This article highlights the preparation of NiFe2O4 nanoparticles by adopting a sol–gel auto-combustion route. The prime focus of this study is to investigate the impact of γ-irradiation on the microstructural, morphological, functional, optical, and magnetic characteristics. The resulting NiFe2O4 products have been characterized employing numerous instrumental techniques such as Field Emission Scanning Electron Microscopy (FESEM), X-ray Diffraction (XRD), UV–visible spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), and Physical Properties Measurement System for a variety of γ-ray doses (0, 25, and 100 kGy). FESEM micrographs illustrate the aggregation of ferrite nanoparticles in the pristine NiFe2O4 product having an average particle size of 168 nm, and the surface morphology is altered after exposure to γ-irradiation. XRD spectra have been analyzed employing the Rietveld method, and the results of the XRD investigation reveal the desired phases (cubic spinel phases) of NiFe2O4 with the observation of other transitional phases. The values of the crystallite size are in the range of 34.04–34.71 nm investigated by the Debye–Scherrer (D–S) method. Several microstructural parameters, such as bond length, bond angle, and hopping length, have been determined from the analysis of the Rietveld method. This study reports that γ-irradiations demonstrate a great influence on optical bandgap energy, and it varies from 1.80 to 1.89 eV evaluated via the Kubelka–Munk function. The FTIR measurement depicts a proof for the persistence of Ni–O and Fe–O stretching vibrations within the respective products at positions 365 and 547 cm−1, respectively, thus indicating the successful development of NiFe2O4. The saturation magnetization (MS) of the pristine Ni ferrite product is noticed to be 28.08 emu/g. A considerable increase in MS is observed in the case of low γ-dose (25 kGy), and a decrement nature is disclosed after the result of high dose of γ-irradiation (100 kGy).

Magnetic nanomaterials have attracted massive attention in generating and emerging innovative technology for real world applications1,2 due to their unique characteristics such as simple controllability, high saturation magnetization, biocompatibility, biological adaptability, low toxicity, chemical stability, and excellent physicochemical and magnetic properties.3–6 In recent years, nanocrystalline spinel ferrites have been investigated immensely owing to their potential applications in chemical sensors, microwave absorbers, permanent magnets, high density recording systems, ferrofluid technology, biomedical, imaging, and high-frequency device applications.7–13 Importantly, they possess excellent electrical and magnetic properties, which are extremely sensitive to many factors such as chemical composition, synthesis method, annealing temperature or temperature treatment, and cation distribution at tetrahedral (A) and octahedral [B] sites.9 Among the various spinel ferrites, however, NiFe2O4 (nickel ferrite) has drawn optimum attention recently due to its various applications in gas sensors,14 spintronics,15 microwave absorption,16 catalysts,17 lithium-ion batteries,18 hydrogen production,19 even in biomedicine,20 etc. As more and more considerations have been devoted keenly to the nano-sized magnetic materials for their inherent unique properties compared to their bulk counterparts, the scientific engrossment on nano-sized NiFe2O4 is expanding in the research community. In this direction, the magnetism of NiFe2O4 is predominantly intriguing due to its substantial saturation magnetization and unique magnetic structures. In general, NiFe2O4 belongs to an inverse spinel structure with Ni2+ ions on octahedral B sites (denoted as Oh sites) and Fe3+ ions on both of the tetrahedral A (denoted as Td site and Oh sites) sites equally.21,22 This is typically maintained by the formation energy in favor of the reverse spinel rather than spinel structure.23 It is also found to have the mixed spinel structure with the inverse one, i.e., some Ni2+ ions may occupy the Td site.21,24,25

A general formula for a nickel ferrite structure is (Ni1−xFex) [NixFe2−x]O4, where x is the degree of inversion. According to the crystal field theory (CFT), magnetic moments arise from the local moments of the Ni2+ with 3d8 as well as Fe3+ with 3d5 electrons. Significantly, the net magnetization comes from the Ni2+ (Oh sites) cations alone (∼2 µB), while Fe3+ moments (∼5 µB) in a high spin state for both Oh and Td sites are antiparallel and cancel each other.21 However, the modification in physical properties (i.e., structural and magnetic properties) of nanoferrites can be justified by instigating radiation damage by means of swift heavy ions,26 laser beams,27 protons,28 and gamma radiation.29 Lately, several irradiation systems are implemented in the cutting-edge world because it is a striking tool for modifying the physical properties of nanoparticles in the research and development in commercial applications and industrial technologies such as aeronautical and satellite communication, pollution control, material development, security systems, and so on.26,30,31 Upon electromagnetic nature, penetrating power, very short wavelength, and all medium propagation properties, the gamma ray (γ) is considered as ionizing radiation.32 Therefore, the present inquiry deals with an interaction of γ with NiFe2O4, which can tune its physical properties since it is known to generate various types of controlled defects, such as point, cluster, and columnar defects in the materials.33 In addition, irradiation with γ rays contain the plausibility of dislocation of Fe3+ ions from tetrahedral A sites to Fe2+ ions at octahedral B sites.30 

