1 Introduction

Y- and Z-type hexaferrite materials and its derivatives were considered as prospective materials for many technological applications. They are the important magnetic parts in various low- and high-frequency devices like inductor cores, microwave absorbers in GHz range, and ultra-high-frequency communications attributed to their particular magnetic properties [1]. Notably, the cutoff frequency of Y- and Z-type hexaferrites is higher than the conventional spinel ferrites because Y- and Z-type hexaferrites have vigorous magnetic planar anisotropy which consequently reduces the required applied magnetic field for the ferromagnetic resonance [2,3,4]. From this point, Co2Y hexaferrite materials could be used as microwave absorbers in GHz frequencies [5]. In particular, Co2Y has a multitude of advantages such as chemical stability, high-saturation magnetization, good coercivity, strong magnetic planar anisotropy, high Curie temperature, corrosion resistance, and high cutoff frequency [4, 5]. In this regards, at room temperature, Y-type hexaferrites have a preferred plane of magnetization perpendicular to the C-axis, but this changes to a cone of magnetization below –58 ∘C. From this temperature to the Curie point, the anisotropy remains in the preferred plane [6, 7]. It is well known that the chemical formula of Y-type hexaferrite is represented as Ba2Me2Fe12O22 where Me equal a small divalent ion such as Co, Ni, and Zn [8]. In this context, the crystal structure of Y-type hexaferrite with the molecular unit cell consisting of the sequence STSTST containing three formula units, tetrahedral and octahedral sites which are occupied by metal ions, with the length of the c-axis being 43.56 Å. The member of the space group is R3m as well as the molecular mass of Ba2Co2Fe12O22 is 1410 g m and its density of 5.40 g cm− 3 [7,8,9,10]. The microstructure and the magnetic properties of Ba2Co2Fe12O22 are strongly depending on the synthesis method, chemical composition, sintering temperature, time, and the precursors used [4]. Significantly, usual method for the preparation of Co2Y is the classical ceramic solid-state method which needs a high annealing temperature (> 1200 ∘C); the yield and abuse of this method is inhomogeneous and with lattice strains in the material [11,12,13]. Otherwise, various synthesis methods are utilized to get extremely homogeneous hexaferrite of Co2Y nanoparticles such as hydrothermal [4], co-precipitation [12, 14], and sol-gel auto-combustion techniques [5, 9, 15]. The most features of these methods are simplicity, excellent chemical homogeneity, uniform microstructure, and a good control of the grain size [9, 13]. Meanwhile, sol-gel is an excellent technique to synthesize hexaferrite at comparatively low temperatures [5, 9, 16,17,18,19,20]. Sol-gel method allows decreasing the sintering temperature of ferrite with the excellent chemical composition in the product materials [17,18,19,20]. Meanwhile, sol-gel technique is preferred in ferrite preparation, where citric acid acts as a fuel or a complexing agent to mix with many multivalent ions to form the chelates [17,18,19,20]. Continuing in this vein, Pullar 2012 [6] demonstrates that a single-phase Y-type hexaferrite can be formed at annealing temperature 1100 ∘C for 3 h using the chemical co-precipitation method whereas the powder was nearly completely CO2Y with a trace of (α-Fe2O3) at 1000 ∘C using sol-gel processing which indicates the proportion of Y ferrite increases with temperature. Iodeh et al. find that the Ba2Y powder is prepared by first preheating at 450 ∘C for 1 h and then sintering at 1100 ∘C for 4 h at heating rate 10 ∘C/min to form pure Y-type hexaferrite with an average crystallite size ranged from 49 to 100 nm as well as magnetic properties including the coercivity Hc = 225 Oe, the saturation magnetization Ms = 31.1 emu/g and remanence magnetization Mr = 14.3 emu/g [5]. Furthermore, Hsing et al. 2006 observe that a single-phase of Co2Y is obtained at annealing temperature 1050 ∘C, whereas intermediate phase, BaFe2O4, is still detected at 1000 ∘C [21]. Notably, in other studies, Hsing et al. 2008 mention that BaM and Co2Y coexisted in the samples at annealed temperatures 1000 ∘C and pure single phase is achieved at 1100 ∘C [22].

Herein, in this paper, in order to further explore the effect of synthesis route and annealing temperature on the properties of Co2Ytype hexaferrite, (Ba2Co2Fe12O22) powders have successfully elaborated using sol-gel auto-combustion (SGA) and coprecipitation (CP) methods. The manipulation of pre-annealing and annealing temperature for different periods of the formed ferrite powders using two strategies on the crystal structure, size, microstructure, and magnetic properties was deduced.

