Structure, magnetostructural transformations and magnetocaloric properties of Heusler alloys and the MnAs compound: a review

Abstract

The review systematises current views on the relationship between the crystal structure, the magnetic ordering and the magnetocaloric effect in two groups of promising functional materials — Heusler alloys of the Ni–Mn–Z (Z = Ga, In, Sn) families and the MnAs compound. Two principal approaches to the quantitative evaluation of the effect are discussed: the indirect one, based on the application of the Maxwell relations to magnetisation and heat-capacity data, and the direct one, which implies an immediate measurement of the adiabatic temperature change. It is shown that indirect methods are convenient for the screening of materials, but in the region of first-order phase transitions, where the conditions of applicability of the Maxwell relations are violated, the error of the calculated quantity may reach tens of per cent. The structural transformations in MnAs are considered separately — the first-order transition from the ferromagnetic hexagonal B8₁-type phase to the paramagnetic orthorhombic B31-type phase near 317 K and the subsequent second-order transition to the hexagonal paramagnetic phase near 394 K — together with the form of the magnetic phase diagram of this compound in steady fields. It is concluded that the combination of a high sensitivity of the magnetostructural transition to the external field with the proximity of the transition temperature to room temperature makes these materials the most realistic candidates for the working body of solid-state magnetic refrigerators.