MODEL FOR STRAIN AND MAGNETIZATION IN MAGNETIC SHAPE-MEMORY ALLOYS

The large magnetic-field-induced strains observed in martensitic phase s based on Ni2MnGa and in other magnetic shape memory alloys are belie ved to arise from a process of twin-boundary motion rather than magnet ostriction. The dependence of strain on magnetization, e(M), generally shows a large component that is linear (rather than quadratic) in M b elow saturation (quadratic dependence being typical of magnetostrictiv e strain). A simple phenomenological model for the magnetization proce ss and field-induced strain by twin-boundary and phase-boundary motion is proposed for both the strong and weak anisotropy cases. The model is shown to account for the nearly linear dependence of strain on magn etization in the martensitic phases of these materials. It shows the f ield dependence of the magnetization and strain to be functions of an effective stiffness constant, C, the transformation strain, e(0), and the magnetic anisotropy of the martensitic phase, K-u, through two red uced field parameters, h(e)=MsH/Ce-0(2) and h(a)=MsH/2K(u). The model also accounts for the magnetization remanence and the nonlinear field dependence closer to saturation (which produces little strain). The cu rvature observed in e(H) at very low fields is not described by this t wo-variant model and may be related to the fact that more Variants exi st which respond to the field with a distribution of susceptibilities. (C) 1998 American Institute of Physics.