Two-Step Calcination-Method-Derived Al-Substituted W-Type SrYb Hexaferrites: Their Microstructural, Spectral, and Magnetic Properties

Abstract

W-type hexaferrites were discovered in the 1950s and are of interest for their potential applications. In this context, many researchers have conducted studies on the partial substitution of Fe sites in order to modify their electric and magnetic properties. In this study, W-type SrYb hexaferrites using Al³⁺ as substitutes for Fe³⁺ sites with the nominal composition Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) were successfully synthesized via the two-step calcination method. The microstructures, spectral bands of characteristic functional groups, morphologies, and magnetic parameters of the prepared samples were characterized using XRD, FTIR, SEM, EDX, and VSM. The XRD results showed that, compared with the standard patterns for the W-type hexaferrite, the W-type SrYb hexaferrites with the Al content (x) of 0.00 ≤ x ≤ 1.25 were a single-W-type hexaferrite phase. SEM images showed the flakes and hexagonal grains of W-type hexaferrites with various Al content (x). The saturation magnetization (M_s) and magneton number (n_B) decreased with Al content (x) from 0.00 to 1.25. The remanent magnetization (M_r) and coercivity (H_c) decreased with Al content (x) from 0.00 to 0.25. Additionally, when the Al content (x) ≥ 0.25, M_r and H_c increased with the increase in the Al content (x). The magnetic anisotropy field (H_a) and first anisotropy constant (K₁) increased with the Al content (x) increasing from 0.00 to 1.25. Al-substituted W-type SrYb hexaferrites with soft magnetic behavior, high M_s, and lower H_c may be used as microwave-absorbing materials.

1. Introduction

The hexaferrites of W-type compounds with the chemical formula AMe₂Fe₁₆O₂₇ (in this formula, A stands for the alkaline earth metal ion, and Me stands for the bivalent transition element ion) were discovered in the 1950s and are of interest for their potential applications [1]. Due to their strong anisotropy and moderate magnetization, W-type hexagonal ferrites have become important magnetic materials. In recent years, the scientific community is keen to develop W-type hexagonal crystals, as manifested by their application in high-density magnetic recording, high-performance permanent magnets, microwave materials, microwave-absorbing materials, etc. [2]. Various synthesis methods have been used to prepare W-type hexaferrites. Some methods have been reported, such as the glass crystallization method [3], the sol–gel method [4], the hydrothermal method [5], a combined method using molten salt and sol–gel [6], the traditional ceramic method [7], the sol–gel self-combustion method [8], chemical coprecipitation [9], and chemical solution deposition [10]. By comparing these methods, it is found that the advantages of the traditional ceramic method are as follows: simplicity, controllable grain size, high productivity, etc.

