# Effects of Chromium(III) Ion Doping on Structural, Optical, and Magnetic Properties of Nickel–Cobalt Ferrite Nanoparticle

Authors: E. K. Senbeto; S. Elangovan
Journal: Journal of Nanomaterials
 (2023)

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## Abstract

Chromium(III)-doped nickel–ferrite nanoparticles, with the general formula Ni0.5Co0.5CrxFe2−xO4 (x = 0.0, 0.10, and 0.20), were prepared by the sol–gel method. The prepared samples were sintered at 700°C for 5 hr. The XRD result showed that the prepared samples are in single-phase partially inverse cubic spinel ferrite structures with space group Fd3m. From the XRD characterization, the average crystallite size for all samples decreased from 39.54 to 30.42 nm and the lattice parameters are found to be 0.8324, 0.8320, and 0.8314 nm for x = 0.0, 0.10, and 0.20, respectively. The energy dispersive X-ray (EDX) spectroscopy analysis confirmed the presence of all ions as per the stoichiometric ratios. The vibrating sample magnetometer measurements revealed that the saturation magnetization (Ms), remnant magnetization (Mr), and coercive fields are found to be decreased with increasing concentration of Cr3+ ions. The UV–vis spectroscopy analysis showed that the energy bandgap (Eg) increased with increasing the concentration of Cr3+ ion from 1.61 to 1.96 eV. The Fourier transform infrared spectra show the two main absorption bands at 407–424 cm−1 and 547–588 cm−1 corresponding to stretching vibrations of metal–oxygen in the octahedral and tetrahedral sites, respectively.


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## Body

## 1. Introduction

Ferrite nanoparticles have been exhaustively studied by many researchers using different preparation techniques due to their remarkable technological applications, such as for high-density magnetic storage devices, targeted drug delivery systems, magnetic resonance imaging enhancement, gas sensors, magnetocaloric refrigerators, medical diagnosis and microwave devices [1–7]. Ferrite nanoparticles with the general formula AB2O4 are cubic spinel structures, where A is a divalent cation and B is a trivalent cation, and oxygen atoms form cubic close packing [8]. The cubic unit cell of this ferrite is formed by 56 atoms; the 32 oxygen atoms are distributed in a cubic closed-packed structure and 24 cations occupy 8 of the 64 available tetrahedral sites and 16 of the 32 available octahedral sites. The cell contains eight formula units (Z = 8) having space group Fd3m, corresponding to Me82+AMe163+BO162−O [9]. The distribution of the cations over the tetrahedral and octahedral sites can greatly influence the physical properties of the material’s structural, electrical, optical, and magnetic properties [10].To attain the materials with desired properties, it is important to get the high-density powder with small and homogenous grain size, it can be attained using wet chemical synthesis techniques, such as the sol–gel method, coprecipitation method, and hydrothermal method [11–13]. These techniques ensure the aggregation of the primary ions that change their properties according to the aging process or a type of annealing.Among spinel ferrite cubic structures, cobalt ferrite (CoFe2O4), hard magnetic material, but nickel ferrite (NiFe2O4) is a soft magnetic material, and both ferrites are in inverse spinel structure because both Co and Ni atoms are predominantly preferred in the octahedral site [14]. But the two Fe3+ cations are equally distributed over octahedral and tetrahedral sites. The nickel–cobalt ferrite nanoparticle has been synthesized by different researchers using different methods.Maaz et al. [15] synthesized NixCo1−xFe2O40.0≤x≤1 Np by coprecipitation method for magnetic analysis. Balideh et al. [16] reported various properties of pure and Dy-doped nickel–cobalt ferrite (Ni0.3Co0.7Dyx Fe2−xO4, x = 0.0, 0.01, 0.02, …, 0.10) Np synthesized using hydrothermal method. Shobana and Choe [17] prepared chromium-doped nickel ferrite nanoparticles using the sol–gel technique for the investigation of structural and electrical properties. Lin et al. [18] reported the structural and magnetic properties of chromium-doped nickel ferrite using the sol–gel autocombustion method.In general, chromium is one of the nonmagnetic ions which contribute to the production of soft magnetic materials leading to reduce magnetic energy losses. The present work is an attempt to study the effect of chromium ion-doped to Co0.5Ni0.5Fe2O4 nanoparticles on structural, optical, and magnetic properties prepared using the sol–gel method.


