Increase of twinning stress in single crystalline Ni-Mn-Ga micropillars

We have investigated the effect of characteristic size decrease on the magneto-mechanical properties of Ni-Mn-Ga single crystalline 10M martensite, using micropillars prepared by focused-ion-beam milling. Following the electro-chemical surface treatment, the twinning stress in a focused-ion-beam milled micropillar with a characteristic size of about 17 µm was reduced from 17 MPa to below 2 MPa, allowing the magnetically induced reorientation of twin variants. We found that twinning stress universally increases with the decrease of characteristic size. In combination with the data for bulk material, our results suggest a power law dependence of twinning stress on characteristic size, with an exponent of -0.5 for both Type I and Type II single twin boundaries. Extrapolation of the fit suggests the characteristic size limit for magnetically induced reorientation to be 30 µm and 2 µm for Type I and Type II twin boundaries, respectively.