2 Experimental

2.1 Synthesis of Co2Y-type Nanopowders by SolGel Auto-combustion Reaction Method

The stoichiometric ratios of metal nitrates, barium nitrate hexahydrate Ba(NO3)2.6H2O, cobalt nitrate hexahydrate Co(NO3)2.6H2O, and ferric nitrate nonahydrate Fe(NO3)3.9H2O with 2:2:12 molar ratios were dissolved in deionized water according to the chemical formula (Ba2Co2Fe12O22) to give a clear aqueous solution. Then, an appropriate amount of citric acid solution acting as a fuel was gradually added to the solution at 70 ∘C to achieve a high homogeneity of metal ions. After that, ammonia solution were carefully added to adjust the pH value to stable 7. The sol was heated and hence converted to a gel. Continuing heating, the gel was automatically burnt and the citrate porous precursors were obtained. The combustion products were then dried at 100 ∘C for 24 h to form a fine dried-precursor. Finally, the precursor products were pre-heated at 600 ∘C with annealing time from 4 to 6 h, then further annealing at different temperatures from 800 to 1000 ∘C at various periods from 1 to 2 h.

2.2 Synthesis of Co2Y-type Powders Using Co-precipitation Route

Similarly, the stoichiometric ratios of metal nitrates were dissolved in deionized water to yield a clear aqueous solution. Then, the mixed solution was precipitated by an aqueous solution of strong base (potassium hydroxide, KOH) at pH 13, to produce a dark-brownish precipitate. Then, the aqueous suspensions were gently stirred at constant 500 rpm for 15 min to achieve a good homogeneity and to attain the stable pH conditions. The precipitates were filtered off and washed with deionized water many times until no residual potassium ions impurities existed. After that, the precipitates were dried in the oven at 100 ∘C overnight, producing brownish-reddish hydroxides precursors. Then, the brownish-reddish solids were pre-annealed at 600 ∘C for various periods and then annealed at high temperatures from 800 to 1000 ∘C for different times to obtain Co2Y single-phase hexaferrite powders.

2.3 Physical Characterization

The different phases formed during different annealing temperatures were recognized using Brucker D8-advance x-ray powder with Cu radiation (λ = 1.5406 Å) at 40 kV and 30 mA. The crystallite sizes of the produced Co2Y type were estimated for the most intense peak (110) plane determined from the x-ray diffraction data using the Debye-Scherrer formula:

$$ D_{\text{hkl}} = k\lambda /\beta \cos\theta $$
(1)

whereDhkl is the crystallite size, k = 0.9 is a shape factor, β is the full width at half maximum (FWHM) of the most intense diffraction peak (110) plane, λ is the wavelength of Cu target = 1.5406 Å, and 𝜃 is the Bragg angle. On the other hand, the microstructure changes accompanying the heat treatment of the mixture were examined by scanning electron microscopy (JEOL-JSM 6400 SEM). Fourier transform infrared spectroscopy (FTIR) was performed at room temperature in transmission mode (Spectrometer JASCO, 6300, Japan) in the range of 400–4000 cm− 1. The magnetic properties of the produced samples were studied as a function of treatment temperatures and time using the vibrating sample magnetometer model (VSM-7410 Lake Shore, USA). The measurement was conducted at room temperature in a maximum applied field of 20 kOe.

3 Results and Discussion

3.1 X-ray Diffraction Analysis

XRD patterns for the Ba2Co2Fe12O22 samples prepared by SGA and CP methods are displayed in Figs. 1 and 2. According to the XRD interpretations, the optimum conditions to obtain a pure single phase of Ba2Co2Fe12O22 hexagonal structure referenced to the standard JCPDS card #82-0472 file were predestined at 1000 ∘C for 2 h using the two synthesis methods. Evidently, for the sol-gel method, at the annealing temperature below 1000 ∘C for 2 h, secondary phases of BaFe2O4 and CoFe2O4 (spinel structure) related to the standard JCPDS card #70-2468 and JCPDS card #79-1744 files, respectively, were disclosed with M-type hexaferrite, JCPDS card #84-0757. Otherwise, single well crystalline Co2Y-type phase was indexed for the precursors pre-annealed at 600 ∘C for periods 4 and 6 h and then annealing at temperature 1000 ∘C for 2 h. Similarity, at low annealing temperature lower than 1000 ∘C, mixtures of three impurity secondary phases including CoFe2O4, BaFe12O19, and BaFe2O4 were obtained using co-precipitation pathway. The crystallite size of Ba2Co2Fe12O22 particles was evaluated from the most intense peak (110) of XRD data based on the Scherer equation. Clearly, the crystallite sizes were found to increase from 55.9 to 100.0 nm in case of sol-gel technique whereas they were increased from 43.6 to 78.4 nm using co-precipitation method with increasing pre-annealing time from 4 to 6 h as listed in Tables 1 and 2. The lattice parameters a and c of the samples were calculated using the following equation [23]:

