13: Review of Magnetic Shape Memory Smart Materials

Chapter 13 reviews magnetic shape memory alloys (MSMAs), often also referred to as ferromagnetic shape memory alloys (FSMAs), which have emerged as an interesting extension of the class of shape memory materials (SMMs). FSMAs combine the attributes and properties of ferromagnetism with a reversible martensitic crystalline solid phase transformation. Magnetically controlled shape memory (MSM) materials present a new way to produce motion and force. MSM phenomena were originally suggested by Ullakko, O'Handley, and Kantner and demonstrated these in a Ni–Mn–Ga alloy in as early as 1996. They suggested a new mechanism based on the magnetic field-induced reorientation of the twin structure of a MSMA. Effectively, the magnetic control of the shape memory effect leads to a much more rapid response of the actuator than the thermal control. The magnetic field controls the reorientation of the twin variants analogous to the way in which twin variants are controlled by stress in classical shape memory alloys. The magnetic shape memory effect has demonstrated that certain shape memory materials that are also ferromagnetic can show very large dimensional changes (6–10%) under the application of a magnetic field. These strains occur within the low-temperature (martensitic) phase.