Rearrangement of variants in Ni2MnGa under magnetic field

Magnetic field-induced strain which appears in association with rearrangement of variants in a stoichiometric Ni2MnGa single crystal exhibiting a martensitic transformation at 202 K has been investigated. The tetragonality of the martensite phase decreases slightly as temperature decreases and its value at 77 K is about 0.940. When the field is applied along the [001](p) ('P' stands for the parent phase) direction after cooling down to 77 K under zero magnetic field, the specimen contracts along this direction. In association with this contraction, the fraction of the variant whose c axis (easy axis of magnetization) lies along the field direction increases and reaches about 100%. When the specimen is cooled under magnetic field of 3.2 MA/m applied along [001](p), this fraction reaches 100% at the martensitic transformation temperature of 202 K. The energy dissipated due to rearrangement of variants by magnetic field is nearly the same as that obtained from its Stress-strain curve. The maximum shear stress by the magnetic field acting on the twinning plane is evaluated to be about 2.5 MPa from its uniaxial magnetocrystalline anisotropy constant (310 kJ/m(3) at 77 K). This is an adequate value because it is larger than the shear stress required for the rearrangement of variants (1.2-2.2 MPa) obtained by compressive tests. (C) 2003 Elsevier Ltd. All rights reserved.