1 Introduction

Ferrimagnetic (ferrites) materials are the salt of some of the transition metals, particularly which crystallize in spinel structure. Ferrite nanoparticles with spinel lattice are one of the most vital and fast-growing areas of the scientists and researchers interest. The spinel ferrites have excellent potential applications in many fields, such as satellite communication, memory storage devices, filters circuit, microwave fascinating materials, computer machinery, satellite dish rod, magnetic soundtrack, transformer cores, high-frequency systems, electronic circuits power delivering devices, electromagnetic interference suppression, cancer disease treatment and magnetic resonance imaging [1]. In general, the molecular formula of ferrites can be written as XY2Z4, where X is the tetrahedral voids and Y is the octahedral sites. Here, X2+ is the divalent positive metal ion similar to Fe, Co, Zn, Ni, Mg, Mn etc., Y3+ is a trivalent cation like In, Al, Ti, V, Cr, Co, Fe, Ni etc. and Z2− is generally the divalent oxygen ion. As an important member of ferrite family, zinc ferrite (ZnFe2O4) has attracted significant research interest due to their distinctive structural and magnetic properties. ZnFe2O4 is a chemically and thermally stable semiconductor material [2]. As one of the magnetic materials, bulk ZnFe2O4 has a normal spinel structure with the tetrahedral (X2+) sites occupied by Zn2+ and the octahedral (Y3+) sites occupied by Fe3+ [3]. However, in the “inverse” spinel structure, more common in spinel ferrites, X2+ cations are replaced by half of Fe3+ ions in the tetrahedral sites and the second half of Fe3+ ions remaining at the octahedral sites [4, 5]. In general, the relative sizes of X2+ and Y3+, understand of the smaller positive ion prefer to occupy the site of lower coordination i.e., tetrahedral site. And the bigger positive ions prefers to occupy the site of higher coordination i.e., octahedral sites. The structures of spinels are affected by the relative ligand-field stabilization energy (LFSE) values of metal ions. These LFSE values of Y3+ positive ions in octahedral sites greater than X2+ ions in tetrahedral sites. The difference in LFSE value between the octahedral and tetrahedral is referred to as octahedral site stabilization energy (OSSE). If the Y3+ ions have more coordination-field stabilization energy (CFSE) gain in the octahedral site than that of X2+ ions, a normal spinel is expected. Whereas, an inverse spinel is formed whenever the divalent X2+ ions have comparatively more CFSE gain in the octahedral geometry than the trivalent Y3+ ions. A normal spinel structure is assumed if both the divalent and the trivalent metals are non-transition metals since no CFSE is involved. And, the formation of inverse spinel structure in some cases which contain transition metal ions. This is because, the transition metal ion may get extra stability in octahedral sites, prefers octahedral voids over tetrahedral voids. Fe3O4 is an inverse spinel since the Fe3+ ions is a high spin d5 system with zero CFSE while, the divalent Zn2+ is a high spin d6 system with more CFSE. Hence, ZnFe2O4 is a normal spinel because both the divalent and trivalent ions are non- transition metal ions. In this present study, PVP functionalized ZnFe2O4 nanoparticles were synthesized using chemical co-precipitation route. Then, the structure, morphology and magnetic domains of ZnFe2O4 nanoparticles were investigated by X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR), Transmission electron microscopy (TEM) with energy dispersive X-ray (EDX) analysis and Vibrating sample magnetometer (VSM).

2 Experimental section

2.1 Materials

Ferric sulfate monohydrate [Fe2(SO4)3·H2O, assay: 99%], ferrous sulfate tetra hydrate [Fe(SO4)·7H2O, assay: 98%], zinc sulfate hepta hydrate [ZnSO4·7H2O, assay: 98%], ethanol, ammonia, polyvinyl pyrrolidone [(C6H9NO)n, assay: 98%] were purchased from Sigma-Aldrich and used without further purification.

2.2 Synthesis of PVP coated ZnFe2O4 nanoparticles

In a typical experiment, aqueous solutions of 100 ml containing 0.017 M of ferric sulfate, 0.033 M of ferrous sulfate,(X = 0.01,0.02, 0.03 and 0.04 M) zinc sulfate hepta hydrate and 0.0025 M (1 g) Polyvinyl Pyrrolidone (PVP) were added into 250 ml conical flask. The mixture was stirred for 45 min to obtain a homogeneous solution and then, 0.25 M of ammonia was dissolved in 50 ml of distilled water. This solution was added slowly to the homogeneous mixture for maintaining the PH level at 11. Under vigorous magnetic stirring for 3 h at room temperature, the solution appears black colour, this is the required condition for precipitation of the nanoparticles. These black powder was dissolved into methanol then centrifuged and separated with a bar magnet. This process was continually repeated for 4 times in order to remove the excess amine molecules. We get a black powder of ZnFe2O4 after drying the precipitate at 100 °C for 6 h. The dried powder was kept at muffle furnace and annealed at 600 °C for 4 h, in order to obtain phase pure ZnFe2O4 nanoparticles.

