  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.
