Field-induced strains of 6% are reported in ferromagnetic Ni-Mn-Ga martensites at room temperature. The strains are the result of twin boundary motion driven largely by the Zeeman energy difference across the twin boundary. The strain measured parallel to the applied magnetic field is negative in the sample/field geometry used here. The strain saturates in fields of order 400 kA/m and is blocked by a compressive stress of order 2 MPa applied orthogonal to the magnetic field. The strain versus field curves exhibit appreciable hysteresis associated with the motion of the twin boundaries. A simple model accounts quantitatively for the dependence of strain on magnetic field and external stress using as input parameters only measured quantities.

Extraordinary magnetostrictive behavior has been observed in Fe-Ga alloys with concentrations of Ga between 4% and 27%. λ 100 exhibits two peaks as a function of Ga content. At room temperature, λ 100 reaches a maximum of 265 ppm near 19% Ga and 235 ppm near 27% Ga. For compositions between 19% and 27%, λ 100 drops sharply to a minimum near 24% Ga and exhibits an anomalous temperature dependence, decreasing by as much as a factor of 2 at low temperatures. This unusual magnetostrictive behavior is interpreted on the basis of a single maximum in the magnetoelastic coupling |b 1 | of Fe with increasing amounts of nonmagnetic Ga, combined with a strongly temperature dependent elastic shear modulus (c 11 −c 12 ) which approaches zero near 27% Ga. λ 111 is significantly smaller in magnitude than λ 100 over this composition range, and has an abrupt change in sign from negative for low Ga concentrations to positive for a concentration of Ga near 21%. Extraordinary magnetostrictive behavior has been observed in Fe-Ga alloys with concentrations of Ga between 4% and 27%. 100 exhibits two peaks as a function of Ga content. At room temperature, 100 reaches a maximum of 265 ppm near 19% Ga and 235 ppm near 27% Ga. For compositions between 19% and 27%, 100 drops sharply to a minimum near 24% Ga and exhibits an anomalous temperature dependence, decreasing by as much as a factor of 2 at low temperatures. This unusual magnetostrictive behavior is interpreted on the basis of a single maximum in the magnetoelastic coupling ͉b 1 ͉ of Fe with increasing amounts of nonmagnetic Ga, combined with a strongly temperature dependent elastic shear modulus (c 11 Ϫc 12 ) which approaches zero near 27% Ga. 111 is significantly smaller in magnitude than 100 over this composition range, and has an abrupt change in sign from negative for low Ga concentrations to positive for a concentration of Ga near 21%.

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