Microstructure of freestanding single-crystalline Ni2MnGa thin films

The complex crystal structure and variant distribution of single-crystalline freestanding Ni–Mn–Ga films was studied in detail using X-ray diffraction in two- and four-circle geometry. Selective chemical etching of a chromium buffer layer facilitates the release of the rigid MgO substrate that would inhibit magnetically induced reorientation (MIR) of variants. The substrate-constrained as well as the freestanding films possess identical crystal structure featuring a seven-layered (pseudo-)orthorhombic modulation (7 M/14 M). Of the 12 different variants observed before and after releasing the film from the substrate, four are predominantly represented. These have the short c-axis and the long a-axis aligned in the film plane. This allows for a reduction of the stray field energy, since the c-axis is the magnetic easy axis. The variant distribution induces a distinct mesoscopic surface pattern that can be observed by light microscopy. On a freestanding microbridge, prepared by photolithography in combination with selective chemical etching, the phase transformation can thus be traced directly. After several heating cycles the surface pattern remains basically unchanged. Application of a magnetic field of 0.5 T while the film transforms from austenite to martensite leads to a considerable change of the structure.