Effect of synthesis methods and a comparative study of structural and magnetic properties of zinc ferrite

Ferrite nanoparticles have generated an extensive research attention due to numerous properties that are superior to its bulk counterpart. Applications of nano ferrite in the areas of MRI contrast agents, magnetic refrigeration, magnetic fluids and targeted drug delivery, data storage, and various biomedical applications^1–7 draw the intense attention of scientists and researchers. The common formula of spinel ferrite is MFe₂O₄, where M is a divalent metal ion. The unit cell of spinel ferrite is composed of 32 oxygen atoms in cubic closed-packed arrangement distributed in tetrahedral (A) and octahedral (B) sites.⁸ The magnetic properties of ferrites are sensitive to the physical and chemical properties of ferrites⁹ and depend on the microstructure, especially on cation distribution¹⁰ and grain size.^11,12 Grain size depends on many factors including the synthesis conditions, sintering temperature, time, and rates of heating and cooling.¹³ Zinc ferrite nanoparticles got a notable consideration due to its outstanding magnetic properties in the equilibrium condition. ZnFe₂O₄ is a normal spinel with tetrahedral sites are favorably occupied by eight Zn²⁺ ions, and octahedral sites are occupied by sixteen anti-parallel Fe³⁺ ions^14,15 distributed in equal moments. Therefore, magnetic moments of Fe³⁺ ions cancel each other and ZnFe₂O₄ in the equilibrium condition possesses zero magnetic moment. Considering the magnetic properties in the nanoscale range superparamagnetism, core/shell structure, spin canting, and metastable cation distribution is some of the phenomena observed in spinel ferrites.¹⁶ Literature study revealed that in the bulk form ZnFe₂O₄ is paramagnetic¹⁵ while at the nanoscale range it is superparamagnetic¹⁷ with the higher magnetic moment.

Research on zinc-substituted ferrites was done extensively.^18,19 However, no detailed work was found in the literature regarding the comparison of magnetic properties of ZnFe₂O₄ at different initial particle sizes. In this work, the structural and magnetic properties of ZnFe₂O₄ synthesized by chemical co-precipitation method and solid state double sintering method followed by subsequent sintering are compared comprehensively.

Analytical grade of Zn(NO₃)₂·6H₂O and FeCl₃ were mixed in the required molar ratio under continuous stirring. NaOH solution of the concentration of 8M was added dropwise under continuous stirring to attain the pH ∼11. The mixture was heated at 80°C for one hour followed by centrifugation at 8000 rpm for 20 min. The particles were then washed ten times by filtration. The particles were dried at 90°C for 36 hours. After drying, the as-synthesized particles were subjected to calcination for 3 hours at 200°C, 400°C, 600°C, 800°C, 1000°C, 1100°C, 1200°C, and 1300°C. Samples prepared by chemical co-precipitation method will be called as Sample X.

Ferrite samples of the chemical formula ZnFe₂O₄ were also prepared by the double sintering ceramic technique. High purity reagent powders of ZnO and Fe₂O₃ were mixed according to their molecular weight and mixed for 6 hours by an agate mortar and pestle. Then the mixture was pressed into the disc shape sample. The disc-shaped sample was pre-sintered at 950°C for 3 hours to form ferrite through a chemical reaction. The pre-sintered material was crushed and milled again for 5 hours. Few drops of a saturated solution of polyvinyl alcohol were added as a binder. The resulting powders were pressed uniaxially under pressure (20 kN/cm²) in the stainless-steel dies to make pellets. The pressed pellets were then finally sintered at 1100°C, 1200°C, 1300°C, and 1400°C for further study. Samples prepared by solid state double sintering method will be called as Sample Y.

All samples were heated slowly in the programmable furnace at the heating rate of about 3°C/min, to avoid the cracking of the samples followed by furnace cooling.

