Time-dependent magnetostrain and thermal phonons in the Ni-Mn-Ga magnetic shape-memory alloys

The isothermal time-dependent deformation of the ferromagnetic Ni-Mn-Ga martensite in the steady magnetic field has been studied both experimentally and theoretically. The role of long wavelength thermal phonons in the deformation process has been disclosed by means of evaluation of the fluctuating mechanical stress, which these phonons induce inside the experimentally observed elements of martensitic structure. A simple relationship between the magnitude of effective fluctuating stress, temperature-dependent elastic modulus and twin width has been derived. In this way, a strong interdependence between the twin width and the effect of thermal phonons on the twin structure is predicted. The fluctuation mechanism of reversibility of the magnetically induced deformation has been deduced from the experimental and theoretical results.