Influence of Constraints and Twinning Stress on Magnetic Field-Induced Strain of Magnetic Shape-Memory Alloys

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Magnetic-field-induced strain of hard 14M and soft 10M martensitic samples of Ni–Mn–Ga single crystals was measured and optically observed in a rotating magnetic field in one- and two-end-constrained conditions. The soft sample showed smooth, continuous deformation of 0.6% when two-end-constrained and 2.7% when one-end-constrained. The hard sample produced no strain when two-end-constrained, and discontinuous deformation of 1.8% when one-end-constrained. Thus, the constraints reduce, and even block, twin boundary motion. This finding must be considered in experiment and applications.