Investigation on the La Replacement and Little Additive Modification of High-Performance Permanent Magnetic Strontium-Ferrite

In this work, an experiment was carried out to investigate the preparation condition of anisotropic, Fe-deficient, M-type Sr ferrite with optimum magnetic and physical properties by changing experimental parameters, such as the La substitution amount and little additive modification during fine milling process. The compositions of the calcined ferrites were chosen according to the stoichiometry LaxSr1-xFe12-2xO19, where M-type single-phase calcined powder was synthesized with a composition of x = 0.30. The effect of CaCO3, SiO2, and Co3O4 inter-additives on the Sr ferrite was also discussed in order to obtain low-temperature sintered magnets. The magnetic properties of Br = 4608 Gauss, bHc = 3650 Oe, iHc = 3765 Oe, and (BH)max = 5.23 MGOe were obtained for Sr ferrite hard magnets with low cobalt content at 1.7 wt%, which will eventually be used as high-end permanent magnets for the high-efficiency motor application in automobiles with Br > 4600 ± 50 G and iHc > 3600 ± 50 Oe.


Introduction
Hexagonal magneto-plumbite-type strontium ferrite (SrFe 12 O 19 ) has material characteristics, such as a stable crystal structure, high coercivity and magnetic energy product, and magnetic anisotropy (K 1 ) [1], so it is widely used in permanent magnet motors and various electronic devices, which is an important basic functional material in the electronics industry. In recent years, with the development of the green economy and environmental protection industry, the miniaturization and high-performance requirements of electronic components have gradually increased, accompanied by strong demand for the miniaturization and high performance of magnets. High-performance permanent magnet ferrites with high remanence (B r ), high coercivity ( i H c ), and high magnetic energy product ((BH) max ) are new market needs for the rapid development of the world's electronic information and automobile industries [2,3]. In order to improve the various physical and magnetic properties of strontium ferrite, research is mainly carried out on two aspects: formula and process conditions. After years of exploration and research, it has been discovered that simply by optimizing the process parameters of ferrite, such as the particle size and distribution after ball milling, the calcination temperature curve, the sintering temperature curve, and the compression molding process, some performance indexes of strontium ferrite can be optimized to a certain level. However, compared with its various theoretical values, there is still a big gap [4][5][6][7][8][9]. In a material, by doping a small amount of other elements or compounds, the material will produce specific optical, electrical and magnetic

Sample Preparation
In this study, Fe-deficient M-type Sr ferrite magnets have been synthesized by the conventional ceramic method. Nonstoichiometric compositions were selected on the basis of the chemical composition La x Sr 1-x Fe 12-2x O 19 , where La content (x) varied from 0.28 to 0.32, and Sr content (1-x) from 0.70 to 0.74. The raw material Fe 2 O 3 is manufactured by the acid recycle Ruthner process of the China Steel Corporation (CSC) in Taiwan [7,24] with high purity (>99.5%) and Specific Surface Area (SSA) = 3.5 ± 0.2 m 2 /g. The SrCO 3 with >98% purity and different particle sizes of 1.0~2.5 µm was used as another starting material to clarify the influence of its particle size on the magnetic properties of Sr ferrite. First, the raw material Fe 2 O 3 , SrCO 3 , and La 2 O 3 powders and the additive CaCO 3 were wet-mixed by an attritor, and then granulated for calcination. A single phase of submicronsized Sr ferrite powder was prepared by calcining in an electric furnace at 1270 ± 5 • C for 1 h in air at increments of 10 • C. The calcination granules were vacillated vigorously in a vibration mill to achieve a powder size of approximately 2.2 ± 0.1 µm on average. Next, the additives CaCO 3 , SiO 2 , and Co 3 O 4 were doped in different concentrations by fine-milling the mixture for about 9 h in a stainless steel ball-miller before sintering. This step guarantees a narrower particle-size distribution with a mean size about 0.70 ± 0.05 µm measured by a Fisher subsieve sizer (Fisher, Waltham, MA, USA), which is important for achieving good performance. Here, we further explain why the two-stage grinding process was performed in the experiment. For the traditional manufacturing process of permanent ferrite materials, the two-stage milling process will be carried out in sequence. These two different milling processes have different purposes for the preparation of permanent ferrite. For the first stage of milling, we can