Development of Multi-Cation-Doped M-Type Hexaferrite Permanent Magnets

Abstract

We report enhanced permanent magnet performance for multi-cation–substituted M-type Sr-hexaferrites (SrM) prepared using conventional ceramic processes. The final cation composition, Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉, could be derived through stepwise and systematic cation composition designs, processing, and characterization. The hexaferrites sample sintered in the temperature range of 1200–1220 °C showed an enhanced coercivity (H_C) of approximately 4.0 kOe and a residual magnetic flux density (B_r) of 2.5–2.6 kG. When samples of the same composition were fabricated into anisotropic magnets through a magnetic-field molding process, performance parameters of B_r = 4.42 kG, H_C = 3.57 kOe, and BH_max = 4.70 M·G·Oe were achieved, a significant improvement over B_r = 4.21 kG, H_C = 3.18 kOe, and BH_max = 4.24 M·G·Oe for the non-substituted SrFe₁₂O₁₉ magnet processed under optimized conditions.

1. Introduction

M-type hexaferrites, which have the basic formula (Ba,Sr)Fe₁₂O₁₉ and were discovered in the early 1950s, have been utilized as permanent magnets owing to their excellent chemical stability and proper hard-magnetic properties, such as strong c-axis magnetic anisotropy and sufficient saturation magnetization (M_S) [1,2]. Ferrite magnets using the M-type hexaferrite phase account for the largest amount of permanent magnet usage worldwide, which makes enhancing magnet performance very important. Many studies have been conducted to enhance the magnetic properties of M-type hexaferrites, such as saturation magnetization (M_S) and the uniaxial magnetocrystalline anisotropy constant (K_u), by substituting various cations. It has been reported that the optimal substitution of La-Zn [3,4], Mn-Zn [5], La-Ce-Zn [6], La-Mn-Zn [7], and Zn-Zr [8] into M-type hexaferrites increases the M_S value. One possible reason for the increase in M_S is that non-magnetic Zn²⁺ ions selectively replace Fe³⁺ ions with spin directions that are antiparallel to the net magnetization direction. It has also been reported that the substitution of Ce-Mn [9], Sm [10], Nd-Nb-Zn [11], Ga-Zn [12], Nb [13], Ce-Dy [14], Co-Al [15], Al [16], and Ho [17] enhances the K_u value or coercivity (H_C) of M-type hexaferrites. However, M_S and K_u do not improve simultaneously in all cases. Generally, in magnetic materials, an increase in M_S causes a decrease in K_u, and an increase in K_u causes a decrease in M_S. It is well-known that La-Ca-Co-substituted M-type hexaferrites exhibit significantly enhanced hard-magnetic properties because their magnetocrystalline anisotropy increases without scarifying the M_S [18,19,20]. Therefore, La-Ca-Co-substituted Sr-hexaferrites (SrM) have been developed into high-grade ferrite magnet products. However, no meaningful progress has been made in the research and development of commercial ferrite magnets for more than 15 years. In addition, owing to the recent surge in the price of cobalt raw materials, the development of ferrite magnets with excellent performance without the use of cobalt is strongly required in the industry.

The goal of this study was to determine the optimal composition for improving the hard-magnetic characteristics of multi-cation-substituted SrM. First, we sought to determine the optimal doping amount of Si-Mg in SrM. Normally, SiO₂ is used as a sintering additive to control grain growth. However, our previous studies [21] have shown that the Si can be applied as substitution elements by co-substituting Si⁴⁺ and Mg²⁺ at Fe³⁺ sites in SrM. The co-substitution of a Si⁴⁺-Mg²⁺ pair for two Fe³⁺ ions in SrM satisfies charge neutrality. To further improve the characteristics, the La-Ca-Co substitution was combined with the optimal level of Si-Mg substitution while controlling the Fe content. Finally, we sought to determine the sintering conditions required for the replacement of Co with Mn to obtain excellent hard-magnetic properties. Based on the obtained optimal cation composition and sintering process conditions, anisotropic magnets were fabricated through magnetic-field molding and their performance as ferrite magnets was evaluated.