Several research studies have focused on different preparation routes to synthesize NiFe2O4 nanoparticles, such as co-precipitation,34,35 sol–gel,36,37 spray pyrolysis,38 mechanical activation,39 hydrothermal method,40 and high energy ball milling.41 Concurrently, to improve the physical properties of nanoferrites using γ rays, various methods have been employed to synthesize ferrite nanoparticles. Recently, Raut30 synthesized ZnFe2O4 using the sol–gel auto-combustion technique and reported that saturation magnetization and magneton number increased by γ radiation doses of 50 and 100 kGy. Raut et al.42 also prepared CoFe2O4 by sol–gel auto-combustion with total radiation doses of 50 and 100 kGy and showed that the lattice parameters decrease with the decrease in coercivity (Hc), remanence magnetization (Mr), and anisotropy field (Hk) as a function of γ radiation doses. Based on the above survey, until now, there has been no study on the impact of γ radiations on NiFe2O4 nanoparticles synthesized by the sol–gel auto-combustion method. Therefore, the aim of this research work is to synthesize and investigate some of the physical parameters of NiFe2O4 before and after γ-irradiation. In order to acquire more detailed crystallographic parameters, such as crystallite size, strain, atomic structure, bond length, and bond angle, the Williamson–Hall (W–H) method and Rietveld refinement (RR) have been executed precisely.

In this experiment, nickel nitrate hexahydrate [Ni(NO3)2·6H2O], ferric nitrate nonahydrate [Fe(NO3)3·9H2O], citric acid (C6H8O7·H2O), and ammonium hydroxide (NH4OH) were used for the sample preparation. The samples were made by mixing the compositions [Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, C6H8O7·H2O, and NH4OH] together. The required materials were employed as received.

A sol–gel auto-combustion route has been employed to prepare the NiFe2O4 nanoparticles as a function of γ-irradiation for 0, 25, and 100 kGy doses involving the metal nitrates of the ingredient components as raw materials in the citric acid matrix. The synthesis route has been explained in Fig. 1:43,44 the metal nitrates and citric acid were dissolved in an appropriate amount to keep the molar ratio of metal ions and used citric acid to 1:1. A little quantity of ammonia was gradually dissolved into the starting solution to balance pH = 7 and stabilize the nitrate–citrate sol. The obtained precursor aqueous solution was stirred vigorously with a magnetic stirrer at 60 °C. After that, the sol was placed into a tray and heated gradually to 120 °C to change into an extremely viscous brown gel. The gel was slowly heated to 250 °C in order to attain dried gel, and after a few minutes, the gel started to completely burn to form a crisp powder. The probable chemical reaction during the synthesis of NiFe2O4 products is given as

NiNO32+2FeNO33+209C6H8O7=NiFe2O4+403CO2+809H2O+4N2.
FIG. 1.

Flowchart of the synthesis of NiFe2O4 nanoparticles.

FIG. 1.

Flowchart of the synthesis of NiFe2O4 nanoparticles.

Close modal

The synthesized NiFe2O4 products have been subjected to γ-irradiation originated from a 60Co source with various doses (25 and 100 kGy) at the Institute of Food and Radiation Biology (IFRB), Atomic Energy Research Establishment, Bangladesh Atomic Energy Commission, Dhaka, Bangladesh. The activity of the 60Co source at the time of exposure is 12.17 kGy/h and liquid phase dosimetry arrangement (cericcerous) has been employed to enumerate the γ-dose rate.

The morphological investigation of the products has been performed by Field Emission Scanning Electron Microscopy (FESEM) (JEOL JSM-7600F, USA). X-ray Diffraction (XRD) (model X’PertPRO XRD Philips PW3040, Netherlands) has been applied on the as-prepared NiFe2O4 and γ-irradiated NiFe2O4 to identify the phases and crystallinity. The XRD structure is equipped with Cu-Kα radiation (λ = 1.5404 Å) maintained in the range of 2θ from 15° to 90°. XRD spectra have been investigated through the Rietveld technique as implemented in FullProf software. To examine the functional groups associated with ferrite samples, Fourier Transform Infrared Spectroscopy (FTIR) equipment has been employed during this experiment. A Physical Properties Measurement System (PPMS), Quantum Design Dyna Cool, under ambient conditions has been used to measure the magnetic hysteresis loop on the obtained products as well as different magnetic parameters such as saturation magnetization (Ms), remnant magnetization (Mr), and coercive field (Hc). Diffuse Reflectance Spectrophotometer (DRS) information has been recorded by a UV–vis device (Model: PerkinElmer UV–Vis–NIR Spectrometer Lambda 1050) at a wavelength of 200–800 nm.

