J. Cent. South Univ. Technol. (2010) 17: 1172-1176

DOI: 10.1007/s11771-010-0614-1

Structures and magnetic properties of nanocomposite CoFe₂O₄-BaTiO₃ fibers by organic gel-thermal decomposition process

ZHOU Zhi(周智), SHEN Xiang-qian(沈湘黔), SONG Fu-zhan(宋福展), MIN Chun-ying(闵春英)

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: The nanocomposite xCoFe₂O₄-(1-x)BaTiO₃ (x=0.2, 0.3, 0.4, 0.5, molar fraction) fibers with fine diameters and high aspect ratios (length to diameter ratios) were prepared by the organic gel-thermal decomposition process from citric acid and metal salts. The structures and morphologies of gel precursors and fibers derived from thermal decomposition of the gel precursors were characterized by Fourier transform infrared spectroscopy, X-ray diffractometry and scanning electron microscopy. The magnetic properties of the nanocomposite fibers were measured by vibrating sample magnetometer. The nanocomposite fibers consisting of ferrite (CoFe₂O₄) and perovskite (BaTiO₃) are formed at the calcination temperature of 900 ℃ for 2 h. The average grain sizes of CoFe₂O₄ and BaTiO₃ in the nanocomposite fibers increase from 25 to 65 nm with the calcination temperature from 900 to 1 180 ℃. The single fiber constructed from these nanograins of CoFe₂O₄ and BaTiO₃ has a necklace-like morphology. The saturation magnetization of the nanocomposite 0.4CoFe₂O₄-0.6BaTiO₃ fibers increases with the increase of CoFe₂O₄ grain size, while the coercivity reaches a maximum value when the average grain size of CoFe₂O₄ is around the critical single-domain size of 45 nm obtained at 1 000 ℃. The saturation magnetization and remanence of the nanocomposite xCoFe₂O₄-(1-x)BaTiO₃ (x=0.2, 0.3, 0.4, 0.5) fibers almost exhibit a linear relationship with the molar fraction of CoFe₂O₄ in the nanocomposites.

Key words: nanocomposite; CeFe₂O₄; BaTiO₃; fiber; organic gel-thermal decomposition process; magnetic property

1 Introduction

Quasi-one-dimensional (Q1D) nanostructured materials have attracted much attention due to their chemical and physical properties and their potential as building blocks for advanced functional devices. In order to obtain Q1D materials, many different preparation techniques, such as arc discharge [1], laser ablation [2], template [3-4], metal organic chemical vapour deposition [5], electrospinning [6] and precursor thermal decomposition [7-8] were developed. The precursor thermal decomposition process due to simple operation and low cost was used to fabricate microscale fibers [9-10]. Among these microscale fibers, most of them were single phase materials [10]. Two and multiphase composite functional materials stimulated interests as these composite materials potentially combine the component characteristics to meet various requirements technologically. In particular, the multiferroic composite Q1D materials consisting of ferromagnetic and ferroelectric phases have been recently reported to show multifunctional features [11]. HUA et al [12] synthesized CoFe₂O₄/Pb (Zr_0.52Ti_0.48)O₃(PZT) nanotubes by the sol- gel template process. The magnetic and electric performances are found to be similar to those of the single CoFe₂O₄ phase and PZT phase, respectively. XIE et al [13] prepared NiFe₂O₄/PZT nanofibers with diameters of 100-400 nm and grain sizes of about 30 nm by electrospinning. In this work, the nanocomposite CoFe₂O₄-BaTiO₃ fibers were prepared by the organic gel-thermal decomposition process, and the structure and magnetic properties of these nanocomposite fibers were investigated.