call it pulverization, or "coarse milling". This coarse milling is usually carried out by a vibrating mill, which is a more vigorous process and able to grind the calcined granules from a centimeter-level size to µm-level size in a shorter time. We usually grind the calcined granules to a powder size of about 2.2 ± 0.1 µm on average through this "coarse milling" process. Because the first stage of the milling process already ground the calcined granules to a relatively smaller size, the second stage of the milling process, that is, the fine milling process, is usually used to grind the magnetic powder to a smaller size of less than 1 µm (0.70 ± 0.5 µm in this study) by a ball mill to fit the permanent magnet process requirements. Because the ball milling process is a relatively mild grinding process, it can achieve a narrow and single-peak magnetic particle size distribution, which is necessary for achieving better magnetic performance, but will take a longer time. However, if we do not grind the calcined granules through the coarse milling process first, but directly use the ball mill to grind, this will cause the required grinding time to become quite long, and it will not be easy to achieve the aforementioned particle size distribution results. The milled slurry was pressed into tablets of Φ26.5 mm in diameter and 13 mm in height under a magnetic field of about 1.5 Tesla and 100 MPa pressure (according to the Japanese Industrial Standard, JIS C2501). The pressed tablets were sintered at a peak temperature of 1230 ± 5 • C for 1 h.

Measurements
X-ray diffraction (XRD, Cu-Kα radiation) was used for the phase identification of the calcined magnetic powder. The powder size distribution of SrCO 3 was measured by the Sympatec laser-scattering system under the sample analysis volume of 0.5 g using a dry dispersion unit (system Rodos). The magnetic properties (B r , i H c , and (BH) max ) of the sintering magnets were measured at room temperature by the NIM-2000 HF Hysteresis graph meter (China National Measuring Science Research Institute) on the polished discs. The b H c describes the magnetic field which makes the B distribution in the magnet change direction. The b H c is smaller than i H c , which is the field necessary to demagnetize the polarization or magnetization to zero. Usually, we use i H c to represent the intrinsic magnetic properties of permanent magnet materials. Scanning electron microscopy (SEM; Zeiss) was used to analyze the ceramic microstructures on the polished and thermally etched faces of the magnets (perpendicular to the applied pressure and field).

Effect of Calcined Ferrite Stoichiometry
The general formula of M-type ferrite is AFe 12 O 19 , where A = Ba or Sr, and a part of these elements can be replaced by rare-earth elements. Among them, a part of Fe could be substituted by cobalt (Co) to obtain better magnetic performance that would result in high intrinsic magnetization M s and anisotropic magnetic field strength H A [13][14][15][16][17][18][19]. Besides, the element at the A position can be substituted by other rare-earth elements, such as La, since La 3+ (1.17 Å), Ba 2+ (1.22 Å), and Sr 2+ (1.32 Å) share almost the same radius. La has the highest solid solution limit for M-type ferrites, and the substitution of La at the A position can enhance the solid content of Co that substitutes Fe, which is critical and beneficial to the improvement of the magnetic characteristics of Sr ferrite [11,13,16,25]. To investigate the influence of the amount of La substituted for Sr in the calcined ferrite on the magnetic properties, a non-stoichiometric formula composed of La x Sr 1-x Fe 12-2x O 19 was selected to form Fe-deficient Sr base M-type ferrite with x = 0.28~0.32. It can be seen from the experimental results in Figure 1 that the substitution amount of La 2 O 3 in the calcined ferrite must be sufficient to react with cobalt added during the fine-milled process to promote the diffusion of Co ions in the sintering reaction to achieve better magnetic properties. When x = 0.32, the level of B r = 4536G and i H c = 4362Oe can be obtained. It can be clearly determined that the higher the amount of La 2 O 3 in the calcined ferrite, the higher the Br and i H c . However, if the amount of La 2 O 3 in the calcined ferrite is too high, it will cause a mismatch with the Co added after calcination, resulting in a decrease in magnetic properties [24]. The X-ray diffraction patterns of the Sr ferrite M-type ferrite La x Sr 1-x Fe 12-2x O 19  intrinsic magnetic properties of permanent magnet materials. Scanning electron microscopy (SEM; Zeiss) was used to analyze the ceramic microstructures on the polished and thermally etched faces of the magnets (perpendicular to the applied pressure and field).

