Working Ni–Mn–Ga Single Crystals in a Magnetic Field Against a Spring Load

This research characterizes ferromagnetic shape memory elements for use as mechanical actuators. A single crystal of Ni–Mn–Ga was pre-strained in compression from 0 to 6 % and then the shape was recovered with a magnetic field perpendicular to the loading direction while working against a pair of springs. The magnetic field was raised from 0 to 0.64 MA/m and then reduced to zero field. Eight pairs of springs with combined spring constants ranging from 14.3 to 269.4 N/mm were used. When the magnetic field was on, the sample expanded against the springs due to magnetic field-induced strain. When the magnetic field was turned off, the springs compressed the sample back to the initial size before the next cycle. During each cycle, force and displacement were measured and the specific work was computed. Specific work increased with the applied magnetic field and the pre-strain, with a maximum of 14 kJ/m³ at 4.5 % pre-strain and 0.64 MA/m. This value is five times less than the values suggested in the literature which were inferred from stress–strain curves measured under various magnetic fields. The spring prescribes the load–displacement path of the magnetic shape memory element and controls the work output of the actuator.