2. Materials and Methods

M-type hexaferrites with nominal compositions of SrFe_12−2xSi_xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.2), Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0, 1, 1.5, 2), and Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) were prepared using solid-state reaction routes. Precursor powders of SrCO₃, CaCO₃, MgO, and Co₃O₄ (all 99.9% purity, Kojundo Chemical Lab. Co., Ltd.), Fe₂O₃ (~99.5%, industrial use), and SiO₂ (99.0%, industrial use) were weighed according to their chemical formulas. Each powder mixture was placed in a polypropylene bottle with yttria-stabilized ZrO₂ balls and deionized (DI) water and balls milled at a rotation speed of 100 rpm for 24 h. Then, the mixed powder was dried in an oven, placed in an alumina crucible, and calcined at a temperature of 1100 °C in air for 4 h. The calcined powders were ground, sieved, ball-milled again for 20 h, and dried. For selected samples, additional powder milling was conducted for 2–6 h with a high-speed (200 rpm) milling machine (PM100, Retsch, Haan, Germany). The ball-milled powders were pressed in a disk-shaped mold with a diameter of 15.0 mm at a pressure of approximately 15 MPa to produce green compact pellets. The pelletized samples were sintered in air in the temperature range of 1200–1230 °C for 2 h. For the selected cation composition, anisotropic sintered magnets with diameters of approximately 35 mm were fabricated at a commercial ferrite magnet manufacturing site (Union Materials Corporation, Seoul, Korea). Here, all process conditions except for the magnetic-field molding process were conducted under the same conditions as those mentioned above. During the molding process for the anisotropic magnet, a magnetic field of 10 kOe was applied to align the hexaferrite particles.

The sample density was calculated based on the weight and geometric dimensions of the disk-shaped samples. X-ray diffraction (XRD; D2 Phaser, Bruker, Mannnheim, Germany) was conducted for phase identification using a Cu-Kα radiation source (λ = 0.15406 nm). Field-emission scanning electron microscopy (FE-SEM; JSM-7610F, JEOL, Tokyo, Japan) was used for phase identification. The magnetic hysteresis curves were measured using a vibrating-sample magnetometer (VSM; Lakeshore 7410, Westerville, OH, USA) and a B-H loop tracer (BH-5501, Denshijiki Industry, Ukima, Japan) at room temperature. For the VSM and B-H loop tracer measurements, fragments of sintered samples weighing approximately 0.1 g and disk-shaped sintered pellets with diameters of approximately 10.5 and 35 mm, respectively, were used.

3. Results and Discussion

3.1. Crystalline Structure and Microstructure Analysis

Figure 1a shows the powder XRD patterns of the SrFe_12–2xSi_xMg_xO₁₉ (x = 0.0, 0.05, 0.1, 0.2) samples sintered at 1230 °C. A secondary phase peak of Fe₂O₃ is observed as the substitution level increases from x = 0.0 to x = 0.1, and it becomes noticeably larger at x = 0.2. The powder XRD patterns of the Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0.0, 1.0, 1.5, and 2.0) samples are shown in Figure 1b. Fe₂O₃ peaks are found at y = 0, and they decrease significantly at y = 1.0; no Fe₂O₃ peak is found at x ≥ 1.5. The diffraction peaks for the M-type hexaferrites were indexed based on hexagonal magnetoplumbite crystal structures with space group P6₃/mmc [International Center for Diffraction Data (ICDD), Powder Diffraction File no. 00-33-1340]. As shown in Figure 1c, no secondary phase peaks were observed in the XRD patterns of the Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) samples. The samples without Fe₂O₃ peaks are considered to have a single-phase M-phase.

Figure 2 shows the magnetic hysteresis curves of the SrFe_12−2xSi_xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.2) samples sintered at 1230 °C measured using VSM. The M_S and H_C values with respect to x are shown in Figure 2b. The sintered sample densities and the M_S and H_C values of the sintered samples are presented in

Table 1. Sintered sample densities (ρ), saturation magnetization (M_S), and coercivity (H_C) values of the SrFe_12−2xSi_xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.2) samples sintered at 1230 °C. The M_S value is defined as the magnetization (M) value at H = 25 kOe.