## 2. Experimental

In this work, the stoichiometry ratio of Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, and Cr(NO3)3·9H2O was used as starting materials or precursors and all are AR grade with purity greater than 99%. Initially, 2 mol of Fe(NO3)3·9H2O and 1 mol of cobalt nitrate and nickel nitrate were dissolved in a beaker containing the amount of doubly deionized water. An equal mole of citric acid with the total mole of metal nitrates was added to the solution as fuel. The mixed solution was stirred with a magnetic stirrer at high speed without a hot plate for more homogenous. In another beaker, chromium nitrate with an appropriate stoichiometry chemical ratio was dissolved in ionized water and stirred using a magnetic stirrer at high speed. Then after, the two solutions were added to one beaker and placed on a hot plate, and stirred well using magnetic stirring from 70 to 100°C. While the magnetic stirrer was on, 25% ammonia solution was added dropwise to the chemical solution to maintain the pH value of 7. As time has gone, the heated solution was changed to a brown viscous networked structure, “gel.” The gel was poured into an evaporating dish and loaded in a hot oven at 180°C to evaporate the water and a fluffy smolder ash was formed. The ignited burnt ash was ground using agate mortar with a pestle several times finely. The ground ash was held in an alumina crucible and loaded in a temperature programmer electric furnace at 650°C for 3 hr to remove the impurities and organic substances. After the temperature of the furnace was dropped to room temperature, the calcined powder was ground again several times thoroughly using agate mortar.The ground calcined powder was divided into two parts: one part of the powder was held in an alumina crucible and loaded in an electric furnace at 700°C for 5 hr for more densification, and with the other part of the powder, a small amount of polyvinyl alcohol (PVA) was mixed as a binder and pressed into a disk-shaped pellet having a thickness of 1.2 mm and a diameter of 13 mm using a hydraulic press of 12.5 tons. The disk-shaped pellet was held on an alumina plate and loaded in an electric furnace at 750°C for 5 hr. The sintered powder was transported for structural and morphological characterization using an X-ray diffractometer (XRD) and scanning electron microscope (SEM), respectively. The elemental composition was analyzed using energy dispersive X-ray (EDX) spectroscopy attached to SEM. The sintered pellet was polished well and transported for magnetic measurement using a vibrating sample magnetometer (VSM). The optical parameters of the sintered powder dissolved in an organic solvent were measured using UV–vis spectroscopy.


### 2.1. Characterization Techniques

The structural and phase analysis of the prepared sample powder (Ni0.5Co0.5CrxFe2−xO4, x = 0.0, 0.10, and 0.20) was characterized by X-ray diffraction ∗XRDML (XRD; X’pert3 PRO) using Cu Kα radiation having a wavelength, λ=1.542A˙ at room temperature. The X-ray reflection was collected at the detector over a range of Bragg’s angle 2θ = 10°–70° with a step size of 0.02° around a goniometric radius of 240 mm (setting generator 40 kV, 30 mA). The surface morphology of the prepared sample was analyzed using a scanning electron microscope (JSM-7610F), which uses the focused beam of an energetic electron to generate signals from the sample. These signals give information about the sample’s external morphology by magnifying the displayed patterns ranging from 500x to 5,000 kx. The elemental composition of the sample can be analyzed using an energy-dispersive X-ray spectroscope (EDX or EDS) attached to the SEM. This combined machine with SEM provides sufficient energy to eject a core electron, creating an electron hole. The displayed peaks from the electron’s excitation can be correlated to the elemental composition of the sample.The magnetic measurement was carried out using LAKESHORE VSM-7410 VSM. The optical property of the prepared sample was analyzed using a UV–vis (Shimadzu UV-1800) spectrometer in radiating light on the sample in the holder (curette), which was dissolved in methanol, in the wavelength range from 225 to 1,000 nm, since methanol does not show any absorbance in this wavelength region. The measurement was carried out in terms of absorbance in the wavelength range corresponding to the transmitted intensity of light. The Fourier transform infrared (FTIR) spectroscopy measurement was carried out at room temperature using a JASCO FTIR spectrometer (Model 6800) in a wave number range from 4,000 to 400 cm−1 by irradiating infrared light in the finely ground KBr pellet sample.


