## 3 Results and Discussion
### Magnetic Property Analysis
These values are also listed in Table 2. The \(M_{\rm S}\), \(M_{\oplus 26 kOe}\), \(H_{a}\), and \(H_{ci}\) values for all samples in this table are plotted as a function of \(x\) as shown in Figure 9.  
The effect of La-Co substitution (\(x\)) on the magnetic properties of as-sintered samples can be understood as follows. As shown in Table 2 and Figure 9a, the values of \(M_{\rm S}\) and \(M_{\oplus 26 kOe}\) gradually decrease from 75.90 to 72.07 emu/g and from 72.27 to 66.80 emu/g, respectively, as \(x\) increases from 0.0 to 0.3. This suggests that Co\({}^{2+}\) replaces Fe\({}^{3+}\) at the 4/2 site, while Co\({}^{2+}\)/Fe\({}^{2+}\) substitution occurs at the 2\(a\) site, leading to the gradual decrease in \(M_{\rm S}\). The \(M_{\rm S}\) dependency of \(x\) for our samples agrees well with that reported for powder samples prepared by the sol-gel process and subsequent firing at 900 \({}^{\circ}\)C for 24 h in air [20]. Specifically, \(M_{\rm S}\) decreases as \(x\) increases. However, the \(M_{\rm S}\) vs. \(x\) behavior reported by [PERSON] et al. [23] differs from ours, despite their use of the sol-gel process and subsequent firing at 900 \({}^{\circ}\)C for 2 h in air to prepare powder samples with \(x\) = 0.0 and 0.05. Lower \(M_{\rm S}\) values for samples with \(x\) = 0.0 and 0.05 may be due to the presence of small nanoparticles (<50 nm) exhibiting superparamagnetism, as observed in their SEM micrographs. In contrast, the larger particles in samples with \(x\)\(\geq\) 0.1 exhibit a decreasing behavior in \(M_{\rm S}\) with increasing \(x\), similar to our sintered bulk samples. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \(x\) & \(M_{\oplus 26 kOe}\) & \(H_{ci}\) & \(M_{s}\) & \(H_{a}\) & \(A\) & **Field Region for** & **R\({}^{2}\)** \\  & **(emu/g)** & **(kOe)** & **(emu/g)** & **(kOe)** & **(Oe)** & **Fitting (kOe)** & **R\({}^{2}\)** \\ \hline
0.0 & 72.27 & 2.68 \(\pm\) 0.14 & 75.90 \(\pm\) 0.04 & 20.1 \(\pm\) 1.3 & 242 & 12–26 & 0.99265 \\
0.1 & 72.04 & 2.99 \(\pm\) 0.09 & 75.46 \(\pm\) 0.04 & 21.5 \(\pm\) 1.2 & 245 & 14–26 & 0.99267 \\
0.15 &Table 2 and Figure 9b show that the value of \(H_{a}\) increases from 20.1 to 21.5 kOe as \(x\) increases from 0.00 to 0.10. It then slightly increases to 21.6 kOe up to \(x\) = 0.20, followed by a further increase to 24.7 kOe up to \(x\) = 0.30. These variations in \(x\) are almost inversely proportional to the variation in the \(c/a\) values with \(x\) shown in Figure 3b, except for the \(H_{a}\) value at \(x\) = 0.3. [PERSON] et al. [19] reported that \(H_{a}\) can be increased due to a gradual decrease in crystallographic symmetry with increasing \(x\) in La-Co co-substituted SrM. However, \(H_{a}\) increases even as \(c/a\) increases at \(x\) = 0.3. This may be due to the increased occupancy of Co\({}^{2+}\) at the 2\(a\) site, as discussed above in relation to the increase in \(c\). The behavior of \(H_{a}\) increasing with \(x\) is similar to that reported by [PERSON] et al. [20]. However, our \(H_{a}\) values are approximately 16% higher than theirs. They evaluated their \(H_{a}\) values not from initial magnetization curves, but from magnetic hysteresis curves measured only up to the applied field of 16 kOe. Furthermore, they did not take the \(A\) factor in Equation (4) into account for their calculation of \(M_{S}\) and \(H_{a}\) values, which might be responsible for their lower \(M_{S}\) and \(H_{a}\) values compared to our data. The \(H_{ci}\) values of our polycrystalline samples were experimentally obtained from the \(M\)-\(H\) hysteresis curves. At least three samples of each composition were measured. The average \(H_{ci}\) values and their standard deviations are shown in Figure 9c. The average \(H_{ci}\) values increase from 2.68 to 3.99 kOe as \(x\) increases. The \(H_{ci}\) values show significant variation with a large standard deviation for the samples with \(x\) = 0.10 and 0.15, resulting in relatively large error bars. However, the standard deviation decreases as \(x\) increases further.