## 3 Results and Discussion
### Magnetic Properties and Magnet Performances  
Figure 5 shows the magnetic hysteresis curves of the SrFe\({}_{12-2x}\)Si\({}_{x}\)Mg\({}_{x}\)O\({}_{19}\) (\(x\) = 0, 0.05, 0.1, 0.2) and SrFe\({}_{12-z}\)(Si\({}_{0.6}\)Li\({}_{0.6}\))\({}_{2}\)O\({}_{19}\) (\(z\) = 0, 0.05, 0.1, 0.2, 0.4, 0.6) samples sintered at 1250 \({}^{\circ}\)C. In the case of the Si-K-substituted SrM samples, a considerable amount of Fe\({}_{2}\)O\({}_{3}\) was formed as a secondary phase even with a small doping level z (Figure 1b; therefore, the magnetic properties were not evaluated. The M\({}_{\text{S}}\) and H\({}_{\text{C}}\) values with respect to \(x\) and \(z\) are plotted in Figure 5c,d. The sintered sample densities as well as M\({}_{\text{S}}\) and H\({}_{\text{C}}\) values of the samples are listed in Table 2.  
In the case of Si-Mg substitution, the M\({}_{\rm S}\) slightly increased at \(x=0.05\) and H\({}_{\rm C}\) increased significantly. Then, M\({}_{\rm S}\) and H\({}_{\rm C}\) tended to decrease as \(x\) further increased to 0.2. As the spin moment contributing to the magnetization value in SrM originated from the Fe\({}^{3+}\) ions, the M\({}_{\rm S}\) decreased when the Fe content decreased owing to a further increase in the Si-Li substitution. The most notable improvement in the magnetic properties was observed in the Si-Li-substituted sample with \(z=0.05\). In this sample, the M\({}_{\rm S}\) value increased significantly from 72.08 to 73.92 emu/g (2.6%), and the H\({}_{\rm C}\) value also increased slightly. This improvement in M\({}_{\rm S}\) is unusual because when Si penetrates the M-type hexaferrite lattice, it is expected to decrease the M\({}_{\rm S}\)[20].The M\({}_{\rm S}\) value is an intrinsic magnetic property that depends on the composition and lattice characteristics, unlike H\({}_{\rm C}\), which is greatly affected by the microstructure. Here, the M\({}_{\rm S}\) values (in emu/g unit) were obtained from samples that had not been densified. A sintered density that is 95% or higher than the theoretical density of approximately 5.1 g/cm\({}^{3}\)[17] is required to become a permanent magnet with excellent B\({}_{\rm F}\). Therefore, in the next experiment, isotropic permanent magnets using various sintering additives were fabricated with the optimized composition of SrFe\({}_{11.95}\)Si\({}_{0.03}\)Li\({}_{0.03}\)O\({}_{19}\), which exhibited the highest M\({}_{\text{S}}\) value with a relatively high H\({}_{\text{C}}\). Figure 6a shows the demagnetization curves of the hexaferrite samples sintered with various additives, including 0.5 wt% SrCO\({}_{3}\), 0.75 wt% SrCO\({}_{3}\), 1 wt% SrCO\({}_{3}\), 0.5 wt% SrCO\({}_{3}\) + 0.5 wt% CaCO\({}_{3}\), 1 wt% SrCO\({}_{3}\) + 0.5 wt% SiO\({}_{2}\), 1 wt% SrCO\({}_{3}\) + 0.5 wt% Co\({}_{3}\)O\({}_{4}\), and 1 wt% CaCO\({}_{3}\) + 0.5 wt% SiO\({}_{2}\), respectively. After measuring the full hysteresis in the range of H = 20 kOe, the demagnetization curves in the second quadrant were obtained. The density (\(\rho\)), remanent magnetization (B\({}_{\text{r}}\)), and coercivity (H\({}_{\text{C}}\)) of the sintered hexaferrite samples are presented in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline
**Composition** & **x, z** & \(\rho\) (g/cm\({}^{3}\))** & **M\({}_{\text{S}}\) (emu/g)** & **H\({}_{\text{C}}\) (Oe)** \\ \hline  & 0 & 2.75 & 72.08 & 3562 \\ SrFe\({}_{12-2x}\)Si\({}_{x}\)Mg\({}_{x}\)O\({}_{19}\) & 0.05 & 2.87 & 72.15 & 3912 \\  & 0.1 & 2.91 & 70.46 & 3402 \\  & 0.2 & 4.07 & 66.49 & 2986 \\ \hline  & 0 & 2.75 & 72.08 & 3559 \\  & 0.05 & 3.09 & 73.92 & 3713 \\ SrFe\({}_{12-z}\)(Li\({}_{z}\)Si\({}_{z}\))\({}_{0.6}\)O\({}_{19}\) & 0.1 & 3.24 & 72.36 & 3393 \\  & 0.2 & 3.23 & 70.5 & 3904 \\  & 0.4 & 3.90 & 70.7 & 3809 \\  & 0.6 & 3.84 & 64.15 & 3216 \\ \hline \end{tabular}
\end{table}