Investigation of microstructural and magnetic properties of Ca²⁺ doped strontium hexaferrite nanoparticles

1 Introduction

In the field of ferrite-based permanent magnetic materials, the discovery of M-type (Magnetoplumbite) hexaferrite in the 1950s marked a watershed moment (Pullar 2012). Due to their critical role in the fabrication of key components for many different devices and machines, magnetic materials are in high demand in the electronics industry. High coercivity, high magnetic saturation and remanence, high magnetocrystalline anisotropy, excellent chemical stability, and a high Curie temperature have all been associated with M-type hexaferrite (Urbano-Peña et al., 2019). A search for low-cost (Mehtab et al., 2022; Ahmad et al., 2013) and high-performance magnetic material leads to the development of new M-type hexaferrite material. Ca is available in abundance in our earth bulk as compared to their Ba and Sr and hence its metal nitrate and oxides are cheaper than other counterparts Ba (Christy et al., 2019). If calcium hexaferrite (CaM) could be synthesized in phase pure form, it would be a low-cost substitute for Ba/Sr - based hexaferrite. Hexaferrites contain one large divalent metal ion (M²⁺=Ba/Sr/Pb/Ca), which disrupts the crystal lattice due to size differences, resulting in high magnetocrystalline anisotropy. Lotgering (Lotgering and Huyberts 1980) described lanthanum ferrite as a material with large magnetocrystalline anisotropy. Ca is least studied as a dopant in M-type hexaferrites. Ca-doped hexaferrites have been shown to improve coercivity, increase the reaction rate, lower phase formation temperature, and thus control grain size more easily (Chauhan et al., 2018; Sable et al., 2010; Ali et al., 2013). As a result, it is worthwhile to investigate the magnetic properties of MFe₁₂O₁₉ by replacing M²⁺ with Ca²⁺. The current study reports on the synthesis of Sr_1-xCa_xFe₁₂O₁₉ (x = 0.0 to 0.5) hexaferrite nanoparticles over a wide concentration range. The magnetic, morphological, and structural characteristics of the prepared samples were studied in depth.

2 Experimental

To synthesize Ca²⁺-doped strontium hexaferrite nanoparticles (by sol–gel auto-combustion method), calcium nitrate, ferric nitrate, strontium nitrate, and citric acid (all the chemicals were AR grade having purity >98%) were used as such without further purification. Double distilled water was used to dissolve metal nitrate salts in the required stoichiometric ratio (as listed in Table 1). Afterward, the citric acid was mixed in the above solution on a magnetic stirrer at room temperature. Ammonia solution (25%) was added drop-wise to maintain a pH of 7. The temperature of the resulting solution was raised to 90–100 °C using a hot plate. With continuous heating for a few hours, the excess water evaporates, leaving a brown gel that looks like honey. The gel was kept in an oven for 1 h at 200 °C to allow auto combustion reactions. Mixed metal oxide precursor material was obtained after the auto-combustion reaction. Impurities were removed from the powder samples using a muffle furnace and calcination for 6 h at 900 °C to convert the amorphous phase to the crystalline phase.

The Rigaku Miniflex-II diffractometer (20-80° range of 2θ utilizing Cu-α emission (λ = 1.54 A), 0.005 step size) was used to study the phase purity, and structure of the materials. FESEM (operating voltage 20 kV) from JEOL, JSM-7100, and energy dispersive X-ray (EDX) analysis were carried out for surface morphology and elemental analysis. Confocal Renishaw Raman Spectrophotometer having laser of 514 nm excitation wavelength was used to obtain structural features in 100–800 cm⁻¹ wavenumber range. VSM (vibrating sample magnetometer, Microsense EV-90) was used to make magnetic measurements.

