With the help of the Ginzburg–Landau theory the temperature–magnetic field phase diagrams of Heusler alloys Ni–Mn–Ga are theoretically investigated. The influence of the parameters of the magnetoelastic constants and elastic moduli on the phase diagrams are discussed. It is shown that with the determined combination of the phenomenological parameters, the critical magnetic field for the phase transition appears. The theoretically predicted value of the critical magnetic field is possible to be achieved in the experiment with the help of modern magnetic field sources.

In Heusler-type alloy Ni2+x−yMn1−xFeyGa, partial substitution of Mn for Ni causes the temperatures of structural (martensitic) TM and magnetic TC (Curie point) phase transitions to converge. Close to the crossover of TM and TC, we have observed the strong strains (Δl/l≈2–4%) induced by the external magnetic field. This effect could be classed with colossal magnetostriction. The system exhibits the magnetic field-controlled one-way shape memory effect at fixed temperature as a result of the magnetic field-induced martensite to austenite structural phase transition.

A reversible field-induced structural phase transition in shape-memory ferromagnetic alloys Ni_{2+x}Mn_{1-x}Ga has been observed at fixed temperature and pressure in magnetic fields about 100 kOe. The theoretical results are in qualitative agreement with experiment.