Relation between structure, magnetization process and magnetic shape memory effect of various martensites occurring in Ni-Mn-Ga alloys

Magnetic properties and magnetic shape memory effect of three Ni-Mn-Ga alloys representing three different martensitic phases; five-layered tetragonal (5M), seven-layered orthorhombic (7M), and non-modulated tetragonal (NM) martensite were studied at room temperature. Magnetization process occurs mostly by magnetization rotation. Magnetic anisotropy energy is high for all three phases ranging from 1.1 to 1.7 x 10(5) J/m(3). The direction of short lattice constant is always a direction of easy magnetization. It is demonstrated that the high magnetic anisotropy and low twinning stress render a MSM effect possible in 5M and 7M martensite. The 5M martensite exhibits more than 6% field induced strain. The giant strain is followed by large magnetization jump. Simultaneous measurement of magnetization changes and field-induced strain as a function of magnetic field demonstrates the nature of the magnetic shape memory effect and validates the proposed mechanism.