Ferromagnetism of thermoelastic martensites: Theory and experiment

A consistent consideration of ferromagnetic alloys undergoing a cubic-tetragonal martensitic transformation on cooling is carried out within the framework of Landau theory. The concept of two different Curie temperatures, corresponding to the parent (austenitic) and resultant (martensitic) phases, is substantiated by taking into account the volume magnetostriction of an alloy. The difference in these two temperatures is evaluated for different alloys from the Ni-Mn-Ga family typifying ferromagnetic martensites. A statistical dispersion of the local Curie temperatures reflecting the heterogeneity of martensitic states is introduced. The qualitatively different temperature dependencies of the magnetization are predicted theoretically and observed experimentally under an applied magnetic field of about 10 kOe. Quantitative agreement between the theoretical and experimental curves is achieved.