Figure 2 reveals the FESEM images of pristine and γ-irradiated NiFe2O4 with comparatively low (25 kGy) and high (100 kGy) doses of γ radiations. It is clearly seen in the FESEM images that the surface morphology of NiFe2O4 is altered with the total dose of γ radiation. The same outcomes have been detected in the previous literature.45–47 According to the FESEM images [Figs. 2(a) and 2(d)], the highest average particle size is found to be 168 nm for the pristine sample. It is seen that as a result of employing a low γ dose, the surface disrupts and forms smaller particles due to the lattice vibration, atomic displacement, and local heating effects.48 The disintegration of particles can also be attributed to induced compressive stress, which causes intrinsic defect recombination or reordering of initially disordered phase and aids the particles to regain their shapes.49,50 In addition, at a low dose, the γ photon has a higher possibility of interaction with the materials and then photoelectric absorption dominates, resulting in a decrease in the average particle size of NiFe2O4 from 168 to 135 nm.51 Conversely, in the case of high dose (100 kGy), the average particle size is slightly increased from 135 to 140 nm as shown in Figs. 2(c) and 2(f). The swelling of the average particle size may occur due to the decrease in photoelectric absorption.52 It is well known that particles amalgamate at higher γ dose owing to no electron transfer, thereby making a sure increase in the average particle size.53 A hypothetical growth mechanism of NiFe2O4 particles in pristine and γ-irradiated samples with different γ doses is depicted in Fig. 3.

FIG. 2.

The FESEM images and particle size distribution of NiFe2O4 samples: (a) and (d) for 0 kGy, (b) and (e) for 25 kGy, and (c) and (f) for 100 kGy doses.

FIG. 2.

The FESEM images and particle size distribution of NiFe2O4 samples: (a) and (d) for 0 kGy, (b) and (e) for 25 kGy, and (c) and (f) for 100 kGy doses.

Close modal
FIG. 3.

A plausible growth mechanism of pristine and γ-irradiated NiFe2O4 samples.

FIG. 3.

A plausible growth mechanism of pristine and γ-irradiated NiFe2O4 samples.

Close modal

Figure 4 depicts that the Rietveld refinement (RR) of the structure by XRD data is processed using FullProf suite software. By a meticulous RR analysis, a dual common inverse spinel cubic structure (Fd3̄m) and hematite phase (R3̄c) is firmly confirmed for the pristine and irradiated samples. It can be evidently indexed to the face centered cubic (fcc) structure of NiFe2O4 (JCPDS Card No. 10-0325)54 as shown in Fig. 4. On the other hand, a nickel phase (Fm3̄m) has appeared only for the irradiated samples. From Fig. 4, it can be seen that the most prominent intense peak is manifested at 2θ = 35.74° corresponding to the (311) plane of NiFe2O4 for the pristine sample. After that, the place of (311) plane orientation has been changed from 2θ = 35.75° to 35.79° by applying gamma radiation. This may be attributed to the formation of Fe3+ ions in place of Fe2+ ions. It is also noticeable that the intensity of the (311) peak increases, while (400) peak intensity decreases after applying 25 kGy gamma radiation. Conversely, owing to 100 kGy gamma radiation, the intensity of the (400) peak rises sharply when the (311) peak falls drastically. Such results show that the shift of the peak intensity is in good agreement with that reported by several authors, and they ascribed that to the lattice distortion arisen after irradiation.55,56

FIG. 4.

The Rietveld refinement of the XRD pattern with different γ irradiations and a face centered cubic (fcc) structure of NiFe2O4.

FIG. 4.

The Rietveld refinement of the XRD pattern with different γ irradiations and a face centered cubic (fcc) structure of NiFe2O4.

Close modal

To know the lattice parameters, we have done the RR analysis using FullProf software. The quality of the RR is determined by means of a set of conventional statistical parameters. Typically, the quality of the RR can be determined using several statistical parameters, such as goodness of fit (χ2), weight profile R-factor (Rwp), and expected R-factor (Rexp), whereas Rwp compares the adjusted data with the experimental data and Rexp estimates the quality of the experimental data and χ2=Rwp/Rexp2.57 During the RR process, χ2 initiates with a maximum value when the goodness of fit model is poor and decreases as the fit data match the experimental data.

