Magnetic-Field Induced Two-Way Shape Memory Effect of Ferromagnetic Ni2MnGa Sputtered Films

The shape memory effect (SME) induced by the magnetic field is interesting and important for physics and application. The ferromagnetic Ni2MnGa films with various compositions were deposited on a poly-vinyl alcohol (PVA) substrate with a radio-frequency (RF) magnetron sputtering apparatus using four kinds of Ni–Mn–Ga targets. After separating from the substrate, the films were heat-treated at 1073 K for 3.6 ks for homogenization and aged at 673 K for 3.6 ks in a constraint condition. The reversible two-way SME by the thermal change was confirmed for the constraint-aged films with various compositions. The gradient of the strain– temperature curve, the amount of strain accompanied by the two-way SME and the width of thermal hysteresis were dependent on the composition of the films. The strain–temperature curve shifted to a high temperature region and the martensitic phase was stabilized by the applied magnetic field. Furthermore, the two-way SME by the magnetic field was observed around the martensitic transformation temperature on cooling for the constraint-aged film, which showed the large gradient and small thermal hysteresis in the strain–temperature curve.