M-type ferrite Sr_0.1Ca_0.4La_0.5Fe₁₂O₁₉ magnetic powders doped with Co_xFe_3-xO₄ (0 ≤ x ≤ 1.25) in precursors were prepared by a ceramics process. The structure of specimens were examined by using an X-ray diffractometer. Only the magnetoplumbite phase was found in the specimens with the cobalt content (x) from 0.25 to 1.25 in precursor. The specimens exhibited a typical hexagonal M-type structure and the particles in specimens were uniformly distributed in size. The specimen with x=1.25 had the maximum saturation magnetization of 67.48 emu/g, while the residual magnetization and the coercivity of the specimen with x=1.00 reached the maximum value of 31.83 emu/g and 1270 Oe, respectively

Keywords: M-type ferrite, Cobalt content, Ceramic method, Magnetic properties

M-type permanent magnetic ferrite (SrFe₁₂O₁₉, SrM) is widely used due to its relatively high saturation magnetization (M_s) and coercivity (H_c). Besides, it has fine magnetocrystal anisotropy constant, excellent corrosion resistance and chemical stability [1-4]. Currently, some metal ions, such as Co²⁺, La³⁺, Ca²⁺, Cu²⁺, La³⁺-Co²⁺, La³⁺-Zn²⁺ and so on, are substituted or doped into the SrM ferrites attempting to improve the magnetic properties for the sake of special application [5-20]. Kools et al. [16] studied the structural and magnetic properties of M-type ferrite doped by La-Co ions. It was found that Co²⁺ ions replaced Fe³⁺ ions on octahedral lattice points 4f₂ and 2a, and the magnetic properties were improved through the 4f₂ substitution. A.L. Xia et al. [17] prepared the SrFe₁₂O₁₉-CoFe₂O₄ nanocomposites by using a hydrothermal method. The study found that compared with single-phase SrFe₁₂O₁₉, when the exchange coupling appeared, with the increasing content of CoFe₂O₄, the M_s of composites increased significantly from 59.2 emu/g to 72.4 emu/g. W.C. Li et al. [18] prepared La-Co substituted Sr_1-xLa_xFe_12-xCo_xO₁₉ ferrite via a sol-gel combustion method. It was found that the M_s reached the maximum value when the La-Co substitution was x=0.3. S.M. Masoudpanah et al. [19] prepared the La-Co substituted M-type ferrite thin films by using laser deposition. The results showed that the M_s increased first and then decreased with the increasing La-Co amount. Y. Liu et al. [20] prepared the La-Co substituted M-ferrite by co-improved precipitation and molten salt method, and the results showed that the M_s and H_c were comparable to those synthesized via a high-temperature method. However, the influence of different cobalt content in precursor on the magnetic properties of SrM ferrites is still not reported. Through this study, it may help enterprises reduce the use of rare earth elements in production
In this study, the SrM magnetic powders were obtained via a ceramic process with different Co_xFe_3-xO₄ (0 ≤ x ≤ 1.25) precursors. The effects of the cobalt content (x) in precursor on the structural and magnetic properties of specimens were studied.

Preparation of Co_xFe_3-xO₄ (0 ≤ x ≤ 1.25) precursors
All specimens with a nominal composition of Co_xFe_3-xO₄ (0 ≤ x ≤ 1.25 with steps of 0.25) were obtained via a traditional ceramic process. All the chemical reagents used were analytically pure (Fe₂O₃ and Co₂O₃). The mixed powders (Fe₂O₃ and Co₂O₃) were ball-milled in water for 2 h, and then the powders obtained were dried, sifted and sintered at 850 ℃ for 3 h in air. Finally, the sintered samples were pulverized to powders with a size of smaller than 100 μm.
Preparation of M-type ferrite Sr_0.1Ca_0.4La_0.5Fe₁₂O₁₉ powders with different cobalt content in precursor
All the Sr_0.1Ca_0.4La_0.5Fe₁₂O₁₉ powder specimens with different Co_xFe_3-xO₄ (0 ≤ x ≤ 1.25) precursors were obtained by using a ceramic process. The analytically pure Fe₂O₃ (98% purity), SrCO₃ (98% purity), CaCO₃ (99% purity) and La₂O₃ (99% purity) were used as raw materials to prepare SrM ferrites. The mixed powders were milled for 2 h, together with Co_xFe_3-xO₄ of 3 wt%. The obtained powders were dried, sifted, and then sintered at 1300 ℃ for 2 h in the air atmosphere in a muffle furnace. Finally, the sintered specimens were pulverized to powders with a size of smaller than 100 μm.
Characterization
The phase identification of specimens was obtained on a PANalytical X’Pert Pro diffractometer (Almelo, Netherlands) using Cu K_α (λ = 1.5406 Å) radiation. The morphologies were examined by a HITACHI S-4800 field emission scanning electron microscopy (FESEM, Tokyo, Japan). The room temperature (RT) magnetic hysteresis loops of specimens were measured on a MicroSense EZ7 vibrating sample magnetometer (VSM) with a maximum external field of 20,000 Oe.

