  In ferromagnetic alloys with shape memory large reversible strains can be
obtained by rearranging the martensitic domain structure by a magnetic field.
Magnetization through displacement of domain walls is possible in the presence
of high magnetocrystalline anisotropy, when martensitic structure rearrangement
is energetically favorable compared to the reorientation of magnetic moments.
In ferromagnetic Heusler alloys Ni$_{2+x}$Mn$_{1-x}$Ga the Curie temperature
exceeds the martensitic transformation temperature. The fact that these two
temperatures are close to room temperature offers the possibility of
magnetically controlling the shape and size of ferromagnets in the martensitic
state. In Ni$_{2+x}$Mn$_{1-x}$Ga single crystals, a reversible strain of $\sim
6$% is obtained in fields of $\sim 1$ T.