Cation Composition	x	ρ (g/cm3)	MS (emu/g)	HC (Oe)
SrFe12−2xSixMgx	0.0	2.75	72.08	3562
	0.05	2.87	72.15	3912
	0.1	2.91	70.46	3402
	0.2	4.07	66.49	2986

. All the samples were sintered without any sintering additives, and thus the densities of the samples were much lower than the theoretical density of SrM (~5.1 g/cm³). Nevertheless, because M_S was measured using VSM in units of emu/g, the densities of the samples did not affect the value of M_S. The M_S value increased slightly at x = 0.05, whereas the H_C value increased significantly. Then, the M_S and H_C values decreased as x increased to 0.2. Because H_C increased without a decrease in the M_S value at a Si-Mg substitution amount of x = 0.05, this value of x is considered to be a very effective substitution composition for a permanent magnet. The M_S value is an intrinsic magnetic property that depends on cation substitution, and the possible mechanism of the M_S increase may have been the preferred substitution of non-magnetic Si⁴⁺ or Mg²⁺ into the down-spin site of Fe³⁺ [4,5,6].

The microstructures of the SrFe_12−2xSi_xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.2) samples sintered at 1230 °C are presented in Figure 3a–d. The samples show fine microstructures at x = 0.05 and 0.1 compared to the x = 0 sample, and they are very porous from x = 0 to 0.1. The Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0, 1, 1.5, 2) samples sintered at 1230 °C showed a much denser microstructure with larger grains (Figure 3e–h). Similar results showing that grain growth and densification are effectively enhanced by doping with Ca [22] or LaCaCo [23,24] have been previously reported. The sintering densities of the samples are presented in

Table 2. Sintering temperature (T_S), sintered sample density (ρ), remanence magnetic flux density (B_r), and coercivity (H_C) of the hexaferrites Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0, 1, 1.5, 2), and Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) samples.

Cation Composition	TS (°C)	y, z, t	ρ (g/cm3)	Br (kG)	HC (kOe)
Sr0.4Ca0.3La0.3Fe11.7-yCo0.2Si0.05Mg0.05	1230	y = 0.0	4.86	2.37	3.78
	1230	y = 1.0	4.82	2.53	3.91
	1230	y = 1.5	4.89	2.69	3.05
	1230	y = 2.0	5.00	2.64	1.89
Sr0.4Ca0.3La0.3Fe10.2Co0.2−zMnzSi0.05Mg0.05	1230	z = 0.0	4.89	2.69	3.05
	1230	z = 0.05	4.94	2.25	1.41
	1230	z = 1.0	4.93	2.40	1.35
	1230	z = 2.0	4.76	2.42	2.85
Sr0.4Ca0.3La0.3Fe10.2Co0.2−zMnzSi0.05Mg0.05	1210	z = 0	4.77	2.64	3.17
	1210	z = 0.05	4.76	2.60	3.89
	1210	z = 1.0	4.73	2.53	4.15
	1210	z = 2.0	4.64	2.44	3.50
Sr0.4Ca0.3La0.3Fe10.2Co0.1Mn0.1Si0.05Mg0.05 (z = 1.0)	1210	t = 2 h	4.81	2.49	3.97
	1210	t = 4 h	4.88	2.53	4.00
	1210	t = 6 h	4.92	2.60	3.95

The error bar for M_S is ± 0.2% and for H_C is ±1% of the original values.

. When the Fe dependency y increased to 2.0 in the Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ samples (Figure 3h), significant grain growth occurred. From the XRD patterns shown in Figure 1b, it was verified that Fe₂O₃ was present at x = 0 and 0.1 but lost at x = 1.5. When a small amount of Fe₂O₃ is present in the M-type hexaferrite, grain growth is well-controlled. However, in the y = 2.0 sample, Fe₂O₃ was exhausted, and thus the Fe content would be insufficient even in the mother phase (M-phase). It is believed that the lack of Fe content promotes grain growth of SrM [23].

3.2. Magnetic Properties and Magnet Performances

The demagnetization curves (4πM vs. H) of the sintered Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0, 1, 1.5, 2) and Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) hexaferrites are presented in Figure 4a–d, and the B_r and H_C values are presented in Table 2. The magnetization curves (4πM–H) were measured using a B-H loop tracer on the disk samples. In the B-H loop tracer measurement, the 4πM data were gathered in Gaussian units, unlike those obtained in emu/g units for the VSM measurements shown in Figure 2. The M (emu/g) values can be converted to 4πM (G) using the following equation:

4πM (G) = 4πM (emu/g) × ρ,

(1)

where ρ is the sintered sample density (g/cm³). The magnetic flux density B (G), magnetization (4πM), and applied magnetic field (H) in the cgs unit system have the following relations [25]:

B (G) = 4πM (G) + H (Oe)