## 2.1. Characterization Techniques

The structural and phase analysis of the prepared sample powder (Ni0.5Co0.5CrxFe2−xO4, x = 0.0, 0.10, and 0.20) was characterized by X-ray diffraction ∗XRDML (XRD; X’pert3 PRO) using Cu Kα radiation having a wavelength, λ=1.542A˙ at room temperature. The X-ray reflection was collected at the detector over a range of Bragg’s angle 2θ = 10°–70° with a step size of 0.02° around a goniometric radius of 240 mm (setting generator 40 kV, 30 mA). The surface morphology of the prepared sample was analyzed using a scanning electron microscope (JSM-7610F), which uses the focused beam of an energetic electron to generate signals from the sample. These signals give information about the sample’s external morphology by magnifying the displayed patterns ranging from 500x to 5,000 kx. The elemental composition of the sample can be analyzed using an energy-dispersive X-ray spectroscope (EDX or EDS) attached to the SEM. This combined machine with SEM provides sufficient energy to eject a core electron, creating an electron hole. The displayed peaks from the electron’s excitation can be correlated to the elemental composition of the sample.The magnetic measurement was carried out using LAKESHORE VSM-7410 VSM. The optical property of the prepared sample was analyzed using a UV–vis (Shimadzu UV-1800) spectrometer in radiating light on the sample in the holder (curette), which was dissolved in methanol, in the wavelength range from 225 to 1,000 nm, since methanol does not show any absorbance in this wavelength region. The measurement was carried out in terms of absorbance in the wavelength range corresponding to the transmitted intensity of light. The Fourier transform infrared (FTIR) spectroscopy measurement was carried out at room temperature using a JASCO FTIR spectrometer (Model 6800) in a wave number range from 4,000 to 400 cm−1 by irradiating infrared light in the finely ground KBr pellet sample.


