Ferroic materials and anomalous strains

Ferroics are materials which become ferro-elastic, -magnetic, -electric at lower temperatures on cooling. They usually show a phase transformation which produces an ordered state (low entropy phase). These properties are partially due to a domain structure. The strain effects are found either by cycling around the transformation temperature (two-way effect, invar), by mechanical straining plus heating above transformation temperature (one-way memory) or by cyclic mechanical loading (pseudo- or rubber elasticity).

Ferromagnetic or -electric materials can be shifted into different shapes by magnetic or electrical fields instead of mechanical stresses. In combination with martensitic transformation considerable strains are obtained in some ferromagnetic alloys: $Ni_2MnGa$, multiferroics.

Polymers can show one-way behaviour like metals, ferro- and piezo-electrical behaviour like ceramics. The largest amounts or reversible strains are obtained with polymers, the smallest with ceramics. Porous charged polymers show hysteresis curves like ceramic ferroelectrics, but larger amounts of strain: ferro electrets. The coiled high entropy conformation corresponds to the high temperature $\beta$-phase (austenite) in alloys, the stretched state to martensite $\alpha$. An entropic force is the cause for a $\alpha\rightarrow\beta$ reverse transformation and the origin of shape memory.

A systematic understanding of this group of materials permits to compare their abilities and limits. This in turn provides a base for the selection of the best material for certain applications.