Influence of constraints and twinning stress on magnetic field-induced strain of magnetic shape-memory alloys

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.