AC performance and modeling of ferromagnetic shape memory actuators

Very large DC field-induced strains (epsilon approximate to 6%) have been reported for Ni-Mn-Ga single-crystal ferromagnetic shape memory alloys (FSMAs) at room temperature. Described here is an AC test system that provides a dynamic bias stress to an FSMA sample. The low-frequency epsilon -H curves show a stress dependence consistent with the DC results, i.e. the maximum output strain peaks for a bias stress of order 1.4 MPa. The AC epsilon -H hysteresis at sub-optimal bias stress can be considerably smaller than that for DC actuation. A thermodynamic model of field-induced twin-boundary motion is expanded to include external stress, threshold field and hysteresis in the twin boundary motion. Twin-boundary motion is driven by the Zeeman energy difference across the domain wall, 2M(s)(.)H, in the high anisotropy limit and is suppressed by domain magnetization rotation in the weak anisotropy limit. The magnitude of the threshold field and hysteresis can be obtained from features on mechanical stress-versus-strain curves. The field dependence and stress dependence of the AC strain are reasonably well accounted for by the model.