Synthesis of zinc–nickel ferrite nanorods and their magnetic properties

Abstract

Zinc–nickel (Zn–Ni) ferrite nanorods were synthesized by a microemulsion-based method in combination with calcination at different temperatures. The morphologies and structures of Zn–Ni ferrite nanorods and their precursors of Zn–Ni–Fe composite oxalate nanorods were characterized using field emission scanning electron microscopy and X-ray diffraction. Magnetization measurements were carried out using a vibrating sample magnetometer at room temperature. It was shown that the Zn–Ni ferrite nanorods were around 50–200 nm in diameter and several micrometers in length. Their saturation magnetization increased with increasing calcination temperature from 350 to 900 °C. In addition, the magnetic properties of the Zn–Ni ferrite nanorods were also affected by their composition.

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1. Introduction

Soft ferrites are commercially important materials on account of their electrical and magnetic properties.¹ Spinel-type ferrites, with general formula AFe₂O₄, are widely used in many electronic and magnetic devices because of their high magnetic permeability and low magnetic loss.² Among them, nickel ferrite (NiFe₂O₄) has been intensively investigated as a magnetic nanomaterial.^3–7 Recently, doping of inverse spinel ferrites with zinc has been used to improve their electrical or magnetic properties.^8–11 Zinc–nickel (Zn–Ni) ferrite is a versatile ferrite from the viewpoint of its large number of applications due to its high magnetic permeability, high electrical resistivity, high Curie temperature, and low power loss at high frequencies.^12–14 At present, many routes are developed to prepare Zn–Ni ferrite nanoparticles such as thermal decomposition,¹³ inert gas condensation,¹⁴ co-precipitation,¹⁵ hydrothermal reaction,¹⁶ sol–gel process^17,18 and ball milling.¹⁹ However, few reports involve the one-dimensional (1D) Zn–Ni ferrite nanomaterials. Wu et al.²⁰ reported that Zn–Ni–Fe composite oxide nanotubes with length of 1 to 5 μm and outer diameter of 200 nm showed 60 Oe of coercivity and 0.6 emu g⁻¹ of largest magnetocrystalline magnetization. The properties of ferrites are known to be strongly influenced by the composition, size, morphology and microstructures.²¹ With no doubt, Zn–Ni ferrite nanorods are expecting to show unique magnetic properties.

In this work, we report the synthesis of Zn–Ni ferrite nanorods with various magnetic properties. This was achieved by calcination of precursors with different molar ratio of Zn, Ni and Fe, prepared using a coprecipitation reaction of Zn²⁺, Ni²⁺ and Fe³⁺ with H₂C₂O₄ in a microemulsion solution.

2. Experimental

2.1. Preparation of Zn–Ni ferrite nanorods

A microemulsion system consisting of cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol was selected for synthesis of Zn_xNi_1−xFe₂(C₂O₄)₃ precursors. The microemulsion was prepared by dissolving CTAB (1.0 g) in a mixture of 75 ml of cyclohexane and 2.5 ml of n-pentanol. The resulting solution was stirred for 30 min. Subsequently, 3.75 ml of an oxalic acid (H₂C₂O₄) aqueous solution (1.2 M) was added into the above solution and the mixture was stirred for an additional 1 h. 1.25 ml of an aqueous solution containing ZnSO₄[0.4 × M], NiSO₄[(1 − x)0.4 M] (x = 0–1) and FeSO₄(0.8 M) was then added to the above microemulsion and stirred for 24 h at room temperature. The as-synthesized solid Zn_xNi_1−xFe₂(C₂O₄)₃ precursor was washed with alcohol and distilled water, centrifuged, and dried. Finally, Zn_xNi_1−xFe₂O₄ nanorods were obtained by calcination of the precursor at different temperatures for 3 h.

