Facile synthesis of hexagonal strontium ferrite nanostructures and hard magnetic poly carbonate nanocomposite

Introduction

In recent years, M-type strontium hexaferrite (Sr-hexaferrite, SrFe₁₂O₁₉, SrFe, SrM) has been the subject of extensive research due to its possible use in different technological applications. These materials are used in the magnetic recording industry, magneto-optic and microwave devices, micro electromechanical systems, transformer cores, and antennas (Nakagawa et al., 2001; Jamalian, 2015; Nabiyouni et al., 2016). M-type hexagonal ferrites, MFe₁₂O₁₉ (Ba, Sr, Pb) has attracted research attention because of their existing and potential applications in permanent magnets, microwave components, and high-frequency devices, to name a few (Canale et al., 2000; Khaleeq-ur-Rahman et al., 2013). This is a hard magnet with high coercivity, which originates from high magneto-crystalline anisotropy with a single, easy magnetization axis (Ohmori and Matijevic, 1993; Chen and Chen, 2002; Duan et al., 2007; Wang et al., 2009; Wong et al., 2014). Among the different chemical routes, the sol–gel method based on the Pechini-type reaction has received considerable attention for its relatively simple synthesis scheme (Suarez et al., 2003; Masoumi et al., 2016a, b). In this method, coordination complexes are formed between a metal ion and citric acid within a solution, to which ethylene glycol (EG) is added to enhance the amount of polymerization reaction, resulting in a highly viscous gel. The pyrolysis of this gel produces a homogeneous mixed oxide. The main advantage of using this method is the lower calcination temperature with smaller hexaferrite crystallites, which is an effective way to increase the recording density of the media (Yongfei et al., 2009; Masoudpanah and Seyyed Ebrahimi, 2011). Therefore, the preparation of SrFe₁₂O₁₉ nanoparticles with high purity, ultrafine size, good dispersion, and excellent magnetism has been the focus of recent studies (Gokon et al., 2002; Pullar and Bhattacharya, 2006; Zhang and Li, 2009). Variations in the synthesis methods reduce the temperature required to obtain the hexagonal hexaferrite. While the ceramic sintering method requires heat treatment at around 1200°C (Haneda and Kojima, 1974; Martinez Garcia et al., 2001, Martinez Garcia et al., 2011), chemical methods, such as sol–gel and co-precipitation, involve thermal treatments between 800°C and 1000°C (Zhong et al., 1997; Estevez Rams et al., 2000; Sivakumar et al., 2004; Alamolhoda et al., 2006; Zaitsev et al., 2006; Hessien et al., 2008). Some variations in these methods allow the synthesis of hexagonal ferrites at temperatures below 800°C (Wang et al., 2006; Primc et al., 2009).

Results and discussion

Figure 1 shows the X-ray diffraction (XRD) pattern of the sample, including SrFe₁₂O₁₉ nanoparticles. The structure and composition of the SrFe₁₂O₁₉ nanocomposites were investigated. The XRD pattern of SrFe reveals the typical diffraction patterns of pure hexagonal phase (JCPDS No.: 24-1207) with the P63-mmc space group, which is consistent with pure strontium hexaferrite. The crystalline sizes can be calculated using the Scherrer equation, D_c=Kλ/βCosθ, where β is the width of the observed diffraction peak at its half maximum intensity (FWHM); K is the shape factor, which takes a value of about 0.9; and λ is the X-ray wavelength (CuK_α radiation, equals to 0.154 nm). The calculated crystalline size was about 28 nm for the SrFe nanoparticles.

Figure 1:

XRD pattern of SrFe₁₂O₁₉ nanoparticles.

To determine the effects of heating on the morphology and size of SrFe nanoparticles, two various reaction times were selected. Figures 2A and B illustrate the scanning electron microscope (SEM) images of the as-synthesized SrFe nanoparticles obtained at 500°C, indicating the preparation of nanoparticles with a mediocre diameter size of 50 nm. SEM images of nanoparticles synthesized at 850°C indicated in Figures 2C and D show a mediocre particle size of about 80 nm.

Figure 2:

SEM images of SrFe₁₂O₁₉ at (A, B) 500°C and (C, D) 850°C.

