Room Temperature Magnetically Ordered Polar Corundum GaFeO₃ Displaying Magnetoelectric Coupling

The
polar corundum structure type offers a route to new room temperature
multiferroic materials, as the partial LiNbO3-type cation
ordering that breaks inversion symmetry may be combined with long-range
magnetic ordering of high spin d5 cations
above room temperature in the AFeO3 system.
We report the synthesis of a polar corundum GaFeO3 by a
high-pressure, high-temperature route and demonstrate that its polarity
arises from partial LiNbO3-type cation ordering by complementary
use of neutron, X-ray, and electron diffraction methods. In situ neutron
diffraction shows that the polar corundum forms directly from AlFeO3-type GaFeO3 under the synthesis conditions. The A3+/Fe3+ cations are shown to be more
ordered in polar corundum GaFeO3 than in isostructural
ScFeO3. This is explained by DFT calculations which indicate
that the extent of ordering is dependent on the configurational entropy
available to each system at the very different synthesis temperatures
required to form their corundum structures. Polar corundum GaFeO3 exhibits weak ferromagnetism at room temperature that arises
from its Fe2O3-like magnetic ordering, which
persists to a temperature of 408 K. We demonstrate that the polarity
and magnetization are coupled in this system with a measured linear
magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling
is a prerequisite for potential applications of polar corundum materials
in multiferroic/magnetoelectric devices.