Effects of La-Co Co-Substitution and Magnetic Field Pressing on the Structural and Magnetic Properties of SrM Hexaferrites

: We carefully investigated the effects of La-Co co-substitution and magnetic field pressing (MFP) on the structural and magnetic properties of SrM hexaferrites. Samples composed of Sr 1 − x La x Fe 12 − x Co x O 19 were sintered at 1230 ◦ C for 2 h in air with sintering additives composed of 0.7 wt% CaCO 3 and 0.7 wt% SiO 2 . A single M-type phase was confirmed to exist up to x = 0.3. Ri-etveld refinement revealed a slight decrease in lattice parameter a and the unit cell volume ( V cell ) with an increasing x , while parameter c showed a significant decrease. The saturation magnetization ( M s ) values increased from 70.90 to 72.40 emu/g with an increasing x from 0.0 to 0.15 and then decreased to 71.38 emu/g with further increasing of x to 0.3, while the anisotropy field ( H a ) increased from 17.7 to 25.9 kOe, leading to a continuous increase in the intrinsic coercivity ( H ci ), from 3.52 to 5.00 kOe, respectively. Using the MFP process, the c -axis of M-type hexaferrite grains could be effectively aligned to the applied field direction, which significantly affected the microstructures and, thus, magnetic properties of samples. Unlike non-MFP samples, exhibiting a significant increase in the average grain size ( d avg ) but almost unaltered average thickness ( t avg ) with an increasing x from 0.0 to 0.3, MFP-processed samples exhibited almost unaltered d avg values but a continuous decrease in t avg . Consequently, the variation in remanent flux density ( B r ) versus x followed that of M s versus x and thus exhibited the highest B r of 4.05 kG for x = 0.15, leading to the highest maximum energy product {( BH ) max } of 3.62 MGOe. With an increasing x from 0.0 to 0.3, the H ci values continuously increased from 3.14 to 3.84 kOe mainly due to a continuous increase in H a , although they were significantly lowered in comparison with those of non-MFP samples because of a large increase in B r for a given composition x. A higher M r / M s ratio always resulted in a larger ( BH ) max in our samples regardless of x . A careful comparison of the microstructures and magnetic properties between MFP and non-MFP samples provided valuable insights into a broad area of permanent magnet optimization.


Introduction
M-type (MeFe 12 O 19 Me: Ba, Sr) hexaferrites have been applied for ceramic permanent magnets due to their cost-effectiveness, chemical stability, and notable hard magnetic characteristics, including high M S and high H a [1][2][3].Thus, these permanent magnets have been used for the motor components of automobiles, household washing machines, air conditioners, etc.Among various fabrication processes employed to prepare M-type hexaferrites [1,4], the solid-state reaction is a commonly employed method in industrial manufacturing due to its cost-effectiveness and suitability for mass production.The performance of a permanent magnet is normally evaluated on the basis of its B r , H ci , and (BH) max values, which are closely related to intrinsic properties, such as M s and H a [1][2][3]5].
The intrinsic properties of M S and H a for M-type hexaferrites depend on the cation substitution composition and site occupancy in the unit cell [1,[3][4][5].M-type hexaferrites consist of the stacked structures of spinel (S, Fe 6 O 8 ) and hexagonal (R, MFe 6 O 11 ) blocks, represented as SRSR, where the asterisk (*) denotes a 180 • rotation around the c-axis.The crystallographic sublattices of Fe 3+ in the M-type structure are categorized into five sublattices: 2a, 2b, 4f 1 , 4f 2 , and 12k sites.The spin magnetic moments of Fe 3+ ions at the 12k, 2a, and 2b sites align parallel to the c-axis, while those at the 4f 1 and 4f 2 sites align antiparallel to it.Consequently, this arrangement results in a net magnetic moment of 20 µ B per unit cell at 0 K [1,6].The M S value can be improved through the substitution of cations like Co 2+ [7], Zn 2+ [8], Al 3+ [9], and Ti 4+ [10] for the Fe 3+ sites.On the other hand, H a can be increased by substituting rare earth elements for the Sr site, such as La [11], Nd [12], and Sm [12].While a simultaneous improvement of both M S and H a values via a single element substitution has never been reported [1,[8][9][10][11][12][13], among various substituents explored, La-Co co-substitution has been reported to be very effective for the enhancement of H a without a significant reduction in M S [14][15][16][17][18][19][20].
The magnetic properties of M-type bulk magnets can be influenced by various factors, such as the sintering density, the average grain size and distribution, and the c-axis alignment of grains, in addition to chemical compositions [21].The magnetic properties of sintered samples are largely influenced by the sintering additives [22,23], sintering temperature and time [24], and atmosphere [25,26].High sintered density and high grain alignment along the c-axis dominantly contribute to an increase in B r , while the size and distribution of grains with multiple domains have an inverse relationship with H ci .The sintering additives of SiO 2 and CaCO 3 have been commonly used for M-type hexaferrites not only to achieve high sintered density through a liquid-phase sintering but also to suppress grain growth [22,23].
Meanwhile, the MFP process is a highly effective technique for the fabrication of high-performance permanent magnets.This is achieved by aligning the magnetic easy axes of individual particles parallel to the external magnetic field during the pressing process [3,27].The c-axis alignment of M-type hexaferrite particles via the MFP method significantly improves the squareness of the demagnetization curve of a sintered bulk sample, resulting in a marked enhancement in both B r and (BH) max .The effectiveness of the MFP method extends to the industrial production of various permanent magnets, such as M-type hexaferrite [28], SmCo 5 [29], Sm 2 TM 17 [30], and Nd-Fe-B magnets [31].Although the MFP process has been widely used in the industry, previous reports on Mtype hexaferrites for permanent magnets mainly focus on non-MFP polycrystalline samples with randomly oriented grains, making it hard to find a comprehensive study for the effect of c-axis alignment on magnetic properties [17,18,32].Therefore, it is critical to conduct a comparative study between non-MFP and MFP samples-as such studies are hardly found in the previous literature-in order to understand their relationship in microstructures and extrinsic magnetic properties, such as H ci , B r , and (BH) max .For this purpose, we carefully investigated the lattice parameters, microstructures, and magnetic properties of both non-MFP and MFP samples.In this study, we carefully investigated the lattice parameters, microstructures, and magnetic properties of La-Co co-substituted Sr M-type hexaferrites with compositions of Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) for a comparative study between bulk samples prepared with and without applying the MFP process.

