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. 2022 Nov 25;7(48):43432–43439. doi: 10.1021/acsomega.2c03256

Improved Electrical Properties of Strontium Hexaferrite Nanoparticles by Co2+ Substitutions

Mah Rukh Rehman 1, Muhammad Aftab Akram 1,*, Iftikhar Hussain Gul 1,*
PMCID: PMC9730768  PMID: 36506130

Abstract

graphic file with name ao2c03256_0017.jpg

In this work, the sol–gel route was employed to synthesize a series of Co2+-substituted strontium hexaferrite nanoparticles (Sr1–xCoxFe12O19, x = 0.0–0.50) to study the effect of cobalt ions doping on the magnetic, electrical, and structural properties of the nanoparticles. The structural analysis of the synthesized nanoparticles, performed by X-ray diffraction, showed the formation of a hexagonal structure having no secondary phases. The morphological analysis, performed through scanning electron microscopy, revealed spherical shaped nanoparticles with uniform distribution. Fourier transform infrared spectra demonstrated two consistent absorption bands indicating the intrinsic stretching vibrations around 600 and 400 cm–1 for tetrahedral and octahedral sites, respectively. It was observed through VSM that with cobalt addition, the saturation magnetization increased and the coercivity decreased. Also, a typical decreasing trend of DC electrical resistivity with increasing temperature measured by a two-probe method confirmed the semiconducting behavior of the synthesized samples. An impedance analyzer was used for the dielectric measurements at room temperature against the alternating frequency range of 250 Hz to 5 MHz, and it was found that the dielectric constant decreased with the increase in cobalt content, suggesting that the doped nanomaterials can be used for microwave absorption, electronics, telecommunication, and other high-frequency applications.

1. Introduction

Based on magnetization, ferrites are broadly divided into two categories, viz., hard ferrites and soft ferrites. Hard ferrites, also known as ceramic magnets, have a high value of magnetic permeability and are highly coercive as the time taken by them to demagnetize is longer than that for soft ferrites. The materials required for the synthesis of hard ferrites are easily available, making them less expensive, and mostly, oxides of barium, iron, and strontium contribute to the making of hard ferrites. Strontium hexaferrite (SrFe12O19), showing attractive electrical and magnetic properties, belongs to the family of M-type hard ferrites and is used as a permanent magnet with the magnetocrystalline anisotropy very large for recording media applications.1−3 Magnetocrystalline anisotropy of hexaferrite has great scientific importance and is used in the biomedical, electronic, and optical fields due to their unique and interesting chemical, physical, and mechanical properties.4 Hexaferrites are of various types depending on their composition; however, all of them have similarities in their crystalline structure. The main sources of the magnetic moment in hexaferrite structure are Fe cations that can be found on any one of the five sites, for example, one tetrahedral, three octahedral, and one trigonal bipyramid sites.5 The properties of Sr-hexaferrite are mainly dependent on the preparation method used, particle size obtained, and the cation distribution in their crystal structure at five crystallographic sites.6

Recently, there has been a renewed interest in mixed nanoferrites for their various applications as the addition of a small amount of dopant enhances their magnetic and structural properties by many folds. Mixed metal nanoferrites have superior properties compared to pure ferrite materials, and the replacement of Fe3+ ions by a divalent cationic substitution have been reported to modify various electric and magnetic properties.7−10 However, the challenges faced while synthesizing these mixed metal nanoferrites are to obtain a well-controlled size and shape, a uniform composition, and lower sintering temperatures for desired magnetic properties to be used in the electronic material industry.11,12 Nanoferrites are mostly prepared by a wet chemical method, such as hydrothermal, solvothermal, sol–gel, and co-precipitation. The wet chemical method is preferred over a solid-state method because it provides low annealing temperature with homogeneous and small-sized nanoparticles.13

In this study, we have employed the sol–gel method to successfully synthesize strontium hexaferrite nanoparticles (Sr1–xCoxFe12O19, x = 0.0–0.50) with Co2+ as the dopant to enhance their electrical and magnetic properties.

