Sol–gel hydrothermal synthesis of strontium hexaferrite nanoparticles and the relation between their crystal structure and high coercivity properties

The samples were prepared using the sol–gel hydrothermal method. Chemical-grade ^Fe(^NO ₃)₃·9^H ₂ ^O, ^Sr(^NO ₃)₂ were used as pre-substances for preparing ^SrFe ₁₂ ^O ₁₉ (SFO) nanoparticles. A suitable amount of ^Fe(^NO ₃)₃·9^H ₂ ^O and ^Sr(^NO ₃)₂ was dissolved in distilled water to obtain ^{Fe 3+} solution and ^{Sr 2+} solution. These solutions were mixed in order to achieved the mole ratios of ^{Fe 3+}/ ^{Sr 2+}=8, 11, 12. Then citric acid (AC) solution was added to the mixture in the mole ratios ^AC/Σ^Me=k (k=1, 2, 3; Σ^Me is total mole of metal ions). The solution was stirred for about 1 h. The pH of the solution was adjusted to 6.5–7.5 with ammonium hydroxide, and subsequently the solution was heated until the volume decreased about 60%. Then sol continued to react in the autoclave at 180 °^C. The achieved particles were dried and calcined at different temperatures in the air to form ^SrFe ₁₂ ^O ₁₉ nanoparticles.

^Sr _(1-x) ^La _x ^Fe ₁₂ ^O ₁₉ (x=0.05; 0.1; 0.2) nanoparticles were prepared by sol–gel hydrothermal method. Chemical-grade ^Fe(^NO ₃)₃·9^H ₂ ^O, ^Sr(^NO ₃)₂ and ^La ₂ ^O ₃ were used as pre-substances. A suitable amount of ^Fe(^NO ₃)₃·9^H ₂ ^O and ^Sr(^NO ₃)₂ was dissolved in distilled water to obtain ^{Fe 3+} and ^{Sr 2+} solution. A suitable amount of ^La ₂ ^O ₃ was dissolved in to ^HNO ₃ solution to obtain ^{La 3+} solution. These solutions were mixed in order to achieved the mole ratio of ^{Fe 3+}/(^{La 3+}+^{Sr 2+})=12. The next steps of the preparation process were the same as for the preparation of ^SrFe ₁₂ ^O ₁₉ nanoparticles.

^CoFe ₂ ^O ₄ nanoparticles were prepared by co-precipitation method. The mixed solution containing 50 ml of ^{Fe 3+} and ^{Co 2+} with the mole ratio ^{Fe 3+}/^{Co 2+}=2 was drop-by-drop added into the reaction vessel containing 100 ml of 1 M NaOH aqueous solution under vigorous stirring. The suspension was stirred for 2 h. The precipitate was then separated by a high-speed centrifuge at 6000 rpm, and then washed with distilled water. The washed sample was dried at 100 °^C in air for 2 h and then was sintered at different temperatures.

The right amount of ^SrFe ₁₂ ^O ₁₉ nanoparticles were dispersed in ethanol and sonicated for 10 min. Then NaOH solution of 1 M was used to activate the ^SrFe ₁₂ ^O ₁₉ nanoparticles by ultrasonic irradiation. After pre-treatment the ^SrFe ₁₂ ^O ₁₉ nanoparticles were separated by centrifugation. Then the ^SrFe ₁₂ ^O ₁₉ nanoparticles were added into the boiling NaOH aqueous solution. The mixed solution containing ^{Fe 3+} and ^{Co 2+} with the mole ratio ^{Fe 3+}/^{Co 2+}=2 was drop-by-drop added into the reaction system and stirred for 2 h. The final product was separated by centrifugation, washed with distilled water, dried and sintered at different temperatures.

The magnetic properties of the products were observed using a vibrating sample magnetometer (VSM) at room temperature under a maximum field of 15 T. The reaction products were identified by x-ray diffraction (XRD) using Cu-^Kα radiation and refined by Rietveld method using Rietan software. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the grain size and morphology of the product.

3.1.1. Influence of the mole ratio Fe³⁺/Sr²⁺

^SrFe ₁₂ ^O ₁₉ nanoparticles were prepared with the mole ratio ^{Fe 3+}/^{Sr 2+}=8, 11, 12. The XRD results showed that the samples with ^{Fe 3+}/^{Sr 2+}=11 and 12 were single phase because we only observed the characteristic peaks of ^SrFe ₁₂ ^O ₁₉. The diffraction peaks at 2θ values of 34.171°, 32.311°, 30.341°, 37.151°, 63.161° and 56.81° corresponded to the strongest diffraction planes (114), (107), (110), (203), (2200) and (2011). These patterns ascribed to hexagonal Sr–M hexaferrite ^SrFe ₁₂ ^O ₁₉ phase. In the XRD patterns of sample with ^{Fe 3+}/^{Sr 2+}=8, besides the peaks of ^SrFe ₁₂ ^O ₁₉, there were peaks of α-^Fe ₂ ^O ₃. The peaks at 2θ values of 33.171°, 35.661°, 54.061°, 49.461° and 24.141° reflected the presence of ^Fe ₂ ^O ₃ phase. Figure 1 showed the XRD patterns of strontium M-ferrite powders with different ^{Fe 3+}/^{Sr 2+} mole ratio (8, 11 and 12) calcined at 1050 °^C.

