Temperature dependence of transformation strain and magnetic-field-induced strain in Ni51Mn24Ga25 single crystal

The martensitic transformation strain up to 0.6% has been obtained in Ni51Mn24Ga25 single crystalline sample, which is attributed to the effect of preferential orientation of the martensite variants, resulted from the internal stress introduced by the directional solidification. An enhanced transformation strain up to 1.1 % is observed with a bias magnetic field of 1.2 T. There is a large difference of MFISmax (λmax) from about 600 to 1300 ppm in the temperature range from 192 to 236 K in martensite phase. Based on the shear actuation model, the magnetocrystalline anisotropy energy and the energy for twin variants reorientation as a function of the temperature have been discussed. We proposed that the rate of the decrement of shear modulus (C′) with respect to the temperature is much larger than that of the magnetocrystalline anisotropy (K) as a result of the constraint λmax = K/C′γt, which has been considered as the reason for the increasing MFISmax with increasing temperature.