Recently, many researchers have conducted studies on the partial substitution of Fe sites in order to modify their electric and magnetic properties [11,12,13,14]. Albanese et al. reported the cation distribution and magnetic properties of BaZn₂Fe_16−xIn_xO₂₇ and BaZn₂Fe_16−xSc_xO₂₇ hexagonal ferrites prepared by using the standard ceramic technique, and they found that the Sc-containing samples (x ≦ 2) had no evidence of iron substitution at 2d position; in the indium series, where the measurement range was extended to x = 4.5, the In ions did not enter the hexahedral position of x = 3.5, and in the In-substituted compounds, at the substitution (x) ≧ 2, the slope of T_C and x became steeper, and a strong dependence of magnetization (σ) on the external magnetic field was observed above and below T_C [11]. The effects of the Sc ion substitution for Fe ions on the resulting microstructure and magnetic properties were investigated by Qi et al. Their results showed that, with the increase in Sc substitution, the squareness ratio (M_r/M_s) of the samples ranged from 0.78 to 0.82, H_a decreased, and M_s increased from 53.61 to 54.04 emu/g with the increase in x from 0 to 0.1, but when x > 0.1, M_s was reduced to 46.50; in addition, the sample also had other excellent magnetic properties, namely an appropriate anisotropy field (about 9 kOe) and high coercive fields (about 1300 Oe) [12]. Ahmadet et al. synthesized the lanthanum-doped W-type hexaferrites BaZn₂La_xFe_16−xO₂₇ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) via coprecipitation and sintered the samples at 1320 °C to study the structural, magnetic, and electrical properties of BaZn₂La_xFe_16−xO₂₇ powders. They found that, as La substitution increased, the grain size decreased due to the fact that La acts as an inhibitor. The decrease in saturation magnetization and remanence is attributed to the spin tilt at the B-site, and the coercivity increase follows 1/r, where r is the radius of the particle. Furthermore, the DC resistivity increased from 0.59 × 10⁷ Ω cm to 8.42 × 10⁷ Ω cm with the increase in La substitution because of the unavailability of Fe³⁺ ions [13]. The impact of Dy addition on the structural, morphological, electrical, and magnetic properties of nanocrystalline W-type hexaferrites prepared using the citrate sol–gel method was reported, and the results exhibited that, with the increase in Dy content, the values of coercivity ranged between 530 and 560 Oe; the magnetization declined as the Dy content increased; a minimum reflection loss of −40 dB was observed at 16.2 GHz for the samples with x = 0.6 and 1.7 mm thickness, and as Dy content increased, the minimum reflection point moved toward high frequency [14]. Aen et al. prepared a series of nanosized single-phase W-type SrGa_xZn₂Fe_16−xO₂₇ (x = 0.0, 0.1, 0.2, 0.3, 0.4) hexaferrites via the sol–gel technique and investigated the role of Ga substitution on the magnetic and electromagnetic properties of nanosized W-type hexagonal ferrites. They found that as the Ga content (x) increased, the saturation magnetization (M_s) decreased, and the coercivity (H_c) was enhanced. All the samples had H_c values in the range of hundreds of oersteds. In the frequency range of 0.5–13 GHz (relative to −20 dB), the microwave absorption performance was improved, and the bandwidth could reach 0.899 GHz [15]. The influence of Al substitution on the magnetic, electrical, and high-frequency dielectric constants of BaCo₂Al_xFe_16−xO₂₇ (x = 0–1) hexaferrites was investigated by Ahmad et al., and the results showed that due to the unavailability of Fe³⁺ ions, the DC resistivity enhanced as the Al content increased; the coercivity (H_c) values of all the samples were in the range of several hundred oersteds. Thus, this is one of the necessary conditions for their use in electromagnetic materials, as well as for safety, switching, and sensing applications; in the frequency range of 0.025–1.5 GHz, the high-frequency permittivity and dielectric loss decreased with the increase in the Al content [16]. Cr-doped W-type hexagonal ferrites with the composition BaNi₂Cr_xFe_16−xO₂₇ (0.0 ≤ x ≤ 0.4) were synthesized using the sol–gel auto-combustion technique, and the structural, magnetic, and dielectric properties of the Cr-substituted W-type hexaferrites were investigated by Rehman et al. Based on their results, the SEM images revealed a platelet-like and hexagonal shape; the hysteresis loops of all the samples were thin and had low coercivity (about a few hundred oersteds) and exhibited soft magnetic properties. This is a prerequisite for high-frequency applications, and in the frequency range of 1–6 ghz, the high-frequency dielectric constants follow Koop’s phenomenological theory, which is consistent with the Maxwell-Wegner model [17]. Iqbal et al. prepared the nanocrystalline W-type hexaferrites of the nominal stoichiometry of BaNi₂Cr_xFe_16−xO₂₇ (x ≤ 1) using the chemical coprecipitation method and investigated their structural, electrical, and magnetic properties. They found that, when Cr was incorporated into BaZn₂Fe₁₆O₂₇, the room temperature resistivity (ρ_RT) for BaZn₂Fe₁₆O₂₇ increased to the order of 10⁹ Ω cm, which is higher than that of W-type hexaferrites (10³–10⁷ Ω cm) previously reported; the dielectric constant (ε′) and dielectric loss (tan δ) decreased with the increase in frequency, and both of them increased with the increase in Cr doping; the improvement in the above parameters results in them having a broad application prospect in high-frequency radar absorption and electromagnetic interference attenuation [18].

