Martensitic transformation and shape memory effect of Ni-rich Ni 2MnGa sputtered films under magnetic field

Abstract

The inference of a magnetic field on the martensitic transformation (MT) temperatures and shape memory effect (SME) was investigated for the Ni-rich Ni 2MnGa sputtered films. The compositions of the films were Ni 51.4Mn 28.3Ga 20.3, Ni 54.1Mn 23.0Ga 22.9 and Ni 54.4Mn 21.3Ga 24.3. The films were heat treated at 1073 K for 3.6 ks and aged at 673 K for 3.6 ks in a constrained condition. Hereafter, the constraint-aged films were described as N51.4(AG), N54.1(AG) and N54.4(AG) using nickel content, respectively. The reversible two-way SME by the thermal change was confirmed for the all constraint-aged films through the MT and its reversion. 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 curves shifted to a high temperature region and the martensitic phase was stabilized by the magnetic field. The strain at the outer surface was given by ε= d/2 r, where d was the thickness of the films and r was the radius of a curvature for the films. The increase of MT temperatures caused by a magnetic field was about 1 K/T. Furthermore, the two-way SME by the magnetic field was observed around the MT temperature on cooling for the N54.4(AG) film with the large gradient and small thermal hysteresis in the strain–temperature curve.