2. Experimental Procedure

2.1. Materials

Strontium nitrate [Sr(NO3)2—Sigma-Aldrich, Germany] was used as a precursor for strontium hexaferrite nanoparticles with deionized water as a solvent. Cobalt(II) nitrate hexahydrate [Co(NO3)2·6H2O], iron(III) nitrate nonahydrate [Fe(NO3)2·9H2O], citric acid, and ammonia solution were purchased from Merck (KGaA Darmstadt—Germany with 98% purity). All these chemicals were of analytical grade and were used as such, with no purification.

2.2. Chemical Synthesis

Sr1–xCoxFe12O19 (x = 0.00–0.50) nanoparticles were synthesized by the sol–gel method. A solution of strontium nitrate, cobalt(II) nitrate hexahydrate, citric acid, and iron(III) nitrate nonahydrate was prepared according to their stoichiometric ratios to form a mixed solution. Ammonia was added dropwise to the solution to obtain a pH of 7. The system was kept under continuous stirring at 350 °C (hotplate) and a solution temperature of 95–100 °C for the gelation to occur, followed by complete combustion to obtain a dark-colored powder. Citric acid was used, which acts as the chelating agent and helps a combustion process to take place.14 The obtained powder was calcined at 1050 °C in a muffle furnace for 8 h to remove carbonaceous impurities, resulting in the formation of a well-distinct hexagonal phase (Figure 1).

Figure 1.

Figure 1

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Chemical synthesis of nanoparticles.

3. Characterization Techniques

The structural and phase analysis of the prepared samples was evaluated by the X-ray diffraction (XRD) technique (STOE, Darmstadt Germany) using Cu Kα radiation. Debye–Scherrer eq 1 was used for the calculation of crystallite size using the most intense peak.15

3. 1

In the above equation, λ is the X-ray wavelength (value = 1.5401 Å), K is the shape factor (value = 0.9), β is the full width at half-maxima, and θ represents the Bragg angle.16

Scanning electron microscopy (SEM) (JEOL JSM-6490A, Japan) was employed to analyze the morphological and topographical features of the as-synthesized samples.

Fourier transform infrared (FTIR) spectroscopy (PerkinElmer Spectrum 100, spectrophotometer) was performed for the analysis of metal–oxygen vibrations in the prepared samples.

An impedance analyzer was used for the dielectric measurements at room temperature against the alternating frequency range of 250 Hz to 5 MHz. The samples for dielectric measurements were prepared by pressing the powder to a circular disc-shaped pellet under a pressure of 3 tons, followed by sintering at 800 °C. Relation 2 was used for the calculation of dielectric constant.

3. 2

where C and d represent the capacitance and thickness, respectively, A denotes the pellet cross-sectional area, and ε0 shows the permittivity of free space. The following relation 3 was used for the calculation of dielectric loss factor (ε″).17

3. 3

Here, tan δ is the tangent loss factor determined by the impedance analyzer directly. The AC conductivity was calculated from the dielectric loss factor with an alternating frequency range using relation 4.15

3. 4

The two-probe method was employed to measure the DC resistivity against high temperatures. eq 5 for the calculation of resistivity is given as

3. 5

where L and A represent the thickness and cross-sectional area of the sample, respectively. Relation 6 was used for the calculation of drift mobility (μd).

3. 6

where n represents the charge carrier concentration, ρ represents the resistivity, and e shows a charge of an electron. The charge carrier concentration was calculated from n = NaDmPFe/M (where Na is Avogadro’s number, Dm is the bulk density, PFe is a number of iron atoms, and M is the molecular weight).18

The magnetic properties such as saturation magnetization (Ms), coercivity (Hc), and remanence magnetization (Mr) were studied through (M–H) hysteresis loop obtained from a vibrating sample magnetometer.

4. Results and Discussion

4.1. Structure and Morphology

The powder XRD patterns of the as-synthesized samples of Sr1–xCoxFe12O19 (x = 0.00–0.50) at room temperature are shown in Figure 2. The nicely separated diffraction peaks appear at planes (1 1 0), (1 1 2), (1 0 7), (1 1 4), (2 0 1), (2 0 3), (2 0 5), (2 0 6), (3 0 0), (2 1 7), (2 0 11), (2 2 0), and (3 1 7) at the respective angles of 2θ = 30.359, 31.358, 32.357, 34.198, 35.42, 37.152, 40.424, 42.552, 55.217, 56.866, 63.16, 67.734, and 72.038. The presence of these peaks at specific angles verifies the generation of nanoparticles to card number JCPDS 01-080-1197. There is no secondary peak for an extra phase of hematite observed in the pure sample.19 However, the XRD patterns of cobalt-substituted strontium hexaferrite (x = 0.15, 0.24, and 0.50) display a desired phase of the strontium M-type hexaferrite and an undesired phase of hematite (Fe2O3). It can be assumed that during the calcination, magnetite got oxidized into a very stable non-magnetic hematite.20 In addition, the shifting of peak position of the doped samples possibly indicates the substitution of Co with the Sr ions because of different ionic radius. Also, this variation is mainly because of the crystallographic site’s occupation in the crystal lattice.21

