Recently, it was found that for ferromagnetic Heusler alloys Ni₂MnSn, Ni₂MnIn, and Ni₂MnSb, the deviation from stoichiometry results in appearance of martensitic transformation [1]. For these systems, the martensitic transformation temperature decreases upon substitution of Mn for Sn, In or Sb; especially drastic decrease was observed in the NiMnIn system. The compositional dependence of the Curie temperature T_c of the austenite phase was found to be weak, while T_c of the martensite phase is strongly depended on the alloy composition and drastically decreases with decreasing In, Sn or Sb content in all the systems.
In this work we report on new experimental results obtained on two series of novel ferromagnetic shape memory alloy, Ni_50+xMn_37-xSn₁₃ and Ni_50+yMn_39-ySn₁₁. They were characterized by differential scanning calorimetry and low-field magnetization measurements. Our DSC measurements revealed that the martensitic transformation temperature T_m in the alloys studied shows a non-monotonous dependence on the Ni excess. Whereas an increase of T_m is observed in both the system as Ni excess increases from 1 at.% to 2 at.%, further substitution of Mn for Ni does not affect martensitic transformation temperature T_m. The low-field magnetic susceptibility measurements were used for determination of Curie temperature T_C of these alloys. The results obtained point to a complex magnetic properties of these materials. For instance, some of the alloys exhibit several anomalies on temperature dependence of susceptibility which gives ground to conclude that, contrary to NiMnGa alloys, in the studied Ni-Mn-Sn compositions Curie temperature of martensite is lower than Curie temperature of austenite.

This work was partially supported by Russian Foundation for Basic Research Grants No. 03-02-17443, 04-02-81058, and 03-02-39006.

[1] Y. Sutou et al., Appl. Phys. Lett. 85 (2004) 4358.

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