Experimental
The La-Co-co-substituted Sr M-type hexaferrites with the nominal compositions of Sr 1−x La x Fe 12−x Co x O 19 , (0.0 ≤ x ≤ 0.4) were prepared via a solid-state reaction in air.We measured the weights of SrCO 3 and Fe 2 O 3 (purity 99.9%, Gojundo, Uiwang-si, Republic of Korea) to have the compositional ratio of Sr:Fe = (1 − x):(12 − x), added 0.3 wt% SiO 2 additive, and ball-milled the powder mixture with zirconia balls in ethanol for 24 h.The powder mixture was dried in an oven at 60 • C for 3 h.Then, the powder mixture was calcined at 1280 • C for 2 h in air.The substituents of La 2 O 3 and Co 3 O 4 powders (99.9% purity, Gojundo, Uiwang-si, Republic of Korea) and the additives of 0.7 wt% CaCO 3 and 0.4 wt% SiO 2 were added to as-calcined powder.The mixed powders were wet-milled with zirconia balls in ethanol for 24 h, dried in an oven at 60 • C for 3 h, and then pelletized with a uniaxial press (10 mm diameter) at a pressure of 220 MPa (Jeil Hydraulics, Seoul, Republic of Korea) to produce randomly oriented samples (Non-MFP1 samples).Also, ballmilled powders were mixed with a distilled water and pelletized with the uniaxial press at 1.5 tons, while applying the external magnetic field of 15 kOe to produce c-axis-aligned samples (MFP samples).We also prepared uniaxially pressed samples (1 inch diameter) at a pressure of 220 MPa and followed by CIP (cold isostatic pressing) with 1.5 tons of non-MFP2 samples.The resulting pellets were sintered at 1230 • C for 2 h, in air.
Powder X-ray diffraction (XRD, D8 Advance, Bruker, Billerica, MA, USA) was performed using a Cu Kα source (λ = 0.15406 nm).Phase identification was conducted with a divergence slit width of 0.2 mm, a voltage of 40V, a current of 40A, and a scanning speed of 2 • /min.The lattice parameters were calculated based on the refined XRD data, using a Rietveld refinement analysis (Bruker TOPAS, V6, software).The microstructure and composition of the samples were analyzed using field emission scanning electron microscopy (FE-SEM, MERLIN Compact, Zeiss, Oberkochen, Germany) with an energy-dispersive spectrometer (EDS).The average grain size was evaluated by applying an intercept line method to the SEM micrographs.To observe the grain boundary, polished samples were thermally etched in a muffle furnace at 1130 • C for 0.5 h, in air.Magnetic hysteresis curves were measured at room temperature, using a vibrating sample magnetometer (VSM-7410, Lakeshore, Westerville, OH, USA) up to the maximum applied field of ±26 kOe.The samples were cut into a cube shape with the dimensions of 2.0 × 2.0 × 2.0 mm for the M-H measurement.The M s and H a values were determined using the law of approach to saturation (LAS) by fitting the initial curves.The B-H hysteresis curves were measured for the samples of 1 inch in diameter at room temperature using a B-H tracer (Permagraph C-300, Magnet-Physik, Cologne, Germany).From this figure, one can see that the M-type single phase is obtainable from x values up to 0.35.Previous studies have demonstrated that La and Co ions exhibit solubility in SrM in the x region of 0.0-0.35[14,16].However, the sample with x = 0.4 exhibits minor peaks of Fe 2 O 3 [ICDD card 04-008-7623], LaFeO 3 [ICDD card 00-037-1493], and CoFe 2 O 4 [ICDD card 00-022-1086], evidencing that this nominal composition is above the solubility limit of La-Co co-substitution in SrM and hence the M-type single phase composed of x = 0.4 is unobtainable at the sintering conditions of 1230 • C for 2 h in air.