Effect of Calcined Ferrite Stoichiometry
The general formula of M-type ferrite is AFe12O19, where A = Ba or Sr, and a part of these elements can be replaced by rare-earth elements. Among them, a part of Fe could be substituted by cobalt (Co) to obtain better magnetic performance that would result in high intrinsic magnetization Ms and anisotropic magnetic field strength HA [13][14][15][16][17][18][19]. Besides, the element at the A position can be substituted by other rare-earth elements, such as La, since La 3+ (1.17 Å), Ba 2+ (1.22 Å), and Sr 2+ (1.32 Å) share almost the same radius. La has the highest solid solution limit for M-type ferrites, and the substitution of La at the A position can enhance the solid content of Co that substitutes Fe, which is critical and beneficial to the improvement of the magnetic characteristics of Sr ferrite [11,13,16,25]. To investigate the influence of the amount of La substituted for Sr in the calcined ferrite on the magnetic properties, a non-stoichiometric formula composed of LaxSr1-xFe12-2xO19 was selected to form Fe-deficient Sr base M-type ferrite with x = 0.28~0.32. It can be seen from the experimental results in Figure 1 that the substitution amount of La2O3 in the calcined ferrite must be sufficient to react with cobalt added during the fine-milled process to promote the diffusion of Co ions in the sintering reaction to achieve better magnetic properties. When x = 0.32, the level of Br = 4536G and iHc = 4362Oe can be obtained. It can be clearly determined that the higher the amount of La2O3 in the calcined ferrite, the higher the Br and iHc. However, if the amount of La2O3 in the calcined ferrite is too high, it will cause a mismatch with the Co added after calcination, resulting in a decrease in magnetic properties [24]. The X-ray diffraction patterns of the Sr ferrite M-type ferrite LaxSr1-xFe12-2xO19 calcined powders with x = 0.28~0.34 are given in Figure     In order to investigate the influence of the main raw material SrCO3 with different particle sizes on the magnetic properties, two different particle sizes of SrCO3 are used for experiments in this work. Table 1 shows the particle size analysis results, and Figure 3 shows the particle size distribution, where both were measured by the Sympatec laser scattering system under the sample analysis volume of 0.5 g. The SEM micrograph of SrCO3 powders is shown in Figure 4. From the above results, it can be found that the morphology and sizes of the two different types of SrCO3 are very different. Figure 5 shows an SEM of the LaxSr1-xFe12-2xO19 calcine powders with two different types of SrCO3 as the raw material. From the results of Figure 6, it can be observed that if the SrCO3 with finer particle size (expressed by the D50 value in Table 1) is used as the main raw material, the Br of Sr ferrite will decrease significantly, while the iHc will increase slightly. Corresponding to the SEM results of the calcined magnetic powder in Figure 5, it could be found that the calcined magnetic powder with a finer particle size of SrCO3 as the main raw material has smaller grains of approximately 1~1.5 μm, which also causes the remanence Br to be relatively low. In contrast, the calcined magnetic powder with a coarser particle size with SrCO3 as the main raw material has a larger size of approximately 1.5~3 μm and tends to have a relative hexagonal plate, and its corresponding magnetic properties are also better.  