Magnetic-field induced shape memory effect in Ni₂MnGa sputtered films

The shape memory effect (SME) by magnetic field is interesting and important for physics and application. The Ni₂MnGa films were deposited on a poly-vinyl alcohol (PVA) substrate with a radio-frequency (RF) magnetron sputtering apparatus using Ni₅₂Mn₂₄Ga₂₄ or Ni_52.5Mn₂₂Ga_25.5 target. After separating from the substrate, the films were heat-treated at 1073 K for 36 ks for homogenization and ordering. They were constrained in a silica tube and heated at 673 K for 14.4 and 57.6 ks. The two-way SME for each constraint-aged film was confirmed by thermal cycling. The shape change of film under magnetic field was observed at constant temperature between A_s^* and A_f^*, or M_s^* and M_f^*. Magnetic field was applied parallel to the film surface up to 5 T using a super-conductor magnet. These films showed the SME by magnetic field. The strain by magnetic field was dependent on the measurement temperature. At temperatures between M_s^* and M_f^*, the strain increased with increasing magnetic field and slightly decreased with decreasing magnetic field. The martensitic transformation was induced by the magnetic field up to 5 T, which corresponds to that observed by cooling down to about 4 K.