Results and Discussion
Although a secondary phase is undetectable in the XRD pattern of the sample with x = 0.35 in Figure 1, we performed a SEM-EDS analysis for this sample to further identify whether it is composed of the M-type single phase or not. Figure 2 presents SEM backscattered electron (BSE) images, along with corresponding EDS element mapping micrographs illustrating the distribution of O, Fe, La, and Sr in the Sr 0.65 La 0.35 Fe 11.65 Co 0.35 O 19 (x = 0.35) samples.In Figure 2, a secondary phase exhibits a brighter contrast compared to the primary phase (SrM).Within this secondary phase, the Fe content is notably lower, while the Sr content is higher compared to the main phase.The phases are presumed to be (La, Sr), (Fe,Co)O 3 , and CoFe 2 O 4 , although it was undetectable in the XRD patterns of the x = 0.35 sample shown in Figure 1.The solubility limit of La-Co in the SrM is believed to exist between x = 0.3 and 0.35 at 1230 • C, in air, from Figures 1 and 2. Therefore, the secondary phases observed for the sample x = 0.4 in Figure 1  No secondary phase was observed in the SEM-EDS analysis for samples with x ≤ 0.3, suggesting that the M-type single phase is achievable from the samples of x ≤ 0.3.The solubility limit of La-Co in the SrM is believed to exist between x = 0.3 and 0.35 at 1230 • C, in air, from Figures 1 and 2.
0.35 sample shown in Figure 1.The solubility limit of La-Co in the SrM is believed to exist between x = 0.3 and 0.35 at 1230 °C, in air, from Figures 1 and 2. Therefore, the secondary phases observed for the sample x = 0.4 in Figure 1 are presumed not to be Fe2O3, LaFeO3, and CoFe2O4 but to be (Fe,Co)2O3, (La, Sr)(Fe,Co)O3, and CoFe2O4 since Sr and Co can substitute for the La and Fe sites, respectively, except the spinel phase of CoFe2O4.No secondary phase was observed in the SEM-EDS analysis for samples with x ≤ 0.3, suggesting that the M-type single phase is achievable from the samples of x ≤ 0.3.The solubility limit of La-Co in the SrM is believed to exist between x = 0.3 and 0.35 at 1230 °C, in air, from Figures 1 and 2.   0.35 sample shown in Figure 1.The solubility limit of La-Co in the SrM is believed to exist between x = 0.3 and 0.35 at 1230 °C, in air, from Figures 1 and 2. Therefore, the secondary phases observed for the sample x = 0.4 in Figure 1 are presumed not to be Fe2O3, LaFeO3, and CoFe2O4 but to be (Fe,Co)2O3, (La, Sr)(Fe,Co)O3, and CoFe2O4 since Sr and Co can substitute for the La and Fe sites, respectively, except the spinel phase of CoFe2O4.No secondary phase was observed in the SEM-EDS analysis for samples with x ≤ 0.3, suggesting that the M-type single phase is achievable from the samples of x ≤ 0.3.The solubility limit of La-Co in the SrM is believed to exist between x = 0.3 and 0.35 at 1230 °C, in air, from Figures 1 and 2.   Rietveld refinement was used to calculate the lattice parameters a and c, with R wp (weighted profile R-value) < 9% and χ 2 (reduced Chi-squared value) smaller than 1.4 accuracy.The lattice parameters a and c were calculated from the d hkl values obtained from powder XRD patterns based on the following equation [14], where d hkl is the inter-planer spacing; and h, k, and l are the Miller indices.The cell volume (V cell ) was calculated from a 2 csin120 • .The lattice parameters of Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.30) samples are shown in Figure 3.As shown in Figure 3, the lattice parameter a gradually decreases with the increasing x in Sr 1−x La x Fe 12−x Co x O 19 from 0.0 to 0.25, and then it turns to slightly increase with a further increasing x, up to 0.30, while the lattice parameter c monotonously decreases up to x = 0.30.The unit cell volume (V cell ) monotonously decreases with the increasing x from 0 to 0.3, as illustrated in Figure 3b.Notably, La 3+ (1.22 Å) and Co 2+ (0.75 Å) can substitute for Sr 2+ (1.31 Å) and Fe 3+ (0.65 Å) sites, respectively [33].The ionic radius of La 3+ is smaller than that of Sr 2+ , while the ionic radius of Co 2+ is larger than that of Fe 3+ .As La-Co is co-substituted into SrM, and Fe 2+ (0.78 Å) is formed due to a local substitution imbalance between La 3+ and Co 2+ [34][35][36].According to Loan et al. [34], with an increasing x, there is a corresponding increase in the concentration of Co 2+ occupying the 2a site of Fe 3+ for x < 0.15.Furthermore, it can be inferred that the occupation of Co 2+ at the 2a site results in the formation of Fe 2+ at the 2a site.Rietveld refinement was used to calculate the lattice parameters a and c, with Rwp (weighted profile R-value) < 9% and χ 2 (reduced Chi-squared value) smaller than 1.4 accuracy.The lattice parameters a and c were calculated from the dhkl values obtained from powder XRD patterns based on the following equation [14], where dhkl is the inter-planer spacing; and h, k, and l are the Miller indices.The cell volume (Vcell) was calculated from a 2 csin120°.The lattice parameters of Sr1−xLaxFe12−xCoxO19 (0.0 ≤ x ≤ 0.30) samples are shown in Figure 3.
As shown in Figure 3, the lattice parameter a gradually decreases with the increasing x in Sr1−xLaxFe12−xCoxO19 from 0.0 to 0.25, and then it turns to slightly increase with a further increasing x, up to 0.30, while the lattice parameter c monotonously decreases up to x = 0.30.The unit cell volume (Vcell) monotonously decreases with the increasing x from 0 to 0.3, as illustrated in Figure 3b.Notably, La 3+ (1.22 Å) and Co 2+ (0.75 Å) can substitute for Sr 2+ (1.31 Å) and Fe 3+ (0.65 Å) sites, respectively [33].The ionic radius of La 3+ is smaller than that of Sr 2+ , while the ionic radius of Co 2+ is larger than that of Fe 3+ .As La-Co is cosubstituted into SrM, and Fe 2+ (0.78 Å) is formed due to a local substitution imbalance between La 3+ and Co 2+ [34][35][36].According to Loan et al. [34], with an increasing x, there is a corresponding increase in the concentration of Co 2+ occupying the 2a site of Fe 3+ for x < 0.15.Furthermore, it can be inferred that the occupation of Co 2+ at the 2a site results in the formation of Fe 2+ at the 2a site.In our previous study on La-Co-co-substituted SrM hexaferrites [37], the M-type single phase could be synthesized at 1300 • C for 2 h in air, without sintering additives.For a comparison with the present results, we performed additional post-annealing at 1230 • C for 2 h in air for those as-sintered samples in Ref. [37].The lattice parameters a and c of the postannealed samples are also represented in Figure 3a.Compared with post-annealed samples without sintering additives, the lattice parameter a showed no significant change, while the decrease in parameter c became more pronounced with the increasing x, particularly up to x = 0.25.Consequently, as shown in Figure 3b, the c/a ratio exhibited a significant decrease with an increasing x up to x = 0.25, while the V cell values are somewhat peculiar variation.It is known that Si and Ca, used as sintering additives, can substitute for Fe and Sr sites, respectively [22,38], which most probably caused the above difference in the lattice parameter c.However, a negligible variation in c for x = 0.3 with sintering additives may be due to a reduced effect of the sintering additives on c at a much higher La-Co concentration relative to the amounts of sintering additives.
Consistent with previous reports on the Sr 1−x La x Fe 12−x Co x O 19 samples with sintering additives of SiO 2 and CaCO 3 [19,20], the lattice constant c continuously decreases while the lattice constant a negligibly changes with the increasing x.Iida et al. [19] and Ogata et al. [20] commonly attribute the variations in the lattice parameters only to differences in ionic radii between La 3+ and Co 2+ substituents and Sr 2+ and Fe 3+ ions in SrM, respectively, but not take the effect of sintering additives into account.However, since the sintering additives of SiO 2 and CaCO 3 contribute to the increase in the lattice parameter c up to x = 0.25, we argue that, when the sintering additives are used, their effects should be taken into account to understand the variation in the lattice parameters as a function of x if they substitute for the cation sites.
Figure 4 shows the SEM micrographs of the Sr 1−x La x Fe 12−x Co x O 19 samples (0.0 ≤ x ≤ 0.3).The average grain size (d avg ) and its standard deviation (SD) are listed in Table 1.The values of both d avg and SD increase from 1.20 ± 0.15 µm to 1.96 ± 0.87 µm as x increases from 0.0 to 0.3, while the average thickness of the hexagonal plate (t avg ) of 0.95 ± 0.34 µm insignificantly changed.To determine the d avg and t avg values from the SEM micrographs composed of randomly oriented grains, we selected about 150 grains with a hexagonal plate shape and also about 150 grains with a rectangular shape, respectively.For this purpose, many different SEM micrographs were used for each composition x.The t avg values from the rectangular grains are surely somewhat overestimated since the cross-sectional cut along the c-axis of each grain is less probable.However, although we take this overestimation into account, this t avg evaluation of randomly oriented grains is very meaningful to compare with that of MFP-processed samples later.For this purpose, many SEM micrographs were used.As shown in Figure 4a, the sample of x = 0.0 exhibits a relatively smaller grain size and more uniform size distribution.However, the grain grows, and its size distribution becomes less uniform with the increasing x, as shown in Figure 4b-f.During the sintering process, a solid-state reaction leads to grain growth and densification, while a liquid-phase sintering can cause a rapid growth and densification even at a relatively lower sintering temperature [39].The solid-state reaction of SrCO 3 and Fe 2 O 3 to form SrFe 12 O 19 is typically completed at the sintering temperature of 1200 • C [23].Sintering additives such as SiO 2 and CaCO 3 are utilized to create a liquid phase during sintering, which might accelerate the formation of the M-type phase.Kang et al. [40] demonstrated that the grain growth behavior in polycrystalline ceramics can be explained by the relative values between the critical driving force and the maximum driving force for growth.The maximum and critical driving forces are influenced by factors such as the grain size, dopants, sintering atmosphere, and temperature.According to Moon et al. [41], the addition of Co 3 O 4 to SrM as a sintering additive promotes the grain growth of M-type hexaferrite [41].Therefore, the increase in the number of relatively larger grains shown in Figure 4b-f can be attributed to the compositional effect of La-Co substitution on these driving forces.The initial magnetization curves and hysteresis loops of the Sr1−xLaxFe12−xCoxO19 (0.0 ≤ x ≤ 0.3) samples are shown in Figure 5a,b, respectively.From the initial magnetization curves shown in Figure 5a, the MS and Ha values of the samples were calculated using the law of approach to saturation (LAS) from the following equation [42]: Table 1.The average grain sizes corresponding to the average length of the hexagonal plate (d avg ) and the average thickness of the hexagonal plate (t avg ), its standard deviation (SD), average aspect ratio, average volume (V avg ), sintered density (D), and relative density (D/D th ) for the non-MFP1 Sr   5a,b, respectively.From the initial magnetization curves shown in Figure 5a, the M S and H a values of the samples were calculated using the law of approach to saturation (LAS) from the following equation [42]: where M represents magnetization induced by an applied magnetic field, H; A is the inhomogeneity parameter associated with the material's inhomogeneity; χ p is the high field susceptibility; and B is the anisotropy parameter linked to magnetocrystalline anisotropy.The reported value for A is approximately 250 Oe in BaM [42], and the parameter χ p becomes negligible in high magnetic fields.Therefore, Equation ( 2) can be approximated by the following equation:    For a hexagonal crystal structure exhibiting uniaxial anisotropy, B can be expressed as follows: The determination of M S involves analyzing the linear portion of the M vs. 1/H 2 curves, with the condition that R 2 is greater than or equal to 0.999.The values for A and H a can be calculated by fitting Equation (3) to the measured initial magnetization curves.It is important to note that R 2 is a statistical parameter reflecting the quality of the curve fit, and for reliable results, it should be at least 0.99.The A, M S , and H a values, along with their R 2 values calculated from Figure 5a and the intrinsic coercivity, H ci , values from Figure 5b, for all samples are listed in Table 2.As shown in Figure 6, Hci increases with the increase in x.This variation in Hci with x can be understood with the following relationship [43,44]: The effect of the microstructure on Hci is characterized by two factors: α and N. The first term in Equation ( 5) represents the magnetic field required to nucleate a reverse domain, and α increases as the grain size decreases.The second term includes N, Mr, and Ms. Here, N is the demagnetization factor, which increases as the grains become more plateletshaped (i.e., with a higher aspect ratio).As shown in Table 1, increasing x results in larger average grain sizes and higher aspect ratios, causing a reduction in α and an increase in N, respectively.On the other hand, the combined value of Mr and Ms in Table 2 slightly increases up to x = 0.15 and then slightly decreases.Therefore, if Ha remains constant with x, the Hci values should be continuously decreased, according to Equation ( 5).However, as shown in Table 2 and Figure 6, the Hci values significantly increase with the increase in x, suggesting that such an increase in Hci originates from the increase in Ha.The increase in Ha with the increasing x is ascribed to a continuous decrease in the crystal symmetry [14].