2.2. Characterization

Energy-dispersive X-ray spectroscopy (EDS) and field emission scanning electron microscopy (FESEM) were performed with a Hitachi S-4800 microscope operated at 15 kV. Thermal gravimetric analysis (TGA) was carried out on a SEIKO TG/DTA6200 thermal analyser at a heating rate of 10 K min⁻¹ from room temperature to 800 °C in air. Powder X-ray diffraction (XRD) patterns were recorded on a X-ray diffractometer (Rigaku Ultima IV) at 40 kV and 40 mA with Cu Kα radiation. The magnetic properties were measured using a Lakeshore 7300 vibrating sample magnetometer (VSM).

3. Results and discussion

3.1. Morphology and structure of Zn–Ni–Fe composite oxalate nanorods

Fig. 1 shows SEM image and EDS spectrum of Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ nanorods. As shown in Fig. 1a, Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ nanorods are around 50–200 nm in diameter and several micrometers in length. The SEM image also reveals that the surface of the rods is very smooth. By observation of the EDS spectrum of the nanorods (Fig. 1b), only Zn, Ni, Fe, C and O are detected, which further confirms the formation of pure Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ nanorods. Fig. 2 shows XRD patterns of the four different oxalate nanorods prepared at similar conditions. The diffraction peaks of Zn–Ni–Fe composite oxalate (Fig. 2a) are different from those of any other three single metal oxalate (Fig. 2b–d) and not the simple superposition of b, c and d, implying that the structure of the precursor is Zn–Ni–Fe composite oxalate like Zn_0.5Ni_0.5Fe₂(C₂O₄)₃.

Fig. 1 Morphology and composition of the Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ nanorods prepared by a microemulsion-based method: (a) SEM image, (b) EDS spectrum.

Fig. 2 XRD patterns of the different oxalate nanorods: (a) Zn_0.5Ni_0.5Fe₂(C₂O₄)₃, (b) ZnC₂O₄, (c)NiC₂O₄, and (d) FeC₂O₄.

3.2. Thermal property of Zn–Ni–Fe composite oxalate nanorods

As shown in Fig. 3, the thermal behavior of Zn–Ni–Fe composite oxalate nanorods is characterized by two transitions: the first one at low temperature corresponds to the loss of surface water; the second one at higher temperature (180–320 °C) is due to decomposition of the oxalate. Therefore Zn–Ni ferrite nanorods can be obtained by calcining oxalate precursors at higher than 320 °C based on this TGA curve.

Fig. 3 TGA curve of Zn–Ni–Fe composite oxalate nanorods.

3.3. Morphology and structure of Zn–Ni ferrite nanorods

SEM images of Zn_0.5Ni_0.5Fe₂O₄ nanorods by calcination of the precursors at different temperatures (350, 500, 650, 800 and 900 °C) are shown in Fig. 4. Fig. 4a shows Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ precursor nanorods which are characterized by the smooth surface morphology. After calcination, the products retain original rodlike morphology of the precursor and the Zn_0.5Ni_0.5Fe₂O₄ nanorods are formed by nanoparticles. It can be concluded that the formation of the Zn_0.5Ni_0.5Fe₂O₄ nanorods during the calcination process was restricted within the Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ template. In other words, this growth process can be viewed as a morphologically templated nucleation process.²² As shown in Fig. 4b, the Zn_0.5Ni_0.5Fe₂O₄ nanorods obtained at 350 °C start to crystallize as the precursor Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ begins to decompose into Zn_0.5Ni_0.5Fe₂O₄ but the nanoparticles are too little to be observed. Fig. 4c shows the SEM image of the nanorods prepared at 500 °C, small Zn_0.5Ni_0.5Fe₂O₄ nanoparticles are clearly visible. At 650 and 800 °C (Fig. 4d and e), the nanoparticles are now much larger and the number of particles is much lower, indicating an Ostwald ripening and “oriented attachment” processes and the rods are still two micrometers in length. At 900 °C, the Zn_0.5Ni_0.5Fe₂O₄ nanorods consist of individual crystals bound to each other.

Fig. 4 (a) SEM image of Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ nanorods; ((b)–(f)) SEM images of Zn_0.5Ni_0.5Fe₂O₄ nanorods obtained at different temperatures, (b) 350 °C, (c) 500 °C, (d) 650 °C, (e) 800 °C and (f) 900 °C.

