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 crysta l of Ni-Mn-Ga having a composition close to the Heusler alloy Ni2MnGa. A tw in boundary was created in a 2 mm x 2 mm x 25 mm single crystal using a per manent 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 magnet ization changes abruptly across this boundary. By moving the sample relativ e to the edge of the magnet, we were able to sweep the boundary back and fo rth along the crystal length. Surface magnetization was measured using a Ha ll probe and the results confirm that the easy axis is the tetragonal c axi s. 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 strai n 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].