Room Temperature Magnetically Ordered Polar Corundum GaFeO Displaying Magnetoelectric Coupling.

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin d cations above room temperature in the AFeO system. We report the synthesis of a polar corundum GaFeO by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO-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 AlFeO-type GaFeO under the synthesis conditions. The A/Fe cations are shown to be more ordered in polar corundum GaFeO than in isostructural ScFeO. 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 GaFeO exhibits weak ferromagnetism at room temperature that arises from its FeO-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.