R. C. O’Handley, M. Marioni, M. Richard, J. Feuchtwanger, B. Peterson, R. Techapiesancharoenkij, D. I. Paul, and S. M. Allen
Massachusetts Institute of Technology, Cambridge MA 02139 USA
Ferromagnetic shape memory alloys such as Ni-Mn-Ga exhibit a martensitic transformation from a cubic to tetragonal or orthorhombic phase that can be highly twinned. The twins can be very mobile, responding with strains up to 6% or 10% by applications of stresses of order 1 - 2 MPa or magnetic fields of 4 to 6 kOe. Data on structure as well as stress-induced and magnetic field-induced strain are presented. The effects of acoustic energy on the field-induced strain are also described. The macroscopic phenomenology and some of the microscopic mechanisms associated with twin boundary motion are described. Some possible applications of these unique materials will be discussed.