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Hysteresis and geometry: a way to search for new materials with unlikely physical properties

Friday, November 5, 2004 - 10:00am - 10:45am
EE/CS 3-176
Richard James (University of Minnesota, Twin Cities)
These thoughts begin with the observation by physicists, 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 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 co-existence 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 additional possibility of controlling the volume fraction of phases with fields or stress. Thus, one could imagine having a strong magnet; apply stress to it and it becomes a strong ferroelectric. The key point is reversibility.

Even big first order phase changes can be highly reversible (liquid water to ice, some shape memory materials), and we argue that it is the nature of the shape change that is critical. We suggest, based on a close examination of measured hysteresis loops in various martensitic systems, that reversibility is governed by the presence of certain special relations among lattice parameters. While these relations are naively geometric, their fundamental status is not clear, but they likely relate to a concept of metastability for an energy functional that includes both interfacial and bulk energy. Fundamentally, we lack the ability to formulate the appropriate concept of metastability because we do not really understand how to model interfacial energy, as we explain.

Acknowledgment: John Ball, Karin Rabe, Jerry Zhang.