Hexagonal ferrite explained

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

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

( 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, Pb,^[6] etc. The point group symmetry of the R block is

\bar{ 6 }m 2 . 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

3 m . 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 nembers

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

P 6 _{3/mmc} space group symmetry. For BaFe₁₂O₁₉, a = 5.89 Å and c = 23.18 Å.^[7] 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

P 6 _{3/mmc} space group symmetry. One example of W-type ferrite is BaFe₁₈O₂₇, with a = 5.88 Å and c = 32.85 Å.^[8]

R-type ferrite

R-type ferrite has a chemical formula of Me^LMe^S₆O₁₁ with a space group of

P 6 _{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\bar{ 3 }m . One example is Ba₂Co₂Fe₁₂O₂₂ with a = 5.86 Å and c = 43.5 Å.^[9] 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

P 6 _{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.^[10] 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\bar{ 3 }m . The stacking order is 3(SRS*S*R*) in one unit cell. One example is Sr₂Co₂Fe₂₈O₄₆ with c = 83.74 Å.^[11]

References

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2. Ishiwata . Shintaro . Terasaki . Ichiro . Azuma . Masaki . Takano . Mikio . 2008-05-01 . High pressure synthesis and structure of a new magnetoplumbite-type cobalt oxide SrCo₁₂O₁₉ . Journal of Solid State Chemistry . en . 181 . 5 . 1273–1278 . 10.1016/j.jssc.2008.02.024 . 2008JSSCh.181.1273I . 0022-4596.
3. Foo . M. L. . Huang . Q. . Lynn . J. W. . Lee . Wei-Li . Klimczuk . Tomasz . Hagemann . I. S. . Ong . N. P. . Cava . R. J. . 2006-02-01 . Synthesis, structure and physical properties of Ru ferrites: BaM Ru₅O₁₁ (M=Li and Cu) and BaM′₂Ru₄O₁₁ (M′=Mn, Fe and Co) . Journal of Solid State Chemistry . en . 179 . 2 . 563–572 . 10.1016/j.jssc.2005.11.014 . 0022-4596.
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