Ferrite Materials Containing Kagomé Layers: Chemistry of Ba₂Fe₁₁Ge₂O₂₂and K₂Co₄V₉O₂₂ Hexaferrites

Hexaferrites have a wide range of technological applications as well as a rich and complicated structural chemistry, with iron oxide layers inducing highly anisotropic ferrimagnetism that remains intact to high temperatures (>400 K). Most hexaferrites (i.e., M-type BaFe12O19) contain the dominant magnetic vectors normal to the hexagonal planes forming hard ferrimagnets, while the much less common Y-type investigated here typically contains the ferrimagnetic vectors in the iron oxide planes, creating soft ferrimagnets suitable for RF applications. To study the structural chemistry and magnetism of this less common hexaferrite phase, single crystals of K2Co4V9O22 (I) and Ba2Fe11Ge2O22 (II) were prepared. The structure of these compounds is a derivative of the sophisticated mineral greenwoodite, having a complex assembly of transition metal octahedra and tetrahedra, notably featuring Kagomé layers in spinel-type blocks that are magnetically isolated from one another. In particular, compound II provides a pathway to developing iron-rich hexaferrites where the magnetic ions are not diluted by site substitution of nonmagnetic ions. This results in an exceptionally high magnetic ordering temperature of 855 K to a canted antiferromagnetic state.