## 3. Results and Discussion

### 3.1. Structural Analysis

Figure1(a)–1(c) shows the XRD result of the prepared sample (Ni0.5Co0.5CrxFe2−xO4, x = 0.0, 0.10, and 0.20) carried out at room temperature. The refined Rietveld Full Proff software XRD diffraction patterns for all samples confirmed that all the prepared samples are in single-phase cubic spinel structure with space group Fd3m which fits the CIF number 1,533,572 [19]. The XRD patterns are well indexed at the plane of reflections (111), (220), (311), (222), (400), (511), and (440) around the corresponding Bragg’s angles 2θ = 19°, 29°, 35.76°, 37.25°, 43°, 56°, and 62°, respectively. The partial substitution of the small amount of Cr3+ ion at the site of Fe3+ ion created no significant phase change. The XRD results showed the broadened peaks with Cr3+ content shifted to a higher angle. The crystallite size, lattice parameters, X-ray density, tolerance factors, and volume of the unit cell of the prepared samples corresponding to the intense peaks (311) are determined from XRD data and calculated using Equations (1)–(5). The values of crystallite size (D), lattice parameters, X-ray density, and tolerance factor are listed in Table 1.Figure 1
(a–c) The XRD pattern of Ni0.5Co0.5CrxFe2−xO4 sample for (a) x = 0.0, (b) x = 0.10, (c) x = 0.20, and (d) the XRD merged intense peak corresponding to (311) plane of reflection.
(a)(b)(c)(d)Table 1
Values of crystallite size (D), lattice parameters, X-ray density, and tolerance factor.
SampleTolerance factor,tCrystallite size,D (nm)Lattice parameters,a = b = c (nm)Volume of a unit cell (nm3)X-ray density (g/cm3)Ni0.5Co0.5Fe2O40.849939.540.83240.57675.322Ni0.5Co0.5Cr0.1Fe1.9O40.846237.570.83200.57595.461Ni0.5Co0.5Cr0.2Fe1.8O40.842230.420.83140.57455.603The crystallite size of the sample can be estimated from Scherrer formula [20, 21]:(1)D=kλβcosθ,where λ is the wavelength of the X-ray radiation, k is Scherrer constant ≈0.9, β is the full width at half maximum in radian, and θ is the Bragg’s angle at which the peaks are observed.For the cubic crystal structure, the lattice constant (a), the volume of the unit cell (V), and the X-ray density can be calculated from the X-ray spectra using the following relations [22]:(2)a=dhkl2h2+k2+l21/2,(3)V=a3,(4)ρX-ray=8MNaV,where M is the molecular mass of the compound and Na is the Avogadro number.The tolerance factor for ferrites of spinel structure can be described to predict the crystal phase stability [23]:(5)t=2RA+RO3RB+RO,where RA, RB, and RO are the average ionic radii of A-site, B-site, and oxygen, respectively.In the zoomed intense peaks in Figure1(d), the incorporation of Cr3+ in the host matrix resulted in the reduction of crystallite size of the sample which is explained the basis of the ionic radii differences of Cr3+ (0.0615 nm) and Fe3+ (0.065 nm) [24–26].Table1 shows that the lattice parameter, volume of the unit cell, and tolerance factor are decreased with increasing chromium ion, which is attributed to the ionic radii difference. This trend indicated that the prepared samples are more strained [27].


### 3.2. Morphological Studies

Figure2 shows the SEM micrographs of Ni0.5Co0.5CrxFe2−xO4 (x = 0.0, 0.10, 0.20) samples characterized at room temperature. The SEM micrographs revealed that the prepared samples are well-defined crystals on a nanometer scale with well-distributed and agglomerated in semispherical shape. These results are similar to the result obtained by Song and Liu [28] and Chandramouli et al. [29].Figure 2
SEM micrograph of Ni0.5Co0.5CrxFe2−xO4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.
(a)(b)(c)


### 3.3. EDX Analysis

Figure3(a)–3(c) shows the energy dispersive X-ray (EDX) spectroscopic analysis of Ni0.5Co0.5CrxFe2−xO4 (x = 0.0, 0.10, 0.20) samples. Table 2 shows that all the elements are present without impurities in the samples as per the stoichiometry ratio.Figure 3
(a–c) EDX spectra of Ni0.5Co0.5Cr0.2Fe1.8O4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.
(a)(b)(c)Table 2
Elemental composition of Ni0.5Co0.5Fe2–xCrxO4 sample according to EDX spectroscopy data.
ElementX = 0X = 0.10X = 0.20Percent (%)wt%Percent (%)wt%Percent (%)wt%O K57.1527.3057.1427.3457.1227.45Fe L28.5747.6325.7243.0622.8638.35Ni L7.1412.517.1412.557.1512.61Co L7.1412.567.1412.607.1512.66Cr L002.864.455.728.93Total100100100100100100


