Nanomechanics and magnetic structure of orthorhombic Ni–Mn–Ga martensite

Atomic-force microscopy (AFM), magnetic-force microscopy (MFM), and nanoindentation experiments were performed on a Ni–Mn–Ga single crystal with orthorhombic (14 M) martensite. The surface relief was characterized and used to identify the orientation of individual twin variants. It was shown that ab- and bc-twinning modes are present. The magnetic domains are elongated along the crystallographic c direction. Twin variants with the c direction perpendicular to the surface exhibit an out-of-plane magnetization which can be identified through a strong contrast in the MFM image. The deformation caused with nanoindentation is recoverable to a large degree. No trace of deformation twinning was found after unloading. The results are discussed in the light of pseudo-elastic twinning and dislocation activity. It is concluded that although the residual deformation is due to dislocation activity, it is likely that pseudo-elastic twinning took place during loading and unloading. The results indicate a size effect on deformation twinning under localized loading.