Magnetic shape memory effect and highly mobile twin boundaries

The magnetic shape memory effect can be classified as an example of the multiferroic effect combining ferro-elasticity and (ferro) magnetism. After short overview of all known effects the focus is on magnetic field induced structure reorientation (MIR) in the martensite of Ni-Mn-Ga. In this material giant deformations of up to 12% have been observed in moderate magnetic fields. The phenomenology of the effect is first discussed and a model presented. The properties of Ni-Mn-Ga relevant to MIR are then considered. One necessary condition for MIR is a highly mobile twin boundary or interface between two differently oriented martensite variants (ferroelastic domains). In 10M modulated martensite, two types of mobile twin boundary (type I and type II) are observed with complex layered microstructures consisting of a hierarchy of twinning systems. The boundaries strongly differ according to the magnitude and temperature dependence of the twinning stress. Finally, the nature of these boundaries and their different behaviour is reviewed.