In martensitic materials, the microstructure consists of differently oriented crystals, which are called martensitic variants. These are connected by twin boundaries as sketched in the figure below. When these materials are ferromagnetic, usually a specific crystallographic direction is favourable for the magnetisation. The arrows mark the magnetization which lies along this so called easy axis. Magnetically induced reorientation (MIR) can occur when additionally the twin boundaries are easily moveable, so an external magnetic field can move them. This results in a change of microstructure and external shape.
In Ni-Mn-Ga single crystals one can obtain an elongation up to 10% by MIR in moderate magnetic fields below 1 T. With this material class one can thus built efficient and compact actuators.
The twin boundary motion and the resulting strain can be observed directly in a polarization microscope when applying a magnetic field.