Crystal structure, magnetic anisotropy and mechanical properties of seven-layered martensite in Ni-Mn-Ga

Magnetic-field-induced strain of approximately 10% is reported in Ni48.8Mn29.7Ga21.5 alloy at ambient temperature in a magnetic field order of 1 T. It was confirmed by different experimental methods that the strain is contributed by twin boundary motion. The crystal structure of thermally-induced martensitic phase in this alloy was found to be close to orthorhombic one in the temperature range from 245 K to 333 K. Lattice parameters are a=0.619 nm, b=0.580 nm, and c=0.553 nm (relating to the cubic parent phase coordinates) at ambient temperature. More detailed x-ray studies revealed seven-layer shuffling-type modulation along [110] and [1 (1) over bar0] directions. It was also found that the orthorhombic phase has low twinning stress and high energy of magnetic anisotropy. The magnetic measurements revealed that the shortest axis (c axis) is the axis of easiest magnetization, the longest (a axis) is the axis of hard magnetization, and b axis is the intermediate one. The compressive stress applied along a axis of a single-variant sample at most 2 MPa is enough to produce approximately 10 % strain realized by twin boundary motion. The necessary conditions for observation a giant magnetic-field-induced strain in non-stoichiometric Ni2MnGa alloys based on the new experimental data are discussed.