M-type hexagonal ferrite (MFe₁₂O₁₉, M is divalent cation, such as Ba²⁺, Sr²⁺, and so on), is one of the most used hard magnetic materials with applications such as permanent magnets, magnetic recording media due to their low cost, good chemical stabilities, high coercivities, high saturation magnetizations, and large magnetocrystalline anisotropies.^[1–5] Recently, the barium ferrite was widely used as magnetic recording media, microwave devices, and electromagnetic wave absorbers.^[3,6,7] Furthermore, various methods of fabricating the M-type hexaferrite have been developed including sol-gel method,^[8,9] co-precipitation,^[10,11] ceramic method,^[12,13] and hydrothermal method,^[14] citrate auto-combustion,^[15] and glycin–nitrate method.^[16]

In order to change physical properties, substituting some cations into SrM structure is an effective method to cope with the different applications. A lot of work has been done to modify the magnetic parameters of SrM by substituting Fe³⁺ with other cations, such as Al³⁺, Cr³⁺, Ga³⁺, and so on.^[17–19] The magnetic properties of substituted hexaferrites directly depend on the electronic configuration of the dopant cations as well as their preference to occupy the different Fe sub-lattices of the magnetoplumbite structure.^[20] In particular, as is well known, Al-substituted SrM has very large coercivity.^[17,21,22] Furthermore, it is important to understand the mechanism for increasing the coercivity by Al³⁺ doping.

SrM with hexagonal structure and molecular formula SrFe₁₂O₁₉ has a magnetoplumbite (M) structure which was determined by Adelsklod.^[23,24] The crystal structure can be separated into two blocks, one block is R Block and another one is S block , R*S* block can be obtained by rotating the corresponding block 180° around the hexagonal c axis, so the chemical composition of the unit cell is Sr₂Fe₂₄O₃₈. The 24 Fe³⁺ ions of the unit cell are among five different crystal sites, i.e., three octahedral sites (12k, 4f₂, and 2a), one tetrahedron site (4f₁) and one trigonal bipyramidal site (2b).^[25] The 16 Fe³⁺ ions with upward spins are located at 2a, 2b, and 12k, whereas the remaining 8 Fe³⁺ ions are located at 4f₁ and 4f₂ with downward spins, which result in a net magnetic moment of 40 μ_B per unit cell.^[26] It has large magnetocrystalline anisotropy by the strong exchange interaction among the Fe³⁺ ions. Therefore the substitution for Fe ions could be an effective way to modify the magnetic properties.

In this study, we prepare Al³⁺-doped SrM by traditional ceramic technology, and the coercivity reaches to 9759 Oe at x = 2.0. We attempt to understand in detail the reasons and the mechanism of enhancing the coercivity and magnetic structure formation of SrAl_xFe_2n−xO₁₉ as a function of the substituted amounts of dopants in a range (0 ≤ x ≤ 2.0).

SrAl_xFe_2n−xO₁₉ (n = 5.9 x = 0–2.0) samples were prepared via ball milling appropriate mixtures of SrCO₃, Fe₂O₃ and Al₂O₃ powders. The amount of each compound was determined for each particular stoichiometry of SrAl_xFe_2n−xO₁₉ and then mixed in a ball milling cylinder. The mechanical alloying was carried out in a traditional ball mill for 4 h and the volume ratio of alcohol to powders was about 1.5:1. After milling, the powders were placed into cylindrical pellets. Calcination was then carried out for 2 h in air at 1200 °C.

The phase composition and crystal structure were analyzed by using Bruker diffractometer with Cu K_α x-ray diffraction measurement (λ = 1.5406 Å); scanning electron microscopy was employed to analyze the morphologies and microstructures of the samples. Magnetic properties could be obtained by analyzing the data of the vibrating sample magnetometer and the Mössbauer spectroscopy. The Mössbauer spectrum at room temperature was recorded using a Mössbauer spectrometer operating in constant acceleration mode (triangular wave) in transmission geometry. The source employed was ⁵⁷Co in Rh matrix of strength 50 mCi. The velocity was scaled by using an enriched a-Fe metal foil under an effective nuclear hyperfine field (H_eff) of 331 kOe at room temperature.