In order to investigate the influence of the main raw material SrCO 3 with different particle sizes on the magnetic properties, two different particle sizes of SrCO 3 are used for experiments in this work. Table 1 shows the particle size analysis results, and Figure 3 shows the particle size distribution, where both were measured by the Sympatec laser scattering system under the sample analysis volume of 0.5 g. The SEM micrograph of SrCO 3 powders is shown in Figure 4. From the above results, it can be found that the morphology and sizes of the two different types of SrCO 3 are very different. Figure 5 shows an SEM of the La x Sr 1-x Fe 12-2x O 19 calcine powders with two different types of SrCO 3 as the raw material. From the results of Figure 6, it can be observed that if the SrCO 3 with finer particle size (expressed by the D 50 value in Table 1) is used as the main raw material, the B r of Sr ferrite will decrease significantly, while the i H c will increase slightly. Corresponding to the SEM results of the calcined magnetic powder in Figure 5, it could be found that the calcined magnetic powder with a finer particle size of SrCO 3 as the main raw material has smaller grains of approximately 1~1.5 µm, which also causes the remanence B r to be relatively low. In contrast, the calcined magnetic powder with a coarser particle size with SrCO 3 as the main raw material has a larger size of approximately 1.5~3 µm and tends to have a relative hexagonal plate, and its corresponding magnetic properties are also better.   . The SEM micrograph of two different raw material SrCO3 powders: (a) coarse-particle SrCO3, (b) fine-particle SrCO3.
(a) (b) Figure 5. The SEM of the LaxSr1-xFe12-2xO19 calcined powders with two different particle sizes of SrCO3 as the raw material: (a) coarse-particle SrCO3 Scheme 3, (b) fine-particle SrCO3.   (a) (b) Figure 5. The SEM of the LaxSr1-xFe12-2xO19 calcined powders with two different particle sizes of SrCO3 as the raw material: (a) coarse-particle SrCO3 Scheme 3, (b) fine-particle SrCO3.   (a) (b) Figure 5. The SEM of the LaxSr1-xFe12-2xO19 calcined powders with two different particle sizes of SrCO3 as the raw material: (a) coarse-particle SrCO3 Scheme 3, (b) fine-particle SrCO3.  The additive CaCO3 could promote densification and uniform grain growth of hard ferrite, resulting in high remanence, but at the same time, iHc must be sacrificed [24,25,26] . Enhancement of Br could also be obtained by using Co3O4 in calcined material, and Co3O4 reduces iHc much better than CaCO3, but the cost of Co3O4 is much higher than adding CaCO3. Therefore, this study discusses the effect of adding CaCO3 before calcination on the improvement of magnetic properties. It can be seen from the comparison of the magnetic properties in Figure 7 that the addition of CaCO3 before calcination can further increase Br, but excessive CaCO3 will cause a decrease in both Br and iHc, especially in iHc. The main reason for the decrease of Br due to excessive CaCO3 is caused by too much non-magnetic phases in the calcined ferrite. Here, it is more appropriate to control the addition of CaCO3 at 0.15% before calcination.   The additive CaCO 3 could promote densification and uniform grain growth of hard ferrite, resulting in high remanence, but at the same time, i H c must be sacrificed [24][25][26]. Enhancement of B r could also be obtained by using Co 3 O 4 in calcined material, and Co 3 O 4 reduces i H c much better than CaCO 3 , but the cost of Co 3 O 4 is much higher than adding CaCO 3 . Therefore, this study discusses the effect of adding CaCO 3 before calcination on the improvement of magnetic properties. It can be seen from the comparison of the magnetic properties in Figure 7 that the addition of CaCO 3 before calcination can further increase B r , but excessive CaCO 3 will cause a decrease in both B r and i H c , especially in i H c . The main reason for the decrease of B r due to excessive CaCO 3 is caused by too much non-magnetic phases in the calcined ferrite. Here, it is more appropriate to control the addition of CaCO 3 at 0.15% before calcination. The additive CaCO3 could promote densification and uniform grain growth of hard ferrite, resulting in high remanence, but at the same time, iHc must be sacrificed [24,25,26] . Enhancement of Br could also be obtained by using Co3O4 in calcined material, and Co3O4 reduces iHc much better than CaCO3, but the cost of Co3O4 is much higher than adding CaCO3. Therefore, this study discusses the effect of adding CaCO3 before calcination on the improvement of magnetic properties. It can be seen from the comparison of the magnetic properties in Figure 7 that the addition of CaCO3 before calcination can further increase Br, but excessive CaCO3 will cause a decrease in both Br and iHc, especially in iHc. The main reason for the decrease of Br due to excessive CaCO3 is caused by too much non-magnetic phases in the calcined ferrite. Here, it is more appropriate to control the addition of CaCO3 at 0.15% before calcination.    