Large field-induced strains in ferromagnetic shape memory materials

We study magnetostriction in ferromagnetic shape memory materials caused by a redistribution of twin bands in response to an applied magnetic field. Recent measurements of strain versus magnetic field in Ni-24 at% Mn-24.7 at% Ga are presented. We report strains of about 0.5% under cyclic application of a field, and field-induced strains of about 4% on specimens that have been previously cooled through the martensitic transformation under stress. A comparison between these measurements and theoretical calculations reveals a discrepancy. We hypothesize that this is due to magnetization rotation, accompanied by the formation of fine magnetic domains within each band of martensite.