Switching the soft shearing mode orientation in Ni–Mn–Ga non-modulated martensite by Co and Cu doping

Abstract

Resonant ultrasound spectroscopy measurements and density functional theory calculations were used to analyze the effect of Co and Cu doping (3–6 at. %) on elastic constants of non-modulated martensite of the Ni–Mn–Ga ferromagnetic shape memory alloy. Due to the doping, the studied alloys exhibited decreased tetragonal ratios c/a ≈ 1.14. Both the experiments and the calculations revealed that the lowering of the c/a ratio resulted in a change of the orientation of the softest shearing modes of the tetragonal lattice. The newly appearing softest shearing modes for the doped materials have approximately orientations and indicate a lattice instability directly related to the particularly low twinning stress for compound twins, needed for the magnetically induced reorientation.