Fundamental aspects of ferromagnetic shape memory effect

The ferromagnetic shape memory effect (FSME) appears as a magnetic field-induced twinning/detwinning in the martensitic phase, resulting in the recoverable strain of the order of martensitic spontaneous distortion.
The underlying physics as well as necessary and sufficient conditions of the effect are overviewed. A magnetostrictive mechanism of the giant magnetomechanical response observed in the Ni–Mn–Ga alloys is substantiated within phenomenological theory of ferromagnetic martensites. Alongside the phenomenological aspect of the FSME, the key role of lattice dynamics and electronic states in lattice instability are discussed. The fundamental aspect is illustrated by the experimental results related to the lattice, mechanical and magnetic instabilities in the prototype Ni-Mn-Ga alloys. The phenomena of the magnetoplasticity, magnetic field-induced superelasticity and strong linear dependence of the magnetization upon the twinning deformation of martensite will be reviewed.