## 4 Summary  
Highly dense La-Co-substituted SrM samples could be prepared using the sintering additives of 0.7 wt% SiO\({}_{2}\) and 0.7 wt% CaCO\({}_{3}\) and the sintering conditions of at 1230 \({}^{\circ}\)C for 2 h, in air. The M-type single phase was obtainable up to \(x\) = 0.3 in \(\text{Sr}_{1-x}\text{La}_{x}\text{Fe}_{12-x}\text{Co}_{x}\text{O}_{19}\). The lattice parameters \(a\), \(c\), and \(V_{cell}\) decrease with an increasing \(x\). For non-MFP samples, both \(H_{a}\) and \(H_{cli}\) showed an increase from 17.7 to 25.9 kOe and from 3.5 to 5.0 kOe, respectively, with an increasing \(x\) up to 0.3, while \(M_{s}\) increases from 70.90 to 72.40 emu/g with an increasing \(x\) from 0.0 to 0.15 and then decreases to 71.38 emu/g up to \(x\) = 0.3. The increase in \(H_{a}\) with an increasing \(x\), due to a decrease in the lattice symmetry, is the primary reason for a continuous increase in \(H_{cli}\). For the undoped sample of \(x\) = 0, there exists a significant difference in \(H_{a}\) and \(H_{cli}\) values between the present sample and the post-annealed sample without sintering additives, which is surely due to the effect of Ca and Si substitution for the Sr and Fe sites in SrM. However, with an increasing \(x\), these values tend to converge. On the other hand, \(H_{a}\) can increase due to a gradual decrease in the lattice symmetry with an increasing \(x\). MFP-processed samples possess much higher \(B_{r}\) and \((BH)_{max}\) values compared with non-MFP samples, and these values are attributable to significantly enhanced grain alignment along the \(c\)-axis. Unlike non-MFP samples, exhibiting a significant increase in \(d_{avg}\) and its standard deviation from 1.20 \(\pm\) 0.15 \(\upmu\)m to 1.96 \(\pm\) 0.87 \(\upmu\)m, but almost unaltered average thickness(\(t_{avg}\)) of \(\sim\)0.95 \(\pm\) 0.34 \(\upmu\)m with increasing \(x\) from 0.0 to 0.3, MFP-processed samples exhibited an almost-unaltered \(d_{avg}\) of \(\sim\)1.35 \(\pm\) 0.29 \(\upmu\)m but a continuous decrease in \(t_{avg}\) from 1.18 \(\pm\) 0.21 to 0.77 \(\pm\) 0.05 \(\upmu\)m. Furthermore, while \(H_{cli}\) values are lowered due to MFP for a given \(x\), they increased with the increasing \(x\), which is similar to the behavior of non-MFP samples. For the samples with the same composition \(x\), both \(B_{r}\) and \((BH)_{max}\) are proportional to the degree of \(c\)-axis orientation \(w(00l)\), squareness \(Q\), and \(M_{r}/M_{s}\) ratio. However, for the samples with different \(x\) values, those depend only on the \(M_{r}/M_{s}\) ratio. In conclusion, compared with non-MFP samples, while the \(B_{r}\) and \((BH)_{max}\) values of MFP samples are greatly enhanced due to grain alignment along the \(c\)-axis (i.e., easy axis), the \(H_{ci}\) values are decreased mainly due to the increase in \(B_{r}\). In addition, referring to previous reports, although the highest \((BH)_{max}\) value of 3.62 MGOe could be achieved from the MFP sample of \(x=0.15\) in this study, further improvement in \(B_{r}\), \(H_{ci}\), and \((BH)_{max}\) is expected not only for this composition but also other compositions by optimizing the MFP process and sintering conditions, and hence leading to a highly dense microstructure consisting of better grain alignment and refined uniform grains. Consequently, the comparative study between MFP and non-MFP samples provides us valuable insights into a broad area of permanent magnet optimization. Conceptualization, K.L. ; funding acquisition, S.-I.Y. ; investigation, K.L., S.L., Y.-M.K., and S.-I.Y. ; methodology K.L. ; project administration, S.-I.Y. ; software, K.L. ; supervision, S.-I.Y. ; writing--original draft, K.L. All authors have read and agreed to the published version of the manuscript. This research was supported by Technology Innovation Program (2010561, Development of manufacture system for 5.5 MGOe high-performance hexaferrite) funded By the Ministry of Trade, Industry, and Energy (MOTIC, Republic of Korea). Not applicable.