3 Results and discussion

XRD data of the Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0–0.5) materials is shown in Fig. 1. According to international standard diffraction data, the peaks of all six compositions correspond to the M-type strontium hexaferrite phase (Godara et al., 2022). At concentration x ≥ 0.3, additional peaks marked with ‘*’, indicating hematite (α-Fe₂O₃) secondary phase was observed. The intensity of the 〈1 0 7〉 and 〈1 1 4〉 planes decreases as dopant ion concentration increases, while the intensity of the α-Fe₂O₃ peak increases. As a result, as the concentration of Ca²⁺ doping increases, the α-Fe₂O₃ phase grows at the expense of the M-type phase. This indicates lower calcium solubility in SrM phase at 900 °C. The lower solubility of calcium is due to its smaller ionic radii, which is responsible for the appearance of the hematite phase. The calcium solubility can be improved by increasing calcination temperature and time (Mohammed et al., 2018; Ahmad et al., 2013).

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Fig. 1

XRD patterns for Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0–0.5).

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Rietveld refinement was performed by Fullprof software to further establish %age phase purity and determine other structural characteristics. For Rietveld refinement, the basic structure (starting lattice parameters and atomic locations) was assumed the same as in ISCD 98-002-6353. The procedure is also reported elsewhere (Godara et al., 2019). The corrected data matches the experimental data fairly well, and the difference data is randomly distributed around zero, as seen in Fig. 2. The Rietveld refined parameters for x = 0.0–0.5 were tabulated in Table 2.

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Fig. 2

Rietveld refinement of Sr_1-xCa_xFe₁₂O₁₉ (x = 0.0–0.5).

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The lattice constants ‘c & a' as well as cell volume decreases with an increase ‘x = 0.2,’ while the lattice constants ‘a & c' as well as cell volume randomly varies for x > 0.2 due to the appearance of the hematite phase. The observed decrease in cell volume and the lattice constants ‘a & c' could be due to the smaller ionic radius of Ca²⁺ (0.99) (Ubaidullah et al., 2020) compared to Sr²⁺ ((1.49) Kaur et al., 2017). As a result of the decrease in lattice parameters, it can be concluded that the incorporation of Ca²⁺ ions into the crystal structure of SrFe₁₂O₁₉ was successful. Verstegen and Stevels have already demonstrated that if the observed c/a ratio is less than 3.98 (Yang et al., 2014), the structure is assumed to be M-type. The observed c/a values in all synthesized samples are less than 3.93, confirming the M-type hexaferrite phase. The presence of secondary phases, bond length, local and dynamic symmetry, local strain, and structural distortion of M-type hexaferrite materials can be investigated using Raman spectroscopy. The Raman spectra of Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0–0.5) are shown in Fig. 3. When compared to the literature (Kaur et al., 2017; Ashima et al., 2012; Alhokbany et al.,2021), our results show that x ≥ 0.3, has two more peaks of α-Fe₂O₃ secondary phase at 224 and 290 cm⁻¹, in addition to peaks corresponding to SrM phase at 85, 183, 314, 337, 411, 527, 615 and 684 cm⁻¹.

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Fig. 3

Raman spectra for Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0–0.5).

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XRD data also exhibits α-Fe₂O₃ secondary phase for x ≥ 0.3, suggesting the existence of secondary phase which agrees well with the results obtained from Raman spectroscopy. The sharpness and the intensity of the secondary phase (α-Fe2O3) peaks increase beyond x = 0.2, which reveals that the concentration of α-Fe₂O₃ continuously increases with calcium doping. Due to E_1g symmetry, the mode at 183 matching to the vibrational mode of the entire spinel block were discovered at lower Raman shift (Mohammed et al., 2020, Musa et al., 2021). The vibrational mode observed around 314 and 337 cm⁻¹ occurs due to the O-Fe-O vibrations. The mode around 411 cm⁻¹ transpires may be due to the Fe(12 k)/Ca(12 k)O₆ octahedra vibrations whereas, the mode around 527 cm⁻¹ arises due to the Fe(4f₂)/Ca(4f₂)O₆ octahedra vibrations. In addition, the Raman active modes around 615 cm⁻¹ and 684 cm⁻¹ may be occurred due to the vibrations of anion sublattices called as, Fe(2b)/Ca(2b)O₅ and Fe(4f₁)/Ca(4f₁)O₄ respectively (Jasrotia et al., 2020).

