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.
Twin relationships and stress-induced reorientation were studied in Ni2Mn1.14Ga0.86 single crystal with five-layered modulated martensite crystal structure. Very low twinning stress of about 0.1 MPa was found for twin boundaries which deviated a few degrees from the (011¯) crystallographic plane. However, twin boundaries oriented exactly parallel to the (011¯) plane exhibited considerably higher level of twinning stress, above 1 MPa. X-ray diffraction experiments and calculations based on approximation of the martensite crystal lattice as a tetragonal lattice with a slight monoclinic distortion identified the two different kinds of twin interfaces as type II and type I twin boundaries.
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