$$ \frac{1}{d^{2}}=\frac{4}{3}\left( \frac{h^{2}+{hk+k}^{2}}{a^{2}} \right)+\frac{l^{2}}{c^{2}} $$
(2)

where a and c are the lattice parameters, d is the interplanar distance and (hkl) are the Miller indices. The change in the lattice parameters, aspect ratio c/a ratio, and unit cell volume by two pathways are displayed in Tables 1 and 2. Of note, the lattice parameters were found to decrease with enhancing the pre-annealing time. Otherwise, c/a ratio was varied from 7.38 to 7.44, which evidenced the ratio locates in the range of Y-type ferrites. Moreover, the unit cell was noticed to decrease with increasing the pre-annealing period which was attributed to decrease of the lattice parameters.

Fig. 1
figure 1

XRD patterns of Co2Y using the sol-gel method with different annealing temperatures

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Fig. 2
figure 2

XRD patterns of Co2Y using co- precipitation method with different annealing temperatures

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Table 1 Variation of the crystallite size, lattice parameters, cell volume, theoretical density of Y-type ferrite synthesized pre-annealed at 600 ∘C for times 4 and 6 h and then annealed at 1000 ∘C for 1 and 2 h using sol-gel combustion method
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Table 2 Variation of the crystallite size, lattice parameters, cell volume, theoretical density at different temperatures, and times of Y-type hexaferrite synthesized using co-precipitation method
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The x-ray theoretical density was estimated using the equation [24]:

$$ \rho_{x} =\frac{nA}{N_{A} V} $$
(3)

where ρx is the x-ray theoretical density, n is the number of atoms in the unit cell with a value equal to 3 because Y-type ferrite structure composes of three overlapping of T and S blocks [3(TS)] and oxygen layers, A is the sum of the atomic weights of atoms in the unit cell, V is volume of the unit cell, and NA is the Avogadro’s number. Evidently, a small diminishing in the theoretical x-ray was observed with the pre-annealing time which was attributed to the lattice parameters. However, the porosity was found to decrease with increasing the annealing time. Besides, the porosity% was smaller for CP method compared with SGA route. Such results may be explained on the basis of the amount of released carbon dioxide, nitrogen gases, and water vapor were increased in case of SGA pathway compared with water vapor and gases for CP technique. The values of x-ray density of Ba2Co2 Fe12O22 nanoparticles were recorded in Tables 1 and 2.

3.2 FT-IR Spectra

The chemical bonding present in Ba2Co2Fe12O22 nanopowders synthesized using two different routes was demonstrated by utilizing FTIR as depicted in Fig. 3. Plainly, all samples possessed two absorption bands in the range 400 and 600 cm− 1, which coincided with the asymmetric stretching vibrations of metal cations at octahedral and tetrahedral lattice sites. Indeed, the band υ1 around 600 cm− 1 was corresponded to the stretching vibration of tetrahedral complexes and υ2 around 400 cm− 1 was related to that of octahedral complexes. Additionally, the observed band at 585 cm− 1 in the spectrum of Ba2Co2Fe12O22 was attributed to the stretching vibration of tetrahedral coordinated Fe3+–O2− bonds [23]. Eventually, the transmission band associated with the OH group was assigned at around 3400 cm− 1 [1, 25].

Fig. 3
figure 3

FTIR spectrum of the Co2Y powders synthesized using co-precipitation and sol-gel strategies pre-annealed at 600 ∘C for 4 h and then annealed at 1000 ∘C for 2 h

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3.3 Microstructure

Figure 4 shows the FE-SEM images of the Co2Y samples synthesized using sol-gel auto-combustion and co-precipitation pathways pre-annealed at 600 ∘C for 4 h and then annealed at 1000 ∘C for 2 h. Distinctly, uncompleted layers of the hexagonal platelets shape exhibited of the particles formed using sol-gel method as represented in Fig. 3a, b. However, different grains with definite grain boundaries as well as distorted hexagonal platelets like structure appeared in the samples synthesized using co-precipitation route as given in Fig. 3c, d.

