Fabrication And Characteristic Of Ferromagnetic Shape Memory Alloy Ni-Mn-Ga Thin Films

Ferromagnetic Shape Memory Alloys (FSMAs) is a new type of intelligent material. Their notable character is shape memory effect which can be induced by magnetic field, and their martensite has large reversible magnetic-field-induced strain. However, the bulk has very high brittleness and is difficult to be machined to small parts. In order to be applied in MEMS such as micro actuators and micro sensors, Ni-Mn-Ga alloys have to be in the form of thin films.In this text, Ni-Mn-Ga thin films were prepared by MW-ECR RF magnetron sputtering on Ni-Mn-Ga bulks, Cu foils and Si wafers. And then the effect of preparation parameters to the films' composition and appearance were discussed. The results are given as below:1. Two main factors that affect films composition are sputtering power and deposition pressure. When the target and substrate distance is 60mm and the air pressure was 0.4Pa, the composition of Ni and Mn increase with the power grows, and Ga decreases. The composition of Ni and Ga are affected clearly by air pressure.2. Subsequently films were annealed in vacuum heat treatment device. Following the increasing of the heating time at 600℃, the Ni content decreases slowly, Mn increases in line with the aging time, and the Ga decreases first then increases little. While the heating time is 60min and the heating temperature between 400℃and 600℃, the Ni content isn't affected as much as Ga and Mn. As the temperature further increases, three elements change slowly.3. The surfaces of the films deposited on silicon wafer are much smoother than others, and in some images can see large grains with the largest diameter about 2μm. After annealed in 400℃about 60min, the film crystallized. With the increasing of the heating temperature, crystal grains grow larger.4. By XRD analyze, the annealed films are polycrystal with main grain orientation (022) (400) (214). The Manganosite phase was found at one time.5. Heating treatment affects the film ferromagnetism greatly. Films have no ferromagnetism until the heating time above 500℃.6. The strain and magnetic field intensity curve shows that the strain grows with the increasing of magnetic field intensity. The largest strain of heated films on copper foil, Si wafer and Ni-Mn-Ga bulk are -98ppm,-85ppm and -142ppm respectively with the field about 4.0KOe.