Unexpected modulation revealed by electron diffraction in Ni-Mn-Ga-Co-Cu tetragonal martensite exhibiting giant magnetic field-induced strain

Abstract

Although the modulation of martensite structure has been considered as a precondition for highly mobile twin boundary and thus a giant magnetic field induced strain, the largest strain to date has been reported in non-modulated tetragonal martensite of Ni-Mn-Ga-Co-Cu. Using high-resolution transmission electron microscopy (HRTEM), we found that nominally non-modulated tetragonal Ni46Mn24Ga22Co4Cu4 single crystal possesses a modulation. The modulation was observed in thin lamellae containing b/c twins prepared by a focused Ga-ion beam and on a sample prepared by a conventional wedge thinning method. The modulation vector q* = (0, 0.2554, 0.4642), was determined by 3D electron diffraction. The simulated HRTEM image of such a structure agrees with the HRTEM micrograph. The presence of modulation in the thin lamellae and not in bulk suggests that Ni-Mn-Ga-Co-Cu martensite is on the edge of the shear instability enabling the high mobility of twin boundaries.