Figure 2.

Figure 2

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XRD pattern of Sr1–xCoxFe12O19.

The XRD spectra of the doped strontium ferrites showed no change in hexagonal structure and geometry, indicating the successful substitution of Co with no definite change in the values of lattice parameters. However, as shown in Figure 2, the extra hematite phase is formed in the doped sample, which increased with the increase in the concentration of doping.22 The phase analysis for the maximum cobalt sample calculated via the Rietveld refinement using an X’Pert HighScore Plus is given in Figure S1 in the Supporting Information.

The SEM images shown in Figure 3 were used to determine the size and morphology of Sr(1–x)CoxFe12O19. All the powders were annealed, and a suspension in deionized water was made by bath sonication that allowed the nanoparticles to arrange in the lowest interface energy configuration. The result shows that no significant change exists in the morphology with increasing cobalt concentration, and the uniformly distributed strontium hexaferrite nanoparticles’ particle size remains 20–25 nm.23 It can also be observed that the particles are fine, having a spherical morphology with minimum agglomeration. The elemental composition analysis of Sr1–xCoxFe12O19 (x = 0.15, 0.24, and 0.5) nanoferrite samples has been studied through the energy-dispersive X-ray (EDX) spectra. The characteristic EDX spectra are shown in Figure S2 of the Supporting Information, and the results of analysis are summarized in Table S1 of the Supporting Information. The traces of impurities and other elements are not identified. The observed composition ratios of metal ions (Sr/Co) and iron to oxygen (Fe/O) are consistent with the expected composition ratio. This indicates that the expected stoichiometry under preparation is well maintained in the samples prepared using the sol–gel technique.

Figure 3.

Figure 3

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Morphological analysis using SEM for Sr1–xCoxFe12O19 (a) x = 0, (b) x = 0.15, (c) x = 0.24, and (d) x = 0.5.

4.2. FTIR Spectra

The structure of the multi-component system was investigated using FTIR spectra at room temperature, and the results are shown in Figure 4. The FTIR spectra obtained confirmed the formation of a hexagonal structure and the distribution of cations. The characteristic absorption bands visible in the frequency range of 370–600 cm–1 agreed with the already reported literature for the ferrite family.24−26 The stretching vibrations of the metal–oxygen bond around 600 cm–1 are for the intrinsic tetrahedral metal complex and around 400 cm–1 is for the octahedral metal complex, confirming the hexagonal ferrite phases.27−29 The shifting of bands toward a higher frequency range by the addition of cobalt is due to the changes in bond length of the metal ions in lattice positions as shown in Figure 4.29,30 The addition of Co ions resulted in reduced mass on sites A and B, and the decrease in reduced mass causes force constant to increase, leading to the frequency shift.31

Figure 4.

Figure 4

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FTIR spectra of the synthesized ferrites.

4.3. Electrical Properties

4.3.1. Dielectric Measurements

The dielectric constant has a real and an imaginary part, and the real part deals with the ability of polarization or the extent to which a material can store energy. The dielectric constant of a material can be the result of dipolar, interfacial, electronic, or ionic polarization.32 The dielectric constant ε′ for Sr1–xCoxFe12O19 (x = 0.00–0.50) with an alternating frequency in the range of 250 Hz to 5 MHz was performed. For the investigation of dielectric properties, pellets of the samples were prepared in a die of 13 mm diameter. All the samples were analyzed at room temperature. Several factors such as synthesis method, calcination conditions, grain size, chemical composition, ionic charge, and so forth affects the dielectric properties of the Sr hexagonal ferrites. It can be observed from Figure 5 that, as the frequency increases, the dielectric constant rapidly decreases for all the samples. The values of dielectric constant for each sample are larger at the lower frequency range and eventually became constant at higher frequencies. As the vibrations of ions are greater in the low-frequency range, a large number of dipoles cannot follow the variation in the electric field; as a result, the dielectric constant decreases as the net polarization is not in phase with the electric field.33

Figure 5.

