Shape memory effect of sputtered Ni-rich Ni₂MnGa alloy films aged in constraint condition

The intermetallic compound Ni₂MnGa has a shape memory effect and a ferromagnetic property. Films of this compound may be applied for an actuator of micro-machines. The Ni-rich Ni₂MnGa films with a nearly 5 μm thickness were deposited on an Al₂O₃ substrate by a radio-frequency magnetron sputtering apparatus with a Ni₅₂Mn₂₄Ga₂₄ target. The sputtering power was 400 W and the substrate temperature was kept at 323 K by cooling water. The composition of the deposited films was Ni-23.4 mol%Mn-23.0 mol%Ga. They were annealed at 1073 K for 36 ks for homogenization and ordering, and slowly cooled down in the furnace. Martensitic transformation temperatures of the films were measured by a differential scanning calorimeter, and the results were M_s = 315 K, M_f = 311 K, A_s = 316 K and A_f = 321 K. To induce a shape memory effect, the annealed films were aged at 573-773 K for 0.9-3.6 ks in a constrained condition. It was found that they showed the two-way shape memory effect by thermal cycling.