# Development of Multi-Cation-Doped M-Type Hexaferrite Permanent Magnets
## 4 Conclusions  
Three series of samples with the chemical formulas of SrFe\({}_{12-2}\)Si\({}_{x}\)Mg\({}_{x}\)O\({}_{19}\) (\(x\) = 0, 0.05, 0.1, 0.2), Sr\({}_{0.4}\)Ca\({}_{0.3}\)La\({}_{0.3}\)Fe\({}_{11.7-y}\)Co\({}_{0.2}\)Si\({}_{0.05}\)Mg\({}_{0.05}\)O\({}_{19}\) (\(y\) = 0, 1, 1.5, 2), and Sr\({}_{0.4}\)Ca\({}_{0.3}\)La\({}_{0.3}\)Fe\({}_{10.2}\) Co\({}_{0.2-z}\)Mn\({}_{z}\)Si\({}_{0.05}\)Mg\({}_{0.05}\)O\({}_{19}\) (\(z\) = 0, 0.05, 0.1, 0.2) were prepared sequentially, and their hard-magnetic properties were evaluated to determine the optimal composition. The optimal  
Figure 7: Plots of magnetization (4\(\pi\)M) and magnetic flux density (\(B\)) vs. \(H\) for anisotropic magnets with compositions of **(a)** SrFe\({}_{12}\)O\({}_{19}\) and **(b–e)** Sr\({}_{0.4}\)Ca\({}_{0.3}\)La\({}_{0.3}\)Fe\({}_{10.2}\)Co\({}_{0.1}\)Mn\({}_{0.1}\)Si\({}_{0.05}\)Mg\({}_{0.05}\)O\({}_{19}\) sintered at 1200 \({}^{\circ}\)C and 1220 \({}^{\circ}\)C with and without sintering additives. **(f)** Demagnetization (4\(\pi\)M vs. \(H\)) curves of all the magnets.  
Si-Mg doping level was \(x\) = 0.05 in the first series of samples, SrFe\({}_{12-2x}\)Si\({}_{x}\)Mg\({}_{x}\)O\({}_{19}\) (\(x\) = 0, 0.05, 0.1, 0.2), because it increased both saturation magnetization (\(M_{\rm S}\)) and coercivity (\(H_{\rm C}\)) and did not produce any second phase at the sintering temperature (\(T_{\rm S}\)) of 1230 \({}^{\circ}\)C. To further improve the characteristics, cation compositions of Sr\({}_{0.4}\)Ca\({}_{0.3}\)La\({}_{0.3}\)Fe\({}_{11.7-y}\)Co\({}_{0.2}\)Si\({}_{0.05}\)Mg\({}_{0.05}\) (\(y\) = 0, 1, 1.5, 2) were designed by combining La\({}_{0.4}\)Ca\({}_{0.3}\)Co\({}_{0.2}\) and Si\({}_{0.05}\)Mg\({}_{0.05}\) substitutions while controlling the Fe content. The optimal \(y\) value was determined to be 1.5 as it resulted in the highest residual magnetic flux density (\(B_{\rm r}\)), and a pure M-type phase without a second phase was confirmed at Fe-deficient \(y\geq\) 1.5. Finally, the effect of the Mn doping level \(z\) in Sr\({}_{0.4}\)Ca\({}_{0.3}\)La\({}_{0.3}\)Fe\({}_{10.2}\)Co\({}_{0.2-z}\)Mn\({}_{z}\)Si\({}_{0.05}\)Mg\({}_{0.05}\)O\({}_{19}\) (\(z\) = 0, 0.05, 0.1, 0.2) samples was evaluated. The doping with Mn (\(z\) = 0.05, 0.1, 0.2) caused abnormal grain growth and decreased the coercivity at \(T_{\rm S}\) = 1230 \({}^{\circ}\)C. When \(T_{\rm S}\) was decreased to 1210 \({}^{\circ}\)C and the ball-milling process was improved, further enhanced hard-magnetic properties were obtained for the \(z\) = 0.1 sample, with relatively high \(B_{\rm r}\) and \(H_{\rm C}\) values simultaneously. Thus, the multi-cation composition optimized in this study was Sr\({}_{0.4}\)Ca\({}_{0.3}\)La\({}_{0.3}\)Fe\({}_{10.2}\)Co\({}_{0.1}\)Mn\({}_{0.1}\)Si\({}_{0.05}\)Mg\({}_{0.05}\)O\({}_{19}\), and when an anisotropic magnet at the pilot level was produced with this composition, a permanent ferrite magnet with excellent performance of \(B_{\rm r}\) = 4.42 kG, \(H_{\rm C}\) = 3.57 kOe, and \(BH_{\rm max}\) = 4.70 M-G-Oe values was achieved. These research results are very promising because they show that it is possible to manufacture magnets with excellent performance at greatly reduced sintering temperatures while also reducing the Co content.  
Conceptualization, Y.-M.K.; Funding acquisition, Y.-M.K.; Investigation, Y.-M.K.; Methodology, J.-P.L. and M.-G.K.; Project administration, Y.-M.K.; Software, J.-P.L. and M.-G.K.; Supervision, Y.-M.K.; Writing--original draft, J.-P.L. and Y.-M.K. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the Research Fund of the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Science and ICT (Grant No. 2022R1F1A1062933).  
Not applicable.  
The authors are grateful to [PERSON] in the R&D Team, Union Materials Corp. for his support of the anisotropic ferrite magnet fabrication.  
The authors declare no conflict of interest.