| Summary: | NiMnSn ferromagnetic shape memory alloys exhibit martensitic transformation at low temperatures, restricting their applications. Therefore, this is a key factor in improving the martensitic transformation temperature, which is effectively carried out by proper element doping. In this research, we investigated the martensitic transformation and magnetic properties of Ni<sub>43</sub>Mn<sub>46-x</sub> Sm<sub>x</sub>Sn<sub>11</sub> (x = 0, 1, 2, 3) alloys on the basis of structural and magnetic measurements. X-ray diffraction showed that the crystal structure transforms from the cubic L2<sub>1</sub> to the orthorhombic martensite and gamma (γ) phases. The reverse martensitic and martensitic transformations were indicated by exothermic and endothermic peaks in differential scanning calorimetry. The martensitic transformation temperature increased considerably with Sm doping and exceeded room temperature for Sm = 3 at. %. The Ni<sub>43</sub>Mn<sub>45</sub>SmSn<sub>11</sub> alloy exhibited magnetostructural transformation, leading to a large magnetocaloric effect near room temperature. The existence of thermal hysteresis and the metamagnetic behavior of Ni<sub>43</sub>Mn<sub>45</sub>SmSn<sub>11</sub> confirm the first-order magnetostructural transition. The magnetic entropy change reached 20 J·kg<sup>−1</sup>·K<sup>−1</sup> at 266 K, and the refrigeration capacity reached ~162 J·Kg<sup>−1</sup>, for Ni<sub>43</sub>Mn<sub>45</sub>SmSn<sub>11</sub> under a magnetic field variation of 0–5 T.
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