Grain Alignments and Microstructures
The magnetic easy axis (c-axis) of M-type hexaferrite grains is aligned parallel to the applied field direction by the MFP process.Figure 7 shows the bulk surface XRD patterns of the MFP-processed Sr1−xLaxFe12−xCoxO19 (x = 0.0, 0.15, 0.2, and 0.3) samples.From this figure, one can see that the bulk surface exhibits grain alignment primarily along the caxis because the peaks of the (0 0 6), (0 0 8), and (0 0 14) planes are much larger than the In order to understand the effect of sintering additives on the intrinsic magnetic properties of M s and H a , we also evaluated those of La-Co-co-substituted SrM bulk samples without additives [37] and post-annealed at the same sintering conditions of 1230 • C for 2 h.Though not presented here, M s values for present samples with additives are lower, i.e., below x = 0.15, but the difference in M s becomes insignificant above x = 0.15.In the case of H a , for the present samples with additives, it is lower only at x = 0.0, but the difference in H a becomes negligible above x = 0.10.The conspicuous difference in both M s and H a at x = 0 most likely originates from the substitution of Ca and Si additive cations for the Sr and Fe sites, respectively.As x increases, the influence of sintering additives on M s and H a seems to become smaller because La-Co substituents affect them dominantly.
The M s and H a values evaluated using LAS have been reported for Sr 1−x La x Fe 12−x Co x O 19 samples by a few research groups [16,19].Unfortunately, it is difficult to compare our M s values with their values because detailed M s evaluation procedures are not described in their papers.Nevertheless, our M s values from x = 0.0 to 3.0, in Table 2, are in good agreement with those reported by Iida et al. [19], who employed the sintering additives of 0.4 wt% SiO 2 and 1.4 wt% CaCO 3 .However, our M s values were higher than those of Sr 1−x La x Fe 12−x Co x O 19 samples reported by X. Liu et al. [16], who employed SiO 2 , CaCO 3 , and Al 2 O 3 (unknown wt%) as the sintering additives below x = 0.1; the difference becomes negligible above x = 0.1.On the other hand, our H a values coincide with those reported only by Iida et al. [19] for the x values ranging from 0.10 to 0.25.Our H a value is smaller for x = 0.0 but it becomes larger for x = 0.30, which may be due to the difference in the amounts of sintering additives.
As shown in Figure 6, H ci increases with the increase in x.This variation in H ci with x can be understood with the following relationship [43,44]: The effect of the microstructure on H ci is characterized by two factors: α and N. The first term in Equation ( 5) represents the magnetic field required to nucleate a reverse domain, and α increases as the grain size decreases.The second term includes N, M r , and M s .
Here, N is the demagnetization factor, which increases as the grains become more plateletshaped (i.e., with a higher aspect ratio).As shown in Table 1, increasing x results in larger average grain sizes and higher aspect ratios, causing a reduction in α and an increase in N, respectively.On the other hand, the combined value of M r and M s in Table 2 slightly increases up to x = 0.15 and then slightly decreases.Therefore, if H a remains constant with x, the H ci values should be continuously decreased, according to Equation (5).However, as shown in Table 2 and Figure 6, the H ci values significantly increase with the increase in x, suggesting that such an increase in H ci originates from the increase in H a .The increase in H a with the increasing x is ascribed to a continuous decrease in the crystal symmetry [14].