The RR process continues until convergence is reached with values less than 2.0 or close to 1.0, which designates a correlation between the experimental data and adjustment model used. The values of Rwp, Rexp, and χ2 are presented in Table I. In addition, the estimated Rietveld refined lattice parameter and volume values are also tabulated in Table I. It can be seen (Table I) that both the lattice parameter and volume decrease simultaneously with the increasing gamma radiations, which signifies that the atomic position is displaced significantly. The decrease in the lattice parameter and volume due to the lattice vacancies produced after γ radiation dose results in distortion and deviation from the spinel cubic structure. In addition, γ radiation typically formed the compressive strain and then generated some disorder into the NiFe2O4 lattice structure.58 

TABLE I.

Several structural parameters of pristine and γ-irradiated samples.

Parameters0 (kGy)25 (kGy)100 (kGy)
Lattice constants (Å) NiFe2O4 (a = b = c8.330 4 8.326 8 8.318 9 
Fe2O3 (a = b, c5.403 3, 5.272 4 5.402 2, 5.304 5 5.030 9, 13.799 8 
Ni (a = b = c⋯ 3.598 0 3.312 9 
Volume (Å3NiFe2O4 578.101 1 577.343 7 575.706 2 
Fe2O3 133.312 5 134.062 5 302.477 5 
Ni ⋯ 46.578 2 36.361 4 
Crystallite size (nm) DD–S 34.04 34.71 34.12 
Dave. 24.55 51.16 42.61 
DW–H 71.23 83.23 77.78 
DRR 70.12 77.78 75.01 
Rietveld refinement Rwp (%) 8.10 8.04 8.07 
Rexp (%) 6.95 7.04 6.98 
G.O.F (χ1.69 1.88 1.56 
Oxygen position (u) 0.229 20 0.253 56 0.397 45 
  Fe–O 1.925 52 2.111 76 3.291 51 
  Fe–Fe 2.945 26 2.943 97 2.941 20 
  Ni1–O 2.103 04 1.751 46 ⋯ 
Bond length (Å) NiFe2O4 Ni2–O 1.504 15 1.854 15 ⋯ 
 Ni1–Ni2 4.165 22 4.163 40 4.159 53 
 Ni1–Fe 3.453 62 3.452 11 3.448 82 
 Ni2–Fe 1.803 59 1.802 81 1.801 10 
Fe2O3 Fe–O 1.470 9, 1,508 8, 1.527 0, 2.258 2 1.801 6, 2.009 8 1.815 4, 2.187 1 
 Fe–Fe 1.527 0 2.137 2 3.406 8, 2.950 7 
Ni2 Ni–Ni ⋯ 2.544 2, 3.598 1 2.342 6, 3.312 9 
 O–Fe–O 79.28, 100.72, 100.75, 100.73 88.40, 91.60 9.20 
Bond angle (deg.) NiFe2O4 O–Ni1–O 109.45, 109.47, 109. 48 109.471 2 109.471 2 
 O–Ni2–O 109.51, 109.48, 109.45 109.471 2 ⋯ 
 Fe–O–Ni1 117.96 126.40 116.67 
 Fe–O–Ni2 62.03 53.60 O–Fe–Ni2 = 98.60 
Fe2O3 Fe–O–Fe 61.641 7 67.958 8 71.498 6 
 O–Fe–O 85.978 3 78.761 0 85.465 3 
Parameters0 (kGy)25 (kGy)100 (kGy)
Lattice constants (Å) NiFe2O4 (a = b = c8.330 4 8.326 8 8.318 9 
Fe2O3 (a = b, c5.403 3, 5.272 4 5.402 2, 5.304 5 5.030 9, 13.799 8 
Ni (a = b = c⋯ 3.598 0 3.312 9 
Volume (Å3NiFe2O4 578.101 1 577.343 7 575.706 2 
Fe2O3 133.312 5 134.062 5 302.477 5 
Ni ⋯ 46.578 2 36.361 4 
Crystallite size (nm) DD–S 34.04 34.71 34.12 
Dave. 24.55 51.16 42.61 
DW–H 71.23 83.23 77.78 
DRR 70.12 77.78 75.01 
Rietveld refinement Rwp (%) 8.10 8.04 8.07 
Rexp (%) 6.95 7.04 6.98 
G.O.F (χ1.69 1.88 1.56 
Oxygen position (u) 0.229 20 0.253 56 0.397 45 
  Fe–O 1.925 52 2.111 76 3.291 51 
  Fe–Fe 2.945 26 2.943 97 2.941 20 
  Ni1–O 2.103 04 1.751 46 ⋯ 
Bond length (Å) NiFe2O4 Ni2–O 1.504 15 1.854 15 ⋯ 
 Ni1–Ni2 4.165 22 4.163 40 4.159 53 
 Ni1–Fe 3.453 62 3.452 11 3.448 82 
 Ni2–Fe 1.803 59 1.802 81 1.801 10 
Fe2O3 Fe–O 1.470 9, 1,508 8, 1.527 0, 2.258 2 1.801 6, 2.009 8 1.815 4, 2.187 1 
 Fe–Fe 1.527 0 2.137 2 3.406 8, 2.950 7 
Ni2 Ni–Ni ⋯ 2.544 2, 3.598 1 2.342 6, 3.312 9 
 O–Fe–O 79.28, 100.72, 100.75, 100.73 88.40, 91.60 9.20 
Bond angle (deg.) NiFe2O4 O–Ni1–O 109.45, 109.47, 109. 48 109.471 2 109.471 2 
 O–Ni2–O 109.51, 109.48, 109.45 109.471 2 ⋯ 
 Fe–O–Ni1 117.96 126.40 116.67 
 Fe–O–Ni2 62.03 53.60 O–Fe–Ni2 = 98.60 
Fe2O3 Fe–O–Fe 61.641 7 67.958 8 71.498 6 
 O–Fe–O 85.978 3 78.761 0 85.465 3 