2 Experimental

The nanocomposite xCoFe₂O₄-(1-x)BaTiO₃ (x=0.2, 0.3, 0.4, 0.5, molar fraction) fibers were prepared by the organic gel-thermal decomposition process, and the process was described in detail in Refs.[9-10]. The starting reagents used were analytical grade Fe(NO₃)₃·

9H₂O, Co(NO₃)₂·6H₂O, BaCO₃, Ti(OC₄H₉) and citric acid. The required metal salts and citric acid were dissolved in an aqueous and ethanol hybrid solution at pH 7.0 with a continuous magnetic stirring. The solution was magnetically stirred at room temperature for 20-  24 h and then surplus water in a vacuum rotary evaporator was removed at 60-80 ℃ until a viscous liquid was obtained. The gel fibers were drawn from the spinnable gels by the domestic machine and dried in a vacuum oven at 80 ℃ for about 24 h. The dried gel fibers were then put in an alumina crucible and subsequently calcined at appropriate temperatures for 2 h under an ambient atmosphere to form the nanocomposite fibers.

The structures, compositions and morphologies of the gel precursors and fibers were examined by Fourier transform infrared spectroscopy (FTIR) using a model of Nexu670 spectrometer, X-ray diffractometry (XRD) using a D/max2500PC diffractometer (RIGAKU), and scanning electron microscopy using a field emission scanning electron microscopy (FESEM, JSM-7001F). Magnetic measurements were carried out at room temperatures by a vibrating sample magnetometer (VSM).

3 Results and discussion

3.1 FTIR spectra of gel precursor and resultant fibers

Fig.1 shows the FTIR spectra of the gel precursor for 0.4CoFe₂O₄-0.6BaTiO₃ fibers and the products derived from calcination of the precursor at different temperatures. From the FTIR spectrum for the gel precursor (Fig.1(a)), two bands at 1 624 and 1 383 cm^-1 owing to the RCOO^- symmetrical and asymmetrical stretching vibration are the characteristic peaks for the citrate. The bands at 1 040, 850, 825, 690 and 640 cm^-1 can be assigned to the vibrations arising from C—OH,  Ti—O, NO₃^-, Co—O and Fe—O bonds [14-16], respectively. This is indicative of the complex formation of metal ions and citric acid.

Fig.1 FTIR spectra of gel precursor and 0.4CoFe₂O₄-0.6BaTiO₃ fibers calcined at different temperatures: (a) Gel precursor;   (b) 300 ℃; (c) 800 ℃; (d) 900 ℃

For the sample obtained at 300 ℃, as shown in Fig.1(b), the characteristic absorption peak for the citrate disappears owing to the decomposition of citrate. Two bands at 1 430 and 859 cm^-1 can be assigned to the characteristic absorption peaks for BaCO₃, while the bands at 1 630, 1 384, 1 050, 690 and 580 cm^-1 are corresponding to the characteristic vibration peaks of H₂O, NO₃^-, Fe₂O₃, Co₃O₄ and TiO₂ [17-19]. After calcination at 800 ℃ (Fig.1(c)), the peaks assigned to Fe₂O₃ and Co₃O₄ disappear and the characteristic peak occurs in the region of 550-600 cm^-1 due to the stretching mode of Fe—O at tetrahedral sites in the spinel structure and the Ti—O stretching mode of BaTiO₃ in the perovskite structure, which indicates that spinel structure CoFe₂O₄ and perovskite structure BaTiO₃ begin to form at the calcination temperature of 800 ℃ [20-21]. As shown in Fig.1(d), when the calcination temperature increases up to 900 ℃, only the characteristic peak at around 580 cm^-1 for CoFe₂O₄ and BaTiO₃ is detected, implying the composite of CoFe₂O₄ and BaTiO₃ formed.

