Effects of bismuth and bismuth-copper substitutions on structure, morphology, and magnetic properties of sol-gel derived barium hexaferrites

1 Introduction

Numerous applications in data storage and high-frequency devices as well as the product development of microwave absorbers and commercial permanent magnets lead to the research on tailoring electromagnetic properties of the M-type barium ferrites (BaFe₁₂O₁₉) (Pullar, 2012; Houbi et al., 2021). Hard/soft magnetic composites consisting of barium ferrites have also attracted the interest of permanent magnet manufacturers (Thirupathy et al., 2020). Besides the microstructure, ferrite properties are inherently related to the exchange interaction between ions occupying the lattice sites. One pathway to increase the magnetization is by the partial substitution of Fe³⁺ with other cations to reduce the magnetic moments coupled in an opposite direction (Pullar, 2012). Spontaneous polarization may also arise from such ion substitutions leading to multiferroic properties (Thirupathy et al., 2020).

Several methods were employed to substitute Fe³⁺ in barium ferrites by different ions. The substitution by scandium (Sc) via the hydrothermal method regulated the size and, hence magnetic properties of barium ferrite nanodiscs (Hähsler et al., 2020). The solid-state reaction method successfully introduced Sc (Wang et al., 2018), titanium (Ti) (Behera and Ravi, 2019), and neodymium (Nd) (Widanarto et al., 2018) into barium ferrites. The substitution by copper (Cu) was also demonstrated via the solid-state mixed oxide route (Rafiq et al., 2018). The co-precipitation was used to synthesize barium ferrites with Cu substitutions (Vadivelan and Jaya, 2016) and modified electrical permittivity as well as ac conductivity by the cobalt (Co) substitution (Shehabi et al., 2020). The substitution by aluminum (Al) in BaFe_12−xAl_xO₁₉ for x = 0.0, 2.0, and 4.0 using high-energy milling changed particle size and magnetic properties and the reflection loss at microwave frequency was optimized in the case of x = 2.0 (Winatapura et al., 2019). The co-substitution by Mn and Ti was achieved by mechanical alloying (Yustanti et al., 2020; Manaf et al., 2017).

The sol–gel method has been used to synthesize barium ferrites with substantial maximum energy products. The method is also effective in partial substitutions of barium hexaferrite by Cu (Asiri et al., 2018; Kumar et al., 2020), Al (Han et al., 2021); Zr-Ni (Zhang et al., 2020), Co-Sm (Faisal et al., 2018; Asghar et al., 2020), Co-Dy-La (Mohammed et al., 2019), Nd-Cd-In (Jasrotia et al., 2019), and Bi (Iqbal et al., 2020). Iqbal and co-workers reported that the Bi-substituted barium ferrite strongly absorbed microwave in a higher frequency regime (8.2–12.4 GHz) than that by spinel ferrites but their magnetizations were moderate below 60 emu/g (Iqbal et al., 2020). From the line broadening in MÖssbauer spectra, Winotai and co-workers concluded that saturation magnetization of BaFe_12−xBi_xO₁₉ was reduced by the substitution of low concentration in the octahedral sites but increased by substituting Bi³⁺ in the tetrahedral sites (Winotai et al., 2000). In this work, the sol–gel auto-combustion is used to synthesize Bi-substituted barium hexaferrite with enhanced magnetization. The co-substitution of Bi-Cu is also explored. Their effects on magnetic properties are related to the variation in phase and morphology of ferrite products.

