Site-selective cobalt substitution in La-Co co-substituted magnetoplumbite-type ferrites: ⁵⁹Co-NMR and DFT calculation study

The La-Co co-substituted magnetoplumbite-type (M-type) ferrites AFe₁₂O₁₉ (A = Ca, Sr and Ba, ion sizes Ca²⁺ $<$ Sr²⁺ $<$ Ba²⁺) with Co compositions around 0.2 have been subjected to ⁵⁹Co-NMR. The results show that Co occupies the 4f₁, 2a and 12k sites, and that the smaller the A ion, the more Co tends to occupy the 4f₁ minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped AFe₁₂O₁₉ and a supercell (2 × 2 × 1 of the unit cell) in which 1/96 of Fe³⁺ is replaced by Co²⁺ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of A, Co is most stable when it occupies the 4f₁ site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f₂ sites. As the A ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f₁ site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the c-axis associated with the difference in A. However, the influence of the A ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La-Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small A ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f₁ site in tetrahedral coordination.