Large field induced strain in single crystalline Ni-Mn-Ga ferromagnetic shape memory alloy

A room temperature free shear strain of 5.7% is reported in a single crystal of Ni-Mn-Ga having a composition close to the Heusler alloy Ni2MnGa. A twin boundary was created in a 2 mm x 2 mm x 25 mm single crystal using a permanent magnet with surface field strength of about 320 000 A/m. A sharp 6.5 degrees bend occurs in the sample at the twin boundary. The surface magnetization changes abruptly across this boundary. By moving the sample relative to the edge of the magnet, we were able to sweep the boundary back and forth along the crystal length. Surface magnetization was measured using a Hall probe and the results confirm that the easy axis is the tetragonal c axis. Powder x-ray diffraction shows that the fcc to body-centered-tetragonal bct martensitic transition of this material involved a 6% reduction of the bct cell c/a ratio, from root 2 to about 1.33. The maximum achievable strain is thus estimated to be 6.2%. The twin planes in the system are the {112}(bct) and were observed to lie almost normal to the long axis of the sample tested. (C) 2000 American Institute of Physics. [S0021-8979(00)16108-0].