Pechini proposes the easy manipulation of particle size and magnetic properties by simple changes in calcination time, precursors, temperature, and surfactants. Figure 3 shows SEM images of SrFe achieved at 700°C, indicating an average particle size of about 30 nm. This result shows that the particle size and morphology of magnetic nanoparticles are easily controlled by temperature. The balance between nucleation rate and growth rate, which determines final particle size and morphology, depends on preparation conditions.

Figure 3:

SEM images of SrFe₁₂O₁₉ synthesized at 700°C.

Figures 4A and B show SEM images of the synthesized products with citric acid at 700°C. Figures 4C and show SEM images of the prepared samples with citric acid at 500°C. Interestingly, results show that an increase in temperature results in preferential growth, suggesting that a higher temperature growth stage is predominant compared with nucleation. At 700°C nano-plates were synthesized compared with nanoparticles (less than 80 nm) prepared at 500°C.

Figure 4:

SEM images of SrFe with citric acid at (A, B) 700°C and (C, D) 500°C.

Figure 5 illustrates the effect of citric acid on particle size at 850°C. SEM images demonstrate the influential role of temperature on morphology; furthermore, the images show that hexagonal plate-like nanostructures are obtained at a temperature of 850°C. Meanwhile, Figure 6 depicts the nanoparticles obtained by both citric acid and EG at 850°C. Mono-disperse nanoparticles with a size of around 20 nm were also synthesized.

Figure 5:

SEM images at 850°C obtained by citric acid.

Figure 6:

SEM images of SrFe prepared with citric acid-EG at 850°C.

SEM images of the prepared samples by citric acid and EG at 500°C are shown in Figures 7A and B, whereas Figures 7C and D show SEM images of ferrite obtained at 700°C. Temperatures less than 850°C were not suitable for the synthesis of nanoparticles. As illustrated in Figure 7, agglomerated nanostructures were achieved at temperatures of 700°C and 700°C. Meanwhile, SEM images of the poly carbonate-SrFe₁₂O₁₉ nanocomposite are illustrated in Figure 8. SEM of the surface confirm the dispersion of magnetic materials in the polymeric matrix.

Figure 7:

SEM images of SrFe₁₂O₁₉ citric acid-EG (A, B) 500°C and (C, D) 700°C.

Figure 8:

Surface SEM images of the nanocomposite.

Figure 9 shows the Fourier transform infrared (FT-IR) spectra of the SrFe nanoparticles; as can be seen, the absorption peaks at 320, 453, and 549 cm⁻¹ are responsible for the stretching modes of Fe-O and Sr-O. The FT-IR spectra of the nanoparticles clearly show the metal–oxygen bonds at around 300–600 cm⁻¹. The spectra exhibit broad absorption peaks between 3500–3500 cm⁻¹, corresponding to the stretching mode of the O-H group of hydroxyl group, and the weak band near 1456 cm⁻¹ is assigned to H-O-H bending vibration mode because of the adsorption of moisture on the surface of nanoparticles (Masoumi et al., 2016a,b). FT-IR spectra of the SrFe nanoparticles at 850°C are shown in Figure 10. As can be seen, the absorptions peaks at 331, 445, and 599 cm⁻¹ are responsible for the stretching modes of Fe-O and Sr-O. FT-IR was used to confirm the existence of SrFe in the nanocomposite. FT-IR spectrum of the as-prepared SrFe-PC nanocomposite is shown in Figure 11. As can be seen, the absorption peaks at 401, 445, and 557 cm⁻¹ are responsible for the stretching modes of Fe-O and Sr-O. The peaks at 1014, 1165, 1195, and 1230 cm⁻¹ are responsible for the stretching mode of C-O, whereas absorptions at 1504 and 1778 cm⁻¹ are responsible for the stretching mode of C=O. Finally, peaks at 2876 and 2970 cm⁻¹ are responsible for the aliphatic carbon–hydrogen bonds.

Figure 9:

FT-IR spectrum of nanoparticles obtained at 500°C.

Figure 10:

FT-IR spectrum of nanoparticles obtained at 850°C.

Figure 11:

FT-IR spectrum of the PC-SrFe nanocomposite.

The room temperature magnetic property of samples was studied using the alternating gradient force magnetometer (AGFM) device. Results obtained at 850°C are shown in Figure 12. The results confirm the preparation of a hard magnet with a high coercivity of about 3200 Oe and saturation magnetization of around 44 emu/g.

