Ferromagnetic SMA (FSMA): background

Martensitic transformation in SMAs can be controlled by stress and temperature. Their relationship can be described by stress-temperature phase diagram. This transformation is reversible. However, a heating or cooling device must be incorporated, when a SMA such as Ni-Ti is used as an actuator. The response of such an actuator depends on heat transfer or conduction which is a rather slow process. Recently, attention is paid to some alloys accompanying a change in ferromagnetism at martensitic transformation and having low hysteresis. This is because the transformation characteristics such as transition point (martensite start temperature, TMs) and macroscopically observed strain, caused by the transformation, are possibly controlled by an applied magnetic field (H). The response of transformation is fast in this case, because the characteristic time is governed by the formation and growth of martensite which is induced by an applied magnetic field. Thus, it is plausible to produce an actuator having reversible straining with quick response to a signal imposed or detected. The alloy with both ferromagnetic properties and shape memory effect is called ferromagnetic shape memory alloy (FSMA) and it is considered as a strong candidate for the fast responsive actuator material.