Fig. 5 shows XRD patterns for the Zn_0.5Ni_0.5Fe₂O₄ nanorods prepared by decomposition of the precursor at 350, 500, 650, 800 and 900 °C respectively. It can be seen that the inverse spinel ferrite of Zn_0.5Ni0_.5Fe₂O₄ is formed as a single phase at all temperatures. All the diffraction peaks can be indexed as pure cubic phase of zinc–nickel ferrite (Zn_0.5Ni_0.5Fe₂O₄). The intensity of the diffraction peaks of Zn_0.5Ni_0.5Fe₂O₄ increases and the peak width (full width at half maximum) decreases with increasing calcinations temperatures. This indicates that, as expected and observed, both the crystallinity and the average size of the crystallites increase with increasing calcination temperatures.

Fig. 5 XRD patterns of Zn_0.5Ni_0.5Fe₂O₄ nanorods prepared by calcination of the corresponding precursors at different temperatures (a) 350 °C, (b) 500 °C, (c) 650 °C, (d) 800 °C and (e) 900 °C.

Fig. 6 shows SEM images of the Zn_xNi_1−xFe₂O₄ nanorods with different x values. They are all obtained by decomposition of the precursors at 500 °C. The SEM images show rodlike morphology formed by nanoparticles and the diameter decreases with increasing Zn content. Chemical formulas for Zn_xNi_1−xFe₂O₄ nanorods are given in Table 1 based on the metal analysis data. Zn–Ni–Fe molar ratios for all samples are consistent with those used in the starting solutions, indicating the complete precipitation of metal ions.

Fig. 6 SEM image of the Zn_xNi_1−xFe₂O₄ nanorods with different x values, (a) 0.1, (b) 0.35, (c) 0.5, (d) 0.65, and (e) 0.9.

Table 1 Measurement ratio of the metal elements of Zn_xNi_1−xFe₂O₄ nanorods

Sample	x value	Zn : Ni : Fe
ZnxNi1−xFe2O4	0.1	0.08 : 0.85 : 2
	0.35	0.31 : 0.62 : 2
	0.5	0.54 : 0.44 : 2
	0.65	0.60 : 0.39 : 2
	0.9	0.86 : 0.07 : 2

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3.4. Magnetic properties of Zn–Ni ferrite nanorods

Fig. 7 shows magnetic hysteresis loops of Zn_xNi_1−xFe₂O₄ (x = 0.5) nanorods prepared by calcination of the precursors at different temperatures. The saturation magnetization (Ms) are summarized in Table 2. The Ms gradually increases with increasing calcination temperatures, or more precisely the crystallite size of the particles. The sample prepared at 350 °C exhibits a narrower hysteresis loop in the M–H curve with Ms of 17.8 emu g⁻¹. While in case of the Zn_xNi_1−xFe₂O₄ (x = 0.5) nanorods obtained at 900 °C, the Ms remarkably increases to 77.1 emu g⁻¹. Combined with XRD and SEM results, we suggest that well crystallinity and large nanoparticle size of the sample due to the higher temperature aroused the significant increase of Ms. The lower Ms associated to the particles with smaller sizes can be attributed to two reasons. First, surface distortions due to the interaction of the transition metal ions with the oxygen atoms in the spinel lattice can reduce the net magnetic moment in the particle. This effect is especially prominent for the ultrafine particles due to their large surface to volume ratio.²³ Second, the magnetocrystalline anisotropy of the particles is dependent on the crystallinity of the nanoparticles. As shown in the XRD patterns, samples prepared by decomposition of the precursor at lower temperatures exhibit the lower intensity and crystallinity. Hence, a large proportion of crystal defects and dislocations can occur within the lattice and this will cause a significant reduction of magnetic moment within the particles, as a result of the magnetocrystalline anisotropy distortion.

Fig. 7 Magnetic hysteresis loops for Zn_xNi_1−xFe₂O₄ (x = 0.5) nanorods prepared by calcinations of the precursor at different temperatures, (a) 350 °C, (b) 500 °C, (c) 650 °C, (d) 800 °C and (e) 900 °C.

