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Abstract:
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. 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 an idea based on ``good fitting of
the phases' governs reversibility. Besides ferroelectricity -- ferromagnetism,
there are many potential property pairs that exhibit lattice parameter
sensitivity and are candidates for the proposed strategy: soluability for H2,
optical nonlinearity, high band gap -- low band gap semiconductors, insulator --
conductor (electrical or thermal), opaque -- transparent (at various
wavelengths), high -- low index of refraction, luminescent -- nonluminescent,
and new kinds of thermoelectric and thermomagnetic materials. Joint work with
Karin Rabe.
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