In the present work, the modified ceramic method, i.e., the two-step calcination method, was used to prepare the samples. Additionally, Al-substituted W-type SrYb hexaferrites were fabricated with the nominal composition Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) via the two-step calcination method. Fe is a strong magnet, and its substitution with Al, which is a weak magnet, modifies the properties of the synthesized W-type SrYb hexaferrites. Thus, the effects of the Al substitution on the microstructure, spectral, and magnetic properties of W-type SrYb hexaferrites were investigated. The novelty of this paper is that the Al-substituted W-type SrYb hexaferrites were synthesized via the two-step calcination method for the first time, and tunable M_s and H_c with Al substitution were obtained.

2. Materials and Methods

2.1. Sample Preparation

In this work, SrCO₃ (99.5 wt.%) powder, Yb₂O₃ (99.9 wt.%) powder, ZnO (99 wt.%) powder, CoO (99 wt.%) powder, Al₂O₃ (99 wt.%) powder, Fe₂O₃ (99.3 wt.%) powder were used as raw materials. All the reagents used in this paper were bought from Aladdin Industrial Corporation. All the starting materials were used as received, i.e., no further chemical decontamination was carried out. W-type SrYb hexaferrites using Al³⁺ as substitutes for Fe³⁺ sites with the nominal composition Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) were successfully synthesized via the two-step calcination method. In this article, in the nominal composition Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25), the subscript of each metal element and oxygen is the ratio of the number of moles. The starting materials were weighed in a stoichiometric composition and were wet-mixed in water for 3 h in a planetary ball mill, with a rotation velocity of 300 rpm and a ball-to-powder of about 6:1. Further, the ground powder was dried and compressed into round pellets. In the first step, the as-prepared pellets were calcined in air for 2.0 h in a muffle furnace at 1300 °C. The heating rate was maintained at 4 °C/min, and cooling was achieved by keeping the furnace closed and cooling the furnace to room temperature. The calcined pellets were pulverized to powder using a vibration mill. In the second step, the pulverized powder was compressed into round pellets and re-calcined in air for 2.0 h in a muffle furnace at 1300 °C. In order to obtain the fine powders, the calcined pellets were crushed using a vibration mill. In order to eliminate the crystal strain and lattice defects, the crushed powder was annealed in air at 800 °C for 2.5 h in a muffle furnace.

2.2. Techniques of Characterization

The X-ray diffraction (XRD) study of all the W-type SrYb hexaferrite powders was carried out by using a Rigaku X-ray diffractometer equipped with Cu K_α (λ = 1.5406 Å) radiation. The infrared spectra were recorded using a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo Scientific, Waltham, MA, USA) for all the samples with KBr pellets in the wavenumber range of 4000 to 400 cm⁻¹. The morphology for all the samples was detected using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, Tokyo, Japan). The elemental compositions of the samples were detected with an energy-dispersive X-ray spectrometer (EDX). The magnetic hysteresis loops for all the samples were measured by using a vibrating-sample magnetometer (VSM) (VSM 3100, Beijing Oriental Chenjing Technology Co., Ltd., Beijing, China) at room temperature under an external magnetic field of 24,000 Oe.

3. Results and Discussion

3.1. XRD Analysis

Figure 1 shows the XRD patterns of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25). As shown in Figure 1, the XRD patterns confirmed that all the samples had a single-phased W-type hexaferrite structure, and the peaks of all the samples matched well with the standard JCPDS card (no. 75-0406); no impurity or undesirable phase was observed for any sample. This means that Al substitution does not change the magnetoplumbite structure of W-type SrYb hexaferrites.

