Phase transformations in ferromagnetic NiMnGa shape memory films

The paper presents recent work on ferromagnetic NiMnGa films sputter deposited on silicon cantilever substrates. The films are partially crystalline if the substrate is held at room temperature (RT) during the deposition. They become fully crystalline after annealing at 400 degrees C. The internal friction (IF) of the RT deposition shows a peak that is attributed to the nanostructure of the him. Films deposited at RT and annealed and films deposited an substrates held above 385 degrees C show a transformation comparable to NiTi shape memory films. However, the damping is one order of magnitude lower. Annealing does not significantly change the phase transformation of films that were deposited at 385 degrees C or above. Measurements of the temperature dependence of the change of the elastic modulus as a function of the magnetic field of a nominally Ni50Mn25Ga25 film suggest that the magnetic anisotropy held increases proportional to the transformation strain, (c/a - 1), from 0.3 T at 50 degrees C to 0.8 T at -150 degrees C.