### 3.4. Optical Properties

Figure4(a) shows the measured values of absorbance corresponding to wavelengths in the range between 220 and 1,000 nm at 300 K. The recorded absorbance (A) and absorption coefficient α can be determined using the following relations [30]:(6)A=Log10IoI,(7)α=2.303AL,where Io and I are incidents and transmitted intensities of beam photons, respectively, and L is the width of the sample holder.Figure 4
(a) UV–vis absorbance spectra vs. wavelength of Ni0.5Co0.5CrxFe2−xO4 sample for x: (1) 0, (2) 0.10, and (3) 0.20; (b) Tauc plot of αhν2eVcm−12 vs. energy, x: 0 (1) Eg = 1.61 eV, 0.10 (2) Eg = 1.83 eV, and 0.20 (3) Eg = 1.96 eV.
(a)(b)The optical direct bandgap can be estimated by transforming the absorption spectral data by using Tauc relation:(8)αhν2=Bhν−Eg,where α is the absorption coefficient, hν is the energy of the incident radiation, B is a constant known as the band tailing parameter, and Eg is the energy bandgap between the top of the valence band and the bottom of the conduction band [27, 31].From Figure4(b), the bandgap was estimated from the plot αhν2 versus photon energy, hν, by extrapolating a straight line to the intercept on the horizontal energy axis, where αhν2=0.The bandgap increases from 1.61 to 1.96 eV with increasing the concentration of chromium ions, which is attributed to the reduction of the size of the prepared sample, as shown in Figure4(b). These results support the effective photocatalytic performance in the visible region for the utility of solar cells and the reduction of harmful organic substances in the water when irradiated.


### 3.5. Fourier Transform Infrared (FTIR) Analysis

Figure5 shows the room temperature FTIR measurement of Ni0.5Co0.5CrxFe2–xO4 (x = 0, 0.1, 0.2) sample, carried out in the wave number region between 4,000 and 400 cm−1. The spectra show two main absorption bands below 1,000 cm−1, which are known common features for all ferrites. The high-frequency absorption band ν2 is observed at 547–588 cm−1 and the lower frequency band ν1 is observed at 407–424 cm−1. The result obtained is in agreement with the previous results [23]. The frequency absorption band ν2 is assigned to the stretching vibration of the metal–oxygen bond in the tetrahedral site while the lower frequency absorption band ν1 is attributed to the stretching vibration of the metal–oxygen bond in the octahedral sites, this conforms to the bond formation and phase stability of the prepared sample [21, 32]. In this work, the absorption band for the two frequency regions is shifted toward the higher wave number as the concentration of the Cr3+ ion increase. The variations of the absorption band obtained from FTIR spectrometry are listed in Table 3.Figure 5
FTIR spectra of Ni0.5Co0.5CrxFe2−xO4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.Table 3
Variations of absorption band obtained from FTIR spectrometry.
SampleAbsorption bandν1 (cm−1)ν2 (cm−1)Ni0.5Co0.5Fe2O4407547Ni0.5Co0.5Cr0.1Fe1.9O4410575Ni0.5Co0.5Cr0.2Fe1.8O4424588


### 3.6. Magnetic Measurements

The M–H hysteresis loop of Ni0.5Co0.5CrxFe2−xO4 (x = 0, 0.1, 0.2) samples measured at room temperature is shown in Figure 6. The magnetic parameters such as saturation magnetization, remnanat magnetization, coercive field, and squareness ratio were obtained from the data of VSM, and the results are tabulated in Table 4.Figure 6
M–H hysteresis loop for Ni0.5Co0.5CrxFe2−xO4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.Table 4
Variation of magnetic parameters with Cr-doped concentrations.
SampleCoercive field,Hc (G)Saturation magnetization,Ms (emu/g)Remnant magnetization,Mr (emu/g)Squareness (Mr/Ms)Ni0.5Co0.5Fe2O41,530.4342.0218.590.442Ni0.5Co0.5Cr0.1Fe1.9O4259.88536.699.530.261Ni0.5Co0.5Cr0.2Fe1.8O4115.532.115.690.177From Table4, it is clear that the magnetic parameter values are decreased with increasing the concentration of chromium ion in the host matrix which is explained on the basis of the antiferromagnetic nature of chromium, the smaller spin magnetic dipole moment of Cr3+ (3.87 μB) when compared to Fe3+ (5.91 μB). Replacement of Cr ion in the interaction of FeA3+−O−FeB3+ with the fixed values of the spin magnetic dipole moment of Ni2+ (2.83 μB) and Co2+ (4.87 μB) resulted in the reduction of the values of magnetic parameters [26].The values of the magnetic dipole moment of the ions can be calculated as follows:(9)MM=nn+2,where MM is the magnetic moment of the ion and n is the number of unpaired electrons in the “3d” orbital which can be deduced from the electronic configuration of the ions in the compound as [Ar]4s°3d3 for Cr3+, [Ar]4s°3d5 for Fe3+, [Ar]4s°3d8 for Ni2+, and [Ar]4s°3d7 for Co2+.The magnetic anisotropy,k, relation to the coercivity, Hc, and magnetization, Ms, through Brown formula can be calculated as follows [33]:(10)k=μBMsHc2.The decreasing value of the coercive field from 1,530.43 to 115.5 G with increasing chromium ion content is attributed to the decreasing anisotropy field that contributes to the decreasing domain wall energy [34]. The calculated value of the squareness ratio (Mr/Ms) is found to be decreased with the increasing Cr content which indicates that the prepared samples are multidomain structures and the achievement of soft magnetic material which is used for high-frequency transformers core, motors, inductors, generators, etc. [35, 36].


