Activation of magnetic shape memory effect in Ni–Mn–Ga alloys by mechanical and magnetic treatment

Mechanical and magnetic training decreased considerably the twinning stress of five-layered modulated, approximately tetragonal martensite (5M) of Ni–Mn–Ga and Ni–Mn–Ga doped with Fe, and of seven-layered modulated, approximately orthorhombic Ni–Mn–Ga martensite (7M). Repeated compressions along two perpendicular directions of single crystal specimens of 5M and 7M reduced the twinning stress approximately threefold. Further reduction of twinning stress of 7M was achieved by the rotation of the specimen in 1 T magnetic field several times. The training resulted in the appearance of the magnetic shape memory effect or giant magnetic-field-induced strain in agreement with a simple energy-based model of the effect. The origin of the decrease in twinning stress is discussed.