(2)

The [G] and [Oe] units can be considered as the same physical quantity in the cgs unit system. Therefore, a sintering density close to the theoretical density is required for a permanent magnet to produce a high magnetic flux density. As shown in Figure 4a, the y = 0 sample exhibited a relatively low B_r value of 2.37 kG owing to the presence of a non-magnetic second phase of Fe₂O₃, which is shown in Figure 1b. The highest B_r value of 2.69 kG was obtained for the y = 1.5 sample with no second phase. The H_C values of the Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ samples increased from 3.78 to 3.91 kOe at y = 1.0, and it decreased with increasing y. The sudden decrease in H_C at y = 2.0 can be attributed to significant grain growth (Figure 3h). As mentioned above, the lack of Fe in the M-phase is believed to promote grain growth.

Figure 4. (a–d) Demagnetization curves (4πM vs. H) of the sintered Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0, 1, 1.5, 2), and Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) hexaferrites.

Figure 4. (a–d) Demagnetization curves (4πM vs. H) of the sintered Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0, 1, 1.5, 2), and Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) hexaferrites.

Next, the magnetic properties were evaluated by substituting an amount of Co with Mn in a fixed composition of y = 1.5, in which a high B_r value was obtained without a Fe₂O₃ secondary phase. Figure 4b,c shows the 4πM-H curves of the Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−xMn_xSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) hexaferrites sintered at 1230 °C and 1210 °C, respectively. The ρ, B_r, and H_C values of the samples are presented in Table 2. In Figure 4b, the Mn-doped samples (z = 0.05, 0.1, 0.2) sintered at 1230 °C exhibit poor demagnetization curves, with significantly decreased B_r and H_C values. However, when these samples were sintered at 1210 °C, the B_r and H_C values of the Mn-doped samples were greatly enhanced. The z = 1.0 sample showed a significantly improved H_C value of 4150 kOe and a relatively high B_r value of 2.53 kG. Here, because the value of B_r is proportional to the sintering density, there is room for improvement in B_r when the sintering density is increased. To improve the sintering density, additional high-energy ball-milling was performed for 2, 4, and 6 h on a sample with a composition of z = 1.0 (Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉) and the resulting 4πM-H curves were measured; the results are shown in Figure 4d. As the ball-milling time increased from 2 to 6 h, the sintering density and B_r value increased. 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_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) samples sintered at 1230 °C and 1210 °C are presented in Figure 5a–d and Figure 5e–h, respectively. Comparing the microstructures of samples with the same composition sintered at 1210 °C and 1230 °C, it can be seen that the grains of the sample sintered at 1230 °C grew slightly more than those of the sample sintered at 1210 °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 °C and 1230 °C are presented in Figure 5i–l. The sample sintered at 1210 °C shows normal grains throughout, whereas the sample sintered at 1230 °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_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0.05, 0.1, 0.2) sintered at 1230 °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_C value of the magnet will be greatly reduced, making it unsuitable for use as a permanent magnet. H_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_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉ (x = 0.05, y = 1.5, z = 1.0) samples sintered at 1210 °C exhibited excellent hard-magnetic properties. To secure process window for sintering temperature, the microstructures of the Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉ samples sintered at 1200 °C and 1220 °C, which is 10 °C lower and higher than the above sintering temperature, were also analyzed. Figure 6a shows the SEM micrographs of the Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉ powders after additional high-energy ball-milling for 6 h, and Figure 6b,c shows the microstructures of the samples sintered at 1200 °C and 1220 °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 °C sintering, which is 20–30 °C lower than the general sintering temperature for hexaferrite magnets, and this without the use of sintering additives.

Finally, anisotropic sintered magnets with the optimal multi-cation composition, Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05, were fabricated at a commercial ferrite magnet manufacturing site with pilot-level processing machines. Here, the differences from fabricating isotropic magnets on a laboratory scale were that the amount of raw material powder used was scaled up by 100 times, the fine milling process was improved, and a wet magnetic-field molding method was applied during the pressing process. The magnetic hysteresis curves, 4πM-H and B-H, for the anisotropic magnets of Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉ sintered at 1200 °C and 1220 °C with and without sintering additives are presented in Figure 7b–e, along with those of a non-substituted SrFe₁₂O₁₉ (SFO) magnet, shown in Figure 7a, for comparison. The SFO sample was a general-grade commercial ferrite magnet manufactured under optimal process conditions. The sintering conditions, sintered sample density, and magnet parameters including 4πM_S, iH_C, bH_C, B_r, and BH_max are listed in

Table 3. Sintered sample density (ρ), saturation magnetization (4πM_S), intrinsic coercivity (kOe), normal coercivity (bH_C), remanent magnetization (B_r), and maximum energy product (BH_max) of hexaferrites magnets with composition of SrFe₁₂O₁₉ (SFO) and Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉ sintered at 1200 °C and 1220 °C with and without sintering additives, respectively.