For permanent ferrite, the cost of La 2 O 3 used for production of the magnet accounts for more than 10% of the total raw material cost. This study wishes to reduce the amount of La 2 O 3 in the calcined ferrite to further reduce the total cost, but the magnetic properties must also be considered. The effect of CaCO 3 added before calcination on the magnetic properties is discussed above. In this section, in order to modify the magnetic properties after reduced La in the calcined formula, the influence of CaCO 3 added after calcination is investigated. The abnormal grain growth with a severe duplex structure is usually observed in M-type ferrites without additives, and is more obvious at a sintering temperature higher than 1200 • C. SiO 2 can be used as a little additive to form a liquid phase for suppressing grain growth and enhancing the coercivity of hard ferrite [27,28]; however, a decreased sintered density results in decreased B r . Thus, the simultaneous addition of CaCO 3 and SiO 2 after calcination is needed for moderating grain growth inhibition and promoting the sintered density [20,27,28]. It can be seen from the experimental results in Figure 8 that while x (La content) decreases from 0.32 to 0.30, the overall magnetic properties are significantly improved by the combined addition of CaCO 3 and SiO 2 after calcination. When CaCO 3 = 0.7 wt% and SiO 2 = 0.25 wt% added after calcination, the overall magnetic properties can reach a better state, which met the requirements of high-efficiency motor applications in automobiles with B r > 4600 ± 50 G and i H c > 3600 ± 50 Oe [29].
Processes 2021, 9, x FOR PEER REVIEW 8 of 12 For permanent ferrite, the cost of La2O3 used for production of the magnet accounts for more than 10% of the total raw material cost. This study wishes to reduce the amount of La2O3 in the calcined ferrite to further reduce the total cost, but the magnetic properties must also be considered. The effect of CaCO3 added before calcination on the magnetic properties is discussed above. In this section, in order to modify the magnetic properties after reduced La in the calcined formula, the influence of CaCO3 added after calcination is investigated. The abnormal grain growth with a severe duplex structure is usually observed in M-type ferrites without additives, and is more obvious at a sintering temperature higher than 1200 °C. SiO2 can be used as a little additive to form a liquid phase for suppressing grain growth and enhancing the coercivity of hard ferrite [27,28]; however, a decreased sintered density results in decreased Br. Thus, the simultaneous addition of CaCO3 and SiO2 after calcination is needed for moderating grain growth inhibition and promoting the sintered density [20,27,28]. It can be seen from the experimental results in Figure 8 that while x (La content) decreases from 0.32 to 0.30, the overall magnetic properties are significantly improved by the combined addition of CaCO3 and SiO2 after calcination. When CaCO3 = 0.7 wt% and SiO2 = 0.25 wt% added after calcination, the overall magnetic properties can reach a better state, which met the requirements of high-efficiency motor applications in automobiles with Br > 4600 ± 50 G and iHc > 3600 ± 50 Oe [29]. In order to improve the magnetic performance of the sintered ferrite magnet, both La and Co must be used at the same time to increase the reactivity of Co 2+ with La 3+ . If only La is contained in the permanent magnet ferrite, the effect of improving Br and iHc will not be obvious. This is because when La and Co undergo a substitution reaction in