Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials (invited)

Ferromagnetic shape-memory alloys have recently emerged as a new class of a ctive materials showing very large magnetic-field-induced extensional strai ns. 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 fie ld of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memo ry alloys strain by virtue of the motion of the boundaries separating adjac ent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnet ic easy axes and large magnetocrystalline anisotropy. The twin boundary con stitutes a nearly 90 degrees domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field an d stress dependence of the strain are reasonably well accounted for by mini mization of a simple free energy expression including Zeeman energy, magnet ic 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 fa miliar phenomenon of magnetostriction. (C) 2000 American Institute of Physi cs. [S0021-8979(00)75208-X].