Twin boundary energy of hierarchically twinned magnetic shape memory alloys

Magnetic shape memory alloys exhibit a hierarchy of twins within twins microstructures, covering all length scale from nanometer to millimeter. Modulated structures as nano-twinned adaptive martensite are the first generation of twinning. Using epitaxial Ni-Mn-Ga films as model system, we observe a regular twinning of 14M modulated variants, which represent the second generation of twinning hierarchy. We analyzed a thickness series from 30 nm to 2 mu m and found a square-root dependency of the twinning period on film thickness. From the Landau-Lifshitz-Kittel scaling law, the twin boundary energy between mesoscopic 14M modulated variants is estimated the value of 0.7 eV/angstrom(2). (C) 2011 American Institute of Physics. [doi:10.1063/1.3635382]