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.

The coupling between the state of magnetization and the shape of a magnetic material is an interesting field, rich in basic science and opportunities for technical applications. Key experimental observations of conventional, anisotropic magnetostriction (the anisotropic strain tensor that rotates with the direction of magnetization in a free magnetic sample) are explained and the theoretical models of magnetostriction and related effects (e.g., the Δ E effect and stress‐induced anisotropy) are described. A new effect, quite distinct from anisotropic magnetostriction, is the large magnetic‐field‐induced strain observed in certain magnetic shape memory alloys. While these materials do exhibit conventional anisotropic magnetostrictive strain (of the order of 10 −4 ), their 6–10% field‐induced strain is due to twin boundary motion, which may or may not occur under magnetization rotation depending on the extent to which the twin boundaries are pinned on defects.

Ferromagnetic shape-memory alloys have recently emerged as a new class of active materials showing very large magnetic-field-induced extensional strains. Recently, a single crystal of a tetragonally distorted Heusler alloy in the NiMnGa system has shown a 5% shear strain at room temperature in a field of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memory alloys strain by virtue of the motion of the boundaries separating adjacent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnetic easy axes and large magnetocrystalline anisotropy. The twin boundary constitutes a nearly 90° domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field and stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression including Zeeman energy, magnetic anisotropy energy, internal elastic energy, and external stress. Models indicate the limits to the magnitude of the field-induced strain and point to the material parameters that make the effect possible. The field-induced strain in ferromagnetic shape-memory alloys is contrasted with the more familiar phenomenon of magnetostriction.

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