A Way to Search for Multiferroic Materials with “Unlikely” Combinations of Physical Properties

The ideas presented in this chapter begin with the observation by physicists (Hill [1, 2] and Hill and Rabe [3]), probing new phenomena through the use of first principles studies, that the simultaneous occurrence of ferromagnetism and ferroelectricity is unlikely. While these studies do not usually consider the possibility of a phase transformation, there is a lot of indirect evidence that, if the lattice parameters are allowed to change a little, then one might have coexistence of “incompatible properties” like ferromagnetism and ferroelectricity. Thus, one could try the following: seek a reversible first-order phase transformation, necessarily also involving a distortion, from, say, ferroelectric to ferromagnetic phases. If it were highly reversible, there would be the interesting possibility of controlling the volume fraction of phases with fields or stress. The key point is reversibility. This chapter is an exploration of these ideas. To use these ideas as the basis for the search for new materials there are two major questions that need to be addressed: