Shape-Memory Alloys, Magnetically Activated Ferromagnetic Shape-Memory Materials

For years it was speculated that the large strains associated with the thermoelastic shape-memory effect, such as in NiTi alloys, could be captured by application of a magnetic field in certain martensites that are also ferromagnetic. Ferromagnetic shape-memory alloys (FSMAs) moved from a hypothetical new class of active materials, to join piezoelectric and magnetostrictive materials, upon observation of a 0.2% magnetic field induced strain in a single crystal of Ni2MnGa in 1996. This article focuses on the application of a magnetic field to a twinned M phase that is ferromagnetic. It is necessary that the magnetic anisotropy of the M phase be large compared to the energy required for twin boundary motion and, further, that the preferred direction of magnetization changes across the twin boundary. When this is the case, application of a magnetic field results in a difference in Zeeman energy, across the twin boundary. This energy difference exerts a pressure on the twin boundary so as to grow the twin variants having the more favorably oriented magnetization. The resulting field-induced twin-boundary motion produces a large strain, fully within the martensitic state of an FSMA. This article describes the crystallography and magnetism of Ni–Mn–Ga in order to explain the very large strains produced by field-induced twin-boundary motion in martensite. Examples of field-induced strain by twin boundary motion in Ni–Mn–Ga FSMA samples having different twin structures are given. Martensitic Fe70Pd30 has also shown field-induced strains of 0.5%, and efforts are under way to develop other iron-base FSMAs. These other materials are not be covered in depth. The state of theoretical modeling of strain and magnetization in FSMAs is reviewed. FSMA field-induced strains are compared and contrasted with the thermoelastic shape-memory effect and magnetostriction. Keywords: Ferromagnetic shape-memory alloys (FSMAs); Field induced strain; Magnetization; Twin rearrangement; Quantitative models; Twin-boundary motion; Micromagnetics; Strain under load