Martensitic and Inter-Martensitic Transformations in Magnetocaloric Ni2.15Mn0.85Ga Heusler Alloy

The entropy changes in successive martensitic phase transformations in Ni-Mn-Ga Heusler alloys can be used to realize enhanced magnetocaloric properties. A detailed study of phase transformations of one such alloy, Ni2.15Mn0.85Ga (Delta Q = 4900 J/kg at 343 K, under 140 kOe), is reported here. Upon cooling, the paramagnetic cubic (L2(1)) austenitic phase transforms into a ferromagnetic 7M modulated monoclinic martensitic phase. This phase is stable in a narrow temperature range, and upon further cooling, it transforms into a non-modulated ferromagnetic tetragonal (L1(0)) phase. The separation between the equilibrium temperatures of the austenitic and tetragonal martensitic phases is only similar to 50 K. The alloy undergoes reversible temperature-induced martensitic and inter-martensitic phase transformations with thermal hysteresis of about 25 K. The conclusions from the detailed study of the phase transformations lead to new possibilities to enhance the magnetocaloric effect (MCE) using the entropy associated with multi-structural transformations.