Figure 1 shows the typical x-ray diffraction (XRD) patterns of the precursor powders without and with various Al concentrations (x = 0–2.0) annealed for 2 h at 1200 °C. All the XRD patterns are similar, and consist of a set of strong diffraction profiles, which are similar to those of the SrFe₁₂O₁₉ (ICCD card No. 79-1411). There is no impurity phase (no additional minor peaks can be observed), thus confirming the formation of pure and well-crystallized phase. It is found that the peak positions systematically shift towards the high values with increasing doping concentration, which shows that there appears to be some change in crystal structure as x increases.

Fig. 1.

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	Fig. 1. XRD patterns of SrAlxFe2n−xO19 at T = 1200 °C.

The lattice parameters a and c of SrAl_xFe_2n−xO₁₉ (n = 5.9, x = 0–2.0) are calculated according to the following formula:

The changes of structural parameters, density and volume with composition are given in Table 1. It can be observed that the values of lattice parameters a and c decrease with x increasing from 5.89 Å to 5.84 Å and from 23.11 Å to 22.89 Å, respectively. This decrease becomes slow with composition rising up to x = 1.4 and then decreases dramatically for x = 2.0. As a result, the volume also decreases, which is attributed to the radius of Al³⁺ (0.535 Å) being smaller than that of Fe³⁺ (0.645 Å).^[29] Furthermore, the density of samples decreases with increasing x.

Table 1.

Table 1.

Table 1. Values of crystal lattice a and c, the unit-cell volume and the density for SrAlxFe2n−xO19. . Composition (x) a/Å c/Å V/10−22 cm3 ρx/(g/cm3) 0 5.89 23.11 6.94 5.03 0.8 5.88 23.05 6.90 4.95 1.4 5.87 23.00 6.86 4.89 2.0 5.84 22.89 6.76 4.88

Table 1. Values of crystal lattice a and c, the unit-cell volume and the density for SrAlxFe2n−xO19. .

Particle size and morphology are studied by SEM as shown in Figs. 2(a)–2(d). It is noteworthy that there is nearly no change in grain size (300 nm to 400 nm) nor morphology in the range of Al³⁺ substitution from x = 0 to x = 2.0 at the same temperature. It could be considered that Al³⁺ substitution does not affect the particle size and morphology, but it makes great contributions to the lattice parameters and the magnetic properties. The influence on the magnetic properties will be discussed in detail later.

Fig. 2.

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	Fig. 2. Scanning electron micrographs of SrAlxFe2n−xO19 magnetic powder particles with (a) x = 0, (b) x = 0.8, (c) x = 1.4, (d) x = 2.0.

