Inclined SEM micrograph of both Ni–Mn–Ga micropillars before load application. Both micropillars have dimensions of . Cartesian coordinates are defined as indicated, i.e., , , and parallel to the short, intermediate, and long edges of the pillars.
In situ SEM micrograph of micropillar 1 being compressed by a truncated cone-shaped microindenter. Note that there are image distortions due to charging of the diamond tip.
Deformation curves for micropillar 1 (solid line, larger loop) and micropillar 2 (dotted line, smaller loop). The arrows indicate the onset of twinning events. At some large twinning events, the slope of the stress-strain curve appears negative because deformation advances so fast that the feedback loop control reduces the applied load to minimize overshooting of the displacement. Besides the linear elastic recovery, there is a nonlinear strain recovery of about 1% for both micropillars. The recovery is assumed to stem from pseudoelastic twinning. The permanent strains are 3.6% for pillar 1 and 0.7% for pillar 2).
(a) AFM height image and (b) MFM magnetic image of the face of micropillar 1. The scan encompasses most of the top area of the micropillar. The left and right edges of the scan leave out approximately of material on each side. The top edge of the scan shows the top end of the micropillar. Twins are visible on the upper section of both images. The boxed area is shown in higher magnification in Fig. 5.
Detailed view of the boxed area in Fig. 4. (a) AFM height image of the surface relief and (b) MFM magnetic image. The strong contrast indicates out-of-plane magnetization, while the weak (neutral) contrast indicates in-plane magnetization. The direction of magnetization is parallel to the crystallographic direction. Every ridge [bright line in (a)] and valley [dark line in (a)] matches with a change in magnetic pattern in (b). (c) A schematic of the MFM magnetic image where the direction is labeled for each magnetic domain. (d) A profile of the surface with a surface relief angle between twins of . The position of the cross section (d) is marked with a white line in (a). The zero position of the cross section corresponds to the on the white line.
(a) AFM height image and (b) MFM magnetic image of the face of micropillar 2. The scan encompasses most of the micropillar. The left and right edges of the scan leave out approximately of material on each side. The top edge of the scan shows the top end of the micropillar. The surface is completely flat with no evidence of twinning.
Schematic cross section of the twin of micropillar 1. The twin sequence is (the letters indicate which crystallographic direction is perpendicular to the surface) and the surface relief angle is . From the left boundary marking a ridge, the right boundary marking a valley, and the intermediate twin being of type A follows that the twin boundaries are inclined to the lower left.
Side view schematic of twins in the micropillar 1 where there is a surface relief on the front and back sides of the micropillar and the twin boundaries (dashed lines) are angled downward and to the right. The sample before compression is shown with dotted lines.
Side view schematic of micropillar 2 at maximum load before unloading. The micropillar deformed via twinning. The twin boundaries (dashed lines) start at the micropillar and extend into the bulk. The pillar before compression is shown with dotted lines.
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