Shape memory and ferromagnetic shape memory effects in single-crystal Ni2MnGa thin films

Epitaxial Ni2MnGa and Ni2Mn1.2Ga0.8 thin films have been grown by molecular beam epitaxy on GaAs (001) substrates with Sc0.3Er0.7As interlayers. Structural characterization of as-grown films confirms the epitaxially stabilized single crystal structure of the films and indicates that the films grow pseudomorphically on GaAs (001) substrates in a tetragonal structure (a=b=5.65 A, c=6.18 A). The films are ferromagnetic at room temperature with coercivity of similar to50 Oe, saturation magnetization of similar to250 emu/cm3, and weak in-plane magnetic anisotropy. The Curie temperature of the films is found to be similar to340 K. However, while the films were attached to the substrate martensitic phase transformations were not observed. In order to observe martensitic phase transformations, free-standing Ni2MnGa bridges and cantilevers were fabricated using front and back side photolithography together with a combination of dry and wet etching. After removal of the substrate, the free-standing bridges and cantilevers showed a unique temperature dependent shape. Observation using a polarized-light optical microscope during repeated thermocycling showed large movement of the cantilevers, confirming a two-way shape memory effect in the free-standing films. Using 100 mum long free-standing bridges, field induced strain or the ferromagnetic shape memory effect was observed in a stoichiometric Ni2MnGa sample at 135 K with the magnetic fields perpendicular to the sample surface. (C) 2004 American Institute of Physics.