Effect of heat treatment on the crystal structure, martensitic transformation and magnetic properties of Mn₅₃Ni₂₅Ga₂₂ ferromagnetic shape memory alloy

In this study, the effect of heat treatment on crystal structure, martensitic transformation, thermodynamic behavior and magnetic properties of polycrystalline Mn₅₃Ni₂₅Ga₂₂ ferromagnetic shape memory alloy was systematically investigated. The results show that the heat treatment has obvious effect on martensitic transformation temperatures, crystal structure and hysteresis loops. Heat treatment greatly effects on transformation temperatures due to modified composition of the matrix. Martensitic transformation temperature, saturation magnetization decreased with the increase heat treatment temperature, reaching their minimum values at the heat treatment temperature of 1173 K for 12 h. Curie temperature of maximum values obtained at solution-treated of 1173 K for 12 h. In other word, increasing heat treatment temperature and time has an effect on Curie temperature. In addition, the annealed alloy Mn₅₃Ni₂₅Ga₂₂ may completely dissolve in vacuum tubes at 1173 K for 12 h. It is found that the studied alloys have some (Mn,Ni)₄ Ga-type compound precipitates, which can be seen dispersing both in grain interiors and on grain boundaries at other heat treatment process. Lastly, Rietveld analysis shows the good agreement between experiment and calculated data of XRD patterns.