A multiferroic material is a material with at least two of the three properties: ferroelasticity, ferroelectricity or ferromagnetism. A natural way to achieve multiferroism is to seek a martensitic phase transformation with a change of lattice parameters (leading to ferroelasticity) and either ferromagnetism or ferroelectricity. Often, in such cases, the ferroelectricity or ferromagnetism can be turned on and off because these properties are sensitive to a change of lattice parameters. Such changes in ferroic properties during a martensitic phase transformation can be utilized to convert heat directly into electricity, without the need of a separate electric generator.We have successfully demonstrated this concept in 2011 on the Heusler alloy Ni45Co5Mn40Sn10. Questions like what materials are most suitable for this energy conversion method, how best to design a device based on this mechanism, or what significantly affects the performance of this method become interesting and urgent. My talk addresses these questions from a thermodynamic point of view, using magnetism as an example. The ideas used are to some extent independent of the ferroic property, and suggest a broad area of study of solid-state energy conversion devices, in which a fraction of the latent heat of a first order phase transformation is converted directly to electricity by methods of induction or charge separation.
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