The lattice constants c and a for the W-type hexagonal ferrites were calculated from the XRD data by using the following formula [19]:

$$\frac{1}{d_{hkl}^2}=\frac{4}{3}\times \frac{h^2+hk+k^2}{a^2}+\frac{l^2}{c^2}$$

(1)

where d_hkl is the inner planner spacing of the XRD pattern, and h, k, and l are Miller exponents. The lattice constants (c and a) for the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) are depicted in Figure 2. It can be clearly seen from the graph that the values of c and a of W-type SrYb hexagonal ferrites with Al substitution (0.25 ≤ Al content (x) ≤ 1.25) are smaller than that of W-type SrYb hexagonal ferrites without Al substitution. The decrease in lattice parameters c and a for the Al-substituted W-type SrYb hexaferrites (0.25 ≤ x ≤ 1.25) is because the ionic radius of Al³⁺ (0.535 Å) is smaller than that of Fe³⁺ (0.645 Å) [20,21,22]. However, as the Al content (x) increased from 0.25 to 1.25, the lattice parameters c and a of the Al-substituted W-type SrYb hexagonal ferrites fluctuated within a certain range.

3.2. FT-IR

The FT-IR spectra of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) are exhibited in Figure 3. Fourier transform infrared spectroscopy is a useful method for representing and recognizing chemical bonds, detecting functional groups, and detecting phase transitions at the tetrahedral and octahedral positions. The absorption bands in the frequency range of 400–800 cm⁻¹ correspond to stretching vibration at tetrahedral and octahedral positions in ferrite structures [23]. The absorption band at about 451 cm⁻¹ is attributed to the stretching vibration of octahedral metal ions and oxygen bonds, and the absorption band at about 594 cm⁻¹ is attributed to the stretching vibration of tetrahedral metal ions and oxygen bonds [24]. The absorption bands at the tetrahedral and octahedral sites showed an intrinsic deviation in behavior, and this may be due to the Al³⁺ (0.535 Å) ion substitution for the Fe³⁺ (0.645 Å) ions. It is obvious from the diagram that the normal vibrational modes of the tetrahedral clusters were higher than those of the octahedral clusters. This is mainly because the bond length of tetrahedral clusters is shorter than that of octahedral clusters [25]. The absorption band at about 673 cm⁻¹ in the spectrum of W-type SrYb hexaferrites with the Al content (x) = 0 could be due to the metal–oxygen stretching vibrations of α-Fe₂O₃ [26]. The absorption band at about 1500 cm⁻¹ in the spectrum of W-type SrYb hexaferrites with the Al contents (x) = 0.5 and 1.0 may be caused by some trace impurities that entered the samples during the mixing of W-type SrYb hexaferrite powder with potassium bromide powder and the subsequent pressing of a very thin pancake sample. The spectra of W-type SrYb hexaferrites with different Al contents (x) indicate that the characteristic absorption bands at about 2921 cm⁻¹ and about 1628 cm⁻¹ are attributed to the presence of the O-H stretching bond because of the moisture content of W-type SrYb hexaferrites during their preparation process [27].

3.3. FE-SEM Images

The FE-SEM images of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) are shown in Figure 4. Figure 4a,b exhibit the FE-SEM fracture surface of the W-type SrYb hexaferrite pellets with the Al content (x) of x = 0.00 and x = 0.25, respectively. The lamellar structure of W-type SrYb hexaferrites can be clearly seen from the lamellar traces left by the fracture surface of the FE-SEM images of these W-type SrYb hexaferrites. Figure 4c–f present the FE-SEM images of the W-type SrYb hexaferrites with the Al contents (x) of x = 0.50, x = 0.75, x = 1.00, and x = 1.25, respectively. It is evident from the FE-SEM images that the morphology of the grains had no obvious change with the change in the Al content (x), and the prepared W-type SrYb hexagonal ferrites exhibited a transparent hexagonal plate.

