Magnetic field-induced strain in single crystal Ni-Mn-Ga

摘要

Field-induced strains up to 10% at room temperature have been observed in magnetic shape memory alloys based on off-stoichiometric compositions of the intermetallic compound Ni_2MnGa. This occurs by the motion of twin boundaries in the ferromagnetic (T_C≈90℃) martensitic (T_(mart) typically 30 to 70℃) state under magnetic fields of a few kOe. Some data illustrating the interdependence of strain, stress, and magnetic field are reviewed. Phenomenological models describe many of these observations by minimization of free energy terms including Zeeman energy, magnetocrystalline anisotropy energy, stored elastic energy and fractional twin-boundary distribution. Two important questions have been raised about field-induced strain in FSMAs. They are 1) the role of body forces (due to action of the field on the sample), and 2) the role of magnetostriction (stress/strain in a single variant under magnetization rotation) in the twin boundary motion. These questions are addressed in light of published data and models.