In the hexagonal structure of strontium ferrite, Fe³⁺ ions occupy five sites: 4f₁, 2a, 4f₂, 12k, and 2b. The contributions of Fe³⁺ ions located on those sites to magnetic properties are different from each other. After Al³⁺ ions enter into the lattice and substitute for some Fe³⁺ ions, the magnetocrystalline anisotropy field is changed. From the Mössbauer spectra as shown in Fig. 3, the substitution sites can be investigated, and then the effect of substitution on anisotropy field can be revealed. The hyperfine parameters including hyperfine field, isomer shift, quadrupole splitting and Fe³⁺ occupation area ratio are shown in Figs. 4–7. It can be observed in Fig. 4 that the isomer shifts of octahedral sites (12k, 4f₂, and 2a) are greater than those of tetrahedron sites, because the electron density of Fe³⁺ ions at octahedral sites is smaller than that at the octahedral sites. The isomer shift value in the range 0.24 mm/s–0.40 mm/s for five sites at x = 0–2.0 implies only the existence of high-spin Fe³⁺ cations.^[30,31] In Fig. 5, the hyperfine fields of five sites are in the following sequence: H_hf (2b) < H_hf (12k) < H_hf (4f₁) < H_hf (2a) < H_hf (4f₂), which is in agreement with the results reported by Evan et al.^[32] The hyperfine field of each site decreases only slightly with increasing x, because the magnetic Fe³⁺ ions are replaced by non-magnetic Al³⁺ ions. The numbers of cations located at sites 12k, 4f₁, 4f₂, 2a, and 2b are 6, 2, 2, 1, and 1 (per formula unit), and the theoretical areas of the sextets are 50, 16.7, 16.7,8.3, and 8.3, respectively. The sextet area is directly proportional to the number of Fe³⁺ ions in the corresponding sites.^[30] In the present study, the corresponding values of relative areas for x = 0 are found to be 51, 11.2, 8.2, 22.4, and 7.2, which is caused by the overlapping of the spectral peaks. From Fig. 6, Al³⁺ mainly replaces Fe³⁺ on 2a and 4f₁ sites for x ≤ 1.4 as reported in Ref. [6]. For x = 2.0, the area of 12k sites has a sharp decline in the relative strength,which implies that Al^{3 +} ions are more inclined to replace Fe³⁺ of 12k site than Fe³⁺ of 4f₁ site. These results indicate that the substitution Al³⁺ ions occur mainly at 2a, 4f₁, and 12k sites, and Fe^{3 +} of 2b site can hardly be substituted for x ≤ 2.0. In Fig. 7, the quadrupole splitting value of 2b site is very large as compared with those of other sites, which is due to the strongly distorted environment around 2b site.^[25,33] After Al³⁺ ions enter into the lattice and substitute for some Fe³⁺ ions, Mössbauer parameters of SrAl_xFe_2−xO₁₉ at room temperature are shown in Table 2.

Fig. 3.

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	Fig. 3. The room temperature Mössbauer spectra of SrAlxFe2n−xO19 with (a) x = 0, (b) x = 0.8, (c) x = 1.4, and (d) x = 2.0.

Fig. 4.

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	Fig. 4. Isomer shifts on five sites of SrAlxFe2n−xO19.

Fig. 5.

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	Fig. 5. Hyperfine fields on five sites of SrAlxFe2n−xO19.

Fig. 6.

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	Fig. 6. Fe3+ ion occupation area ratio on five sites of SrAlxFe2n−xO19.

Fig. 7.

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	Fig. 7. Plots of quadrupole splitting versus Al doping amount on five sites of SrAlxFe2n−xO19.

Table 2.

Table 2.

Table 2. Mössbauer parameters of SrAlxFe2n−xO19 at room temperature. . Site Hhf/T QS/(mm/s) IS/(mm/s) Spectral area/% x = 0 4f2 51.73 0.28 0.40 8.2 4f1 48.65 0.18 0.26 20 12k 41.30 0.40 0.35 51 2b 40.94 2.17 0.29 7.2 2a 50.19 0.16 0.32 13.6 x = 0.8 4f2 51.46 0.28 0.38 12.3 4f1 47.82 0.20 0.27 17.7 12k 41.11 0.41 0.35 50.8 2b 40.54 2.14 0.25 9.3 2a 50.08 0.12 0.36 9.9 x = 1.4 4f2 50.66 0.28 0.35 14.2 4f1 47.14 0.31 0.25 16.2 12k 40.95 0.43 0.34 51 2b 39.93 2.11 0.33 12 2a 49.27 0.16 0.38 6.6 x = 2.0 4f2 49.85 0.16 0.35 15.6 4f1 44.18 0.35 0.27 8.2 12k 40.64 0.47 0.34 37.6 2b 36.32 2.07 0.38 22.7 2a 47.20 0.17 0.38 15.9

Table 2. Mössbauer parameters of SrAlxFe2n−xO19 at room temperature. .