Fig 1(a) shows the XRD patterns of sample X in the as-synthesized condition and subsequent sintering at different temperatures that are 200°C, 400°C, 600°C, 1100°C, 1200°C, and 1300°C for 3 hours. XRD analysis of sample X has revealed sharp and well-defined peaks, which were indexed by comparing standard patterns. Prominent diffraction planes of spinel structure are (220), (311), (400), (422), (511), and (440).²⁰ The peaks and unmixed hkl value indicates the formation of single phase spinel structure of ZnFe₂O₄. Fig 1(b) shows XRD patterns of sample Y in the pre-sintered condition and subsequent sintering at different temperatures that are 1100°C, 1200°C, 1300°C, and 1400°C for 3 hours. XRD patterns of sample Y demonstrate that ZnFe₂O₄ obtained single phase structure except in pre-sintered sample. Sample Y, pre-sintered at 950°C exhibit an intense peak at 44.75^o that is the presence of multiple phases. This peak exists due to unreacted ZnO. No secondary phase was observed in the XRD analysis of sample Y at the other sintering temperatures, which ensures the single-phase formation of ZnFe₂O₄ in sample Y.

The crystallite size of sample X was determined from the FWHM of (311) peak using Scherrer’s formula. The particle size of Sample X was found to be 5 nm in the as-synthesized condition. Note that, the particle size increases with the increase of sintering temperature which is manifested by the reduction of FWHM in the XRD pattern of sample X. Lattice parameter was found higher in sample Y than sample X. Lattice parameter was found to be 8.48 Å for sample X and 8.57 Å for samples Y. The different lattice parameters indicate larger lattice defects at the surface area of the nanoparticles and the different inversion ratio of nano and micro particles.

Fig. 2(a) and (b) shows SEM micrographs of ZnFe₂O₄ sintered at 1200°C synthesized by chemical co-precipitation method and solid state double sintering method, respectively.

Initial particle size is one of the important factors that affect the microstructure of the final sintered product.²¹ Fig. 2(a) shows the microstructure with coaxial homogeneous grains of sample X, while Fig 2(b) shows the microstructure with non-homogenous grains of sample Y.

Proper densification was high in sample X, compared to sample Y. Comparing two microstructures, it is clear that the use of ultrafine nanoparticle initially has a definite advantage in the sintering technique than the use of a micro sized powder of analytical grade ZnO and Fe₂O₃.

Fig. 3 (a) shows the plots of magnetization (M) as a function of applied field (H) of samples X sintered at 200°C, 400°C, 600°C, 800°C, 1000°C, 1100°C, 1200°C, and 1300°C for 3 hours. Field dependence of magnetization did not attain saturation, demonstrating the noncollinear behavior of magnetization in this range of sintering temperature. No appreciable hysteresis was observed in these M-H curves. Maximum magnetization (M_max) increases suddenly in the samples sintered at 200°C and 400°C. However, ZnFe₂O₄ is paramagnetic at bulk level, but at the nanoscale, there was a large change in the magnetic moment due to the change in the cation distribution. At the nanoscale, some of the Fe atoms transferred to A site leading to the decoupling of antiferromagnetically ordered Fe atoms on B site. Therefore, at the nanoscale, there is an enhancement of the magnetic moment of ZnFe₂O₄ is observed in the as dried condition and sintered at 200°C and 400°C. There is a sudden decrease of M_max at 600°C and onwards. As-dried sample and samples sintered in the temperature range from 200°C to 600°C the M-H curve hold the shape of the superparamagnetic nature. The shape of the M-H curve indicates magnetically isotropic nature of ferrite at the sintering temperature of 800°C. Sintering leads to the change of cation distribution that results in the onset of mixed spinel phase. The magnetic moment decreases drastically at the sintering temperature of 800°C and onwards due to the change in cation distribution again leading back to the normal spinel structure. M-H curve of sample X indicates an onset of ferrimagnetic phase at the sintering temperature of 600°C. This ferrimagnetic phase increases with the increase of sintering temperature from 600°C to 1200°C and decreased from 1300°C. Fig. 3 (b) shows M-H curve of samples Y sintered at 1100°C, 1200°C, 1300°C and 1400°C for 3 hours. All the samples demonstrate normal spinel structure, which means that the tetrahedral sites are exclusively occupied by Zn²⁺ ions and octahedral sites by an equal number of Fe³⁺ ions with opposite spin. Note that, ZnFe₂O₄ shows paramagnetic nature due to B-B interaction and absence of superexchange interaction between the two sublattices. The existence of considerable ferrimagnetic phase in the M-H curve of sample Y sintered at 1100°C, indicates non-paramagnetic nature at low field. That reveals a cluster with higher magnetic moment inside the paramagnetic matrix. M-H curve of samples Y, sintered at 1200°C, 1300°C, and 1400°C shows the increment of ferrimagnetic phase with temperature. This unexpected nature is due to the different sintering conditions as discussed.

