# Development of Multi-Cation-Doped M-Type Hexaferrite Permanent Magnets
## 3 Results and Discussion
### Magnetic Properties and Magnet Performances
Much of the change in the demagnetization curves shown in Figure 4b,c can be understood from the microstructure observations. The SEM micrographs of the 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 sintered at 1230 \({}^{\circ}\)C and 1210 \({}^{\circ}\)C are presented in Figure 5a-d and Figure 5e-h, respectively. Comparing the microstructures of samples with the same composition sintered at 1210 \({}^{\circ}\)C and 1230 \({}^{\circ}\)C, it can be seen that the grains of the sample sintered at 1230 \({}^{\circ}\)C grew slightly more than those of the sample sintered at 1210 \({}^{\circ}\)C, as expected. However, this is insufficient to explain the collapse of the demagnetization curve of the Mn-doped samples (\(z\) = 0.05, 0.1, 0.2) shown in Figure 4b. The SEM micrographs with low magnetization for the \(z\) = 0.05 and 0.2 samples sintered at 1210 \({}^{\circ}\)C and 1230 \({}^{\circ}\)C are presented in Figure 5i-l. The sample sintered at 1210 \({}^{\circ}\)C shows normal grains throughout, whereas the sample sintered at 1230 \({}^{\circ}\)C shows the coexistence of a region of normal grains and a region with abnormally large grains. Therefore, the poor demagnetization curves for the Mn-doped samples 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\({}_{19}\) (\(z\) = 0.05, 0.1, 0.2) sintered at 1230 \({}^{\circ}\)C (Figure 4b) are attributed to the presence of abnormally large grains. Abnormal grain growth (AGG) can occur when a few large grains grow faster than normal grains [22]. The occurrence of AGG depends on the sintering temperature and substitution composition [17]. A similar effect of doping level on the occurrence of AGG has been reported in hexaferrite materials, wherein abnormal grain growth occurred depending on the doping level of Ca or Si [7; 22; 23; 24]. If the sample contains large grains owing to the occurrence of AGG, the \(H_{\rm C}\) value of the magnet will be greatly reduced, making it unsuitable for use as a permanent magnet. \(H_{\rm C}\) is greatly reduced in samples where AGG occurs because a reverse magnetic domain is easily generated within a large grain, and demagnetization occurs quickly owing to the easy movement of the magnetic domain wall. Based on the experimental results, the cation substitution composition and sintering temperature were optimized. The 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\({}_{19}\) (\(x\) = 0.05, \(y\) = 1.5, \(z\) = 1.0) samples sintered at 1210 \({}^{\circ}\)C exhibited excellent hard-magnetic properties. To secure process window for sintering temperature, the microstructures of the 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\({}_{19}\) samples sintered at 1200 \({}^{\circ}\)C and 1220 \({}^{\circ}\)C, which is 10 \({}^{\circ}\)C lower and higher than the above sintering temperature, were also analyzed. Figure 6a shows the SEM micrographs of the 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\({}_{19}\) powders after additional high-energy ball-milling for 6 h, and Figure 6b,c shows the mi crostructures of the samples sintered at 1200 \({}^{\circ}\)C and 1220 \({}^{\circ}\)C. The sintered samples exhibit dense microstructures with no AGG grains. Low-magnification images of these samples are presented in Figure 6c-e. It is a very encouraging that such a dense microstructure was obtained even at 1200 \({}^{\circ}\)C sintering, which is 20-30 \({}^{\circ}\)C lower than the general sintering temperature for hexaferrite magnets, and this without the use of sintering additives.