Ferromagnetic shape memory in the NiMnGa system

Strain versus field measurements for a ferromagnetic shape memory alloy in the NiMnGa system demonstrate the largest magnetostrictive strains to date of nearly 1.3%. These strains are achieved in the martensitic state through field-induced variant rearrangement, An experimental apparatus is describe d that provides biaxial magnetic fields and uniaxial compressive prestress with temperature control while recording microstructural changes with optic al microscopy, The magnetostrictive response is found to be sensitive to th e initial state induced by stress-biasing the martensitic variant structure , and exhibits rate effects related to twin boundary mobility. Experiments performed with constant stress demonstrate work output capacity, Experiment al results are interpreted by using a theory based on minimization of a mic romagnetic energy functional that includes applied field, stress, and demag netization energies, It is found that the theory provides a good qualitativ e description of material behavior, but significantly overpredicts the amou nt of strain produced. Issues concerning the martensitic magnetic anisotrop y and variant nucleation are discussed with regard to this discrepancy.