## 3 Results and Discussion
### Magnetic Properties and Magnet Performances

Table 2: The sintered sample density (\(\rho\)), saturation magnetization (M\({}_{\text{s}}\)), and coercivity (H\({}_{\text{C}}\)) of hexaferrite samples, SrFe\({}_{12-2x}\)Si\({}_{x}\)Mg\({}_{x}\) and SrFe\({}_{12-z}\)(Li\({}_{z}\)Si\({}_{z}\))\({}_{0.6}\) sintered at 1250 \({}^{\circ}\)C. The error bar for M\({}_{\text{S}}\) is \(\pm\) 0.2% and for H\({}_{\text{C}}\) is \(\pm\)1% of the original values. The sintering density (\(\rho\) = 4.7 \(\sim\) 5.0 g/cm\({}^{3}\), Table 3) significantly increased when the sintering additives were used compared to when they were not (\(\rho\) = 2.7\(\sim\)3.9g/cm\({}^{3}\), Table 2). Notably, the sintering temperature was set to 1230 \({}^{\circ}\)C (20 \({}^{\circ}\)C lower than the case in Table 2). We also found that a sufficient H\({}_{\rm C}\) value could not be obtained without adding 0.5 wt% SiO\({}_{2}\). In addition, a sample composition of La-Co-Si-Li-substituted SrM, Sr\({}_{0.8}\)La\({}_{0.2}\)Fe\({}_{11.75}\)Co\({}_{0.02}\)Si\({}_{0.03}\)Li\({}_{0.03}\)O\({}_{19}\) was synthesized using the same procedure and sintered with two set of additives: 1 wt% CaCO\({}_{3}\) + 0.5 wt% SiO\({}_{2}\) and 1 wt% SrCO\({}_{3}\) + 0.5 wt% SiO\({}_{2}\). Since it is well-known that La-Co substitution into SrM enhances the hard magnetic properties [7; 8; 9], the composition Sr\({}_{0.8}\)La\({}_{0.2}\)Fe\({}_{11.75}\)Co\({}_{0.02}\)Si\({}_{0.03}\)Li\({}_{0.03}\)O\({}_{19}\) was specifically designed for producing high-performance permanent magnets with the expectation of combining the effects of La-Co and Si-Li substitution. The demagnetization curves of La-Co-Si-Li-substituted SrM are presented in Figure 6b, and the magnetic parameters are listed in Table 3. The La-Co-Si-Li-added SrM samples showed significantly improved H\({}_{\rm C}\) values compared with the Si-Li-substituted SrM. This is because La-Co substitution increases the K\({}_{\rm u}\) of SrM. Finally, anisotropic sintered magnets with the compositions SrFe\({}_{12}\)O\({}_{19}\), SrFe\({}_{11.95}\)Li\({}_{0.03}\)Si\({}_{0.03}\)O\({}_{19}\), and Sr\({}_{0.8}\)La\({}_{0.2}\)Fe\({}_{11.75}\)Co\({}_{0.2}\)Si\({}_{0.03}\)Li\({}_{0.03}\)O\({}_{19}\) were fabricated by adding a magnetic field pressing process. Sintering additives 0.5 wt% SiO\({}_{2}\) + 1.0 wt% CaCO\({}_{3}\) are commonly used. The demagnetization curves (4\(\pi\)M-H and B-H) of the anisotropic magnets are shown in Figure 7, and the magnet parameters are presented in Table 4. The B\({}_{\rm r}\) of Si-Li-substituted SrM (4314 G) increased by 2.5% compared to that of unsubstituted SrM (4207 G), and the H\({}_{\rm C}\) also slightly increased. Thus, the maximum energy product (BH\({}_{\rm max}\)) value increased from 4.24 to 4.46 M-G-Oe. The 2.5% increase in B\({}_{\rm r}\) can be attributed to the increase in the M\({}_{\rm S}\) of the Si-Li-substituted SrM (\(z\) = 0.05) sample, as shown in Figure 3d. La-Co-Si-Li-substituted SrM exhibited a high H\({}_{\rm C}\) value; however, the \({}_{\rm p}\)H\({}_{\rm C}\) and BH\({}_{\rm max}\) values were significantly lower than those of Si-Li SrM due to the poor squareness of the demagnetization curve. The squareness of the demagnetization curves is closely related to the grain orientation, which is determined during the magnetic field pressing or sintering processes. Therefore, the performance of the magnet (B\({}_{\rm r}\) and BH\({}_{\rm max}\)) could be improved by further process optimization. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Magnet Composition** & \(\rho\) & **iH\({}_{\rm C}\)** & **bH\({}_{\rm C}\)** & **B\({}_{\rm r}\)** & **BH\({}_{\rm max}\)** \\  & **(g/cm\({}^{3}\))** & **(Oe)** & **(Oe)** & **(Oe)** & **(G)** & **(M-G-Oe)** \\ \hline SrFe\({}_{12}\)O\({}_{19}\) & 4.95 & 3182 & 3002 & 4207 & 4.24 \\ \hline SrFe\({}_{11.95}\)Li\({}_{0.03}\)Si\({}_{0.03}\)O\({}_{19}\) & 4.94 & 3206 & 3009 & 4314 & 4.46 \\ \hline Sr\({}_{0.8}\)La\({}_{0.2}\)Fe\({}_{11.75}\)Co\({}_{0.2}\)Li\({}_{0.03}\)Si\({}_{0.03}\)O\({}_{19}\) & 5.02 & 3897 & 3138 & 4293 & 3.98 \\ \hline \end{tabular}
\end{table}