Transformation of twinned Ni52.0Mn24.4Ga23.6 martensite in a rotating magnetic field: Theory and experiment

The influence of a rotating magnetic field on disk-shaped twinned Ni-Mn-Ga single crystals is studied theoretically and experimentally. A magnetoelastic model of ferromagnetic martensite is used for the comprehension of experimental results. The model considers the magnetic field influence on each twin component in terms of the magnetically induced mechanical stress (magnetostress). The angular dependence of magnetostress and the correspondence between the directions of the magnetic field and magnetization vector are obtained. The magnetically induced transformation of twin structure of the specimen is observed experimentally in a Ni52.0Mn24.4Ga23.6 single-crystalline disk by magnetic measurements performed in a two-dimensional vibrating sample magnetometer. The threshold character of the transformation process is stated. The threshold angles between the [100] crystallographic direction and the directions of magnetic field and magnetic vector of the transformed twin component were measured for the different magnetic field values. The comparison of experimental values with the theoretical ones points to the comparatively low value of the magnetic anisotropy constant (50 kJ m(-3)). The obtained results disclose the possibility of obtaining large magnetically induced strains in ferromagnetic-shape memory alloys with reduced magnetocrystalline anisotropy.