The hysteresis loops of powder samples are measured at room temperature by using a vibrating sample magnetometer in Fig. 8. The H_c can be directly obtained from the loop. The magnetocrystalline anisotropy constant (K₁) and σ_s can be obtained by using the law of approach to saturation which is the sole static method to determine them for non-oriented polycrystalline materials. For the stage of approach to saturation in the hysteresis loop, the magnetization behavior can be described as^[34–36]

Fig. 8.

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	Fig. 8. M–H loops for SrAlxFe2n−xO19 at 1200 °C.

The saturation magnetization M_s and magnetic anisotropy constant (K₁) can be obtained by fitting the hysteresis loop using Eqs. (4) and (5) as shown in Table 3. It is noticeable that the value of M_s decreases with increasing Al³⁺ doping, while the coercivity increases.

Table 3.

Table 3.

Table 3. The room temperature magnetic parameters of Al3+ substituted SrAlxFe2n−xO19. . Composition x Hc/Oe Ms/(emu/cm3) B/107 Oe2 K1/106 (erg/cm3) 0 5748.7 380.57 2.97 4.02 0.8 7031.9 297.16 3.97 3.98 1.4 8736.9 253.94 5.91 3.78 2.0 9759.2 183.20 8.64 3.30

Table 3. The room temperature magnetic parameters of Al3+ substituted SrAlxFe2n−xO19. .

The coercivity of M-type ferrite can be expressed as^[37]

In Fig. 9, it is obvious that the saturation magnetization M_s decreases more rapidly than the magnetic anisotropy constant (K₁) with the increase of Al³⁺ ion content, which can cause the magnetic anisotropy field to increase. As for the results of Mössbauer spectra, the reduction of saturation magnetization is attributed to the fact that the Al³⁺ ions preferentially occupy 2a, 4f₁, and 12k sites for x ≤ 2.0, and a large number of Fe³⁺ on 12k sites, which are replaced by Al³⁺ contributed to the saturation magnetization, drastically decrease for x = 2.0. In addition, super-exchange interaction of Fe³⁺–O²⁻–Fe³⁺ is also among the factors that can influence the magnetic properties. Super-exchange interaction decreases with non-magnetic Al³⁺ replacing magnetic Fe³⁺ sites that can lead to the decrease of saturation magnetization. Furthermore, Xu et al.^[39] found that the Fe³⁺ ions on 12k sites make a negative contribution to magnetocrystalline anisotropy constant, whereas Fe³⁺ ions on 2b sites result in a large positive contribution.^[40] In this work, Al³⁺ ions mainly replace Fe³⁺ ions of 2a, 4f₁, and 12k sites and Fe³⁺ ions on 2b sites can hardly be substituted by Al³⁺ ions, as evidenced from the Mössbauer results for x ≤ 2.0, which also causes the enhancement of magnetic anisotropy field.

Fig. 9.

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	Fig. 9. Dependencies of saturation magnetization, Ms, and magnetic anisotropy constant, K1, on Al3+ ion substitution in SrAlxFe2n−xO19.

A series of SrAl_xFe_2n−xO₁₉ powders (n = 5.9, 0 ≤ x ≤ 2.0) are synthesized by conventional ceramic method. At low substitutions, Al³⁺ ions mainly occupy the 2a, 4f₁, and 12k sites of Fe³⁺. At x = 2.0, the decrease of magnetizations is attributed to the fact that Fe³⁺ on 2b sites can hardly be substituted. The magnetic moment per unit cell, saturation magnetization and magnetic anisotropy constant decrease with increasing Al³⁺ substitution, while the coercivity increases. The enhancement of coercivity is caused by increasing magnetic anisotropy field, which is attributed to the fact that the saturation magnetization (M_s) decreases more rapidly than the magnetic anisotropy constant (K₁) with the increase of Al³⁺ substitution.