However, the crystallite size of the samples is calculated from the linewidth of the (311) peak using the following Debye–Scherrer (D–S) equation:59DDS=kλβcosθ, where “DD–S” is the crystallite size, “λ” is the wavelength of CuKα radiation (1.5404 Å), “β” is the full width at half maximum (FWHM), and “θ” is the diffraction angle of the strongest characteristic peak.

After that, we have also estimated the average crystallite size (Dave. using the Lorentz function by Origin pro-2018. From Table I, it is clearly evident that in both cases, the DD–S and Dave. increase drastically at 0 and 25 kGy samples and decrease for the 100 kGy sample. To get more accurate crystallographic parameters such as crystallite size, strain, atomic structure, bond length, and bond angle, the Williamson–Hall (W–H) method and Rietveld refinement (RR) are performed precisely. Initially, the W–H method gives the values of the crystallite size and strain concurrently.

The equation of the W–H method is60βcosθ=4εWHsinθ+KλDWH, where KλDWH is an intercept in the W–H plot that corresponds to the crystallite size (DW–H) and slope εW–H corresponds to the strain as shown in Fig. 5. It can be seen that DW–H lies between 71.23 and 83.23 nm, which emulates the DD–S and Dave trend. Additionally, the Rietveld refined (RR) crystallite size is found to ranging from 70.12 to 77.78 nm. It is observed that the crystallite size increases after 25 kGy γ radiation doses, which is ascribed to the following: γ radiation interacts with the particles or grains and ionizes Fe3+ ions to Fe2+ ions61 as follows:

Fe3++γFe2++e.
(1)

However, the crystallite size decreases at a 100 kGy γ radiation dose due to the modification of the ratio of Fe2+/Fe3+ ions in the octahedral B site that can be expressed by the following equation:62 

Fe2++γFe3++e.
(2)

The dislocation density (δ) values have been evaluated using Eq. (3) having the values of the crystallite size,63,64

δ=1D2.
(3)

The following relation has been employed to determine the lattice strain (ε) of the synthesized nickel ferrites:63 

ε=βcosθ4.
(4)

The values of δ are noticed to be 8.63 × 1014, 8.30 × 1014, and 8.60 × 1014 (lines/m2) for 0, 25, and 100 kGy γ-radiation doses, respectively. The strain values are 0.001 018, 0.000 998, and 0.001 016 belonging to 0, 25, and 100 kGy γ-radiation doses, respectively. Both the values of δ and ε follow the opposite fashion of D evaluated using the Debye–Scherrer (D–S) formula.

FIG. 5.

Williamson–Hall plot for NiFe2O4 with different γ irradiations. A comparison study of the crystallite size among various methods.

FIG. 5.

Williamson–Hall plot for NiFe2O4 with different γ irradiations. A comparison study of the crystallite size among various methods.

Close modal

According to the cation distribution of NiFe2O4, the ionic radius of the A-site (rA) and B-site (rB) can be theoretically calculated using the following relations:65,66

rA=CAFerFe3+,
rB=12CBNirNi2++CBFerFe3+,

where r(Ni2+)and r(Fe3+) are ionic radii of Ni2+ (0.69 Å) and Fe3+ (0.645 Å), respectively, while CAFe′ are the concentrations of Fe3+ ions on A sites and CBNi and CBFe are the concentrations of Ni2+ and Fe3+ ions on B sites. Using these formulas, the ionic radius of the A-site (rA) and B-site (rB) was calculated. The proposed cation distribution for NiFe2O4 is Fe13+ANi12+Fe13+BO42. The theoretical lattice parameter of Ni ferrite is calculated for NiFe2O4 using rA and rB,

rA=CAFerFe3+=1×0.645=0.645,
rB=12CBNirNi2++CBFerFe3+,
rB=121×0.69+1×0.645=120.69+0.645=0.6675,
ath=833rA+R0+3rB+R0=8.3688Å.

