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.

All the hexaferrite members are constructed by stacking a few building blocks in a certain order.

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 or noble metals. 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⁺.

The R block has a chemical formula of Me^LMe^S₆O₁₁^2-. Me^L are larger metal cations, for example, alkaline earth metals (Ba, Sr,) rare earth metals, Pb, 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.

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.

M-type ferrite is made up of alternating S and R blocks in the sequence of SRS*R*. (* denotes rotating that layer around the c axis by 180°.) The chemical formula of M-type ferrite is Me^LMe^S₁₂O₁₉. Common examples are BaFe₁₂O₁₉, SrFe₁₂O₁₉. It exhibits ${\displaystyle P6_{3}/mmc}$space group symmetry. For BaFe₁₂O₁₉, a = 5.89 Å and c = 23.18 Å. M-ferrite is a very robust ferrimagnetic material, thus widely used as fridge magnets, card strips, magnets in speakers, magnetic material in linear tape-open.

W-type ferrite, like the M-type, consists of S and R blocks, but the stacking order and the number of blocks are different. The stacking sequence in a W-ferrite is SSRS*S*R* and its chemical formula is Me^LMe^S₁₈O₂₇. It exhibits ${\displaystyle P6_{3}/mmc}$space group symmetry. One example of W-type ferrite is BaFe₁₈O₂₇, with a = 5.88 Å and c = 32.85 Å.

R-type ferrite has a chemical formula of Me^LMe^S₆O₁₁ with a space group of ${\displaystyle P6_{3}/mmc}$. Unlike other hexaferrites, R-type ferrite doesn't have an S block. Instead, it only has single B layers extracted from the S block. The stacking sequence is BRB*R*.

Y-type ferrite has a chemical formula of Me^L₂Me^S₁₄O₂₂ with a space group of ${\displaystyle R{\bar {3}}m}$. One example is Ba₂Co₂Fe₁₂O₂₂ with a = 5.86 Å and c = 43.5 Å. Y-type ferrite is built up with S and T blocks with an order of 3(ST) in one unit cell. There is no horizontal mirror plane in a Y-type ferrite.

Z-type ferrite has a chemical formula of Me^L₃Me^S₂₆O₄₁ with a space group of ${\displaystyle P6_{3}/mmc}$. It has a complicated stacking of SRSTS*R*S*T* in one unit cell. Some Z-type members may have sophisticated magnetic properties along different directions. One example is Ba₃Co₂Fe₂₄O₄₁ with a = 5.88 Å and c = 52.3 Å.

X-type ferrite has a chemical formula of Me^L₂Me^S₃₀O₄₆ with a space group of ${\displaystyle R{\bar {3}}m}$. The stacking order is 3(SRS*S*R*) in one unit cell. One example is Sr₂Co₂Fe₂₈O₄₆ with c = 83.74 Å.