Magnet ID	Sintering Additives	TS (°C)	ρ (g/cm3)	4πMS (kG)	iHC (kOe)	bHC (kOe)	Br (kG)	BHmax (M·G·Oe)
SFO	Optimized	1230	4.92	4.39	3.18	3.00	4.21	4.24
No_1220	No additives	1220	5.04	4.55	3.31	2.89	4.41	4.44
No_1200		1200	5.01	4.51	3.40	3.17	4.37	4.55
Ad_1220	SiO2 0.4wt% + CaCO3 0.8wt%	1220	5.00	4.54	3.53	3.21	4.40	4.64
Ad_1200		1200	5.01	4.54	3.57	3.29	4.42	4.70

, where iH_C and bH_C are the absolute values of H when 4πM = 0 and B = 0, respectively, in the 4πM-H and B-H curves. Refer to Table 3 for process conditions, such as sintering additives and sintering temperature, for each sample ID. The demagnetization curves of all the samples are plotted in Figure 7f. First, when comparing the overall demagnetization curve of the isotropic magnet without the magnetic-field molding process, as shown in Figure 4d, the squareness and B_r values increased significantly. It can also be seen that the performance of the Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉ magnets significantly improved compared to the SFO magnets. The best performance of BH_max = 4.70 M·G·Oe was obtained when the Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉ sample was sintered at 1200 °C using sintering additives of 0.4 wt.% SiO₂ + 0.8 wt.% CaCO₃. It is very promising that high magnet performance can be achieved even at a significantly lower sintering temperature, and that Co can be partly replaced with the much less expensive Mn.

4. Conclusions

Three series of samples with the chemical formulas of SrFe_12−2xSi_xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.2), Sr_0.4Ca_0.3La_0.3Fe_11.7-yCo_0.2Si_0.05Mg_0.05O₁₉ (y = 0, 1, 1.5, 2), and Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) were prepared sequentially, and their hard-magnetic properties were evaluated to determine the optimal composition. The optimal Si-Mg doping level was x = 0.05 in the first series of samples, SrFe_12−2xSi_xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.2), because it increased both saturation magnetization (M_S) and coercivity (H_C) and did not produce any second phase at the sintering temperature (T_S) of 1230 °C. To further improve the characteristics, cation compositions of Sr_0.4Ca_0.3La_0.3Fe_11.7−yCo_0.2Si_0.05Mg_0.05 (y = 0, 1, 1.5, 2) were designed by combining La_0.4Ca_0.3Co_0.2 and Si_0.05Mg_0.05 substitutions while controlling the Fe content. The optimal y value was determined to be 1.5 as it resulted in the highest residual magnetic flux density (B_r), and a pure M-type phase without a second phase was confirmed at Fe-deficient y ≥ 1.5. Finally, the effect of the Mn doping level z in Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.2−zMn_zSi_0.05Mg_0.05O₁₉ (z = 0, 0.05, 0.1, 0.2) samples was evaluated. The doping with Mn (z = 0.05, 0.1, 0.2) caused abnormal grain growth and decreased the coercivity at T_S = 1230 °C. When T_S was decreased to 1210 °C and the ball-milling process was improved, further enhanced hard-magnetic properties were obtained for the z = 0.1 sample, with relatively high B_r and H_C values simultaneously. Thus, the multi-cation composition optimized in this study was Sr_0.4Ca_0.3La_0.3Fe_10.2Co_0.1Mn_0.1Si_0.05Mg_0.05O₁₉, and when an anisotropic magnet at the pilot level was produced with this composition, a permanent ferrite magnet with excellent performance of B_r = 4.42 kG, H_C = 3.57 kOe, and BH_max = 4.70 M·G·Oe values was achieved. These research results are very promising because they show that it is possible to manufacture magnets with excellent performance at greatly reduced sintering temperatures while also reducing the Co content.