2 Materials and methods

Six barium ferrite samples including a pristine sample (A) were synthesized by the sol–gel auto-combustion technique. The BaFe_12−xBi_xO₁₉ samples with x = 0.1 and 0.5 are respectively referred to as B and C. Samples coded D, E, and F are Ba_1−xFe_12−xBi_xCu_xO₁₉ with x = 0.1, 0.2, and 0.3 respectively. Bi and Bi-Cu ions were introduced by adding bismuth(III) nitrate pentahydrate (Bi(NO₃)₃·5H₂O) and copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O) in the reaction between iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and barium nitrate (Ba(NO₃)₃). The starting materials were dissolved in 30 mL of deionized water. Then, 1.441 g of citric acid was added to the solution and completely dissolved under constant stirring. Citric acid is a fuel in the gel combustion commonly used for barium ferrite syntheses (Mohammed et al., 2019; Jasrotia et al., 2019). The pH of the solution was adjusted to 7 by adding ammonium hydroxide (NH₄OH) solution (25%). After heating at 90 °C for 230 min, the solution turned into a viscous gel. The next heating step at 150 °C for 190 min enabled the gel combustion resulting in a brown product. The ground product was subsequently heated in a furnace at 450 °C for 2 h and at 1050 °C for 3 h with a heating rate of 4.5 °C/min.

Crystalline phases were examined by X-ray diffractometer (XRD, Philips X’PERT MPD) using 1.5406 Å Cu-k_α radiation. The diffraction was measured at varying angles (2θ) from 20° to 80° with a scan rate of 0.5°/min. The unit cell volume (V_cell) was calculated from;

Morphologies of ferrite samples were examined by a scanning electron microscope (SEM, FEI Quanta 450 FEG) using the accelerating voltage of 10 kV. Before the imaging, the samples were coated with Pd-doped Au by sputtering. The particle size in SEM micrographs were analyzed by the ImageJ program and the size distributions were presented in histograms. Room temperature magnetic properties were characterized by a vibrating sample magnetometer (VSM, VersaLab Quantum Design). To complete each hysteresis loop, the magnetic field was swept from –20 kOe to +20 kOe. The coercivity and remanence were respectively determined from the x- and y- intercept of the hysteresis loop.

3 Results and discussion

The XRD pattern of the pristine sample A in Fig. 1(a) exhibits characteristic peaks of barium hexaferrites with the magnetoplumbite structure (JCPDS: 43-0002). Diffractions from the (1 1 4) and (1 0 7) give rise to sharp peaks of high intensity. Besides, the trace of hematite (α-Fe₂O₃, JCPDS: 24-0081) is present as the second phase. Also shown in Fig. 1(a), the Bi substitution of x = 0.1 (sample B) gives rise to comparable barium hexaferrite peaks but the intensity of the hematite peak is reduced. The secondary hematite phase entirely disappears as x  is increased to 0.5 (sample C). The effect of this Bi substitution is similar to the Cu substitution in enhancing the sintering and suppressing iron oxide formation (Rafiq et al., 2018). By contrast, the simultaneous substitutions by Bi and Cu in Ba_1−xFe_12−xBi_xCu_xO₁₉ exhibits the opposite effect on the secondary phase formation. As shown in Fig. 1(b), the intensity of the hematite phase is increased with increasing x from 0.1 to 0.3. The presence of hematite is a result of competing reaction which is not only dependent on the substitution but also the fuel used in the sol–gel auto-combustion (Sankaranarayanan and Khan, 1996; Godara et al., 2021). For the charge neutrality, the hematite formation can be increased with increasing ion substitutions (Katlakunta et al., 2015).

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Fig. 1

XRD patterns of (a) BaFe_12−xBi_xO₁₉ (A (x = 0), B (x = 0.1), C (x = 0.5)) and (b) Ba_1−xFe_12−xBi_xCu_xO₁₉ (D (x = 0.1), E (x = 0.2), F (x = 0.3)) samples compared to the barium hexaferrite standard.

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As listed in Table 1, the lattice parameter a is increased by only 0.034 % with the Bi substitution whereas the c value is decreased by 0.17% with the highest Bi-Cu co-substitution (sample F, x = 0.3). The c/a ratio does not significantly vary with the substitution and remains around 3.94. The value of less than 3.98 is consistent with the M-type hexaferrites (Chawla et al., 2014). The V_cell is expanded by only 0.029% by and reduced from 700.6 to 699.1 Å³ only by the Bi-Cu co-substitution. The crystallite size of the pristine sample A is 63.88 nm and significantly rise with the increasing Bi substitution. However, the co-substitution of Bi and Cu does not monotonically increase the crystallite size and the reduction to 62.92 nm is observed in the case of sample E (x = 0.2). The irregular changing crystallite size was also observed in the case of co-substitution by Co-Zr ions with smaller average sizes (Chawla et al., 2014; Mudsainiyan et al., 2014).