Here, R0 is the ionic radii of oxygen (1.32 Å).

Through the R-R analysis, we have determined the bond length and bond angle to know the gamma radiation effect on the NiFe2O4 structure. First, in the case of NiFe2O4, the Fe–O and Ni2–O bond lengths increase abruptly, while Fe–Fe, Ni1–O, Ni1–Ni2, Ni1–Fe, and Ni2–Fe bond distances decrease after using gamma radiations as shown in Table I and Fig. 6.

FIG. 6.

Several Rietveld refined structures with denoting bond lengths and bond angles.

FIG. 6.

Several Rietveld refined structures with denoting bond lengths and bond angles.

Close modal

Interestingly, there is no bond length between Ni1–O and Ni2–O at the 100 kGy sample, which may be attributed to the development of the Fe–O bond distance. Furthermore, multiple bond distances of Fe–O, Fe–Fe, and Ni–Ni are detected for Fe2O3 and Ni structures as shown Table I. Likewise, in the event of a bond angle, we have found multiple bond angles in the NiFe2O4 and Fe2O4 structures. The O–Ni1–O and O–Ni2–O bond angles are almost analogous; on the contrary, Fe–O–Ni1 and Fe–O–Ni2 bond angles are completely different in the NiFe2O4 structure. What is more, an interesting point is that the Ni2 atom does not show the bond length and bond angle as well at the 100 kGy sample. It can be speculated that owing to the higher gamma radiation, the Ni2–O bond splits up and then forms a new bond only with the Fe atom as shown in Fig. 6.

However, the hopping length (distance between magnetic ions in tetrahedral site-LA and octahedral sites-LB) is another important structural parameter, which is estimated by the following equations:67LA=14a3 and LB=14a2, where “a” is the lattice parameter of the NiFe2O4 structure. The calculated values of the LA and LB are presented in Table II. It is seen (Table II) that the hopping lengths in LA and LB sites decrease concurrently with changing gamma radiations. However, other groups showed that in the case of ZnFe2O4 and CoFe2O4, hopping lengths increased after using gamma radiations as they have found greater values of lattice parameters.30,42

TABLE II.

The estimated hopping lengths and average bond length of NiFe2O4 for various γ irradiations.

Parameters0 (kGy)25 (kGy)100 (kGy)
Hopping length (Å) La 3.6072 3.6056 3.6022 
Lb 2.9445 2.9440 2.9412 
Average bond length (Å) Tetrahedral site (RA1.5035 1.8542 3.9257 
Octahedral sites (RB2.2691 2.0525 1.9331 
Parameters0 (kGy)25 (kGy)100 (kGy)
Hopping length (Å) La 3.6072 3.6056 3.6022 
Lb 2.9445 2.9440 2.9412 
Average bond length (Å) Tetrahedral site (RA1.5035 1.8542 3.9257 
Octahedral sites (RB2.2691 2.0525 1.9331 

In the present study, the lattice parameter, volume, and bond length decrease with altering gamma radiations, which firmly confirms that these structural parameters control the hopping lengths. Furthermore, the inter-ionic distances (average bond lengths) at tetrahedral-RA and octahedral sites-RB are calculated using the following relations:68 

RA=a3u14,
(5)
RB=a3u22u+116,
(6)

where “u” represents the Rietveld refined oxygen positional parameter whose values are 0.229 20, 0.253 56, and 0.397 45 for 0, 25, and 100 kGy gamma radiation samples, respectively. Our refined “u” values are analogous to previously reported data.69 However, other groups have taken the theoretical value of the oxygen position parameter (u = 0.381 Å). Patange et al. have chosen the “u” value for Al3+ substituted NiFe2O4 nanoparticles despite the Rietveld refinement analysis.70 It is observed that “u” is increased with rising gamma radiations, and RA is increased while decreasing RB as shown in Table II.

The change of RA and RB can be explained on the basis of the drastic movement of oxygen position parameters. Since the oxygen position is developed considerably with rising gamma radiations, the NiFe2O4 structure moves slowly close to the fcc structure and oxygen ions move toward the octahedral coordinate at the end. However, several studies investigated on spinel ferrite by gamma doses. Here, we have reported the effect of the gamma dose over several spinel ferrites in Table III.

TABLE III.

A comparison study for various spinel ferrites under γ irradiations.