3.4. EDX Analysis

Figure 5 and Figure 6 illustrate the EDX spectra and elemental mapping images of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ with the Al content (x) ranging from x = 0.00 to x = 0.50, and the Al content (x) ranging from x = 0.75 to x = 1.25, respectively. As shown in Figure 5 and Figure 6, the suitability of replacing Al for Fe in W-type SrYb hexaferrites is confirmed. The EDX spectra in Figure 5 and Figure 6 for W-type SrYb hexaferrites with different Al contents (x) reveal the presence of Sr, Yb, Zn, Co, Al, Fe, and O, and their contents, which are in close agreement with the nominal elemental compositions. The inset of each figure exhibits the molecular and atomic weight of the element that was used in the preparation process. The increase in Al content and decrease in the host elements mean that the materials keep their stoichiometric contents. This confirms that Al ions replace Fe ions in the W-type SrYb hexaferrites.

3.5. Magnetic Properties

The magnetic hysteresis loops with the maximum magnetic field of ±24000 Oe, and the magnified charts of magnetic hysteresis loops within the range of ±750 Oe, for the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ are exhibited in Figure 7a,b, respectively. From the magnetic hysteresis loops, the saturation magnetization (M_s), remanent magnetization (M_r), and coercivity (H_c) for W-type SrYb hexaferrites with the Al content (x) ranging from 0.00 to 1.25 were calculated.

Figure 8 illustrates the effects of the Al content (x) on the saturation magnetization (M_s) and remanent magnetization (M_r) for the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇. As can be seen from Figure 8, the value of M_s gradually decreased from 72.779 emu/g at x = 0.00 to 61.882 emu/g at x = 1.25. Additionally, M_r slightly decreased from 3.161 emu/g at x = 0.00 to 3.058 emu/g at x = 0.25 and increased by degrees as the Al content (x) increased from 0.25 to 1.25. This changing trend of M_s is basically consistent with that of BaCo₂Fe_16−xAl_xO₂₇ synthesized using the traditional sol–gel method, which was studied by Ahmad et al. [16]. In the unit cell of a W-type hexaferrite that is composed of two hexagonal blocks (R block) and four spinel blocks (S block), there are seven kinds of crystal sites, detailed as follows: one trigonal bipyramidal site (2d), two tetrahedral sites (4 e and 4 f_IV), and four octahedral sites (12 k, 4 f_VI, 4 f, and 6 g) [28]. In W-type hexaferrites, the spin directions of the 12 k, 4 f, 6 g, and 2 d sublattices are spin-up, whereas the spin directions of the 4 e, 4 f_VI, and 4 f_IV sublattices are spin-down, with the nearest cations coupled via superexchange interactions through the O²⁻ ions [28]. The total magnetic moment (i.e., 27.4 μ_B) of W-type hexaferrites is due to their uncompensated upward spins [16]. Thus, the magnetic properties of W-type hexaferrites can be affected by the occupation of their sites by different cations [12,14,16]. The magnetic moments of Fe³⁺ and Al³⁺ ions are 5.0 μ_B and about 0.0 μ_B, respectively [16,29]. As the Al content (x) increased from 0.00 to 1.25, M_s decreased, which is attributed to the following factor: Albanese et al. studied the Al-substituted Zn₂-W hexaferrites and Al-substituted M-type Ba hexaferrites and found that Al³⁺ ions preferentially enter into the octahedral sites 2 a and 12 k [30,31]. The nonmagnetic Al³⁺ ions having a magnetic moment of 0.0 μ_B, which is less than that of Fe³⁺ ion (5.0 μ_B), replace the iron ions from the sites having spins in an upward direction, and this results in a decrease in the total magnetic moment. Therefore, the values of the saturation magnetization (M_s) decrease in the Al-substituted W-type SrYb hexaferrites.

