J. Phys. IV France 112 (2003) 925
Microstructure change in Ni-Mn-Ga seven-layered martensite connected with MSM effectN. Lanska and K. Ullakko
Helsinki University of Technology, Department of Engineering Physics and Mathematics, Laboratory of Biomedical Engineering, P. O. Box 2200, 02015 Hut, Finland
Single crystal samples of near-stoichiometric Ni 2MnGa alloy with orthorhombic seven-layered martensitic structure have been investigated. Measured lattice parameters are a=0.619 nm, b=0.580 mn and c=0.553 nm (in coordinates of parent cubic phase) at ambient temperature. Giant magnetic-field-induced strain (MSM effect) of about 10% was observed in this phase. Microstructure change under applied magnetic field was studied by means of X-ray diffraction and optical microscopy methods. It was shown that the mechanism of MSM effect in sevenlayered orthorhombic martensitic phase is similar to the mechanism found before in five-layered tetragonal martensite. Giant magnetic-field-induced strain in orthorhombic phase is caused by sample transformation from one single-variant state ( a axis parallel to the field) by twinning ((101) [ 10 ] system) to another single-variant state ( c axis parallel to the field).
© EDP Sciences 2003