Minimizing hysteresis in martensite phase transforming magnetocaloric Heusler alloys

The large magnetocaloric effect in Heusler alloys showing martensite phase transformation puts them forward as efficient materials for magnetic refrigeration. However, irreversibility of the magnetocaloric cooling cycle is a major challenge for real applications. This irreversibility is directly linked to the thermal hysteresis at the first-order martensite phase transition. Therefore, minimizing the hysteresis is essential in order to achieve reversibility. Here we show a large reduction in the thermal hysteresis at the martensite transition in the Ni$_{2}$Mn$_{1.4}$In$_{0.6}$ and Ni$_{1.8}$Co$_{0.2}$Mn$_{1.4}$In$_{0.6}$ Heusler alloys upon the application of hydrostatic pressure. Our pressure dependent X-ray diffraction study on Ni$_{2}$Mn$_{1.4}$In$_{0.6}$ reveals that with increasing pressure the lattice parameters of the two crystallographic phases (austenite and martensite) change in such a way that they increasingly satisfy the geometric compatibility (co-factor) condition. These results provide an opportunity to overcome the hysteresis problem and hence the irreversible behavior in Heusler materials using pressure as an external parameter.