The effect of Bi substitution on the morphology of barium hexaferrite is notable in Fig. 2. Similar to previous reports (Faisal et al., 2018; Asghar et al., 2020; Mohammed et al., 2019; Jasrotia et al., 2019), the agglomeration by magnetic interaction leads to the particle size much larger than the crystallite size. Also in Fig. 2, the particle size distributions are shown by histograms in the insets. The pristine sample is composed of irregular submicron particles with rough morphology. With increasing Bi substitution, the particles evolve into microscale with flat hexagonal facets reflecting the magnetoplumbite structure. The distinctive feature in the case of the highest Bi substitution x = 0.5 (sample C) is a highly compact stacking of microparticles. The particle growth is consistent with the increase in crystallite size of Bi-substituted barium ferrites. The size of hexagonal particles is even larger by the Bi-Cu substitutions. The trend is consistent with the barium hexaferrites substituted by Cu (Rafiq et al., 2018; Vadivelan and Jaya, 2016) and Ti (Behera and Ravi, 2019). In addition to ion substitutions, the sintering is clearly promoted.

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Fig. 2

Morphology of BaFe_12−xBi_xO₁₉ ((a) sample A (x = 0), (b) sample B (x = 0.1), (c) sample C (x = 0.5)) and Ba_1−xFe_12−xBi_xCu_xO₁₉ ((d) sample D (x = 0.1), (e) sample E (x = 0.2), (f) sample F (x = 0.3)) imaged by SEM. Histograms of particle size distribution are shown in the insets.

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The Bi and Bi-Cu substitutions influence magnetic properties as respectively shown by hysteresis loops in Fig. 3(a) and (b). Magnetic parameters deduced from these hysteresis loops are compared in Table 1. The coercivity (H_c in Table 1) is decreased with increasing Bi substitution similar to a previous report on the Cu substitution (Asiri et al., 2018). The Fe³⁺ ions in the 12k, 4f₂, and 2b sites predominantly contribute to the magnetocrystalline anisotropy. Because the ion substitution does not drastically affect the c/a ratio, the coercivity change is not due to the anisotropy. Instead, such reduction is related to the increase in particle size observed in Fig. 2. The multi-domain structure, following the increase in particle size, requires a smaller magnetic field to switch its magnetization direction. It is also consistent with the larger particle size that the Bi-Cu substituted barium hexaferrites exhibit even lower values of coercivity. Such a trend was also observed by the co-substitution by Bi-Ti (Belous et al., 2006). The reduction in coercivity of barium hexaferrites with high magnetizations could be exploited in producing microwave absorbing and data storage materials. The remanence (M_r in Table 1) is also decreased from 34 to 27 emu/g with increasing Bi-Cu from x = 0.1 to 0.3. On the other hand, the increase in Bi substitution from 0.1 to 0.5 slightly affects the remanence of BaFe_12−xBi_xO₁₉.

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Fig. 3

Magnetic hysteresis loops of (a) BaFe_12−xBi_xO₁₉ and (b) Ba_1−xFe_12−xBi_xCu_xO₁₉ samples.

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By increasing an applied magnetic field in the VSM measurement from 10 kOe in Fig. 3 to the maximum of 20 kOe, the hysteresis loops are still not saturated. The saturation magnetization (M_s in Table 1) is then approximated from the law of approach saturation. Magnetizations of hard ferrites approach the saturation value according to Eq. (3) (Bate, 1991).

In Fig. 4, the M values from the hysteresis loops in Fig. 3 are plotted against 1/H². The linear trends are obtained in the case of Bi substitution with R² around 0.987–0.988. For Bi-Cu substitution, the R² is increased to 0.993–0.995 corresponding to lower magnetizations. The saturation magnetization, determined from the y-intercept in the plot of M against 1/H², is 85 emu/g in the case of the pristine sample A. This saturation magnetization is much larger than typical values in the range of 42–63 emu/g (Chawla et al., 2014; Mudsainiyan et al., 2014), but comparable to those in some previous reports (Rafiq et al., 2018; Winatapura et al., 2019). In barium hexaferrites, Fe³⁺ ions in the 12k, 2a and 2b sites have their magnetic moments in parallel to the crystallographic c-axis, but those occupying the tetrahedral 4f₁ and octahedral 4f₂ sites are antiparallel. A higher occupancy of Fe³⁺ ions in the 12k sites lead to a larger magnetization (Rane et al., 2013).