Note that, magnetic behavior of ferrites depend on the cation distribution²² and change of oxidation state of the cation. Cation distribution and change of oxidation state are dependent on the sintering temperature.²³ Fig. 3 (c), (d) and (e) shows the maximum magnetization (M_max), the ratio of remanence to maximum magnetization (M_r/M_max) and coercivity (H_c) of sample X and Y as a function of sintering temperature. Fig. 3 (a) indicates an onset of magnetic phase transition of both samples at the sintering temperature 1100°C. Comparable particle size and similar cation distribution in both the samples are the key facts of the observed phase transition. The values of M_r and H_c in the superparamagnetic state are zero while in the ferromagnetic state it is non-zero. Acquired M_r and H_c of both the samples agree with the above statement and simplify the evolution of the magnetic phase.

Mössbauer analysis at room temperature of sample X and Y sintered at 1100°C are presented in Fig. 4 (a) and (b). Both the spectra show excellent fitting between experimental and theoretical curves with chi² value 0.629 and 0.493.

Both the spectra show central doublets, which are recognized for the superparamagnetic nature and discussed in Refs. 24–26 comprehensively. The broadening for sample X is attributed due to the mixed spinel phase as (Zn_xFe_1−x)[Zn_1−xFe_1+x]O₄, where x is the inversion parameter and also probably due to the smaller average grain size. This justifies different cationic state is present in sample X. Mössbauer spectra of sample X shows a sextet subsite that is the presence of Fe in the tetrahedral site. The subsite spectrum is presented inset of Fig. 4 (a). According to XRD analysis, there is no evidence of Fe³⁺ state in sample X sintered at 1100°C. This indicates that the decoupled larger nanoparticles also present at the tetrahedral site (due to sintering) and did the major contribution in the magnetic properties. Fig. 4 (b) shows no such evidence of sextet that is no significant magnetic contribution is observed in sample Y. Inversion parameter of sample X and Y was found 0.057 and 0 respectively. Different parameters and site occupancy of both the samples are presented in Table I.

Physical properties of ferrite products depend on the sintering temperature. The initial particle size of the material is important in sintering technology.

Therefore, comparison of the magnetic properties of ZnFe₂O₄ synthesized by different preparation technique could be of interest. Fig. 5(a), (b), and (c) shows the comparison of M-H curve of ZnFe₂O₄ synthesized by chemical co-precipitation and the conventional double sintering method. The earlier section already revealed that the magnetization value of ZnFe₂O₄ synthesized by chemical co-precipitation was high. Fig 3(a) shows that magnetization and the volume fraction of the ferrimagnetic phase reduced by the increase in sintering temperature in sample X. Fig. 5 revealed that the magnetization of ZnFe₂O₄ synthesized by the chemical method and subsequent sintering from 1100°C to 1300°C is higher compared to the magnetization of ZnFe₂O₄ synthesized by the double sintering method. The degree of inversion in ZnFe₂O₄ synthesized by chemical co-precipitation was higher than the solid-state double sintering technique which enhances the magnetization in sample X. However, at the sintering temperature of 1300°C both the curves tend to converge.

The comparison of magnetic properties between two samples showed that ZnFe₂O₄ with initial particle size at nano metric scale synthesized by chemical co-precipitation has better magnetic properties. Two different synthesis methods reveal different cation distribution that has a significant effect on magnetic properties. In conclusion, cation distribution and subsequent sintering are the key factors that determine the magnetic properties of ZnFe₂O₄.

Author’s acknowledge Materials Science Division, BAEC, and Ministry of Science and Technology, GOB greatly.