3 Results and discussion

3.1 Structural analysis

From XRD graph (Fig. 1), it is evident that there is no detectable trace of extra peaks corresponds to impurity and amorphous phase in the patterns. For a small amount of Zn doped with Fe3O4, it affects the tetrahedral and octahedral structure, on the other hand, magnetic properties increases. On increasing the doping ratio in terms of 0.01 mg, the Zn2+ ions promoted to grain growth while the grains evolving from spherical smooth surfaces with peaks become sharper. Beyond the concentration, it has been noticed that there is an increment in grain size may be due to an increase in the density of the pores. The crystallite size decreases significantly when the sintering temperature reaches around 600 °C with different doping ratio of Zn2+ ion, due to that Zn2+, Fe3+ cations shared between tetrahedral and octahedral voids, respectively. Since, the ionic radius of Zn2+ (74 pm) is greater than Fe3+ (60 pm) ion [3]. So, the ZnFe2O4 nanoparticles exhibit normal spinel structure. The crystallite size of the sample (11.14 nm) is estimated using Scherrer’s equation [2],

Fig. 1
figure 1

XRD patterns of a Fe3O4, b Zn0.01 MFe2O4, c Zn0.02 MFe2O4, d Zn0.03 MFe2O4 and e Zn0.04 MFe2O4 nanoparticles

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$${\text{D}}=0.9\uplambda /\upbeta \cos \uptheta$$

where λ, β and θ be the X-ray wavelength, the full width of half maximum (FWHM) of the peak, the Bragg angle, respectively. All the observed peaks were indexed to cubic Zn-ferrite spinel structure using JCPDS data (Card No: 87-0713). The reflections (220), (110), (311), (400), (024), (422), (333), (440) and (144) showed that the formation of single phase cubic normal spinel structure. The lattice parameter was calculated by using the following relation [5],

$${\text{a}}={\left[ {{{\text{d}}^2}\left( {{{\text{h}}^2}+{{\text{K}}^2}+{{\text{l}}^2}} \right)} \right]^{1/2}}.$$

A lattice constant deals with physical dimension of all crystal systems and structure of magnetic materials. The composition, crystallite size and lattice constant for all the samples are summarized in Table 1. From this table, the average lattice constant is ~ 8.4 Å. It is revealed that the nanoparticles may have cubic structure because of all the values of lattice constants (a) are almost equal. We conclude from the above said analysis that the structural length lies between tetrahedral and octahedral sites within the unit cell. Therefore, the Zn materials are grown on the Fe materials may lead to the super paramagnetic behavior of an entire system.

Table 1 Structural properties of ferrite nanoparticles
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3.2 Functional group analysis

The FT-IR spectroscopy indicates the positions of the ions involved in the crystal lattice through their vibration modes. The spectra as shown in Fig. 2 were recorded at room temperature in the frequency range between 4000 and 400 cm−1. The shift in the band position was observed by increasing Zn2+ concentration in Fe3O4 because of the variation in the cation–oxygen bond length at the octahedral (Y3+) and tetrahedral sites (X2+). This may be interpreted by the stronger binding of Fe3+ ions at the X-sites than at the Y-sites. The bands between 3563 and 3218 cm−1 could be attributed to the O–H stretching vibration of H2O absorbed by the sample [6] and the bands at 1112–1100 cm−1 ascribed for C–H bending mode. The peaks with a wave number of 1101 and 1102 cm−1, are due to stretching vibration of C–O bond [7, 8]. The bands at 1618 and 1619 cm−1 correspond to the asymmetric stretching vibrations of C=O bond [9]. The peaks at 2924 cm−1 are due to stretching vibrations of C=H bond of the CH3 functional group [6]. The bands in the range 457–470 and 540–549 cm−1 correspond to the vibration of the chemical O\Moct\O bond at octahedron position and the vibration of the chemical bond O\Mtet\O at tetrahedron position, respectively [10] and all the frequencies data are showed in Table 2.

