Polar Magnets in Double Corundum Oxides

Polar magnets are promising materials for multiferroic and magnetoelectric applications in spintronic devices, owing to the coexistence of electrical polarization and magnetization and hence magnetoelectric effect. However, the design and preparation of polar magnets is a challenge due to the incompatible requirements for ferroelectricity and ferromagnetism on chemical bonding and electronic configurations. Corundum derivatives can incorporate magnetic ions into all the octahedral metal sites to enhance magnetic interactions accompanied by large spontaneous polarization if polar polymorph structures are adopted, providing an ideal platform for polar-magnet design. Considering the variable cationic combinations in A 2 BB′O 6 -type double corundum derivatives, a large number of new polar magnets are anticipated. However, so far only 14 compounds in this family have been reported, including 11 experimentally prepared and 3 theoretically predicted. The crystal structure types and physical properties of these compounds are found to largely depend on the synthesis conditions, chemical–geometrical factors, electronic configurations of cations, and spin structures, or a combination of any of the above. Therefore, it is essential to review the crystallization and transforming rules of known polar magnets and guide further exploration of new multifunctional materials in double corundum derivatives.