SampleRietveld refinementNo. of phasesLattice constantDetermined bond length and bond angleDetermined RA and RBHopping lengthReferences
ZnFe2O4 No Single Increase No No Increase 30  
CoFe2O4 No Single Increase No No Increase 42  
NiFe2O4 Yes Three Decrease Yes Yes Decrease Present 
SampleRietveld refinementNo. of phasesLattice constantDetermined bond length and bond angleDetermined RA and RBHopping lengthReferences
ZnFe2O4 No Single Increase No No Increase 30  
CoFe2O4 No Single Increase No No Increase 42  
NiFe2O4 Yes Three Decrease Yes Yes Decrease Present 

The optical bandgap (Eg) of the γ pristine and γ-irradiated NiFe2O4 samples has been calculated from the Kubelka–Munk (K–M) function through the following equation:71 

FRα=(hϑEg)2hϑ,
(7)

where “R” is the reflectance, “α” is the absorption coefficient, “” is the incident light energy, and “Eg” is the optical bandgap. Figure 7 shows the Eg plots of the NiFe2O4 treated with different γ irradiation doses. Eg is calculated by plotting a graph between (FR×hϑ)2 and . In pristine NiFe2O4, the Eg is 1.85 eV. Subsequently, for 25 and 100 kGy irradiation doses, the Eg is found to be 1.80 and 1.89 eV, respectively. Thus, we observe that the Eg decreases when low doses of γ irradiation are used, while with high dose γ irradiation, the Eg is increased. The change in Eg can be expounded by the following ways: (i) quantum-size effect may be accountable for altering the Eg because it results from the strong interaction between the surface of NiFe2O4 and γ photons.72 (ii) Decrease of Eg may be ascribed to the creation of localized states into the NiFe2O4 structure due to structural defects.73 (iii) Enhancement of Eg due to the optical scattering at the particle or grain boundaries and intrinsic absorption.32 In addition, it is anticipated that V̈O is created within the NiFe2O4 structure during a higher dose of γ irradiation that will play a significant role in increasing the Eg.74 However, it is manifested that the γ irradiation significantly affects the crystallite size and, hence, alters the optical bandgap of NiFe2O4.

FIG. 7.

The Eg of NiFe2O4 with various γ irradiation doses and with a comparison of the Eg study.

FIG. 7.

The Eg of NiFe2O4 with various γ irradiation doses and with a comparison of the Eg study.

Close modal

Figure 8 displays the FTIR diagram of pristine NiFe2O4 and NiFe2O4 irradiated with γ-doses for analysis of chemical bonds within the resulting products. The prominent bands with sharp peaks have been observed at 365 and 547 cm−1, which are assigned to stretching vibrations of the Ni–O bond in octahedral complexes and tetrahedral Fe–O vibrations, respectively.75,76 This analysis of FTIR presents the consistency with the published results in numerous bodies of literature77–79 and ensures the construction of the spinel NiFe2O4 nanoparticles identifying characteristic peaks in the FTIR diagram. The FTIR studies clearly support the formation of Ni ferrite as observed in XRD analysis. The presence of band positions in the ranges of 1000–1300 and 2000–3000 cm−1 demonstrates the survival of O–H, C–O, and C=H stretching modes of organic compounds.80 Two peaks appear at 1381 and 3754 cm−1 in the case of the pristine and γ-irradiated products, which belong to stretching vibration of C–H and the contribution of N–H bonds, respectively.80 

FIG. 8.

FTIR spectra of pristine and γ-irradiated NiFe2O4.

FIG. 8.

FTIR spectra of pristine and γ-irradiated NiFe2O4.

Close modal

Figure 9 shows the room temperature magnetization (M) for the applied field (H) of the as-prepared NiFe2O4 nanoparticles before (0 kGy) and after irradiation (25 and 100 kGy). The applied field ranged from −20 to +20 kOe, and it is apparent that the magnetization is not saturated at 20 kOe. Low coercivity and low hysteresis indicate the ferrimagnetic behavior of the samples. The magnetic parameters, such as saturation magnetization (Ms), remanence (Mr), and coercivity (Hc), are summarized in Table IV. The saturation magnetization (Ms) for the pristine sample was found to be ∼28 emu/g, which is less than that for the bulk NiFe2O4. It can be attributed to the fact that NiFe2O4 becomes the mixed spinel from the inverse spinel structure at the nanoscale.81 

FIG. 9.

M–H curves of the pristine and γ-irradiated samples of NiFe2O4.

FIG. 9.

M–H curves of the pristine and γ-irradiated samples of NiFe2O4.

Close modal
TABLE IV.

Magnetic parameters of pristine and γ-irradiated samples of NiFe2O4.

