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 {1 a 0 1 c} bcta-1 a 0 1 cnbct orientations and indicate a lattice instability directly related to the particularly low twinning stress for compound {101} a1 01n twins, needed for the magnetically induced reorientation.