Heusler alloys in general have the chemical formula X2YZ, where X and Y are transition metals and Z is usually a main group element from the columns III-V of the periodic table. The first Heusler alloy attracted attention because it (Cu2MnAl) showed ferromagnetic properties even though none of the alloyed elements is ferromagnetic. The explanation for this phenomenon is based on the magnetic properties of Mn, which plays a crucial role in many Heusler alloys.
A coupled magnetic and martensitic transition in Ni(-Co)-Mn-X (Ni50-xCoxMn50-yXy) Heusler alloys leads to a magnetocaloric effect and to a magnetic shape-memory effect. Especially Ni-Mn-Ga alloys show large strains and are considered as shape-memory alloys and as elastocaloric materials. Especially Ni(-Co)-Mn-In alloys show the most promising magnetocaloric properties with highest ΔTad for a Heusler alloy of −8K in fields of around 2T. The corresponding ΔTad upon magnetic field cycling is reduced to −3T. These large losses of further field cycles are due to the large thermal hysteresis that is inherent in the structural change of crystal symmetry in combination with the incommensurability of the two lattice structures. In addition, the martensitic transition in Heusler alloys is accompanied by a volume change of approximately 0.75% to 1%.
The main characteristic for Heusler alloys lies in the possibility of sensitively tailoring the phase transition. By stoichiometric variances and substitutions, the transition temperature, thermal hysteresis width and the magnetic-field sensitivity as well as the pressure sensitivity of the phase transition can be tuned according to the needs. The microstructure plays an important role for the properties of the phase transition since the compatibility of the martensite and austenite structures determines the thermal hysteresis and the transition width as well as the kinetics of the phase transformation.