Figure 12:

Room temperature hysteresis loop achieved at 850°C.

The hysteresis loop for SrFe₁₂O₁₉magnetite nanoparticles in 500°C is shown in Figure 13. Interestingly, we observe that lower temperature coercivity is less than 850°C. The outcomes show the direct effect of temperature on the magnetic property of the prepared ferrite. Super paramagnetic at 500°C was converted to ferromagnetic at 850°C. In addition, saturation magnetization is also lower than 850°C and is around 1.4 emu/g.

Figure 13:

Room temperature hysteresis curve at 500°C.

Figure 14 shows the magnetization curve of SrFe₁₂O₁₉ by citric acid in 850°C. As can be seen, the ferrite exhibits ferromagnetic behavior with a lower coercivity compared with the other samples. Coercivity is near 4700 Oe, and saturation magnetization is 32 emu/g. Meanwhile, Figure 15 shows the magnetization curve of SrFe₁₂O₁₉ obtained with EG-citric acid in 500°C; the ferrite also exhibits ferromagnetic behavior with coercivity at around 160 Oe and a saturation magnetization of 27 emu/g.

Figure 14:

Magnetization loop of SrFe₁₂O₁₉ by citric acid at 850°C.

Figure 15:

Hysteresis loop of product obtained with citric acid-EG at 500°C.

Figure 16 shows the magnetization curve of the PC-SrFe₁₂O₁₉ 20% nanocomposite. This magnetization indicates that SrFe 20% nanocomposites inherit the magnetic property from the SrFe₁₂O₁₉ NPs; however, the magnetization is lesser (0.72 emu/g) because of the presence of non-magnetic poly carbonate. This reduction in saturation magnetization is a result of the interfacial effect of the typical polymer matrix nanocomposite. The ferromagnetic property of the prepared nanocomposites is an essential characteristic of a heterogeneous nanocomposite, because materials with this magnetic behavior tend to have a low tendency to experience inter-particle agglomeration caused by dipole–dipole interaction compared with ferromagnetic nanocomposites. Moreover, better dispersion of the magnetic nanocomposites ensures mechanical stability (Saffari et al., 2014a, b, 2015; Ghanbari and Salavati-Niasari, 2015a, b). The results also indicate that nanocomposite formation decreases coercivity. This finding can be attributed to the addition of polymeric matrix on the surface of the magnetic nanostructure, which changes its magnetic features (Saffari et al., 2014a, b; Ghanbari and Salavati-Niasari, 2015a, b; Nabiyouni et al., 2016). The magnetic moments of magnetic nanoparticles are agglomerated by the polymeric chains; hence, a lower magnetic field (2650 Oe) is required to align the single domain nanoparticles in the field direction (Ghanbari and Salavati-Niasari, 2015a, b).

Figure 16:

Magnetization loop of PC-SrFe₁₂O₁₉ 20% nanocomposite.

Conclusions

In conclusion, the synthesis, characterization, and magnetic property of SrFe₁₂O₁₉ nanocomposite were reported. The effects of temperature, precursors, and concentration on the morphology and particle size of the products were investigated. AGFM results confirmed that nanoparticles and nanocomposite exhibit ferromagnetic behavior. However, by adding the polymer matrix, the coercivity of ferromagnetism was decreased. Results show that the sol–gel method is a suitable approach for the preparation of SrFe and SrFe-PVA nanocomposites, which are promising candidates for a wide range of industrial applications.

Experimental section

Synthesis of the SrFe₁₂O₁₉ (SrFe) nanoparticles

About 0.012 mol of Fe(NO₃)₃ 9H₂O and 0.001 mol of SrCl₂ were dissolved in 50 mL of distilled water. Then, 1 g of citric acid was slowly added to the solution. The simultaneous effect of citric acid and EG was also investigated. (In this reaction 1 g of citric acid and 3 mL of EG were dissolved in the solution and then stirred for 1 h). Finally, 20 mL of sodium hydroxide solution (1 M) was slowly added to the solution until it reached a pH level of 10. A brown-black precipitate was then centrifuged and washed with deionized water. The obtained precipitate was calcinated at 500°C–800°C. Figure 17 illustrates the schematic diagram for the sol–gel method.

Figure 17:

(A) Schematic of ferrite preparation (B) nanocomposite preparation.