## 3.1. Structural Analysis

Figure1(a)–1(c) shows the XRD result of the prepared sample (Ni0.5Co0.5CrxFe2−xO4, x = 0.0, 0.10, and 0.20) carried out at room temperature. The refined Rietveld Full Proff software XRD diffraction patterns for all samples confirmed that all the prepared samples are in single-phase cubic spinel structure with space group Fd3m which fits the CIF number 1,533,572 [19]. The XRD patterns are well indexed at the plane of reflections (111), (220), (311), (222), (400), (511), and (440) around the corresponding Bragg’s angles 2θ = 19°, 29°, 35.76°, 37.25°, 43°, 56°, and 62°, respectively. The partial substitution of the small amount of Cr3+ ion at the site of Fe3+ ion created no significant phase change. The XRD results showed the broadened peaks with Cr3+ content shifted to a higher angle. The crystallite size, lattice parameters, X-ray density, tolerance factors, and volume of the unit cell of the prepared samples corresponding to the intense peaks (311) are determined from XRD data and calculated using Equations (1)–(5). The values of crystallite size (D), lattice parameters, X-ray density, and tolerance factor are listed in Table 1.Figure 1
(a–c) The XRD pattern of Ni0.5Co0.5CrxFe2−xO4 sample for (a) x = 0.0, (b) x = 0.10, (c) x = 0.20, and (d) the XRD merged intense peak corresponding to (311) plane of reflection.
(a)(b)(c)(d)Table 1
Values of crystallite size (D), lattice parameters, X-ray density, and tolerance factor.
SampleTolerance factor,tCrystallite size,D (nm)Lattice parameters,a = b = c (nm)Volume of a unit cell (nm3)X-ray density (g/cm3)Ni0.5Co0.5Fe2O40.849939.540.83240.57675.322Ni0.5Co0.5Cr0.1Fe1.9O40.846237.570.83200.57595.461Ni0.5Co0.5Cr0.2Fe1.8O40.842230.420.83140.57455.603The crystallite size of the sample can be estimated from Scherrer formula [20, 21]:(1)D=kλβcosθ,where λ is the wavelength of the X-ray radiation, k is Scherrer constant ≈0.9, β is the full width at half maximum in radian, and θ is the Bragg’s angle at which the peaks are observed.For the cubic crystal structure, the lattice constant (a), the volume of the unit cell (V), and the X-ray density can be calculated from the X-ray spectra using the following relations [22]:(2)a=dhkl2h2+k2+l21/2,(3)V=a3,(4)ρX-ray=8MNaV,where M is the molecular mass of the compound and Na is the Avogadro number.The tolerance factor for ferrites of spinel structure can be described to predict the crystal phase stability [23]:(5)t=2RA+RO3RB+RO,where RA, RB, and RO are the average ionic radii of A-site, B-site, and oxygen, respectively.In the zoomed intense peaks in Figure1(d), the incorporation of Cr3+ in the host matrix resulted in the reduction of crystallite size of the sample which is explained the basis of the ionic radii differences of Cr3+ (0.0615 nm) and Fe3+ (0.065 nm) [24–26].Table1 shows that the lattice parameter, volume of the unit cell, and tolerance factor are decreased with increasing chromium ion, which is attributed to the ionic radii difference. This trend indicated that the prepared samples are more strained [27].