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3.1.2. Influence of the mole ratio AC/ΣMe

^SrFe ₁₂ ^O ₁₉ samples (the ratio of ^{Fe 3+}/^{Sr 2+}=11 and 12) were prepared with different ratios of ^AC/Σ^Me=k (k=1, 2 and 3).

XRD patterns (figure 2) showed that all samples with ^AC/Σ^Me=k=1, 2, 3 had high purity, ^SrFe ₁₂ ^O ₁₉ phase wasn't mixed with any different phase. So, the mole ratio ^AC/Σ^Me did not affect the singles phase of ^SrFe ₁₂ ^O ₁₉ samples which were prepared by sol–gel hydrothermal method. However, the mole ratio ^AC/Σ^Me affecting the magnetic properties of ^SrFe ₁₂ ^O ₁₉ samples and will be presented in VSM measurement results.

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3.1.3. Influence of the sintering temperature

The sintering temperature was determined by the thermal analysis. The thermal analysis result of ^SrFe ₁₂ ^O ₁₉ (^{Fe 3+}/^{Sr 2+}=11, k=3) in the range 100–1200 °^C is presented in figure 3.

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The result shows that in the range of temperature of 200–500 °^C the burning process of organic substances took place, at 500 °^C, the burning process almost completed and total mass of sample reduced 72.45%. So the samples were preliminarily calcined at 500 °^C. After the preliminary calcination, samples were sintered at temperatures of 550, 750 and 1050 °^C. In the XRD patterns (figure 4) at 550 °^C the only characteristic peak of α-^Fe ₂ ^O ₃ was observed. At the higher temperature, 750 °^C, ^SrFe ₁₂ ^O ₁₉ phase was formed, but a small amount of α-^Fe ₂ ^O ₃ was still observed. At the sintering temperature of 1050 °^C, single phase of ^SrFe ₁₂ ^O ₁₉ was formed. Figure 4 presents the XRD patterns of particle samples obtained after sintering at 550, 750 and 1050 °^C.

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Figure 5 is the SEM micrograph of ^SrFe ₁₂ ^O ₁₉ phase (^{Fe 3+}/^{Sr 2+}=11, k=3, sintered at 1050 °^C). One can observe that most of the particles are of hexagonal shape.

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The average crystallite size was calculated by the Scherrer equation

where d is average crystallite size; β—full width at half maximum (FWHM), λ=1.5406 Å—wavelength of X radiation. The average crystallite size of ^SrFe ₁₂ ^O ₁₉ is presented in table 1.

Table 1. Lattice parameters and average crystallite size of ^SrFe ₁₂ ^O ₁₉ samples.

Sample a (Å) b (Å)c (Å) V (Å)³R ^_p (%) R ^_wp (%)Average crystallite size (nm)

SFO11, k=2	5.87	5.87	23.00	686.88	7.13	9.47	7.4
SFO11, k=3	5.87	5.87	23.01	687.38	8.05	10.95	7.8

In order to optimize the process of preparation of ^SrFe ₁₂ ^O ₁₉ nanoparticles and study the influences of preparation condition to material's magnetic properties, we varied the mole ratio ^{Fe 3+}/^{Sr 2+}, the mole ratio ^AC/Σ^Me and the calcination temperature.

The Rietveld method of structure refinement with powder diffraction patterns using Rietan software is used to refine the ^SrFe ₁₂ ^O ₁₉ structure. Calculated results showed that ^SrFe ₁₂ ^O ₁₉ with structure belongs to the P6₃/^mmc space group, α=β=90 °, γ=120 ° and lattice parameters a, b, c and V are reported in table 1. The R ^_p and R ^_wp values under 10% show that the product prepared by sol–gel hydrothermal method has single phase and no other phase can be observed.

Figure 6(a) is the XRD result which was refined by Rietveld method and the simulated structure of ^SrFe ₁₂ ^O ₁₉ nanoparticles sintered at 1050 °^C and kept for 2 h. The positions of atoms in the unit cell were calculated and are reported in table 2. The result of particles sample is close to the calculated result for bulk sample [14].

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Table 2. The atomic positions in the unit cell of ^SrFe ₁₂ ^O ₁₉.