The Bohr magneton number (n_B) was calculated for the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) using the relation given below [32]:

$$n_B=\frac{M_W\times M_S}{5585}$$

(2)

where M_W stands for the molecular weight. The values of the M_r/M_s ratio for the W-type SrYb hexaferrites are calculated from M_r and M_s. The effects of the Al content (x) on the magneton number (n_B) and M_r/M_s of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ are presented in Figure 9. From Figure 9, it is found that as the Al content (x) increased, the value of n_B decreased, from 20.245 μ_B at x = 0.00 to 16.814 μ_B at x = 1.25. The trend of the Bohr magneton number (n_B) is in agreement with that of M_s, as shown in Figure 8. This means that the magnetic moment is responsible for the change in M_s. Additionally, the M_r/M_s ratio is called the squareness ratio. As seen from Figure 9, the value of the M_r/M_s ratio basically remained constant at about 0.0434 as the Al content (x) increased from 0.00 to 0.25 and then gradually increased from 0.0434 at x = 0.25 to 0.0710 at x = 1.25. These results indicate that the W-type SrYb hexaferrites with the Al content (x) ( 0.00 ≦ x ≦1.25) have a multidomain structure.

The magnetic anisotropy field (H_a) and magnetocrystalline anisotropy constant (K₁) were determined through the law of saturation approach [33]. The relationship between the magnetization (M) and high magnetic fields (H) can be expressed in the following formula [34]:

$$M=M_S\left(1-\frac{A}{H}-\frac{B}{H^2}\right)+\chi H$$

(3)

Here, A is the constant caused by inhomogeneity, H is the external magnetic field, and X is susceptibility. In a strong magnetic field, constant A is approximately zero, and χ is ignored. Therefore, Equation (3) is simplified to the following equation [35]:

$$M=M_S\left(1-\frac{B}{H^2}\right)$$

(4)

For a strong magnetic field, the plots of M vs. 1/H² should give a straight line, as evident from Equation (4). Figure 10 exhibits the plots of the magnetization (M) versus 1/H² of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ with the Al content (x) (0.00 ≦ x ≦ 1.25). The slope gives the value of B, which denotes the magnetocrystalline anisotropy constant. Additionally, K₁ represents the first anisotropy constant, which can be calculated using Equation (5), and the magnetic anisotropy field (H_a) is calculated from the value of K₁ using Equation (6) [36]:

$$K_1=M_S{\left(\frac{15B}{4}\right)}_{}^{0.5}$$

(5)

$$H_a=\frac{2K_1}{M_S}$$

(6)

The effects of the Al content (x) on the first anisotropy constant (K₁) and magnetic anisotropy field (H_a) of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25) are presented in Figure 11. As seen from this figure, the value of K₁ increased from 2.384 × 10⁵ erg/cm³ at x = 0.00 to 2.644 × 10⁵ erg/cm³ at x = 1.25. Additionally, the value of H_a increased from 6.546 kOe at x = 0.00 to 8.502 kOe at x = 1.25. Compared with Figure 9, the changing trends of the first anisotropy constant (K₁) and magnetic anisotropy field (H_a) with the increase in the Al content (x) from x = 0.00 to x = 1.25 are basically consistent with those of the M_r/M_s ratio, which is called the squareness ratio. The increase in the value of H_a can be attributed to the following fact: From Figure 8, it is evident that the value of M_s gradually decreased with an increase in the Al content (x) from x = 0.00 to x = 1.25, while based on Figure 11, it is deduced that the value of K₁ increased with an increase in the Al content (x) from x = 0.00 to x = 1.25. According to Equation (6), H_a is inversely proportional to M_s and positively proportional to K₁. Therefore, the increase in the Al content (x) from x = 0.00 to x = 1.25 resulted in the increase in H_a.