Grain Alignments and Microstructures
The magnetic easy axis (c-axis) of M-type hexaferrite grains is aligned parallel to the applied field direction by the MFP process.Figure 7 shows the bulk surface XRD patterns of the MFP-processed Sr 1−x La x Fe 12−x Co x O 19 (x = 0.0, 0.15, 0.2, and 0.3) samples.From this figure, one can see that the bulk surface exhibits grain alignment primarily along the c-axis because the peaks of the (0 0 6), (0 0 8), and (0 0 14) planes are much larger than the main peak of the (1 1 4) plane.However, other peaks, such as the (1 0 7), (1 1 4), and (2 0 11) planes, are not negligible, implying that the c-axis alignment is insufficient.
by ∑I(00l) and ∑I (hkl), respectively.Similarly, the X-ray-integrated intensities of the (00 plane and the (hkl) plane of the non-oriented powder sample are denoted by ∑I(00l) θ an ∑I(hkl) θ , respectively.The w(00l) values evaluated for the MFP samples using Equation (6 are presented in Figure 7.As shown in this figure, the w(00l) increases from 42.4 to 55.9% with the increasing of x from 0.0 to 0.3.The average grain sizes (davg), corresponding to the average length of the hexagona plate, and the average thickness of the hexagonal plate (tavg) were determined from th micrographs shown in Figure 8.The values and their standard deviation (SD) of davg an tavg are listed in Table 3.The average aspect ratio calculated by davg/tavg and the averag volume (Vavg) are also listed in Table 3, together with the measured sintered density (D and evaluated relative density (D/Dth).As shown in Table 3, the davg value slightly increase from 1.30 to 1.38 µm as x increases from 0.0 to 0.3, while the SD values are decreased from ± 0.35 to 0.20 µm.Also, the tavg values of samples decreased from 1.18 to 0.77 µm with th increasing x value, and their SD values also decreased from ± 0.21 to 0.05 µm.The degree of orientation along the c-axis, w(00l), was calculated according to the following formula [45]: where the X-ray-integrated intensities of the (00l) plane and the (hkl) plane are represented by ∑I(00l) and ∑I (hkl), respectively.Similarly, the X-ray-integrated intensities of the (00l) plane and the (hkl) plane of the non-oriented powder sample are denoted by ∑I(00l) θ and ∑I(hkl) θ , respectively.The w(00l) values evaluated for the MFP samples using Equation ( 6) are presented in Figure 7.As shown in this figure, the w(00l) increases from 42.4 to 55.9% with the increasing of x from 0.0 to 0.3.
Figure 8 shows the SEM micrographs of the MFP-processed Sr 1−x La x Fe 12−x Co x O 19 (x = 0.0, 0.15, 0.2, and 0.3) samples.Figure 8a-d show the plane view perpendicular to the applied field, while Figure 8e-h show cross-sectional view parallel to the applied field.Consequently, the grains shown in Figure 8a-d predominantly represent the basal planes of hexaferrites, while those in Figure 8e-h represent cross-sectional views of hexagonal plates.Non-MFP1 samples exhibit non-uniform grains due to the random orientations of adjacent grains (see Figure 4).
The average grain sizes (d avg ), corresponding to the average length of the hexagonal plate, and the average thickness of the hexagonal plate (t avg ) were determined from the micrographs shown in Figure 8.The values and their standard deviation (SD) of d avg and t avg are listed in Table 3.The average aspect ratio calculated by d avg /t avg and the average volume (V avg ) are also listed in Table 3, together with the measured sintered density (D) and evaluated relative density (D/D th ).As shown in Table 3, the d avg value slightly increases from 1.30 to 1.38 µm as x increases from 0.0 to 0.3, while the SD values are decreased from ± 0.35 to 0.20 µm.Also, the t avg values of samples decreased from 1.18 to 0.77 µm with the increasing x value, and their SD values also decreased from ± 0.21 to 0.05 µm.M-type grains have been reported to have a hexagonal plate shape, with the basal plane perpendicular to the c-axis [1], which can also be observed in the planar microstructure in Figure 8.To grow into a hexagonal plate shape, the a-axis growth rate must be faster than the c-axis growth rate.Grain growth occurs via the diffusion of components through the liquid phase formed at the grain boundary during the sintering stage.Through the MFP process, the c-axis of the plate-shaped M-type grains can be dominantly aligned parallel to the applied field.Since the basal plane orientation is in the same direction as other grains during sintering, growth may be hindered, resulting in a slight increase in x.
The d avg , t avg , and aspect ratios listed in Tables 1 and 3 are represented as a function of x for non-MFP1 and MFP samples, respectively, in Figure 9. Interestingly, as shown in Figure 9a,b, with the increasing x, while the d avg values continuously increase and the t avg values slightly decreases for non-MFP1 samples, the d avg values remain almost constant, and t avg values continuously decrease for the MFP samples.In addition, except for x = 0.0, both the d avg and t avg values of the MFP samples become smaller than those of the non-MFP1 samples with the increasing x.These differences result in an opposite variation in the average grain volumes with the increasing x, as shown in Figure 9c, while the aspect ratios of MFP samples are relatively lower than those of non-MPF1 samples; however, both increase with the increasing x, as shown in Figure 9c.
Appl.Sci.2024, 14, x FOR PEER REVIEW 13 of 19 M-type grains have been reported to have a hexagonal plate shape, with the basal plane perpendicular to the c-axis [1], which can also be observed in the planar microstructure in Figure 8.To grow into a hexagonal plate shape, the a-axis growth rate must be faster than the c-axis growth rate.Grain growth occurs via the diffusion of components through the liquid phase formed at the grain boundary during the sintering stage.Through the MFP process, the c-axis of the plate-shaped M-type grains can be dominantly aligned parallel to the applied field.Since the basal plane orientation is in the same direction as other grains during sintering, growth may be hindered, resulting in a slight increase in x.
The davg, tavg, and aspect ratios listed in Tables 1 and 3 are represented as a function of x for non-MFP1 and MFP samples, respectively, in Figure 9. Interestingly, as shown in Figure 9a,b, with the increasing x, while the davg values continuously increase and the tavg values slightly decreases for non-MFP1 samples, the davg values remain almost constant, and tavg values continuously decrease for the MFP samples.In addition, except for x = 0.0, both the davg and tavg values of the MFP samples become smaller than those of the non-MFP1 samples with the increasing x.These differences result in an opposite variation in the average grain volumes with the increasing x, as shown in Figure 9c, while the aspect ratios of MFP samples are relatively lower than those of non-MPF1 samples; however, both increase with the increasing x, as shown in Figure 9c.The increase in Hci for MFP samples listed in Table 4 is primarily influenced by th increase in Ha listed in Table 2, according to Equation (5).While the increment in Ha from x = 0.2 to 0.3 is about twice as much as that from x = 0.0 to 0.15, the increment in Hci fo MFP samples is reduced by about 0.76.Therefore, it is difficult to explain the variation i Hci solely by the variation in Ha.According to Equation ( 5), Hci is dependent on the mag netic properties, such as Ha, Ms, and Mr, as well as the microstructure factors of α and N Meanwhile, the MFP process can largely enhance the grain alignment, and thereby Mr i abruptly increased, which may cause a significant decrease in Hci, according to Equation (5) for a given composition, x.Interestingly, however, after MFP, the 4πMr(or Br) value increase from x = 0.0 to 0.15 and then decrease to x = 0.3, while Hci continuously increase with the increasing x, suggesting that the 4πMr increase is not always accompanied by a Hci decrease for different x values.Furthermore, as shown