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.

Basic building blocks

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 or noble metals. The S block is essentially a slab cut along the $(111)$ 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, Sr,) rare earth metals, Pb, etc. The point group symmetry of the R block is $\barm2$. 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 $3m$. 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.

Family members

M-type ferrite

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 $P6_3/mmc$ 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

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 $P6_3/mmc$ space group symmetry. One example of W-type ferrite is BaFe₁₈O₂₇, with ''a'' = 5.88 Å and ''c'' = 32.85 Å.

R-type ferrite

R-type ferrite has a chemical formula of Me^LMe^S₆O₁₁ with a space group of $P6_3/mmc$. 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

Y-type ferrite has a chemical formula of Me^L₂Me^S₁₄O₂₂ with a space group of $R\barm$. 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

Z-type ferrite has a chemical formula of Me^L₃Me^S₂₆O₄₁ with a space group of $P6_3/mmc$. 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

X-type ferrite has a chemical formula of Me^L₂Me^S₃₀O₄₆ with a space group of $R\barm$. The stacking order is 3(SRS*S*R*) in one unit cell. One example is Sr₂Co₂Fe₂₈O₄₆ with ''c'' = 83.74 Å.{{Cite journal , last=Wu , first=Meixia , title=Single-Crystal Growth and Room-Temperature Magnetocaloric Effect of X-Type Hexaferrite Sr₂Co₂Fe₂₈O₄₆ , url=https://doi.org/10.1021/acs.inorgchem.9b03724 , journal=Inorganic Chemistry, year=2020 , volume=59 , issue=10 , pages=6755–6762 , doi=10.1021/acs.inorgchem.9b03724 , pmid=32364708 , osti=1631933 , s2cid=218504683

References

Ferromagnetic materials
Ceramic materials