Transformation behaviour of freestanding epitaxial Ni-Mn-Ga films

We analyze the transformation behaviour of a 2 mu m thick epitaxial Ni-Mn-Ga film by combining temperature dependent measurements of magnetization, electrical resistance, X-ray diffraction (XRD) and tensile stress-strain characteristics. While the magnetization measurements hint for a simple austenite-martensite transformation below the Curie temperature at about 90 degrees C, resistivity measurements reveal a two-stage transformation in the temperature regimes (I) of 40-80 degrees C and (II) of 140-160 degrees C. XRD and pseudoplastic behaviour prove the presence of martensite well above the Curie point. The combination of four independent methods suggests that the transformation at (II) may originate from a weakly first order transformation followed by an intermartensitic transformation at (I). This interpretation is in line with the large superplastic strain observed for the tensile direction parallel to the [100] direction of the Ni-Mn-Ga unit cell. The strain increases from about 10% at 110 degrees C to 14% at room temperature suggesting an increase in tetragonal distortion. (C) 2012 Elsevier B.V. All rights reserved.