in Figure 9, after MFP, the in crease in aspect ratio with x leads to an increase in N and decrease in α, which is presum ably responsible for the additional reducing effect on Hci.Consequently, since the increas ing effect of Ha is dominant compared with the decreasing effects of Mr and the micro structure factors α and N, Hci is considered to increase with the increasing x for MFP sam ples.The increase in H ci for MFP samples listed in Table 4 is primarily influenced by the increase in H a listed in Table 2, according to Equation (5).While the increment in H a from x = 0.2 to 0.3 is about twice as much as that from x = 0.0 to 0.15, the increment in H ci for MFP samples is reduced by about 0.76.Therefore, it is difficult to explain the variation in H ci solely by the variation in H a .According to Equation ( 5), H ci is dependent on the magnetic properties, such as H a , M s , and M r , as well as the microstructure factors of α and N.Meanwhile, the MFP process can largely enhance the grain alignment, and thereby M r is abruptly increased, which may cause a significant decrease in H ci , according to Equation (5) for a given composition, x.Interestingly, however, after MFP, the 4πM r (or B r ) values increase from x = 0.0 to 0.15 and then decrease to x = 0.3, while H ci continuously increases with the increasing x, suggesting that the 4πM r increase is not always accompanied by an H ci decrease for different x values.Furthermore, as shown in Figure 9, after MFP, the increase in aspect ratio with x leads to an increase in N and decrease in α, which is presumably responsible for the additional reducing effect on H ci .Consequently, since the increasing effect of H a is dominant compared with the decreasing effects of M r and the microstructure factors α and N, H ci is considered to increase with the increasing x for MFP samples.11a, H ci increases from 3.14 to 3.84 kOe in the MFP samples with increasing x from 0.0 to 0.3.This increasing tendency of H ci with x is also commonly observed for non-MFP samples in this figure.However, the H ci values for a given x decrease with increasing grain alignment.The w(00l) values for non-MFP2 samples evaluated by Equation (6) were 33.4,35.3, 33.9, and 41.2% for x = 0.0, 0.15, 0.20, and 0.30, respectively, while those could not be evaluated for non-MFP1 samples.Somewhat enhanced grain alignments for non-MFP2 samples might be induced by the higher pressure of the CIP.As shown in Figure 11b, with the increasing grain alignment for a given composition, x, the increase in M r always results in the decrease in H ci for all compositions according to Equation (5).As shown in Figure 11a, Hci increases from 3.14 to 3.84 kOe in the MFP samples with increasing x from 0.0 to 0.3.This increasing tendency of Hci with x is also commonly observed for non-MFP samples in this figure.However, the Hci values for a given x decrease with increasing grain alignment.The w(00l) values for non-MFP2 samples evaluated by Equation (6) were 33.4,35.3, 33.9, and 41.2% for x = 0.0, 0.15, 0.20, and 0.30, respectively, while those could not be evaluated for non-MFP1 samples.Somewhat enhanced grain alignments for non-MFP2 samples might be induced by the higher pressure of the CIP.As shown in Figure 11b, with the increasing grain alignment for a given composition, x, the increase in Mr always results in the decrease in Hci for all compositions according to Equation (5).The serious decease in Hci with improving the grain alignment have also been reported for M-type [46] and Nd-Fe-B [47] magnets.Since it has been suggested that Hci can be enhanced through simulations on grain boundary diffusion in SrM [48], it may be possible to further improve the Hci values of MFP samples by using more appropriate sintering additives to ensure smaller average grains and also the presence of a secondary phase at the grain boundary.
Figure 12 shows the Br, Hci, and (BH)max values versus x and also Br versus Mr/Ms for MFP-processed Sr1−xLaxFe12−xCoxO19 samples (x = 0.0, 0.15, 0.2, and 0.3).The data of MFPprocessed samples from Refs.[16] and [19] are also plotted on this figure for a comparison.As shown in Figure 12a, our Br values are comparable to those from Liu et al. [16], who used different sintering additives (SiO2, CaCO3, and Al2O3), but much smaller than those from Iida et al. [19], who employed the same kinds of sintering additives but different amounts (0.4 wt% SiO2 and 1.4 wt% CaCO3); however, their Ms values are almost identical to our Ms values.As previously mentioned, the degree of grain alignment can be evaluated by several different parameters, such as degree of c-axis orientation, w(00l); squareness, Q; and the Mr/Ms ratio.All of these parameters are proportional to Br for a given x.However, for different compositions, only the Mr/Ms ratio is proportional to Br.
Thus, the much higher Br values reported by Iida et al. are attributable to the higher degree of alignment indicated by higher Mr/Ms values in Figure 12d.Figure 12b shows that our Hci values are much lower compared with those of previous reports, which are primarily due to the much larger average grain size in our samples.As shown in Figure 10 and Table 4, the highest (BH)max was achievable from the sample of x = 0.15 among all of our MFP samples.Compared with the previously reported (BH)max values in Figure 12c, we can see that our values are similar to those from Liu et al. [16] but much lower than those from Iida et al. [19], and this is also the case for Br values.Consequently, the highest (BH)max is obtainable from the highest Br.As shown in Figure 12d, both the Br and (BH)max The serious decease in H ci with improving the grain alignment have also been reported for M-type [46] and Nd-Fe-B [47] magnets.Since it has been suggested that H ci can be enhanced through simulations on grain boundary diffusion in SrM [48], it may be possible to further improve the H ci values of MFP samples by using more appropriate sintering additives to ensure smaller average grains and also the presence of a secondary phase at the grain boundary.
Figure 12 shows the B r , H ci , and (BH) max values versus x and also B r versus M r /M s for MFP-processed Sr 1−x La x Fe 12−x Co x O 19 samples (x = 0.0, 0.15, 0.2, and 0.3).The data of MFP-processed samples from Refs.[16,19] are also plotted on this figure for a comparison.As shown in Figure 12a, our B r values are comparable to those from Liu et al. [16], who used different sintering additives (SiO 2 , CaCO 3 , and Al 2 O 3 ), but much smaller than those from Iida et al. [19], who employed the same kinds of sintering additives but different amounts (0.4 wt% SiO 2 and 1.4 wt% CaCO 3 ); however, their M s values are almost identical to our M s values.As previously mentioned, the degree of grain alignment can be evaluated by several different parameters, such as degree of c-axis orientation, w(00l); squareness, Q; and the M r /M s ratio.All of these parameters are proportional to B r for a given x.However, for different compositions, only the M r /M s ratio is proportional to B r .
Thus, the much higher B r values reported by Iida et al. are attributable to the higher degree of alignment indicated by higher M r /M s values in Figure 12d.Figure 12b shows that our H ci values are much lower compared with those of previous reports, which are primarily due to the much larger average grain size in our samples.As shown in Figure 10 and Table 4, the highest (BH) max was achievable from the sample of x = 0.15 among all of our MFP samples.Compared with the previously reported (BH) max values in Figure 12c, we can see that our values are similar to those from Liu et al. [16] but much lower than those from Iida et al. [19], and this is also the case for B r values.Consequently, the highest (BH) max is obtainable from the highest B r .As shown in Figure 12d, both the B r and (BH) max values are also proportional to the M r /M s ratio for all samples with different compositions.Consequently, in comparison with previous reports on MFP-processed

