Synthesis and magnetic properties of nanocomposite Ni_1−x Co _x Fe₂O₄–BaTiO₃ fibers by organic gel-thermal decomposition process

Figure 1 shows the FTIR spectra of the gel precursor for 0.5Ni_0.5Co_0.5Fe₂O₄–0.5BaTiO₃ fibers and the products derived from calcination of the precursor at different temperatures. From the FTIR spectrum for the gel precursor in Fig. 1a, the two bands at 1,614 and 1,384 cm⁻¹ arise from the RCOO⁻ symmetrical and asymmetrical stretching vibration, which are the characteristic absorption peaks for the citrate. The bands at 1,072, 845, 690, 637 and 555 cm⁻¹ can be assigned to the characteristic vibration peaks of C–OH, Ti–O, Co–O, Ni–O and Fe–O bonds [14‐16], respectively. This is indicative of the complex formation of metal ions and citric acid.

For the samples obtained at 300 °C, as shown in Fig. 1b, the characteristic absorption peak for the citrate at around 1,614 cm⁻¹ disappears owing to the decomposition of citrate. Two bands at 1,470 and 859 cm⁻¹ can be assigned to the characteristic absorption peaks for BaCO₃. The bands at 1,386 and 1,060 cm⁻¹ are corresponding the characteristic vibration peaks of NO₃ ⁻ and Fe₂O₃, and the band at about 582 cm⁻¹ is assigned to NiO₂, Co₃O₄ and TiO₂ [17‐19]. With the calcination temperature increased to 900 °C, as shown in Fig. 1c, the peaks assigned to BaCO₃ and metal oxides disappear and the characteristic peak at around 580 cm⁻¹ is detected due to the stretching mode of Fe–O at tetrahedral sites in the spinel structure and the stretching mode of Ti–O in the perovskite BaTiO₃, which indicates that Ni_0.5Co_0.5Fe₂O₄ and BaTiO₃ would be formed at this calcination temperature [20, 21].

Figure 2 shows the TG-DSC curves of the 0.5Ni_0.5Co_0.5Fe₂O₄–0.5BaTiO₃ gel precursor and the thermal decomposition process roughly consists of the following three stages. The first stage takes place at the temperature range of 50 ~ 300 °C. The DSC curve exhibits a broad endothermic event corresponding to a weight loss of about 10% at low temperatures 50 ~ 180 °C and this is attributed to the loss of free water and bound water from the gel precursor. Then a large and sharp exothermic event occurs at around 210 °C in the DSC curve correspondingly a weight loss of about 27%, owing to the initial break-down of the complexes and a spontaneous combustion. The spontaneous combustion is induced by the in situ oxidizing interactions of citrate, nitrate ions and ammonium nitrate in the gel accompanying with liberation of H₂O, CO₂, NO _X [22, 23].

The second stage at the temperature range of 300 ~ 900 °C, the DSC curve exhibits a broad exothermic event and a series of small endothermic peaks. The broad exothermic event at around 380 °C is accompanied by a weight loss of 23% due to continuing oxidation of the organic matters and formation of metal oxides. A series of small endothermic peaks at around 800 °C correspond to a weight loss of about 10%, and it is believed that at this temperature range the formation of Ni_0.5Co_0.5Fe₂O₄ and BaTiO₃ takes place.

The third stage at the temperature range of 900 ~ 1,180 °C, the TG curve shows almost no changes in weight loss and some small endothermic events occur in the DSC curve owing to the crystallization and grain growth of the formed Ni_0.5Co_0.5Fe₂O₄ and BaTiO₃, which is evidenced by the above FTIR (Fig. 1c) analysis and confirmed by the following XRD data.