Radiation (kGy)Ms(Ex) (emu/g)Remanence, Mr (emu/g)Coercivity, Hc (Oe)
28.08 4.87 0.0022 
25 40.63 8.44 0.0303 
100 20.00 4.87 0.0216 
Radiation (kGy)Ms(Ex) (emu/g)Remanence, Mr (emu/g)Coercivity, Hc (Oe)
28.08 4.87 0.0022 
25 40.63 8.44 0.0303 
100 20.00 4.87 0.0216 

The tetrahedral A-site consists of ferromagnetically ordered Fe3+ ions, and the octahedral B-site consists of Fe3+ and Ni2+ ions. The magnetic contribution in the inverse spinel arises from Ni2+ in the octahedral B-site. Due to antiferromagnetic ordering, the Fe3+ moments from both the A-site and B-site cancel each other. Considering that the magnetic moments of Fe3+ and Ni2+ ions are 5 μB and 2 μB, the net magnetic moment is 2 μB by Neel’s two sublattice model.82 Therefore, the proposed change in the cation distribution at the nanoscale can be interpreted as Fe13+ANi12+Fe13+BO42. However, the presence of Ni2+ might not be that dominant in the tetrahedral A-site for the pristine samples. In addition, the synthesis methods significantly impact the magnetic properties on spinel ferrites.83 

There is an increase in the Ms from 28 to 41 emu/g after the γ-irradiation of 25 kGy. After the γ-irradiation, the degree of the mixed-phase may have increased, and more Ni2+ has been transferred to the A-site. In addition, the irradiation releases the pinned domains on the surface, and the significant increase in magnetization was observed.84,85 Furthermore, the onset of crystallite size growth was observed after the irradiation. Crystallite size growth may accompany the migration of Fe3+ ions to the B-site.8,86 The decreasing trend in the lattice parameter also established the emergence of strong ionic interactions among the lattice sites. Therefore, the enhanced saturation magnetization was observed.

Consequently, it will be interesting to investigate the magnetic properties on exposing the samples to higher γ-radiation. However, a strong γ-irradiation of 100 kGy decreased the Ms drastically to 20 emu/g. The strong γ-irradiation causes the adverse effect on magnetic ordering by the ion-induced disorder. In addition, high energy ions may penetrate the sample. The host atoms and molecules interact with the irradiated gamma photons via inelastic collision.

The interaction causes energy loss and introduces defects or partial amorphization depending on the amount of energy lost.87 Hence, a decrease in crystallite size was observed. The reduction in crystallite size introduces spin canting, magnetic dead layers, and weakening of super-exchange interaction. Consequently, a substantial reduction in saturation magnetization has emerged.

Nanoparticle NiFe2O4 ferrites in the spinel cubic structure have been synthesized via the sol–gel auto-combustion route. We have investigated the structural, morphological, magnetic, and optical properties of the synthesized ferrite nanoparticles with the variation of γ-doses. FESEM micrographs clearly indicate the variation in morphology and aggregation nature within the bare and γ-irradiated Ni ferrite products. According to the XRD investigation of the conducted research, it is concluded that an irregular variation in the crystallite size results from the results of the Scherrer method. In addition, analysis of the W–H plot and RR exhibits the identical trend of variation upon incorporation of γ-irradiation into the NiFe2O4 crystal network. The crystallite size of the pristine Ni ferrite sample is observed as 24.55–71.23 nm. The result after γ-irradiation with 25 kGy shows the increment nature of the crystallite size and then decreases after exposure to high γ-dose (100 kGy). From the Rietveld analysis, numerous structural parameters, such as bond length, bond angle, and hopping length, are estimated in this study. The optical bandgap energy is estimated to be 1.85 eV in the pristine product, and consequently, in the case of low (25 kGy) and high (100 kGy) γ-doses, it corresponds to 1.80 and 1.89 eV, respectively. Two absorption bands located at 365 and 547 cm−1 have been detected from the FTIR measurement, which indicates the stretching vibrations at the octahedral site of Ni–O and tetrahedral Fe–O vibrations, respectively. This obtained observation confirms the formation of a spinel cubic phase in both pristine and γ-irradiated Ni ferrite nanoparticles supported by XRD results. The PPMS measurement reveals that the MS value for the pristine product is noticed as 28.08 emu/g. A considerable increase in saturation magnetization from 28.08 to 40.63 emu/g was observed in the low γ-irradiated compound (25 kGy), which may be due to the surface canting effect affected by the agglomeration nature of the ferrite particle and then decreasing nature after exposure to high γ-dose. The sol–gel fabrication of the NiFe2O4 nanoparticle along with the use of γ-radiation may improve the morphological, structural, and magnetic properties of NiFe2O4 NPs, which can make it technologically more applicable for gas sensing devices.

The authors declare no conflict of interest.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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