Structural and morphological properties
Fig. 1 shows the XRD patterns of SrM specimens obtained with different cobalt content (x) from 0 to 1.25 in precursors. As shown, in the specimens with x from 0.25 to 1.25, only typical peaks from single-phase magnetoplumbite SrM was found, which implies that the Co²⁺ ions entered into the magnetoplumbite lattice without producing any second phase.
The lattice constants a and c can be calculated using the values of Miller indices (h, k, l) and the inter-planer spacing d_hkl corresponding to (107) and (114) peaks according to the following formula (1):



The variations of lattice constants a and c of the specimens with different cobalt content (x) are shown in Fig. 2. It can be found that the c decreases slightly from 22.9725 Å (x=0.00) to 22.9616 Å (x=0.25). When the x ranges from 0.25 to 0.50, the c increases to 22.9847 Å. With a further increase of x from 0.50 to 1.00, the c decreases to 22.9624 Å. From 1.00 to 1.25, the c increases to 22.9689 Å. However, compared with c, the variation of a is not obvious, suggesting that the x affected c more greatly. This may result from the different distribution of Co ions.
Fig. 3 indicates the change of crystal axis ratio of c/a in different specimens. The c/a is approximately the same in Fig. 3. On the basis of Verstegen and Stevels [26], the value of c/a ratio may present the structure type. For the M-type structure, the ratio was assumed to be smaller than 3.98. Herein, the values of c/a ratio for the specimens range from 3.8954 to 3.9018, which seems to be in well accord with the reported range of M-type structure.
Figure 4shows the typical FESEM images of specimens. As shown in Fig. 4, all the specimens consist of the relatively uniform particles with typical hexagonal structure. It can be found that the average particle size of specimen with x = 0 is obviously smaller than those of specimens with x > 0. However, with the increasing x, the typical morphology and the average particle size of specimens did not change obviously.
Magnetic properties
Fig. 5 shows the RT magnetic hysteresis loops of specimens with different x. The M_s, H_c and residual magnetization (M_r) of specimens with different x were determined from Fig. 5 and are listed in Table 1. What should be pointed out is that since the magnetic hysteresis loops were roughly saturated at a field of 20 kOe, the value of magnetization obtained at 20 kOe can approximately be used as M_s.
As can be seen from Table 1 and Fig. 6, the M_s of specimens increases with the increase of x from 0 to 1.25, and it reaches 67.48 emu/g when x=1.25. The M_r of specimens increases with the increase of x from 0 to 1.00. When x increased from 1.00 to 1.25, the M_r decreases, and the maximum value is 31.83 emu/g (x=1.00). It is evident that the H_c has the same feature as M_s as shown in Table 1 and Fig. 7. The specimen with x=1.00 exhibits the best magnetic properties, including the M_s (66.2 emu/g), the M_r (31.83 emu/g) and the H_c (1270 Oe). The variation of M_s and H_c with different x is in agreement with those reported in Ref. [17] and Refs. [20-21], respectively. However, the M_s, the M_r and the H_c of specimens are lower than those reported by Rashad et al. [27]. The Mössbauer investigations of La–Co-substituted M-type ferrites revealed that La³⁺ ions can replace the most Sr²⁺ ions, and a valence change of some Fe³⁺ to Fe²⁺ ions occurred in the 2a and 4f₂ sites, while Co²⁺ ions mainly replaced Fe³⁺ at 4f₂ octahedral sites [22-24]. When x is smaller (larger) than 1.0, the magnetic moment difference caused by Co substitution is positive (negative), respectively, resulting in the M_r of specimen increases first and then decreases with the increase of x.
G. Asti et al. [25] found that the magnetocrystal anisotropy field of ferrite increased with the increase of cobalt content, and the Mössbauer spectrum analysis of ferrite revealed that when the content of cobalt substitution is within a certain range, most of the Co²⁺ ions will substitute Fe³⁺ ions in 4f₂ and 2a octahedral sites. The magnetocrystal anisotropy constant K₁ increases with the increase of cobalt content (x), and consequently the H_c was enhanced. With the increasing ratio of x, Co²⁺ replaces Fe³⁺ ions at 2b octahedral sites, thus reducing K₁ and H_c.

The M-type Sr_0.1Ca_0.4La_0.5Fe₁₂O₁₉ ferrite powders were obtained via a ceramic process with different Co_xFe_3-xO₄ (0 ≤ x ≤ 1.25) precursors. The XRD patterns showed that the single magnetoplumbite phase was obtained in all the specimens. The SEM study manifested that the particles were distributed uniformly in the specimens. The VSM study indicated the M_s of specimens increased with the increasing x from 0 to 1.25, and reached the maximum value at x=1.25, while the M_r of specimens increased with the increasing x from 0 to 1.25. The specimen with x=1.00 exhibited the best magnetic properties of M_s=66.2 emu/g, M_r =31.83 emu/g and H_c=1270 Oe. The cost of SrM may be reduced through tailoring the content of Co to reduce the usage of rare earth elements.