Magnetoelastic Coupling in Ni-Mn-Ga Magnetic Shape Memory Alloy

The role of magnetoelastic coupling in the mechanism of magnetically induced reorientation or redistribution (MIR) of twin variants is still a matter of some controversy. To evaluate this role ordinary magnetostriction of different Ni-Mn-Ga single crystals transforming to 5M (exhibiting MIR) and NM (no MIR) martensite were measured. The magnetostriction of Ni-Mn-Ga austenite is relatively low and steeply increases when approaching to martensite transformation. This is correlated to the softening of elastic modulus. Observed high field contribution of opposite sign may be due to the dependence of higher order elastic constant on magnetic field. The magnetostriction of martensite is difficult to determine as it is masked by much stronger MIR effect and indirect method must be used. The results are discussed in the frame of magnetoelastic model for MIR and compared with magnetic energy model.