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 …