the permanent ferrite, the ratio of La to Co needs to reach a certain value, which is not suitable for values which are too large or too small [3,[11][12][13][14][15][16][17][18]. Past studies usually designed a calcining La-Co system material formula to simultaneously achieve high Br and high iHc [3,[11][12][13][14][15][16][17][18]. It should be noticed that this design of the formula requires the participation of the precious metal element of cobalt to be doped in the calcining formula, which may result in lower production flexibility, big mass-production hazards, and greater costs. In this study, cobalt was added as a little additive after the calcination process, and replaced Fe during the sintering process. From the result shown in Figure 9, the magnetic properties of Sr ferrite are moderately modified by Co3O4 simultaneously added with CaCO3 and  In order to improve the magnetic performance of the sintered ferrite magnet, both La and Co must be used at the same time to increase the reactivity of Co 2+ with La 3+ . If only La is contained in the permanent magnet ferrite, the effect of improving B r and i H c will not be obvious. This is because when La and Co undergo a substitution reaction in the permanent ferrite, the ratio of La to Co needs to reach a certain value, which is not suitable for values which are too large or too small [3,[11][12][13][14][15][16][17][18]. Past studies usually designed a calcining La-Co system material formula to simultaneously achieve high B r and high i H c [3,[11][12][13][14][15][16][17][18]. It should be noticed that this design of the formula requires the participation of the precious metal element of cobalt to be doped in the calcining formula, which may result in lower production flexibility, big mass-production hazards, and greater costs. In this study, cobalt was added as a little additive after the calcination process, and replaced Fe during the sintering process. From the result shown in Figure 9, the magnetic properties of Sr ferrite are moderately modified by Co 3 O 4 simultaneously added with CaCO 3 and SiO 2 after calcination. At a lower amount of Co 3 O 4 added after calcination (Co 3 O 4 = 1.7 wt%), the goal of B r > 4600 ± 50 G and i H c > 3600 ± 50 Oe could be achieved at the same time.
SiO2 after calcination. At a lower amount of Co3O4 added after calcination (Co3O4 = 1.7 wt%), the goal of Br > 4600 ± 50 G and iHc > 3600 ± 50 Oe could be achieved at the same time. Figure 9. Dependence of Br and iHc of the sintered magnet on the amount of Co3O4 added after the calcination process. Figure 10 presents the micromorphology of the ferrite La0.3Sr0.7Fe11.4O19 sintered magnet at 1230 °C with additive CaCO3 = 0.7 wt%, SiO2 = 0.25 wt% and Co3O4 = 1.7 wt% added after calcination. The grains in the magnet are plate-shaped, and the normal direction of plate-like grains harmonizes with the easy magnetization axis (i.e., C-axis). The alignment of the pressed powder was not annihilated during the sintering process and has good uniformity of grain size approximately smaller than 2 μm; thus, the grains would be oriented and grown, bringing about the high performance of the sintering ferrite magnets, which can also be observed from the B-H hysteresis curves shown in Figure 11. The magnetic results obtained in this study are better than traditional Sr-La-Co ferrites [3,[16][17][18], and help to achieve the purpose of high-end permanent magnets and large-scale industrial production.  The alignment of the pressed powder was not annihilated during the sintering process and has good uniformity of grain size approximately smaller than 2 µm; thus, the grains would be oriented and grown, bringing about the high performance of the sintering ferrite magnets, which can also be observed from the B-H hysteresis curves shown in Figure 11. The magnetic results obtained in this study are better than traditional Sr-La-Co ferrites [3,[16][17][18], and help to achieve the purpose of high-end permanent magnets and large-scale industrial production.
