Magnetoplasticity

Magnetocrystalline anisotropy and twinned microstructure are at the origin of the appreciable magnetic-field-induced strains found in some ferromagnetic thermoelastic martensites. The change of magnetization across the twinning plane leads to a force on the twinning dislocations when a magnetic field is applied. The sensitivity of this basic microscopic mechanism not only to crystallographic and magnetic properties of the material, but also to microstructural details, leads to a large variance in the macroscopically attainable magnetic-field-induced strains, while the maximum strain is fixed for any given martensite structure. The magnetic-field-induced deformation of Ni–Mn–Ga single crystals was studied in uniaxial (“static”) and rotating (“dynamic”) magnetic fields. The spatial and orientational distribution of martensite domains, twin thickness and mutual interactions between twinning dislocations all contribute to the macroscopic result.