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 char-acteristic size of about 17 mu m 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 char-acteristic 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 mu m and 2 mu m for Type I and Type II twin boundaries, respectively.