FESEM micrographs of Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0, 0.1,0.3,0.5) are shown in Fig. 4. In FESEM micrographs, particles are highly agglomerated (as a result of magnetic interaction between the individual grains of the samples) [Trudel et al., 2019] and the average particle size continuously increases from 125 nm (x = 0.0) to 240 nm (x = 0.5). Godara et al. also reported similar trends in literature at higher calcination temperature 1200 °C (Godara et al., 2021, Mohammed et al., 2018, Mohammed et al., 2019). The grain size increases upon replacing Sr²⁺ ion with a smaller radius of Ca²⁺ ion in samples from x = 0.0 to 0.5. The smaller size of Ca²⁺ ion facilitates ionic diffusion and thus promotes the growth of grains (Godara et al., 2021; Blanco and Gonzalez, 1989). The small spherical-shaped particles on the surface of agglomerated grains in the significant number were observed only in the x = 0.5 sample, which may be secondary hematite phase present in a considerable amount as also revealed in XRD plots. The elemental data for samples (x = 0.1, 0.3, and 0.5) are tabulated in the inset of EDX spectra in Fig. 5. The EDX spectra show the presence of Ca, Sr, Fe and O atoms. The observed increase in Ca content confirms the successful replacement of Strontium ions by calcium ions in prepared samples.

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Fig. 4

FESEM micrographs and histograms for Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0, 0.1, 0.3, and 0.5).

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Fig. 5

EDX spectra of Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.1, 0.3, and 0.5).

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The M-H plots were recorded at room temperature in pellet form for Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0–0.50) is shown in Fig. 6a and 6b. M-H plots revealed the non-saturating behavior of all the synthesized samples in the applied field range. To obtain M_s, saturation law i.e plotting M vs. 1/H² data, and then a straight line was fitted to it, which was then used in data of M vs. H in the range 8 to 18 kOe (Godara et al., 2021) using equation (3). The fitting plot of a straight line is shown in Fig. 6c.

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Fig. 6

Magnetic measurement of Sr_(1-x)Ca_xFe₁₂O₁₉ (x = 0.0–0.5) samples: a) M vs. H graphs, b) zoomed view of M vs. H graphs, c) linear fitting of M vs. 1/H² to determine M_s and d) graphs of H_a, M_s, M_r and H_c vs. composition.

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Here M_s stands for saturation magnetization, A stands for inhomogeneity, χ_p stands for high field susceptibility, and B stands for anisotropy. A linear relationship between M and 1/H² from 8 to 18 kOe was observed. As a result, the A/H and χ_p terms in Eq. (3) can be ignored. The magnetic parameters derived from M vs. H graphs are provided in Table 3 for all the studied samples. The M_s value increases from 67.87 for x = 0.0 sample to 72.38 emu/g (theoretical limit of pure SrM is 72 emu/g (Shirk and Buessem, 1970) for x = 0.20 sample afterward decrease with a calcium content (due to the formation antiferromagnetic secondary phase (α-Fe₂O₃) beyond x = 0.2). The increase M_s can be discussed as follows: (a) With the increase in dopant Ca²⁺ concentration upto x = 0.20 (Hooda et al., 2015), the particle size increases. This is because larger particles have a more extensive domain size and therefore align many atomic spins in a fixed direction.

The overall magnetic moment increases due the alignment of atomic spins in a particular direction when the magnetic field is applied. (b) At a lower substitution level, some of the Fe³⁺ sites and Sr²⁺ sites are occupied by Ca²⁺ ions because of their relatively small ionic radius at 4f₂ spin down and 2a spin-up position (Hooda et al., 2015). When the substitution level is higher, as communicated earlier (Godara et al., 2021) 12 k up spin site and 4f₂ down spin sites are also occupied by Ca²⁺ ions. Ca²⁺ ions successfully substituted upto x = 0.20 without forming any secondary phase, therefore, these ions are expected to occupy a significant part of the down spin sites. It results in the fall in the oppositely oriented Fe³⁺ (down spin) ions concentration, consequently an increase in the values of M_s. Conversely to our observation of M_s. A continuous decrease in M_s values in Sr_1-xCa_xFe_11.5Co_0.5O₁₉ (x = 0.10–0.50) samples have been reported by Ali et al. (Ali et al., 2013). Whereas H_c values increased to x = 0.20 and then decreased afterward. Other researchers working in M-type hexaferrite groups have also been reported a similar trend (Hooda et al., 2015; Yuping et al., 2018; Kumar et al., 2018). The variation of M_s, M_r, H_c and H_a as a function of ‘x’ for Sr_1-xCa_xFeO₁₉ (x = 0.0–0.5) is shown in Fig. 6d.

