A phenomenological theory is presented to study the multiferroic system in the shape memory alloys Ni2Mn1+xIn1-x with the Heusler-type structure, where the phase transitions are characterized by the two order parameters, i.e., the martensitic distortion e3 and the magnetization M. The Landau free energy is expanded in powers of e3 and M with including the Zeeman energy and the energy increase by the uniaxial force on e3. The resultant free energy is applied to analyze the experimental results, such as the phase diagram in the temperature-concentration plane and the reentrant ferromagnetism by increasing temperature. The magnetic fields are shown to cause the ferromagnetic-ferromagnetic or metamagnetic transitions as observed. Further, it is predicted that the uniaxial force can dominate the appearance of the ferromagnetism in some alloys. It is found that these exotic behaviors of this alloy system are all ascribed to the repulsive interaction between e3 and M, which originates from their biquadratic term. Through the present analyses, it is verified that this repulsive interaction is expected in other alloy systems Ni2Mn 1+xX1-x (X=Sn, Sb) and makes a contrast to the attractive interaction realized in another group of the alloy systems including Ni 2+xMn1-xGa and Ni2Mn1-xCu xGa.
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