Abslruct-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 described that provides biaxial magnetic flelds and uniaxial compressive prestress with temperature control while recording microstructural changes with optical microscopy. The magnetostrictive

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... nse is found to he sensitive to the initial state induced by stressbiasing the martensitic variant structure, and exhibits rate effects related to twin boundary mobility. Experiments performed with constant stress demonstrate work output capacity. Experimental results are interpreted by using a theory based on minimization of a micromagnetic energy functional that includes applied field, stress, and demagnetization energies. It is found that the theory provides a good qualitative description of material behavior, hut significantly overpredicts the amount of strain produced. Issues concerning the martensitic magnetic anisotropy and variant nu. cleation are discussed with regard to this discrepancy.