Giant magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase. The strain proved to be caused by magnetic-field-controlled twin boundary motion. According to an analysis of x-ray diffraction data, the crystal structure of this phase is nearly orthorhombic, having lattice parameters a=0.619 nm, b=0.580 nm, and c=0.553 nm (in cubic parent phase coordinates) at ambient temperature. Seven-layer shuffling-type modulation along the (110)[11̄0]p system was recorded. The results of mechanical tests and magnetic anisotropy property measurements are also reported.

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Magnetic field-induced strain (MFIS) of 12% is reported in ferromagnetic Ni46Mn24Ga22Co4Cu4 martensite exhibiting non-modulated (NM) tetragonal crystal structure with lattice parameter ratio c/a>1. The strain was measured at ambient temperature in a magnetic field of the order of 1 T. The twinning stress σTW and the magnetic stress σMAG were also measured and the condition for a giant MFIS observation σTW<σMAG was confirmed. The MFIS was achieved in NM Ni46Mn24Ga22Co4Cu4 martensite by considerable lowering of the σTW value as compared to the values for NM martensites in ternary Ni-Mn-Ga system.

The crystal structure of ferromagnetic near-stoichiometric Ni 2 MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered ͑5M͒ and seven-layered ͑7M͒ martensitic phases, exhibit the martensitic transformation temperature (T M ) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, T M and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or T M has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni-Mn-Ga system has been confirmed.

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