Single phase boundary actuation of a ferromagnetic shape memory foil

The novel mechanism of temperature-gradient-induced single phase boundary actuation is presented for a single crystalline ferromagnetic shape memory alloy (FSMA) foil. It is shown that applying a temperature gradient along the FSMA foil specimen results in the formation and propagation of a martensite-austenite phase boundary from the hot to the cold side, allowing for reproducible strain-temperature characteristics. The selection of martensite variants upon phase transformation is controlled by simultaneously applying a bias magnetic field, which determines the maximum strain response. Single phase boundary actuation is demonstrated for a Ni-Mn-Ga foil of 100 μm thickness with 10 M martensite structure at room temperature. A small temperature gradient of 5 K mm-1 and a bias field along the temperature gradient of 120 mT are sufficient to achieve the maximum possible strain of 4.1%, corresponding to the length difference of the short c-axis of tetragonal martensite and the axis of cubic austenite. For a bias magnetic field in the perpendicular direction, the maximum strain change is −1.9%.