The martensitic transition and magnetocaloric properties of Ni51Mn49−xSnx

The martensitic transition and magnetocaloric effects in Ni51Mn49−xSnx (x=13.4, 13.8, 14.0, 14.2, 14.5, 14.8 and 15.0) Heusler alloys were investigated by X-ray powder diffraction, differential scanning calorimetry and magnetic measurements. X-ray diffraction analysis reveals that the alloys exhibit austenitic state for x=15.0, the coexistence of austenitic and martensitic phases for 14.5≥x≥13.8, but martensitic state for x=13.4 at room temperature. The austenite has L21 Heusler structure with the space group Fm-3m, while the martensite has an orthorhombic structure (Pmma) with a=8.614 Å, b=5.655 Å and c=4.369 Å. The martensitic transformation is observed in all the alloys during the temperature decreasing. With the increase of x from 13.4 to 15.0, the martensitic transformation temperature TA→M decreases from 308 to 208 K, due to the decrease of valence electron concentration e/a. The magnetocaloric effects are investigated in the vicinity of the martensitic transition. The largest magnetic entropy change ΔSM of 10.77 J kg−1 K−1 at about 268 K was obtained in Ni51Mn34.5Sn14.5 alloy with a magnetic-field change of 1.5 T.