Table 2 Magnetic parameters for Zn_xNi_1−xFe₂O₄ (x = 0.5) nanorods prepared by calcination of the precursor at different temperatures

Temperature (°C)	Ms (emu g^−1)
350	17.8
500	51.1
650	56.3
800	63.6
900	77.1

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Fig. 8 shows hysteresis loops of Zn_xNi_1−xFe₂O₄ nanorods with different x values obtained at 500 °C. As sown in Table 3, Zn_xNi_1−xFe₂O₄ nanorods with different x values show Ms of 9.3–51.1 emu g⁻¹ due to different compositions. When Zn doping level (x value) increases from 0.1 to 0.5, the Ms of samples increases. However with further increasing x value to 0.9, the Ms of Zn_xNi_1−xFe₂O₄ nanorods decreases. The Zn_0.5Ni_0.5Fe₂O₄ nanorods show a maximum Ms of 51.1 emu g⁻¹.

Fig. 8 Magnetic hysteresis loops for Zn_xNi_1−xFe₂O₄ nanorods with different x values, (a) 0.1, (b) 0.35, (c) 0.5, (d) 0.65, and (e) 0.9.

Table 3 Magnetic parameters for Zn_xNi_1−xFe₂O₄ nanorods

Sample	Ms (emu g^−1)
ZnxNi1−xFe2O4 (x = 0.1)	25.6
ZnxNi1−xFe2O4 (x = 0.35)	32.6
ZnxNi1−xFe2O4 (x = 0.5)	51.1
ZnxNi1−xFe2O4 (x = 0.65)	32.3
ZnxNi1−xFe2O4 (x = 0.9)	9.4

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When non-magnetic divalent Zn²⁺ ions are introduced, they tend to occupy tetrahedral (A) sites by transferring Fe³⁺ ions to octahedral (B) sites due to their favoritism by polarization effect. However, site preference of cations also depends upon their electronic configurations. Zn²⁺ ions show marked preference for A sites where their free electrons respectively can form a covalent bond with the free electrons of the oxygen ion. This forms four bonds oriented towards the corners of a tetrahedron. Ni²⁺ ions have marked preference for an octahedral environment due to the favorable fit of the charge distribution of these ions in the crystal field at B sites. In view of the above considerations the cation distribution can be written as (Zn_x²⁺Fe_1−x³⁺)^A(Ni_1−x²⁺Fe_1+x³⁺)^B.^24,25 As the content of Zn²⁺ ions increases, the number of Fe³⁺ ions on B-sites increases. Therefore the magnetic moment on B-site increases and the magnetic moment on A-sites decreases. The net value of Ms increases up to x = 0.5. When ‘x’ exceeds 0.5, Ms starts decreasing because magnetic moment of the few Fe³⁺ ions on the A-site are no longerable to align all the moments on the B-sites antiparallel to themselves, since this is opposed by the negative B–B exchange interaction.^26–29

4. Conclusions

In summary, Zn_0.5Ni_0.5Fe₂O₄ nanorods were successfully prepared by calcination of the Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ nanorods which were synthesized by a microemulsion method. The Zn_0.5Ni_0.5Fe₂O₄ nanorods retain the original rodlike morphology of the Zn_0.5Ni_0.5Fe₂(C₂O₄)₃ nanorods through the whole calcination process and they have the same dimension of 50–200 nm in diameter and several micrometers in length. The magnetic properties of the Zn_0.5Ni_0.5Fe₂O₄ nanorods are largely influenced by calcination temperatures. As the calcination temperatures increase from 350 to 900 °C, the Ms of the Zn_0.5Ni_0.5Fe₂O₄ nanorods increase. The Zn_0.5Ni_0.5Fe₂O₄ nanorods obtained at 900 °C show a maximum Ms of 77.1 emu g⁻¹. When the Zn doping level (x value) increases from 0.1 to 0.5, the Ms of samples increases. However with further increasing x value to 0.9, the Ms of Zn_xNi_1−xFe₂O₄ nanorods decreases.

Acknowledgements

This work was supported by National Natural Science Foundation of China (no. 21071017 and no. 21376029).

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