic field at a constant temperature, has been observed. This effect has been attributed to the irreversible magnetic-field-induced structural transformation. It has been shown that the small value of Δ Tad in Ni2.19 Mn0.81 Ga is not due to the kinetics of the transformation but originates from other factors which are intrinsic to first-order magnetic phase transitions. Relevance of these factors to other giant magnetocaloric materials has been outlined.