Figure 5

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Dielectric constant variation with frequency.

The variation of dielectric constant can be explained according to the Maxwell and Wagner model and agrees with Koop’s theory.34,35 Maxwell and Wagner’s model of space charge polarization can narrate the amplification of dielectric constant values at low frequencies.36 The model describes the separation of the dielectric material by two layers, extremely resistive grain boundaries and effectively conducting grains. The electrons can effortlessly proceed toward the grain boundaries in the presence of an external field. The extremely resistive nature of grain boundaries results in charge accumulation that results in large polarization.37 At low frequencies, these charge carriers provide fast reactions to variations in an electric field. This response contributes to polarization, increasing the dielectric constant.

The imaginary part known as dielectric loss deals with the amount of energy dissipated.29 The dielectric loss at an alternating frequency is shown in Figure 6. It can be observed from Figure 6 that there is a dielectric loss for all the samples as the frequency increases. This dielectric loss is attributed to the Maxwell–Wagner type of interfacial polarization and also in accordance with Koop’s theory.33,37 Ashiq et al. studied the effect of Al–Cr doping on the dielectric properties of strontium hexaferrites and found the rapid decrease in dielectric loss by increasing the frequency, which also conforms with our results.33 By the increase in cobalt content, the values of dielectric loss showed an increasing trend. The Koop’s phenomenological theory states that in the low-frequency regions, the grain boundaries are extra resistive. As a result, polarization requires a greater amount of energy. This will cause high energy losses. The grain boundaries are not so much resistive in the regions having high frequencies. The energy loss is slight as polarization requires less amount of energy. Thus, these materials are fit for energy storage devices because of their dielectric loss and dielectric constant behaviors in the regions of high frequency and compact values of resistivity.38

Figure 6.

Figure 6

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Dielectric loss variation with frequency.

The dielectric tangent loss for Sr1–xCoxFe12O19 (x = 0.00–0.50) against the increasing frequency range is shown in Figure 7. It has been observed that the tangent loss decreased with an increase in frequency. When an externally applied field frequency resonates with the ion frequency, then the resonance peaking behavior arises, and this behavior was shown by all the samples except the one with x = 0.15. More energy is required for electron exchange between Fe3+ and Fe2+ due to the high resistance of grain boundaries causing high energy loss.17 With the addition of cobalt, the hopping probability increases, resulting in a peak shift toward higher frequency. The presence of a homogeneous structure is verified by small values of tan δ, making material rendering for microwave applications.39

Figure 7.

Figure 7

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Dielectric tangent loss variation with frequency.

The AC conductivity was measured against the alternating frequency at room temperature, and the results are shown in Figure 8. In the case of cobalt-doped strontium hexaferrite, the AC conductivity showed an increasing trend; that is, by increasing the concentration of cobalt, the AC conductivity also increases. The value of AC conductivity decreased first for a small concentration of cobalt and then started to increase. For the maximum concentration of cobalt, the value was higher than that of pure strontium hexaferrite. The increase in the AC conductivity with the increase in the frequency is due to the hopping mechanism of charge carriers. Another reason can be the increase in bandgap width at the nanosize level, as the size of the particle decreases, the width of the bandgap increases.40 A similar trend of increase in AC conductivity has been observed by Kaur et al.36,41

Figure 8.

Figure 8

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AC conductivity variation with frequency.

The size, microstructure, and shape of nanomaterials have a direct effect on thermal properties. The information about microstructure can be obtained using impedance analysis. The imaginary and real components of conductive materials can be studied using impedance analysis. The impedance analysis of all the synthesized samples was recorded in response to frequency, and the results are shown in Figures 9, 10, and 11. All of these measurements were performed at room temperature.18 By increasing the concentration of cobalt, hopping of charges increases, and as a result, impedance decreases. The real and imaginary part of the impedance shows a decreasing trend. By increasing the value of x in Sr(1–x)CoxFe12O19, the impedance decreases. A similar trend by doping the pure material with the different elements has been reported earlier.33 The decreasing trend in the value of Z proclaims an increase in conductivity, which resulted in a decrease in resistance. The lower values of frequencies correspond to high impedance values. This is due to the factor of space charge polarization. It will render electron hopping inept to continue with the alternations of an electric field. The impedance values became constant due to the reduction in space charge polarization effect at higher frequency.42,43

Figure 9.