Figure 12 depicts the influence of the Al content (x) on the coercivity (H_c) of the W-type SrYb hexaferrite Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇. As seen from Figure 12, with the increase in the Al content (x), the value of H_c slightly decreased from 75 Oe at x = 0.00 to 73 Oe at x = 0.25 and then gradually increased from 73 Oe at x = 0.25 to 105 Oe at x = 1.25. All the samples had Hc values ranging from 70 Oe to 110 Oe. This low coercivity makes them ideal for microwave equipment, safety, switching, and sensing applications [37]. The trend in H_c is not in agreement with that of BaCo₂Fe_16−xAl_xO₂₇, which was studied by Ahmad et al. [16]. However, the H_c values in this study are slightly lower than those reported by Ahmad et al. [16]. The main reason is that the preparation method used in this study is different from that of the previous study [16]. The coercivity mechanism hinges on the internal and external properties of the material. The intrinsic properties of the material are related to its crystal structure and chemical composition, such as M_s, T_c, anisotropic energy E_k, and anisotropic field H_a. The external properties of the material are related to its microstructures, such as defects, grain size, and grain shape. In theory, the coercivity (H_c) of the particle assembly can be expressed in the following formula [38]:

$$H_c=\alpha \left(\frac{2K_1}{M_s}-N_d{M}_S\right)=\alpha \left(H_a-H_d\right)$$

(7)

where α is a constant, N_d is the shape-demagnetizing factor, and H_d is the shape-demagnetizing anisotropy field. As shown in Figure 4, the platelet-like shape and the average grain size basically remained unchanged with the Al substitution, and thus, N_d remained basically the same. From Equation (7), the increase in coercivity (H_c) with the Al content (x) increasing from 0.25 to 1.25 mainly arises from the increase in the magnetic anisotropy field (H_a), as shown in Figure 11. According to the coercivity mechanism, the slight decrease in the value of H_c, from 75 Oe at x = 0.00 to 73 Oe at x = 0.25, is mainly due to the external properties of the material, such as slight changes in defects, grain size, and grain shape [16].

4. Conclusions

In this study, the modified ceramic method, i.e., the two-step calcination method, was used to synthesize Al-substituted W-type SrYb hexaferrites with the nominal composition Sr_0.85Yb_0.15Zn_1.5Co_0.5Al_xFe_16−xO₂₇ (0.00 ≤ x ≤ 1.25). The microstructures, spectral bands of characteristic functional groups, morphologies, and magnetic parameters of the prepared samples were characterized using XRD, FTIR, SEM, EDX, and VSM. The XRD results show that, compared with the standard patterns for the W-type hexaferrite, the W-type SrYb hexaferrites with the Al content (x) of 0.00 ≤ x ≤ 1.25 are a single W-type hexaferrite phase. The absorption band at about 451 cm⁻¹ is attributed to the stretching vibration of the octahedral metal ions and oxygen bonds, and the absorption band at about 594 cm⁻¹ is attributed to the stretching vibration of the tetrahedral metal ions and oxygen bonds. As shown in SEM images, the grains of the W-type hexaferrites with various Al contents (x) have hexagonal platelet-like shapes. The EDX spectra for the W-type SrYb hexaferrites with the Al content (x) (0.00 ≦ x ≦ 1.25) revealed the presence and contents of Sr, Yb, Zn, Co, Al, Fe, and O and are closely aligned with the nominal elemental compositions. M_s and n_B decreased with the Al content (x) increasing from 0.00 to 1.25. M_r and H_c decreased with the Al content (x) increasing from 0.00 to 0.25. Additionally, when the Al content (x) ≥ 0.25, M_r and H_c increased with an increase in the Al content (x). H_a and K₁ increased with the Al content (x) increasing from 0.00 to 1.25. The Al-substituted W-type SrYb hexaferrites exhibited a soft magnetic behavior because of their high saturation magnetization and low coercivity; thus, they may be used as microwave-absorbing materials.