Processes 2021, 9, x FOR PEER REVIEW 9 of 12 SiO2 after calcination. At a lower amount of Co3O4 added after calcination (Co3O4 = 1.7 wt%), the goal of Br > 4600 ± 50 G and iHc > 3600 ± 50 Oe could be achieved at the same time. Figure 9. Dependence of Br and iHc of the sintered magnet on the amount of Co3O4 added after the calcination process. Figure 10 presents the micromorphology of the ferrite La0.3Sr0.7Fe11.4O19 sintered magnet at 1230 °C with additive CaCO3 = 0.7 wt%, SiO2 = 0.25 wt% and Co3O4 = 1.7 wt% added after calcination. The grains in the magnet are plate-shaped, and the normal direction of plate-like grains harmonizes with the easy magnetization axis (i.e., C-axis). The alignment of the pressed powder was not annihilated during the sintering process and has good uniformity of grain size approximately smaller than 2 μm; thus, the grains would be oriented and grown, bringing about the high performance of the sintering ferrite magnets, which can also be observed from the B-H hysteresis curves shown in Figure 11. The magnetic results obtained in this study are better than traditional Sr-La-Co ferrites [3,[16][17][18], and help to achieve the purpose of high-end permanent magnets and large-scale industrial production.

Conclusions
In this study, the ferrite formula and process technique of high-performance anisotropic Fe-deficient Sr base M-type ferrite magnets were successfully developed, and the magnetic characteristics, such as Br and iHc, were obviously ameliorated. We especially emphasize that the calcining material formula of LaxSr1-xFe12-2xO19 developed by this research does not include the precious metal cobalt, the amount of La2O3 in the calcined ferrite is small, and a little amount of the additive Co3O4 is added after the calcination process, which overcomes high-cost technical bottlenecks, such as excessive La2O3 and Co3O4 contained in the magnet in order to achieve high characteristics in the past, and we comprehensively improve magnetic characteristics and production efficiency. Besides, the incorporation of CaCO3, SiO2, and Co3O4 in the fine-milling step is well-suited for promoting the intrinsic magnetic properties, densification, and possible grain uniformity of Mtype Sr ferrite without allowing for too much grain growth and making the magnetic powder more easily aligned by the applied field. The extreme utilization of these three additives added after calcination presents the opportunity to achieve high magnetic properties (i.e., Br = 4608 Gauss, bHc = 3650 Oe, iHc = 3765 Oe, and (BH)max = 5.23 MGOe) under the little Co3O4 addition of 1.7 wt%, which are desirable for higher-grade brushless DC motor applications, especially in modern electric vehicles.

Conclusions
In this study, the ferrite formula and process technique of high-performance anisotropic Fe-deficient Sr base M-type ferrite magnets were successfully developed, and the magnetic characteristics, such as B r and i H c , were obviously ameliorated. We especially emphasize that the calcining material formula of La x Sr 1-x Fe 12-2x O 19 developed by this research does not include the precious metal cobalt, the amount of La 2 O 3 in the calcined ferrite is small, and a little amount of the additive Co 3 O 4 is added after the calcination process, which overcomes high-cost technical bottlenecks, such as excessive La 2 O 3 and Co 3 O 4 contained in the magnet in order to achieve high characteristics in the past, and we comprehensively improve magnetic characteristics and production efficiency. Besides, the incorporation of CaCO 3 , SiO 2 , and Co 3 O 4 in the fine-milling step is well-suited for promoting the intrinsic magnetic properties, densification, and possible grain uniformity of M-type Sr ferrite without allowing for too much grain growth and making the magnetic powder more easily aligned by the applied field. The extreme utilization of these three additives added after calcination presents the opportunity to achieve high magnetic properties (i.e., B r = 4608 Gauss, b H c = 3650 Oe, i H c = 3765 Oe, and (BH) max = 5.23 MGOe) under the little Co 3 O 4 addition of 1.7 wt%, which are desirable for higher-grade brushless DC motor applications, especially in modern electric vehicles.

Conflicts of Interest:
The authors declare no conflict of interest.