Analytical and experimental issues in Ni-Mn-Ga transducers

Ferromagnetic shape memory martensites in the Ni-Mn-Ga system have been demonstrated to achieve a number of the criteria required for next generation actuators including the production of large theoretical strains up to 6%. The large strain originates in the rotation of twin variants and associated twin boundary motion which occurs in response to magnetic fields. The magnetic activation holds promise in actuator design because it can lead to higher bandwidths than those achieved through pure martensite-austenite phase transformation, as is the case with thermally-activated shape memory alloys. In this paper, we report on experimental measurements collected from a cylindrical Ni49.0Mn30.0Ga21.0sample alloy, driven as cast by a collinear magnetic field-stress pair. Despite the lack of a known restoring force and the fact that no training procedures are applied, quasi-static strains as large as 4300 micro-strain are shown. Furthermore, dynamic results in the DC-20kHz range are presented which would suggest the presence of a Delta-E effect similar to that seen in Terfenol-D but exhibiting an opposite dependence of stiffness with DC field. The potential implications of the results for the design and control of dynamic structures based on Ni-Mn-Ga are very significant