Hexagonal ferrite

Hexagonal ferrites or hexaferrites are a family of ferrites with hexagonal crystal structure. The most common member is BaFe₁₂O₁₉, also called barium ferrite, BaM, etc. BaM is a strong room-temperature ferrimagnetic material with high anisotropy along the c axis.^[1] All the hexaferrite members are constructed by stacking a few building blocks in a certain order.

Basic building blocks

S block structure extracted from an M-type ferrite.

The A layer of S block composed of Me^S-centered tetrahedron and Me^S-centered octahedron.

R block structure extracted from an M-type ferrite.

R block structure, view along -c ferrite direction.

The A layer of S block, view along -c ferrite direction.

The B layer of S block made up of edge-sharing Me^S-centered octahedron.

T block structure extracted from a Y-type ferrite.

T block structure, view along -c ferrite direction.

The B layer of S block, view along -c ferrite direction.

S block

The S block is very common in hexaferrites, which has a chemical formula of Me^S₆O₈²⁺. Me^S are smaller metal cations, for example, Fe and other transition metals^[2] or noble metals.^[3] The S block is essentially a slab cut along the ${\displaystyle (111)}$ plane of an AB₂O₄ spinel. Each S block has one A layer and one B layer. The A layer features Me^S-centered tetrahedron and Me^S-centered octahedron, while the B layer is made up of edge-sharing Me^S-centered octahedron. Both A and B layers have the same chemical formula of Me^S₃O₄²⁺.

R block

The R block has a chemical formula of Me^LMe^S₆O₁₁^2-. Me^L are larger metal cations, for example, alkaline earth metals (Ba,^[4] Sr,^[5]) rare earth metals,^[5] Pb,^[6] etc. The point group symmetry of the R block is ${\displaystyle {\bar {6}}m2}$. The large metal cations are located in the middle layer of the three hexagonally packed layers. This block is also composed of face-sharing Me^S-centered octahedra and Me^S-centered trigonal bipyramids.

T block

The T block has a chemical formula of Me^L₂Me^S₈O₁₄^2-. The point group symmetry of the T block is ${\displaystyle 3m}$. One T block consists of 4 oxygen layers with the two Me^L atoms substituting two oxygen atoms in the middle two layers. In one T block, there are both Me^S-centered octahedra and Me^S-centered tetrahedra.