Impact of post-heat treatment on functionality of multiple-site compounds: A case study of magnetic anisotropy in La–Co co-substituted magnetoplumbite-type ferrite

Abstract

In compounds with multiple crystallographically inequivalent transition metal sites, the transition metals occupying each site may play different roles. Therefore, when aiming to improve functionality through element substitution, particularly when the substituent is distributed across multiple sites, differences in the substituent’s distribution among sites may significantly affect the material properties. This study examined this phenomenon in magnetoplumbite-type (M-type) ferrites, representative multiple-site compounds. It is known that co-substitution of La and Co in M-type ferrites enhances uniaxial magnetic anisotropy. Furthermore, experimental and DFT calculation results have shown that Co occupies multiple crystallographic sites. Additionally, only Co occupying the tetrahedrally coordinated 4f₁ site has been shown to contribute to the enhancement of uniaxial anisotropy. This study compared the effects of two post-heat treatments, immediate water quenching and slow cooling (with a cooling rate of −50 K/h), on the magnetic anisotropy of the La–Co co-substituted BaFe₁₂O₁₉ system. The results revealed that the anisotropy field value at room temperature for the slowly cooled sample with the highest Co substitution rate was 22.4 % higher than that of the water quenched sample. Drawing on DFT calculation results, this increase can be attributed to the relative rise in Co ions occupying 4f₁ sites under slow cooling conditions. These results demonstrate that controlling cation distribution by adjusting post-heat treatment conditions is a viable option for tuning the functionality of multi-site compounds.