Fig. 2
figure 2

FTIR spectra of a Fe3O4, b Zn0.01 MFe2O4, c Zn0.02 MFe2O4, d Zn0.03 MFe2O4, and e Zn0.04 MFe2O4 nanoparticles

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Table 2 Tentative vibrational assignments of ferrite nanostructures
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3.3 Morphological analysis

Figures 3 and 4a, b depict TEM photographs of Fe3O4 and ZnxFe2O4 (x = 0.03 M), nanoparticles, respectively. The TEM images show that the particles with the nearly spherical shape having sizes in the range between 20 and 50 nm. These particles are found to be stacked on top of each other due to their mutual magnetic attraction. The observed particle sizes were consistent with the crystallite size determined by the XRD technique. The selected area electron diffraction (SAED) analysis of the distinctive samples was shown in Figs. 3 and 4c. The SAED patterns specify that the Zn2+ substituted ferrite nanoparticles were originated in well-crystalline nature. The super imposition of the bright spot with Debye ring pattern focused on polycrystalline nature of the samples. Chemical purity and stoichiometry of the Fe3O4 and ZnFe2O4 nanoparticles were investigated by EDAX spectroscopy. The strong peaks related to Fe, Zn and O were found in the Figs. 3 and 4d. The EDAX results also confirmed that the precursors used in the synthesis have fully undergone the chemical reaction to form the single phase nanocrystalline zinc ferrite as shown in Tables 3, 4.

Fig. 3
figure 3

a, b TEM images, c SAED pattern and d EDAX spectrum of Fe3O4 (x = 0.00 M) nanoparticles

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Fig. 4
figure 4

a, b TEM images, c SAED pattern and d EDAX spectrum of ZnxFe2O4 (x = 0.03 M) nanoparticles

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Table 3 EDAX spectrum analysis of Fe3O4 nanoparticles
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Table 4 EDAX spectrum analysis of ZnFe2O4 nanoparticles
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3.4 Magnetic characterization

The magnetic properties are the most important properties of ferrites, which are depending on the processing conditions, nanostructure, chemical composition and the type of the additives. Figure 5 shows a typical M-H diagram for (a) Fe3O4 and (b) ZnFe2O4 nanoparticles. As the applied field is increased to a positive value large enough to saturate the polarization of the material, M (moments) then reduced to a negative value large enough to produce saturation in the reverse direction then increased back to zero once again. A symmetrical closed loop known as hysteresis is thus formed. The magnetic induction is remaining when the applied field is reduced from saturation to zero is called the remnant (Mr). Here, the retentivity value of ZnFe2O4 is 571.58E−6 emu. This magnetic retentivity results showed that the ZnFe2O4 nanoparticles annealed at 600 °C temperature exhibit, decreased hysteresis loop area when an increase of Zn2+ ions. The saturation magnetization (Ms) increases into 23.451E−3 emu and then particle size is calculated to be 11.14 nm (from XRD). This shows that the ZnFe2O4 is normal spinel, with Zn2+ in tetrahedral sites and Fe3+ in octahedral sites with anti-parallel arrangement of magnetic moments of super paramagnetic materials. Hence, the magnetization behavior depends on the crystallite size and cation distribution. The most common characteristic of fumets is their super-paramagnetic behavior and the decrease of the saturation magnetization in comparison to the related bulk material (55 emu g−1). The average crystallite size was 12 nm and the magnetic saturation (Ms) 41.70 emu g−1 [11]. For the present system, the values of remanence magnetization (Mr) decreases from 17 to 10 emu g−1 with increasing Zn2+ substitution The remnant ratio (R) = Mr/Ms is a characteristic parameter of a material. Higher and lower remnant ratios denote different material responses and accordingly determine where the material will be applied. It is desirable to have higher remnant ratios for magnetic recording and memory devices [12]. The magnetic moment (μB) per atom in Bohr magneton for each sample is calculated using the following equation and represented in Table 5.

Fig. 5
figure 5

VSM images of Fe3O4 and Zn0.03 MFe2O4 nanoparticles at room temperature

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Table 5 VSM analysis of Fe3O4 and Zn0.03 MFe2O4
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$$\upmu {\text{B}}={\text{M}} \times \left[ {{{\text{M}}_{\text{S}}}/5585} \right]$$

where M is molecular weight of ZnFe2O4 [13].

4 Conclusions

In summary, a simple and economical che mical co precipitation method to synthesize ZnFe2O4 nanoparticles has been developed. The prepared ferrite samples were characterized by XRD, FT-IR, TEM with EDAX and VSM analyses. The XRD patterns confirmed that the particle size of the powders annealed at 600 °C was found to be ∼ 11.14 nm. This crystallite size was also correlated with the particle size observed from TEM images. It was found that a zinc ferrite nanoparticle has the normal spinel structure with Zn2+ and Fe3+ cations distributed between tetrahedral and octahedral sites. The TEM photographs showed that the particles having spherical shape. The SAED patterns indicated that the zinc substituted ferrites were in well –crystalline nature. The strong peaks related to Fe, Zn and O were identified by the EDAX spectra. The value of coercivity 210.03 G and saturation magnetization 23.451E−3 emu were obtained at room temperature. The better crystallinity can improve the saturation magnetization will opens up ZnFe2O4 nanoparticles have utilized for various promising applications.