3 . 1 .
Characteristics of Randomly Oriented Polycrystalline Samples 3.1.1.Lattice Parameters and Microstructures The powder XRD patterns of the Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.4) samples sintered at 1230 • C for 2 h in air are shown in Figure 1.The diffraction peaks corresponding to the M-type hexaferrite phase are indexed with reference to the ICDD cards [00-033-1340].

Figure 1 .
Figure 1.The powder XRD patterns of Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.4) samples sintered at 1230 • C for 2 h, in air.

Figure 2 .
Figure 2. SEM-BSE, and the corresponding EDS element mapping micrographs for O, Fe, La, and Sr for the Sr 0.65 La 0.35 Fe 11.65 Co 0.35 O 19 sample.

Figure 3 .
Figure 3.The lattice parameters (a, c) (a) and unit cell volumes (V cell ) and the c/a ratio (b) for the Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) samples sintered at 1230 • C (filled square) and post-annealed at 1230 • C (open triangle) for samples in Ref. [37].

Figure 5 .
Figure 5.The Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) samples sintered at 1230 • C for 2 h in air, presenting the initial magnetization curves (a) and M-H curves with the demagnetization curves in the inset (b).

Figure 6
Figure6shows the dependencies of M S , H ci , and H a on x from the data in Table2.The M S values increase from 70.90 to 72.40 emu/g as x increases from 0.0 to 0.15, and then they decrease to 71.38 emu/g up to x = 0.3.On the other hand, the H a values increase from 17.7 to 25.9 kOe as x increases from 0.0 to 0.3.The H ci values increase from 3.517 to 4.294 kOe as x increases from 0.0 to 0.15.The H ci values increase slightly to 4.385 kOe at x = 0.2 and then increase to 5.001 kOe at x = 0.3.

Figure 6 .
Figure 6.The Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) samples sintered at 1230 • C for 2 h in air, presenting: plots of M s , H ci , and H a vs. x.

Figure 8
Figure 8 shows the SEM micrographs of the MFP-processed Sr1−xLaxFe12−xCoxO19 (x 0.0, 0.15, 0.2, and 0.3) samples.Figure 8a-d show the plane view perpendicular to the ap plied field, while Figure 8e-h show cross-sectional view parallel to the applied field.Con sequently, the grains shown in Figure 8a-d predominantly represent the basal planes o hexaferrites, while those in Figure 8e-h represent cross-sectional views of hexagona plates.Non-MFP1 samples exhibit non-uniform grains due to the random orientations o adjacent grains (see Figure 4).The average grain sizes (davg), corresponding to the average length of the hexagona plate, and the average thickness of the hexagonal plate (tavg) were determined from th micrographs shown in Figure8.The values and their standard deviation (SD) of davg an tavg are listed in Table3.The average aspect ratio calculated by davg/tavg and the averag volume (Vavg) are also listed in Table3, together with the measured sintered density (D and evaluated relative density (D/Dth).As shown in Table3, the davg value slightly increase from 1.30 to 1.38 µm as x increases from 0.0 to 0.3, while the SD values are decreased from ± 0.35 to 0.20 µm.Also, the tavg values of samples decreased from 1.18 to 0.77 µm with th increasing x value, and their SD values also decreased from ± 0.21 to 0.05 µm.

Figure 7 .
Figure 7.The bulk-surface XRD patterns of the MFP-processed Sr 1−x La x Fe 12−x Co x O 19 (x = 0.0, 0.15,0.2and 0.3) samples sintered at 1230 • C for 2 h in air.The degrees of orientation, w(00l), are also represented on the plot.

