Giant magneto-elastic coupling in multiferroic hexagonal manganites

In order to investigate a possible structural change of RMnO$_3 $ at the magnetic transition temperature, we have carried out high-resolution structural studies using neutron diffraction. Here we show that the hexagonal manganites RMnO$_3$ undergo an isostructural transition at T$_N$ with unusually large atomic displacements: two orders of magnitude larger than those seen in any other ordinary materials, resulting in a uniquely strong magneto-elastic coupling. For the first time, we could follow the exact atomic displacements of all the atoms in the unit cell as a function of temperature and found consistency with theoretical predictions based on group theories. We argue that this gigantic magneto-elastic coupling of RMnO$_3$ arises from geometrical frustration, and holds the key to the recently observed magnetoelectric phenomenon in this intriguing class of materials.