## 3.2. Morphological Studies

Figure2 shows the SEM micrographs of Ni0.5Co0.5CrxFe2−xO4 (x = 0.0, 0.10, 0.20) samples characterized at room temperature. The SEM micrographs revealed that the prepared samples are well-defined crystals on a nanometer scale with well-distributed and agglomerated in semispherical shape. These results are similar to the result obtained by Song and Liu [28] and Chandramouli et al. [29].Figure 2
SEM micrograph of Ni0.5Co0.5CrxFe2−xO4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.
(a)(b)(c)


## 3.3. EDX Analysis

Figure3(a)–3(c) shows the energy dispersive X-ray (EDX) spectroscopic analysis of Ni0.5Co0.5CrxFe2−xO4 (x = 0.0, 0.10, 0.20) samples. Table 2 shows that all the elements are present without impurities in the samples as per the stoichiometry ratio.Figure 3
(a–c) EDX spectra of Ni0.5Co0.5Cr0.2Fe1.8O4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.
(a)(b)(c)Table 2
Elemental composition of Ni0.5Co0.5Fe2–xCrxO4 sample according to EDX spectroscopy data.
ElementX = 0X = 0.10X = 0.20Percent (%)wt%Percent (%)wt%Percent (%)wt%O K57.1527.3057.1427.3457.1227.45Fe L28.5747.6325.7243.0622.8638.35Ni L7.1412.517.1412.557.1512.61Co L7.1412.567.1412.607.1512.66Cr L002.864.455.728.93Total100100100100100100


## 3.4. Optical Properties

Figure4(a) shows the measured values of absorbance corresponding to wavelengths in the range between 220 and 1,000 nm at 300 K. The recorded absorbance (A) and absorption coefficient α can be determined using the following relations [30]:(6)A=Log10IoI,(7)α=2.303AL,where Io and I are incidents and transmitted intensities of beam photons, respectively, and L is the width of the sample holder.Figure 4
(a) UV–vis absorbance spectra vs. wavelength of Ni0.5Co0.5CrxFe2−xO4 sample for x: (1) 0, (2) 0.10, and (3) 0.20; (b) Tauc plot of αhν2eVcm−12 vs. energy, x: 0 (1) Eg = 1.61 eV, 0.10 (2) Eg = 1.83 eV, and 0.20 (3) Eg = 1.96 eV.
(a)(b)The optical direct bandgap can be estimated by transforming the absorption spectral data by using Tauc relation:(8)αhν2=Bhν−Eg,where α is the absorption coefficient, hν is the energy of the incident radiation, B is a constant known as the band tailing parameter, and Eg is the energy bandgap between the top of the valence band and the bottom of the conduction band [27, 31].From Figure4(b), the bandgap was estimated from the plot αhν2 versus photon energy, hν, by extrapolating a straight line to the intercept on the horizontal energy axis, where αhν2=0.The bandgap increases from 1.61 to 1.96 eV with increasing the concentration of chromium ions, which is attributed to the reduction of the size of the prepared sample, as shown in Figure4(b). These results support the effective photocatalytic performance in the visible region for the utility of solar cells and the reduction of harmful organic substances in the water when irradiated.