Figure 3 shows XRD patterns of the nanocomposite 0.5Ni_0.5Co_0.5Fe₂O₄–0.5BaTiO₃ fibers obtained at various calcination temperatures for 2 h. After calcination at 900 °C, the diffraction collections are indexed just to both the spinel ferrite NiFe₂O₄ (JCPDS No. 10-0325) and perovskite BaTiO₃ (JCPDS No. 05-0626) phases, which is in agreement with the above FTIR and TG/DSC analyses. As the ferrite NiFe₂O₄ has the same structure with Ni_0.5Co_0.5Fe₂O₄, it can be deduced that the composite consisting of the ferrite Ni_0.5Co_0.5Fe₂O₄ and perovskite BaTiO₃ is formed at this calcination temperature. With the calcination temperature increasing from 900 to 1,180 °C, the corresponding peaks become sharper and narrower, and the crystallization of Ni_0.5Co_0.5Fe₂O₄ and BaTiO₃ is improved and the consequent crystalline grains grow.

The average crystalline size (D) of Ni_1−x Co _x Fe₂O₄ and BaTiO₃ phases in the fibers can be calculated from the full width at half maximum (FWHM) of the reflection peaks of (311) and (101) using Scherrer’s equation. The calculated average crystalline grain size D of Ni_1−x Co _x Fe₂O₄ and BaTiO₃ phases in 0.5Ni_1−x Co _x Fe₂O₄–0.5BaTiO₃(x = 0.2, 0.3, 0.4, 0.5) fibers with the calcination temperatures is plotted in Fig. 4. The crystalline grain sizes are influenced largely by the calcination temperature and the cobalt content in Ni_1−x Co _x Fe₂O₄. For both Ni_1−x Co _x Fe₂O₄ and BaTiO₃ phases at low cobalt content 0.2, D values increase from about 15 to 55 nm with the calcination temperature from 900 to 1,180 °C. By substitution of Ni with Co in Ni_1−x Co _x Fe₂O_4, the grain sizes of Ni_1−x Co _x Fe₂O₄ and BaTiO₃ tend to increase at various calcination temperatures, whilst it finds that the average grain size of BaTiO₃ in 0.5Ni_0.5Co_0.5Fe₂O₄–0.5BaTiO₃ composite has almost a same size when the calcination temperature over 1,000 °C.

The lattice parameter of Ni_1−x Co _x Fe₂O₄ was calculated by use of a special software (MDI Jade 5.0). The average grain size and lattice parameter of Ni_1−x Co _x Fe₂O₄ with various Co content obtained at 1,180 °C are represented in Table 1. It can be seen that from Table 1 that the grain size and lattice parameter of Ni_1−x Co _x Fe₂O₄ increase with Co content, owing to the crystal lattice inflation induced by the substitution of Ni cations (0.78 Å) with larger Co cations (0.82 Å) [24]. The grain sizes and the coupling of ferromagnetic Ni_1−x Co _x Fe₂O₄ and ferroelectric BaTiO₃ nanophases so that can be tailored by controlling the calcination process and chemical composition.

The SEM graphs of the typical nanocomposite 0.5Ni_0.5Co_0.5Fe₂O₄–0.5BaTiO₃ fibers obtained at 1,180 °C for 2 h are showed in Fig. 5. It can be seen that the fibers are characterized with a diameter range of 3 ~ 7 μm, a high aspect ratio up to 1 × 10³ and a dense surface. These fibers are composed of nanosized particles of Ni_0.5Co_0.5Fe₂O₄ and BaTiO₃. In comparison with the average crystalline size estimated by Scherrer’s equation, the particle size observed is in a range 50 to 300 nm and generally in a nano-scale, whilst some particles are larger possibly owing to particle aggregations. The morphologies of the other nanocomposite 0.5Ni_1−x Co _x Fe₂O₄–0.5BaTiO₃(x = 0.2, 0.3, 0.4, 0.5) fibers are very similar.