3.2 Structural characterization of nanocomposite fibers

Fig.2 shows XRD patterns of the gel precursor and nanocomposite 0.4CoFe₂O₄-0.6BaTiO₃ fibers obtained at various calcination temperatures for 2 h. It can be seen that the gel precursor (Fig.2(a)) is basically amorphous and does not contain crystalline inorganic matters. After calcination at 800 ℃ (Fig.2(b)), CoFe₂O₄ ferrite phase and BaTiO₃ perovskite phase are basically formed whilst some BaCO₃ still exists, which is verified by the above FTIR analysis (Fig.1(c)). When the calcination temperature increases up to 900 ℃ (Fig.2(c), the reflections are indexed just to both CoFe₂O₄ and BaTiO₃ phases. With the calcination temperature increasing from 900 to 1 180 ℃ (Figs.2(c)-(f)), the diffraction peaks for the CoFe₂O₄ and BaTiO₃ phases correspondingly become narrower and sharper, which means that the crystallites building the composite fibers gradually increase and the crystallization is improved.

Fig.2 XRD patterns of gel precursor and 0.4CoFe₂O₄- 0.6BaTiO₃ fibers calcined at different temperatures: (a) Gel precursor; (b) 800 ℃; (c) 900 ℃; (d) 1 000 ℃; (e) 1 100 ℃; (f) 1 180 ℃

The average crystalline grain size (D) of CoFe₂O₄ and BaTiO₃ phases in the composite fibers can be calculated from the diffraction data in the XRD pattern using Scherrer equation. Fig.3 shows the calculated grain size D of CoFe₂O₄ and BaTiO₃ phases in xCoFe₂O₄-  (1-x) BaTiO₃ (x=0.2, 0.3, 0.4, 0.5) fibers with the calcination temperature. It is found that the grain sizes are influenced largely by the calcination temperature. For both CoFe₂O₄ and BaTiO₃ phases in the molar fraction range of 0.2-0.5, D values increase from 25 to 65 nm with the calcination temperature from 900 to 1 180 ℃. So, the crystalline grain sizes of CoFe₂O₄ and BaTiO₃ in nanocomposites can be tailored by controlling the thermal decomposition process.

Fig.3 Grain size of xCoFe₂O₄-(1-x)BaTiO₃ fibers with calcination temperature: (a) For CoFe₂O₄ grain size; (b) For BaTiO₃ grain size

The SEM images of the nanocomposite 0.4CoFe₂O₄-0.6BaTiO₃ fibers obtained at 1 180 ℃ are shown in Fig.4. The fibers (Fig.4(a)) are characterized with a diameter range of 1-7 mm, a high aspect ratio up to 1×10⁵ and a dense surface. The morphology of a single nanocomposite fiber is shown in Fig.4(b), which is constructed by nanoparticles. These nanoparticles of CoFe₂O₄ and BaTiO₃ are arranged along the fiber with a necklace-like structure, which allows a maximum magnetoelectric coupling without a substrate constraint and magnify the mechanical displacement arising from the piezoelectric or magnetostrictive effect due to a high aspect ratio [13]. The morphologies of other nanocomposites xCoFe₂O₄-(1-x)BaTiO₃ fibers with various molar fractions (x=0.2, 0.3, 0.5) are very similar.

Fig.4 SEM images of 0.4CoFe₂O₄-0.6BaTiO₃ fibers: (a) Multi- fibers; (b) Single fiber

3.3 Magnetic properties

The hysteresis loops were measured to determine the saturation magnetization (M_s) and coercivity (H_c). Fig.5 shows the hysteresis loops of the randomly oriented nanocomposite 0.4CoFe₂O₄-0.6BaTiO₃ fibers obtained at different calcination temperatures for 2 h. These hysteresis loops exhibit soft magnetic characteristics of CoFe₂O₄ ferrites, implying a magnetic ordering in these nanocomposites. The magnetic parameters of 0.4CoFe₂O₄-0.6BaTiO₃ fibers with calcination temperature are represented in Table 1. It can be observed that M_s increases from 21.5 to 35.5 A?m²/kg as the calcination temperature increases from 900 to    1 180 ℃. This phenomenon may be attributed to the crystallization improvement and grain growth, which can reduce the crystal defects and enhance a net magnetic moment [22].

