Phase transitions in ferromagnetic Ni2+xMn1-xGa alloys with regard for the modulation order parameter

The phase diagram of ferromagnetic alloys Ni2+xMn1- xGa is reconstructed on the basis of temperature dependences of the resistance. It is seen from this diagram that for small x, structural transitions from the cubic to the tetragonal phase are preceded by structural transformations in the cubic phase. In the framework of the phenomenological Landau theory of phase transitions, phase diagrams of the structural and magnetic phase transitions in these alloys are analyzed with regard for the modulation order parameter. It is shown that premartensitic and postmartensitic phase transitions related to the appearance of the modulated structure can occur along with martensitic transformations. The strain and modulation order parameters substantially affect the magnetic phase transitions via the interaction with the magnetic order parameter. (C) 2001 MAIK Nauka/ Interperiodica.