  Multiferroic rare earth manganites attracted recent attention because of the
coexistence of different types of magnetic and ferroelectric orders resulting
in complex phase diagrams and a wealth of physical phenomena. The coupling and
mutual interference of the different orders and the large magnetoelectric
effect observed in several compounds are of fundamental interest and bear the
potential for future applications in which the dielectric (magnetic) properties
can be modified by the onset of a magnetic (dielectric) transition or the
application of a magnetic (electric) field. The physical mechanisms of the
magnetoelectric effect and the origin of ferroelectric order at magnetic
transitions have yet to be explored. We discuss multiferroic phenomena in the
hexagonal HoMnO3 and show that the strong magneto-dielectric coupling is
intimately related to the lattice strain induced by unusually large spin-phonon
correlations.