Fig.5 Hysteresis loops of randomly oriented 0.4CoFe₂O₄- 0.6BaTiO₃ fibers obtained at different calcination temperatures

As shown in Table 1, the coercivity initially increases with the increase of the calcination temperature, reaches a maximum value of 88.4 kA/m at 1 000 ℃, and then decreases with the further increase of calcination temperature. For the randomly oriented nanocomposite CoFe₂O₄-BaTiO₃ fibers, the coercivity should be primarily due to the magnetocrystalline anisotropy but the shape anisotropy. According to the STONER- WOHLFARTH single domain theory [23], the magnetocrystalline anisotropy energy of a nanocrystal single-domain is proportion to the nanocrystal particle volume. At low calcination temperatures the average crystallite size of CoFe₂O₄ in CoFe₂O₄-BaTiO₃ fibers is supposed within the critical single-domain size, and the magnetocrystalline anisotropy increases with the increase of the grain size. With a further increase of calcination temperature over 1 000 ℃, the coercivity exhibits a reduction tendency as the CoFe₂O₄ grain size is larger than the critical single-domain size, and the grains become multi-domains. The domain-wall movements taking place in these multi-domain grains result in the coercivity reduction [24]. It can be deduced that the critical single-domain size of CoFe₂O₄ grains in the 0.4CoFe₂O₄-0.6BaTiO₃ fibers is around 45 nm, which is close to 40 nm for the CoFe₂O₄ nanoparticles reported by CHINNASAMY et al [25].

Table 1 Effect of calcination temperature on CoFe₂O₄ grain size (D) and magnetic properties of 0.4CoFe₂O₄-0.6BaTiO₃ fibers

Fig.6 shows the hysteresis loops of the randomly oriented nanocomposite xCoFe₂O₄-(1-x)BaTiO₃ (x=0.2, 0.3, 0.4, 0.5) fibers obtained at 1 180 ℃ for 2 h. Based on the magnetic properties estimated from the hysteresis loop measurements, the relationships between saturation magnetization (M_s) and remanent magnetization (M_r) of these nanocomposite CoFe₂O₄-BaTiO₃ fibers with the molar fraction of CoFe₂O₄ ferrite phase are plotted in Fig.7. It can be seen that M_s and M_r show almost a linear relationship with the molar fraction of CoFe₂O₄ ferrite, which implies a high magnetic ordering arising from the CoFe₂O₄ ferrite in these nanocomposite fibers, and a tight coupling between the ferromagnetic CoFe₂O₄ ferrite and ferroelectric BaTiO₃ perovskite can be expected. The magnetoelectric effect for these nanocomposite CoFe₂O₄-BaTiO₃ fibers is under investigation.

Fig.6 Hysteresis loops of randomly oriented xCoFe₂O₄- (1-x)0.6BaTiO₃ fibers obtained at 1 180 ℃ for 2 h

Fig.7 M_s and M_r of xCoFe₂O₄-(1-x)BaTiO₃ fiber with molar fraction (x) of CoFe₂O₄

4 Conclusions

(1) Nanocomposite xCoFe₂O₄-(1-x)BaTiO₃ (x=0.2, 0.3, 0.4, 0.5) fibers are prepared by the organic gel-thermal decomposition process using citric acid and metal salts. These fibers are composed of nanosized ferrite CoFe₂O₄ and ferroelectric BaTiO₃ and have diameters as thin as 1 ?m, aspect ratios as high as    1× 10⁵ and a dense surface.

(2) The average grain sizes of CoFe₂O₄ and BaTiO₃ in the nanocomposite fibers increase from 25 to 65 nm with the calcination temperature increasing from 900 to  1 180 ℃. These nanosized grains fabricate the single nanocomposite fiber with a necklace-like structure.

(3) The magnetic properties for the nanocomposite fibers are largely influenced by the grain size and molar fraction of CoFe₂O₄ ferrite phase.

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(Edited by CHEN Wei-ping)