Figure 9 .
Figure 9.The average grain sizes of the Sr1−xLaxFe12−xCoxO19 (x = 0.0, 0.15, 0.2, and 0.3) sample sintered at 1230 °C.The average length of the hexagonal plate (davg) (a), the average thicknesses of the hexagonal plate (tavg) (b), average aspect ratios (c), and average grain volumes (d) were separately evaluated from the micrographs shown in Figures 4 and 7.
Figure 9.The average grain sizes of the Sr1−xLaxFe12−xCoxO19 (x = 0.0, 0.15, 0.2, and 0.3) sample sintered at 1230 °C.The average length of the hexagonal plate (davg) (a), the average thicknesses of the hexagonal plate (tavg) (b), average aspect ratios (c), and average grain volumes (d) were separately evaluated from the micrographs shown in Figures 4 and 7.

3. 2 . 2 .
Figure 10a-c show the demagnetization (second quadrant of 4πM-H and B-H) curves and plots of B vs. (BH)max for Sr1−xLaxFe12−xCoxO19 samples (x = 0.0, 0.15, 0.2, and 0.3) processed with and without MFP process and sintered at 1230 °C for 2 h.Here, the direction in which the magnetic field is applied during the B-H measurement coincides with the applied field direction during MFP.The 4πMr, Hci, and (BH)max values for these samples are listed in Table 4. Squareness, Q, is defined by Q = Hk/Hci, where Hk is the knee field

Figure 9 .
Figure 9.The average grain sizes of the Sr 1−x La x Fe 12−x Co x O 19 (x = 0.0, 0.15, 0.2, and 0.3) sample sintered at 1230 • C. The average length of the hexagonal plate (d avg ) (a), the average thicknesses of the hexagonal plate (t avg ) (b), average aspect ratios (c), and average grain volumes (d) were separately evaluated from the micrographs shown in Figures 4 and 7.
Figure 9.The average grain sizes of the Sr 1−x La x Fe 12−x Co x O 19 (x = 0.0, 0.15, 0.2, and 0.3) sample sintered at 1230 • C. The average length of the hexagonal plate (d avg ) (a), the average thicknesses of the hexagonal plate (t avg ) (b), average aspect ratios (c), and average grain volumes (d) were separately evaluated from the micrographs shown in Figures 4 and 7.

3. 2 . 2 .
Figure 10a-c show the demagnetization (second quadrant of 4πM-H and B-H) curves and plots of B vs. (BH) max for Sr 1−x La x Fe 12−x Co x O 19 samples (x = 0.0, 0.15, 0.2, and 0.3) processed with and without MFP process and sintered at 1230 • C for 2 h.Here, the direction in which the magnetic field is applied during the B-H measurement coincides with the applied field direction during MFP.The 4πM r , H ci , and (BH) max values for these samples are listed in Table4.Squareness, Q, is defined by Q = H k /H ci , where H k is the knee field corresponding to a magnetization of 90% of B r .As shown in Table4, the highest induction

Figure 10 .
Figure 10.Second quadrant of (a) B-H curves, (b) 4πM-H curves and plots of (c) B versus (BH) max of MFP-processed Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) samples sintered at 1230 • C for 2 h in air.

Figure 11
Figure11shows the H ci values versus x in (a) and also versus 4πM r in (b) for Sr 1−x La x Fe 12−x Co x O 19 samples (x = 0.0, 0.15, 0.2, and 0.3) with and without the MFP process.As shown in Figure11a, H ci increases from 3.14 to 3.84 kOe in the MFP samples with increasing x from 0.0 to 0.3.This increasing tendency of H ci with x is also commonly observed for non-MFP samples in this figure.However, the H ci values for a given x decrease with increasing grain alignment.The w(00l) values for non-MFP2 samples evaluated by Equation (6) were 33.4,35.3, 33.9, and 41.2% for x = 0.0, 0.15, 0.20, and 0.30, respectively, while those could not be evaluated for non-MFP1 samples.Somewhat enhanced grain alignments for non-MFP2 samples might be induced by the higher pressure of the CIP.As shown in Figure11b, with the increasing grain alignment for a given composition, x, the increase in M r always results in the decrease in H ci for all compositions according to Equation (5).
Figure11shows the Hci values versus x in (a) and also versus 4πMr in (b) for Sr1−xLaxFe12−xCoxO19 samples (x = 0.0, 0.15, 0.2, and 0.3) with and without the MFP process.As shown in Figure11a, Hci increases from 3.14 to 3.84 kOe in the MFP samples with increasing x from 0.0 to 0.3.This increasing tendency of Hci with x is also commonly observed for non-MFP samples in this figure.However, the Hci values for a given x decrease with increasing grain alignment.The w(00l) values for non-MFP2 samples evaluated by Equation (6) were 33.4,35.3, 33.9, and 41.2% for x = 0.0, 0.15, 0.20, and 0.30, respectively, while those could not be evaluated for non-MFP1 samples.Somewhat enhanced grain alignments for non-MFP2 samples might be induced by the higher pressure of the CIP.As shown in Figure11b, with the increasing grain alignment for a given composition, x, the increase in Mr always results in the decrease in Hci for all compositions according to Equation(5).

Figure 11 .
Figure 11.The H ci values versus x (a) and versus 4πM r (b) for Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) samples sintered at 1230 • C for 2 h in air with and without MFP process.
are presumed not to be Fe 2 O 3 , LaFeO 3 , and CoFe 2 O 4 but to be (Fe,Co) 2 O 3 , (La, Sr)(Fe,Co)O 3 , and CoFe 2 O 4 since Sr and Co can substitute for the La and Fe sites, respectively, except the spinel phase of CoFe 2 O 4 .
The initial magnetization curves and hysteresis loops of the Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) samples are shown in Figure

Table 2 .
The Ms, Ha, Mr, Hci, A, field region for fitting, and R 2 values for the Sr1−xLaxFe12−xCoxO19 (0.0 ≤ x ≤ 0.3) samples sintered 1230 °C for 2 h in air.The calculated values are presented with the error range.xMs (emu/

Table 2 .
The M s , H a , M r , H ci , A, field region for fitting, and R 2 values for the Sr 1−x La x Fe 12−x Co x O 19 (0.0 ≤ x ≤ 0.3) samples sintered 1230 • C for 2 h in air.The calculated values are presented with the error range.

Table 4 .
The 4πM r , H c , (BH) max , and squareness Q values for the Sr 1−x La x Fe 12−x Co x O 19 (x = 0.0, 0.15, 0.2, and 0.3) samples sintered at 1230 • C with and without MFP process.