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.