Effect of post-annealing on magnetic anisotropy in La–Co co-substituted M-type strontium ferrite

We investigated the effect of post-annealing on the magnetic anisotropy in La– Co co-substituted magnetoplubite-type strontium ferrite, a matrix phase of commercial high-performance ferrite magnets. Post-annealing was found to be effective in controlling the distribution of substituted Co2+. The Co2+ distribution obtained by lower-temperature annealing shows higher magnetic anisotropy, suggesting that uniaxial magnetic anisotropy is enhanced only when Co2+ occupies the most stable occupied site, i.e., the 4f1 site in the tetrahedral coordination. Furthermore, the slower the cooling rate, the higher the anisotropy. The temperature-dependent Co2+ distribution predicted from the reported density functional theory calculations was compared with the actual anisotropy after annealing, yielding +2.3 meV/ion as the Co2+ anisotropy constant at the tetrahedrally coordinated site and −1.2 meV/ion as that at the octahedrally coordinated sites. Careful selection of heat treatment conditions leads to efficient use of cobalt, an element with supply risks.