Mechanical and shape memory properties of ferromagnetic Ni$_2$MnGa sputter-deposited films

The ternary intermetallic compound Ni$_2$MnGa is an intelligent material, which has a shape memory effect and a ferromagnetic property. Use of shape memory alloy films for an actuator of micro machines is very attractive because of its large recovery force. The data of mechanical and shape memory properties of the films are required to use for the actuator. The purpose of this study is to investigate the effects of fabrication conditions and to clarify the relationships between these properties and fabrication conditions of the Ni$_{2}$MnGa films. The Ni$_{2}$MnGa films were deposited with a radio-frequency magnetron sputtering apparatus using a Ni$_{50}$Mn$_{25}$Ga$_{25}$ or Ni$_{52}$Mn$_{24}$Ga$_{24}$ target. After deposition, the films were annealed at $873\sim 1173$ K. The asdeposited films were crystalline and had columnar grains. After the heat treatment, the grains widened and the grain boundary became indistinct with increasing heat treatment temperature. MnO and Ni$_{3}$ (Mn, Ga) precipitations were observed in the heat-treated films. The mechanical properties of the films were measured by the nanoindentation method. Hardness and elastic modulus of as-deposited films were larger than those of arcmelted bulk alloys. The hardness of the films was affected by the composition, crystal structure, microstructure and precipitation, etc. The elastic modulus of the films was also changed with the heat treatment conditions. The heat-treated films showed a thermal two-way shape memory effect