Figure 9

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Impedance real part Z′ variation with frequency.

Figure 10.

Figure 10

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Impedance imaginary part Z″ variation with frequency.

Figure 11.

Figure 11

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Cole–Cole plot Z′ vs Z″.

4.3.2. DC Resistivity

The DC resistivity was measured using the “two-probe method” at an alternating temperature, and the results are shown in Figure 12. The method of synthesis, sintering, chemical composition, porosity, grain size, density, and so forth are factors affecting the electrical resistivity of ferrites. The resistivity and conductivity are electrical properties governed by hopping of electron between ions of the same element having one or more valance states.44 A linear decreasing trend in DC resistivity by the addition of cobalt was observed with increasing temperature. At high temperature, high lattice vibrations resulted in ease of electron transfer, thus resulting in maximum conduction. Thus, an increase in temperature results in a decrease in resistivity.45Figure 13 shows the DC conductivity trend with the increasing temperature.

Figure 12.

Figure 12

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DC resistivity variation with temperature.

Figure 13.

Figure 13

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DC conductivity variation with temperature.

The drift mobility with variation in temperature in Figure 14 shows an increase in the drift mobility of electrons as the temperature increases, as charge carriers get more energy to overcome the activation energy barrier, thus resulting in an increase of mobility and conductivity by addition of cobalt.

Figure 14.

Figure 14

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Electron mobility variation with temperature.

4.4. Magnetic Properties

The hysteresis loops for Sr(1–x)CoxFe12O19 (x = 0–0.5) sample series were measured at room temperature. The saturation magnetization increases with the increase in the concentration of cobalt as shown in Figure 15. The magnetic properties of hexaferrites depend on the formation conditions and the site occupation among five crystallographic sites of the hexaferrite tetrahedral, octahedral, and trigonal bipyramid.36 The pure strontium hexaferrite possesses the largest coercive force (Hc), which is due to the strong uniaxial anisotropy along the c-axis of M-hexaferrite. The coercivity decreased by the addition of cobalt. As the concentration of cobalt increases, the coercivity decreases probably due to the appearance of soft magnetic properties. Cobalt is less coercive than Sr; that is why, by the addition of Co, the coercivity decreased.33

Figure 15.

Figure 15

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Hysteresis loop of Sr1–xCoxFe12O19 (x = 0–0.5).

5. Conclusions

The cobalt-doped strontium hexaferrite nanoparticles with the general formula Sr1–xCoxFe12O19 (where x = 0.0, 0.15, 0.24, and 0.50) were successfully prepared by the sol–gel route. XRD indicated the formation of a hexagonal structure with no impurity. The formation of spherical shaped nanoparticles was confirmed from SEM analysis with no visible porosity, which verified the successful synthesis of the mixed ferrite nanoparticles. FTIR analysis having distinct absorption bands in the range of 400–600 cm–1 indicates the characteristic features of hexagonal ferrites. The increase in bond strength can cause a peak shift toward a higher frequency. The dielectric constant variation with an alternating frequency is in accordance with the Koop’s theory. The higher value of dielectric loss (ε″) can be attributed to the higher energy losses. The minimum values of tan δ at high frequency make the cobalt-doped strontium hexaferrite nanoparticles a potential candidate for microwave applications. A unique trend of increasing AC conductivity (σAC) with the increase in frequency was explained on the translation hopping of electrons. The DC resistivity, electron drift, velocity, and DC conductivity confirm the semiconducting nature of strontium hexaferrite nanoparticles by the addition of cobalt. The technique used for the synthesis proved to be environment-friendly, inexpensive, and without harmful byproducts.

Acknowledgments

The authors would like to acknowledge the NRPU Project nos. 6018 and 9998 of Higher Education Commission Islamabad.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c03256.

  • Rietveld phase analysis and EDX elemental composition (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c03256_si_001.pdf (199.2KB, pdf)

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