Gamma irradiated nanostructured NiFe₂O₄: Effect of γ-photon on morphological, structural, optical, and magnetic properties

Magnetic nanomaterials have attracted massive attention in generating and emerging innovative technology for real world applications^1,2 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 O_h sites) and Fe³⁺ ions on both of the tetrahedral A (denoted as T_d site and O_h 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 T_d site.^21,24,25

A general formula for a nickel ferrite structure is (Ni_1−xFe_x) [Ni_xFe_2−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⁺ (O_h sites) cations alone (∼2 µB), while Fe³⁺ moments (∼5 µB) in a high spin state for both O_h and T_d 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 (H_c), remanence magnetization (M_r), and anisotropy field (H_k) 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

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 (M_s), remnant magnetization (M_r), and coercive field (H_c). 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.

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 (Fd$3̄$m) and hematite phase (R$3̄$c) 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 (Fm$3̄$m) 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

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 (R_wp), and expected R-factor (R_exp), whereas R_wp compares the adjusted data with the experimental data and R_exp 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 R_wp, R_exp, 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 

However, the crystallite size of the samples is calculated from the linewidth of the (311) peak using the following Debye–Scherrer (D–S) equation:⁵⁹ $DD−S=kλβcosθ$⁠, where “D_D–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 D_D–S and D_ave. 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 is⁶⁰ $βcosθ=4εWHsinθ+KλDWH$⁠, where $KλDWH$ is an intercept in the W–H plot that corresponds to the crystallite size (D_W–H) and slope ε_W–H corresponds to the strain as shown in Fig. 5. It can be seen that D_W–H lies between 71.23 and 83.23 nm, which emulates the D_D–S and D_ave 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:

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 

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

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

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.

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

where r(Ni²⁺)and r(Fe³⁺) are ionic radii of Ni²⁺ (0.69 Å) and Fe³⁺ (0.645 Å), respectively, while C_AFe′ are the concentrations of Fe³⁺ ions on A sites and C_BNi and C_BFe are the concentrations of Ni²⁺ and Fe³⁺ ions on B sites. Using these formulas, the ionic radius of the A-site (r_A) and B-site (r_B) 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 r_A and r_B,

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.

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-L_A and octahedral sites-L_B) is another important structural parameter, which is estimated by the following equations:⁶⁷ $LA=14a3$ and $LB=14a2$⁠, where “a” is the lattice parameter of the NiFe₂O₄ structure. The calculated values of the L_A and L_B are presented in Table II. It is seen (Table II) that the hopping lengths in L_A and L_B 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

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-R_A and octahedral sites-R_B are calculated using the following relations:⁶⁸ 

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 R_A is increased while decreasing R_B as shown in Table II.

The change of R_A and R_B 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.

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

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

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 literature^77–79 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 

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 (M_s), remanence (M_r), and coercivity (H_c), are summarized in Table IV. The saturation magnetization (M_s) 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 

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 μ_B and 2 μ_B, the net magnetic moment is 2 μ_B 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 M_s 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 M_s 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 M_S 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.