Furthermore, for crystals with hexagonal symmetry, B can be written as:.

H_a and K₁ represent to anisotropy field, and anisotropy constant respectively. The value of B can be calculated by plotting M = M_s(1-B/H²) against 1/H². Furthermore, the anisotropy field, H_a, can be calculated by plugging the value of B into Eq. (4) (Singh et al., 2008). The coercivity (H_c) value is decreased from 5788 to 5292 Oe with an increase in x from 0.0 to 0.3 increases afterward till x = 0.5. It's linked to (a) a decrease in the anisotropy field (H_a) (Iqbal et al., 2009; Abdellah et al., 2018), (b) oxygen vacancies that act as domain wall pinning centers (Kumar et al., 2015), (c) presence of secondary phases and (d) Ca dopant-induced grain growth [Godara et al., 2021]. The presence of Fe³⁺ on sites of hexagonal unit cells contributing to H_a with a trend of 12 k < 4f1 < 2a < 4f2 < 2b (Li et al., 2000; Singh et al., 2008) causes a decrease in H_a, and different authors have reported significant contributions. From x  = 0.0 (18.87 kOe) to 0.3 (17.23 kOe), there is a ≈8 percent drop in the anisotropy field (H_a) as shown in Table 3. On the other hand, doping results in a significant increase in grain size (from 125 (x = 0.0) to 240 nm (x = 0.5)), as shown in Fig. 3. Coercivity has an inverse relationship with grain size (above critical grain size). In the present study, It has been observed that both grain size and anisotropy field (H_a) contribute towards the reduction of coercivity with an increased dopant ion concentration upto x = 0.30. However, the anisotropy field has a greater influence on the mechanism of coercivity than grain from x = 0.0 to x = 0.5. Single domains, highly anisotropic, and magnetically hard materials have a squareness ratio greater than 0.5. However, the material is multi-domain and randomly oriented in nature if the squareness ratio is less than 0.50. Therefore, a high squareness ratio is beneficial to permanent magnets and magnetic recording (Habanjar et al., 2020). The value of squareness ratio is ≈ 0.5 in all the synthesized samples suggesting that the hexaferrite obtained are randomly oriented single domain particles (Chauhan et al., 2018).

4 Conclusion

Sr_1-xCa_xFe₁₂O₁₉ (x = 0.0–0.5) materials were synthesized from the sol–gel auto-combustion method. Phase purity in Sr_1-xCa_xFe₁₂O₁₉ samples upto x = 0.20 is confirmed by XRD data beyond which α–Fe₂O₃ secondary phase is observed. FESEM indicates that grain size monotonically increases with an increase in Ca²⁺ content. The enhanced grain size as the amount of Ca doping increases is because Ca acts as a grain growth promoter. Non-linear M vs. H loops show the existence of strong magnetic ordering in all the samples. The enhancement in M_s from 67.87 emu/g with x = 0.0 to 72.63 emu/g with x = 0.2 is the replacement of non-magnetic Ca²⁺ in place of Fe³⁺ ions at spin down (4f₂) sites along with Sr²⁺ site and increase in domain size with grain growth. The reduction in coercivity from 5799 Oe (at x = 0.00) to 5292 Oe (x = 0.30) has been related to the corresponding decrease in H_a. The large values of M_s (72.63 emu/g) and H_c (5388 Oe) in the x = 0.20 sample are useful for low-cost permanent magnet devices.