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 $\ensuremath{\Delta}{T}_{\text{ad}}$ in the vicinity of a first-order magnetostructural phase transition has been carried out on a ${\text{Ni}}_{2.19}{\text{Mn}}_{0.81}\text{Ga}$ Heusler alloy. It was found that the directly measured $\ensuremath{\Delta}{T}_{\text{ad}}\ensuremath{\sim}1\text{ }\text{K}$ 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 $\ensuremath{\Delta}{T}_{\text{ad}}$ 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 $\ensuremath{\Delta}{T}_{\text{ad}}$ in ${\text{Ni}}_{2.19}{\text{Mn}}_{0.81}\text{Ga}$ 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.