Enhanced magnetocaloric effects driven by two successive magneto-structural transformations in Ni55.5 Mn17.8Ga26.7 alloy under hydrostatic pressure

The crystal structure, transformation property and magnetocaloric effect (MCE) were investigated in Ni-55(.5) Mn17.8Ga26.7 alloy. The sample at room temperature exhibits an unmodulated martensitic structure with a small residual trace of 7 M modulated martensitic structure. Two successive magneto-structural transformations (MSTs) consisting of martensitic and intermartensitic transformations are observed. It is found that the maximum isothermal entropy change vertical bar Delta S vertical bar(max) of 7.65 J/kg K at 309 K and refrigeration capacity (RC) of 78.84 J/kg are attained for a field change mu(0)Delta H = 3T under ambient pressure (p = 0 GPa). The enhanced MCE can be ascribed to the two successive MSTs. Much importantly, the vertical bar Delta S vertical bar(max) is basically independent of hydrostatic pressure and equals to 7.74 J/kg K at 314 K for p = 0.622 GPa. Besides, the application of hydrostatic pressure improve the width of phase transition and a large RC of 101.57 J/kg has been observed for p = 0.622 GPa under mu(0)Delta H = 3T. Consequently, the response of RC to magnetic field increases from 26.86 J/kg T for p = 0 GPa to 34.13 J/kg T for p = 0.622 GPa. The reversible RC is also improved by pressure. Such an enhanced MCE and the non-toxic raw materials make this alloy particularly appealing for solid-state cooling technologies.