Effect of Microstructure in the Vicinity of Ferrite Grain Surface on Coercivity in Ca-La-Co M-type Ferrite Magnets

Yamamoto, Masaki; Kawata, Tsunehiro; Kobayashi, Yoshinori

doi:10.2497/jjspm.66.201

Cited by 1 publication

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“…In fact, 59 Co-NMR signals of various secondary phases have been observed in NMR experiments on La-Co co-substituted M-type ferrite magnets, making identification of the essential signal difficult. It is also known that Co segregates on the grain surface depending on the heat treatment conditions and plays a different role from that in the grain [27]. More importantly, the valence of transition metals in M-type ferrite is generally unstable.…”

Section: Samplementioning

confidence: 99%

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

Nakamura,

Ohta,

Kobayashi

et al. 2024

J. Phys. Mater.

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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.

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Section: Samplementioning

confidence: 99%

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

Nakamura,

Ohta,

Kobayashi

et al. 2024

J. Phys. Mater.

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Effect of Microstructure in the Vicinity of Ferrite Grain Surface on Coercivity in Ca-La-Co M-type Ferrite Magnets

Cited by 1 publication

References 8 publications

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

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

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Effect of Microstructure in the Vicinity of Ferrite Grain Surface on Coercivity in Ca-La-Co M-type Ferrite Magnets

Cited by 1 publication

References 8 publications

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

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

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

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