Fig. 4
figure 4

SEM micrographs of the Co2Y samples pre-annealed at 600 ∘C for 4 h and then annealed at 1000 ∘C, in air for 2 h (a, b) co-precipitation method and (c, d) sol-gel method

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3.4 Magnetic Properties

Magnetic properties of the samples were studied at room temperature using the vibrating sample magnetometer (VSM). Figures 5 and 6 depict the M − H magnetization curves of the samples pre-annealed at 600 ∘C for 4 and 6 h and then annealed at temperatures from 800 to 1000 ∘C for 1 and 2 h using both strategies. Moreover, the magnetic parameters are listed in Tables 3 and 4. The saturation magnetization (Ms) of the samples was ranged from 20.68 to 34.5 emu/g whereas the coercivity (Hc) was changed from 195 to 1756 Oe, respectively, depending on the annealing temperature. It was observed that the samples annealed at 800 ∘C showed a maximum Hc value of 1756 Oe. The high saturation magnetization and coercive force at low temperature 800 ∘C were explicated on basis of the formation of barium M-type hexaferrite BaFe12O19 and cobalt spinel ferrite CoFe2O4 phases as intermediate, which are well known to exhibit high saturation magnetization and large coercivity. However, the saturation magnetization was found to decrease from 20.7 to 33.9 emu/g as the result of the formation of Ba2Co2Fe12O22 phase. Notably, the sample pre-annealed at 600 ∘C for 6 h and then annealed at 1000 ∘C for 1 h presented a good saturation magnetization Ms = 30.5 emu/g, probably because of the presence of the traces of BaFe12O19 and CoFe2O4 magnetic intermediates. High-saturation magnetization Ms = 33.9 emu/g was attained for the Co2Y sample produced by pre-annealed at 600 ∘C for 6 h and then annealed at 1000 ∘C for 2 h using co-precipitation method. For comparison, the saturation magnetization of the crystalline Co2Y, prepared by the classical solid-state reaction pathway [26], recorded elsewhere was 34.0 emu/g. The change in the saturation magnetizations of the powders obtained by CP and SGA may be attributed to the spin canting and the size effect. In this regards, the magnetic order of Y-type hexaferrite is a result of the super exchange interaction mechanism of metal ions between A (tetrahedral) and B (octahedral) sites. Meanwhile, it should be recognized that the coercivity was increased with increasing the lattice imperfections, voids, and porosity. Subsequently, the coercivity of the produced powders by SGA route was larger than those formed by CP strategy as the result of smaller particle size, lower particles homogeneity magnetic anisotropy of SGA sample [5, 18, 24].

Fig. 5
figure 5

Magnetic properties of Co2Y using the sol-gel method with different annealing temperatures and times

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Fig. 6
figure 6

Magnetic properties of Co2Y fabricated using co-precipitation method at different annealing temperatures and times

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Table 3 Magnetic properties of Ba2Co2Fe12O22 nanopowders prepared using sol-gel method at different annealing temperatures and times
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Table 4 Magnetic properties of Ba2Co2Fe12O22 nanopowders synthesized using co-precipitation method
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4 Conclusion

In summary, Co2Y nanopowders have been successfully fabricated using two chemical strategies including sol-gel auto-combustion SGA and co-precipitation CP methods. X-ray diffraction analysis indicated that well crystalline single Y-type hexaferrite phase was acquired by pre-annealing the formed precursors by both techniques at 600 ∘C for 4 and 6 h and then annealed at 1000 ∘C for 2 h. The aspect ratio c/a ratio was varied from 7.38 to 7.44, which manifested the ratio locates in the range of Y-type ferrites. The porosity was found to decrease for Co2Y powders obtained by CP route compared by such formed by SGA route. FT-IR spectrums confirmed all samples have two absorption bands in the range of 400 and 600 cm− 1, which identified to the asymmetric stretching vibrations of metal cations at octahedral and tetrahedral lattice sites. FE-SEM photos indicated that the synthesis conditions played a role for change the microstructure of Co2Y type. VSM results described that a good saturation magnetization Ms = 33.9 emu/g was achieved for Co2Y type pre-annealed at 600 ∘C for 6 h and then annealed at 1000 ∘C for 2 h using co-precipitation strategy as the results of increasing of the size and super exchange interaction mechanism of metal ions between A (tetrahedral) and B (octahedral) sites. Wide coercivities (195 to 1756 Oe) were attained at different synthesis conditions. Overall, such synthesized Co2Y nanoparticles are quite encouraging for their utilizing in magnetic recording media and high frequency applications.