On the stress-assisted magnetic-field-induced phase transformation in Ni₂MnGa ferromagnetic shape memory alloys

The effect of magnetic field on the martensitic phase transformation in Ni₂MnGa single crystals was investigated under compression. Reversible and one-way stress-assisted field-induced phase transformations were observed under low field magnitudes. The total work output levels achieved during reversible stress-assisted field-induced phase transformation are similar to that attained using field-induced martensite reorientation in NiMnGa magnetic shape memory alloys (MSMAs). However, the actuation stress levels are an order of magnitude higher. Possible magneto-microstructural mechanisms and necessary magnetic and mechanical conditions to accomplish field-induced phase transformation are discussed. A thermodynamical description is introduced to understand magnetic energy contributions to trigger the phase transformation. Materials design and selection guidelines are proposed to search for this new mechanism in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation. The present work output levels achieved in the Ni₂MnGa MSMA and the possibility of further increase place MSMAs above many currently available high frequency active materials.