6.11.2 Hexagonal Ferrites

Hexagonal ferrites are based on hexagonal magnetoplubite and are often called M-type ferrites. The model compound of this family is barium ferrite with formula BaFe₁₂O₁₉. The large hexagonal unit-cell contains 64 atoms, i.e. two formula units. The structure is basically a mixture of cubic closed packed and hexagonal closed packed layers formed by barium and oxygen ions. Chemical substitution of Ba  sites is usually done with Sr atoms while Fe atoms are substituted by Al atoms, based on the size and valence, resulting in a change in the magnetic behavior.

Out of 12 iron atoms of one formula unit, 9 occupy the octahedral sites, two occupy tetrahedral sites and the remaining one is 5-fold coordinated.  Out of these, 7 atoms on the octahedral site and 1 with 5-fold coordination have their spins in one direction while spins of the rest of the atoms are oriented oppositely i.e. say 8 atoms with spins up and 4 atoms with spin down. As we saw earlier, each Fe³⁺ ion has spin magnetic moment of 5μ_B simple math gives a net magnetic moment of 20μ_B  per formula unit leading to a magnetic moment of 40μ_B  per unit cell.

This material has a high degree of magnetic anisotropy and it magnetizes relatively easily along [0001]-direction or c-axis of its unit cell. The material is typically categorized as a hard ferrite with coercivity between 50-100 kA.m^­1 depending upon the microstructure and composition.