Atom xyzB

Sr	0.66667	0.33333	0.25000	0.605
Fe(1)	0	0	0	−0.941
Fe(2)	0	0	0.25825	−2.278
Fe(3)	0.33333	0.66667	0.02808	−1.019
Fe(4)	0.33333	0.66667	0.18991	−0.585
Fe(5)	0.16727	0.33824	−0.10942	−0.427
O(1)	0	0	0.15237	−0.783
O(2)	0.33333	0.66670	−0.05789	1.450
O(3)	0.16222	0.35362	0.25000	−2.107
O(4)	0.15907	0.31305	0.05385	0.015
O(5)	0.47891	1.00758	0.14970	−0.397

Because of the same ion radius between ^{Sr 2+} and ^{La 3+}, when ^{La 3+} is introduced into ^SrFe ₁₂ ^O ₁₉ system it tends to substitute the ^{Sr 2+} position. Figure 6(b) is the XRD result which was refined by Rietveld method of the ^Sr _0.8 ^La _0.2 ^Fe ₁₂ ^O ₁₉ nanoparticles sintered at 1050 °^C and kept for 2 h. No peak of any phases was observed except the characteristic peak of hexagonal structure of P6₃/^mmc space group. The bond length of ^{Fe 3+} ion and ^{O 2−} in the crystal does not change equally, but may increase or decrease depending on whether the tetrahedron or octahedron contains ^{Fe 3+} ions distortion. However, the decrease of bond length of ^{Fe 3+}(2^b)–^{O 2−} (2b position is located in the same plane with ^{M 2+} in crystal) can be observed when ^{La 3+}, whose ion radius is smaller than that of ^{Sr 2+}, substitutes the ^{Sr 2+} position (table 3).

Figure 7 is the XRD results of the shell sample (^CoFe ₂ ^O ₄), core sample (^SrFe ₁₂ ^O ₁₉) and core/shell sample (^SrFe ₁₂ ^O ₁₉/^CoFe ₂ ^O ₄). On the XRD pattern of core/shell sample (^SrFe ₁₂ ^O ₁₉/^CoFe ₂ ^O ₄) the existence of two phase ^SrFe ₁₂ ^O ₁₉ and ^CoFe ₂ ^O ₄ was observed.

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The core/shell structure of ^SrFe ₁₂ ^O ₁₉/^CoFe ₂ ^O ₄ and the morphologies of hexaferrite phase and spinelferrite phase can be observed in the TEM and SEM photographs in figure 8.

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On the TEM of ^SrFe ₁₂ ^O ₁₉/^CoFe ₂ ^O ₄ samples, one can observe the core–shell structure. The TEM results also show the dimension of the core–shell particles of ^SrFe ₁₂ ^O ₁₉/^CoFe ₂ ^O ₄. The diameters of the core–shell particles are in the range of 265.86–354.22 nm, the diameters of the cores are in the range of 140.20–195.53 nm.

Figure 9 shows the VSM results at room temperature of ^SrFe ₁₂ ^O ₁₉ samples with ^{Fe 3+}/^{Sr 2+}=11, 12; k=1, 2, 3 and sintered at 1050 °^C. The magnetic properties of ^SrFe ₁₂ ^O ₁₉ samples are reported in table 4.

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Saturation magnetization of SFO samples with different mole ratios ^{Fe 3+}/^{Sr 2+} and ^Ac/Σ^Me are not very different, but the SFO sample with the mole ratio ^{Fe 3+}/^{Sr 2+}=11 and k=3 has the highest saturation magnetization (^Ms=66 ^{emu  g −1}). This sample also has the highest coercivity (H ^_c =6315 ^Oe). Other samples have high coercivity in the range of 4911–5665 Oe. The coercivity of ^SrFe ₁₂ ^O ₁₉ depends on the anisotropy of the grain, and coercivity increases with the increase of anisotropy. It can be observed that the higher the mole ratio ^AC/Σ^Me=k, the higher are the coercivity values.

Figure 10(a) is the hysteresis loops of ^SrFe ₁₂ ^O ₁₉ and ^Sr _0.8 ^La _0.2 ^Fe ₁₂ ^O ₁₉ nanoparticles prepared by hydrothermal method in the same conditions. The substitution of ^{Sr 2+} by ^{La 3+} created the change of coercivity. H ^_c value of ^Sr _0.8 ^La _0.2 ^Fe ₁₂ ^O ₁₉ is higher than that of ^SrFe ₁₂ ^O ₁₉. Figures 10(b) and (c) are the hysteresis loops of the ^SrFe ₁₂ ^O ₁₉/^CoFe ₂ ^O ₄ nanoparticles after calcination at 950 °^C (figure 10(b)) and 1050 °^C (figure 10(c)) for 2 h.

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