The hysteresis loops were measured to determine saturation magnetization (M _s) and coercivity (H _c). Figure 6 shows the hysteresis loops of the randomly oriented nanocomposite 0.5Ni_0.5Co_0.5Fe₂O₄–0.5BaTiO₃ fibers obtained at different calcination temperatures for 2 h. These loops are characterized with typical soft magnetic properties of Ni_0.5Co_0.5Fe₂O₄ ferrite, implying a magnetic ordering in the as-prepared nanocomposite fibers. The magnetic parameters of the Ni_0.5Co_0.5Fe₂O₄–BaTiO₃ fibers with calcination temperature are represented in Table 2. It can be seen that M _s and magnetic remance (M _r) increase from 7.69 to 20.62 Am²/kg and 0.70 to 7.05 Am²/kg, respectively corresponding the calcination temperature from 900 to 1,180 °C. This phenomenon can be explained due to the crystallization improvement and grain growth of the ferrite [25].

As showed in Table 2, the coercivity increases initially with the calcination temperature and reaches a maximum value at around 1,100 °C due to the increasing magnetocrystalline anisotropy. According to the Stoner–Wohlfarth single-domain theory [26], when the crystalline grain size of Ni_0.5Co_0.5Fe₂O₄ is within the single-domain size, the magnetocrystalline anisotropy energy of a nanocrystal single domain Ni_0.5Co_0.5Fe₂O₄ is proportion to the volume of nanocrystal particles and the magnetocrystalline anisotropy of Ni_0.5Co_0.5Fe₂O₄ increases with the increasing grain size from about 34 to 65 nm corresponding the calcination temperature from 900 to 1,100 °C as showed in Table 2. With a further increase of calcination temperature over 1,100 °C, the coercivity then exhibits a reduction tendency as the Ni_0.5Co_0.5Fe₂O₄ grain size is larger than the single-domain size and the particles become multi-domains. The domain-wall motions taking place in these multi-domain particles will result in the coercivity reduction [27]. It can be reasoned that the single-domain size of Ni_0.5Co_0.5Fe₂O₄ in the nanocomposite Ni_0.5Co_0.5Fe₂O₄–BaTiO₃ fibers is around 65 nm, which is close to the value 70 nm for CoFe₂O₄ particles reported by Rashada et al. [28] and 100 nm for NiFe₂O₄ particles reported by Yao et al. [29]. The effect of calcination temperature on the squareness (M _r/M _s) of Ni_0.5Co_0.5Fe₂O₄ in the nanocomposite fibers is similar to that of coercivity.

Figure 7 shows the hysteresis loops of the randomly oriented nanocomposite 0.5Ni_1−x Co _x Fe₂O₄–0.5BaTiO₃(x = 0.2, 0.3, 0.4, 0.5) fibers obtained at 1,180 °C for 2 h. The saturation magnetization of Ni_1−x Co _x Fe₂O₄–BaTiO₃(x = 0.2, 0.3, 0.4, 0.5) fibers obtained at 1,180 °C for 2 h are represented in Table 1. It can be seen that M _s vaules increase from 12.95 to 20.62 Am²/kg with the increase of Co content, which is mainly due to the substitution of Ni²⁺ions by Co²⁺ ions in the octahedral sites and higher magnetic moment of Co²⁺ ions [30]. In order to compare the magnetic properties, the hysteresis loops of CoFe₂O₄ and NiFe₂O₄ fibers calcined at 1,180 °C are also showed in Fig. 7. Compared with the saturation magnetization of the single phase CoFe₂O₄ fibers (87.77 Am²/kg) and NiFe₂O₄ fibers (24.85 Am²/kg), the 0.5Ni_1−x Co _x Fe₂O₄–0.5BaTiO₃(x = 0.2, 0.3, 0.4, 0.5) nanocomposite fibers exhibit lower M _s values owing to non-magnetic BaTiO₃ phase. The H _c values for these nanocomposite fibers are distributed between the coercivity value of CoFe₂O₄ fibers (52.42 kA/m) and NiFe₂O₄ fibers (6.65 kA/m), implying the magnetization behaviour and magnetic ordering for the nanocomposite fibers can be tailored by design of the chemical composition.