Theory of magnetization: Twin boundary interaction in ferromagnetic shape memory alloys

Abstract

Much interest has been recently given to the development and understanding of ferromagnetic shape memory alloys. In particular, the interaction of crystallographic twin boundaries with an external magnetic field has been put forth as a mechanism for shape deformation and applied to the alloy Ni2MnGa. In this article, we examine this interaction analytically. We consider two crystallographic variants wherein the magnetization directions at their extreme boundaries are along their easy axes. Further, we have included the magnetic exchange energy and have postulated a Gaussian strain distribution localized in one of the variants. Using the basic micromagnetic equations, we determine the magnetization distribution throughout the variants as well as the equilibrium positions of the twin boundary. We show that the resulting magnetic domain wall leads the twin boundary position as long as the magnetic driving force is insufficient to overcome the strain energy impediment. For the reverse situation, both the twin boundary and magnetic domain wall move through the opposing variant. The magnetic coercivity is correspondingly reduced.