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Fig. 4

Plots of M as a function of 1/H² using the values from hysteresis loops of BaFe_12−xBi_xO₁₉ (A (x = 0), B (x = 0.1), C (x = 0.5)) and Ba_1−xFe_12−xBi_xCu_xO₁₉ (D (x = 0.1), E (x = 0.2), F (x = 0.3)) samples.

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The saturation magnetization is even higher by the Bi substitution and the maximum value of 99 emu/g obtained in sample C (x = 0.5) is likely due to the high packing density and purity of barium hexaferrite. Magnetic properties are changed as a result of partial substitutions in five sublattices of barium hexaferrites by di-, tri- and tetravalent ions (Chawla et al., 2014; Mudsainiyan et al., 2014; Vinnik et al., 2015). The increase in saturation magnetization is due to the substitution of Fe³⁺ in the tetrahedral 4f₁ site by Bi³⁺. Consistent with increasing hematite formations by the Bi-Cu addition, the increases of x to 0.2 and 0.3 in Ba_1−xFe_12−xBi_xCu_xO₁₉ yield lower saturation magnetizations than 97 emu/g by the lowest substitution of x = 0.1 (sample D). Besides the effect of phase impurity, the saturation magnetization is increased if cations substitute the Fe³⁺ in the tetrahedral sites. The magnetization in the case of substitution by Bi-Cu is likely reduced because the Fe³⁺ substitutions in the crystals can also occur at other sites depending on the type and concentration of ions (Belous et al., 2006). The ionic radii of Fe³⁺ in octahedral coordination (0.67 Å) and Cu²⁺ (0.78 Å) ions are comparable and Cu²⁺ tends to occupy the octahedral 4f₂ site (Vinnik et al., 2015). The substitutions in octahedral sites tend to weaken exchange interactions of Fe³⁺-O²⁻-Fe³⁺ and therefore reduce the magnetization (Jasrotia et al., 2019; Godara et al., 2021). The ionic radii of Bi³⁺ (1.1 Å) are larger than those of Cu²⁺ and Fe³⁺ and the partial substitution of Ba²⁺ whose ionic radii is 1.5 Å (Barsoum, 2020), cannot be ruled out.

Finally, the squareness ratio (M_r/M_s in Table 1) of all ferrites are compared. The highest value of 0.45 is observed in the pristine sample, also with the highest coercivity, is attributed to particles with a single magnetic domain. The Bi substitution reduces the squareness ratio of barium hexaferrite. The squareness ratio is further reduced to 0.32–0.35 by the Bi-Cu substitution indicating multi-domain structures (Chawla et al., 2014; Mudsainiyan et al., 2014; Kagdi et al., 2018). The results are consistent with the increase in particle size and the reduction in coercivity. Interestingly, the values of squareness ratio over 0.6 were reported in co-substituted BaFe₁₂O₁₉ (Belous et al., 2006) and BaFe₁₂O₁₉/CaCu₃Ti₄O₁₂ composites (Mohammed et al., 2019).

4 Conclusions

The substitution of Bi in sol–gel derived BaFe_12−xBi_xO₁₉ increased the size of hexagonal particles and the substitution of x = 0.5 inhibited the hematite formation as the second phase. Such an increase in particle size reduced the coercivity whereas the remanence remained rather constant. Interestingly, the saturation magnetization consistent with the law of approach saturation was as high as 99 emu/g in the case of x = 0.5. The particle size of Ba_1−xFe_12−xBi_xCu_xO₁₉ is even more enhanced by the substitution by Bi-Cu and it follows that the coercivity was decreased below 700 Oe. The magnetizations were also reduced by the high substitution by Bi-Cu.