J. Phys. IV France 112 (2003) 959
Relation between structure, magnetization process and magnetic shape memory effect of various martensites occurring in Ni-Mn-Ga alloysO. Heczko, L. Straka and K. Ullakko
Helsinki University of Technology, Department of Engineering Physics and Mathematics, Laboratory of Biomedical Engineering, P.O. Box 2200, 02015 Hut, Finland
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 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 SM 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.
© EDP Sciences 2003