Large cyclic magnetic-field-induced deformation in orthorhombic (14M) Ni–Mn–Ga martensite

Magnetomechanical experiments were performed with a ferromagnetic Ni–Mn–Ga single crystal consisting of thermoelastic orthorhombic martensitic phase at room temperature. The crystal was deformed in uniaxial compression along 〈100〉 with an orthogonal magnetic field and without a magnetic field. The sample deforms due to motion of twin boundaries. When compressed with a magnetic field, twinning occurs at higher stress than without a magnetic field, and the twinning is reversible upon unloading. The stress–strain curves exhibit two plateaus which are related to two different twinning systems, namely (110)[11̄0] and (011)[011̄]. During cyclic experiments in a rotating magnetic field, the magnetic-field-induced strain increases from initially 6% to 9.7%. The latter value was repeatedly measured upon more than 1000 rotations of the field. The increase of magnetic-field-induced strain during magnetomechanical cycling is related to a transition from combined partial (110)[11̄0] and (011)[011̄] twinning to complete (101)[101̄] twinning. The magnetic anisotropy constants are obtained from an analysis of the magnetomechanical properties.