Functional principle

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.

change of a magnetic shape memory alloy after rotation of an external magnetic field

Deformation of a magnetic shape memory alloy by moving a twin boundary by rotation of an external magnetic field.

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.

Straining of a 5M Ni-Mn-Ga single crystal observed in a polarization microscope. The contrast originates from variants with different orientation, hence the diagonal lines are twin boundaries which are moved by the applied field (up to 0.4 T). By a vertical magnetic field variants with their easy axis in this direction are favoured and grow on expense of the horizontally aligned variants. In this direction the unit cell is shorter, which results in a shrinking of the actuator. For reversible action a spring, perpendicular to the field, compresses the actuator to the original state once the field is turned off.

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