A model is proposed for describing the experimentally observed martensite and magnetic domain structures in Heusler ferromagnetic alloys Ni_(2+x)Mn_(1-x)Ga. On the basis of this model, the field dependences of magnetization and deformation of the alloys are calculated numerically and an expression for the maximum attainable strains induced by external magnetic fields in these alloys is derived. It is shown that for small values of the effective elastic modulus and demagnetizing factor of alloys, the strains induced by the magnetic field may attain maximum possible values of approximately 5%, which are determined by lattice distortions as a result of the martensite transition in fields of about 1 kOe.
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