## 3.5. Fourier Transform Infrared (FTIR) Analysis

Figure5 shows the room temperature FTIR measurement of Ni0.5Co0.5CrxFe2–xO4 (x = 0, 0.1, 0.2) sample, carried out in the wave number region between 4,000 and 400 cm−1. The spectra show two main absorption bands below 1,000 cm−1, which are known common features for all ferrites. The high-frequency absorption band ν2 is observed at 547–588 cm−1 and the lower frequency band ν1 is observed at 407–424 cm−1. The result obtained is in agreement with the previous results [23]. The frequency absorption band ν2 is assigned to the stretching vibration of the metal–oxygen bond in the tetrahedral site while the lower frequency absorption band ν1 is attributed to the stretching vibration of the metal–oxygen bond in the octahedral sites, this conforms to the bond formation and phase stability of the prepared sample [21, 32]. In this work, the absorption band for the two frequency regions is shifted toward the higher wave number as the concentration of the Cr3+ ion increase. The variations of the absorption band obtained from FTIR spectrometry are listed in Table 3.Figure 5
FTIR spectra of Ni0.5Co0.5CrxFe2−xO4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.Table 3
Variations of absorption band obtained from FTIR spectrometry.
SampleAbsorption bandν1 (cm−1)ν2 (cm−1)Ni0.5Co0.5Fe2O4407547Ni0.5Co0.5Cr0.1Fe1.9O4410575Ni0.5Co0.5Cr0.2Fe1.8O4424588


## 3.6. Magnetic Measurements

The M–H hysteresis loop of Ni0.5Co0.5CrxFe2−xO4 (x = 0, 0.1, 0.2) samples measured at room temperature is shown in Figure 6. The magnetic parameters such as saturation magnetization, remnanat magnetization, coercive field, and squareness ratio were obtained from the data of VSM, and the results are tabulated in Table 4.Figure 6
M–H hysteresis loop for Ni0.5Co0.5CrxFe2−xO4 sample, x: (a) 0, (b) 0.10, and (c) 0.20.Table 4
Variation of magnetic parameters with Cr-doped concentrations.
SampleCoercive field,Hc (G)Saturation magnetization,Ms (emu/g)Remnant magnetization,Mr (emu/g)Squareness (Mr/Ms)Ni0.5Co0.5Fe2O41,530.4342.0218.590.442Ni0.5Co0.5Cr0.1Fe1.9O4259.88536.699.530.261Ni0.5Co0.5Cr0.2Fe1.8O4115.532.115.690.177From Table4, it is clear that the magnetic parameter values are decreased with increasing the concentration of chromium ion in the host matrix which is explained on the basis of the antiferromagnetic nature of chromium, the smaller spin magnetic dipole moment of Cr3+ (3.87 μB) when compared to Fe3+ (5.91 μB). Replacement of Cr ion in the interaction of FeA3+−O−FeB3+ with the fixed values of the spin magnetic dipole moment of Ni2+ (2.83 μB) and Co2+ (4.87 μB) resulted in the reduction of the values of magnetic parameters [26].The values of the magnetic dipole moment of the ions can be calculated as follows:(9)MM=nn+2,where MM is the magnetic moment of the ion and n is the number of unpaired electrons in the “3d” orbital which can be deduced from the electronic configuration of the ions in the compound as [Ar]4s°3d3 for Cr3+, [Ar]4s°3d5 for Fe3+, [Ar]4s°3d8 for Ni2+, and [Ar]4s°3d7 for Co2+.The magnetic anisotropy,k, relation to the coercivity, Hc, and magnetization, Ms, through Brown formula can be calculated as follows [33]:(10)k=μBMsHc2.The decreasing value of the coercive field from 1,530.43 to 115.5 G with increasing chromium ion content is attributed to the decreasing anisotropy field that contributes to the decreasing domain wall energy [34]. The calculated value of the squareness ratio (Mr/Ms) is found to be decreased with the increasing Cr content which indicates that the prepared samples are multidomain structures and the achievement of soft magnetic material which is used for high-frequency transformers core, motors, inductors, generators, etc. [35, 36].


## 4. Conclusion

Ni0.5Co0.5CrxFe2−xO4 (x = 0, 0.1, 0.2) samples were prepared using the sol–gel technique. The XRD powder confirms that all the samples are in single-phase crystalline structure with space group Fd3m without any observed impurity. The crystallite size decreased from 39.54 to 30.42 nm, lattice parameter also decreased from 0.8324 to 0.8314 nm but the X-ray density which is related to the theoretical density of the sample increased from 5.322 to 6.603 g/cm3 with increasing the concentration of the Cr3+ ions due to its smaller ion radii when compared to the ionic radii of Fe3+ ions. The SEM micrograph indicated that the grains in the sample are homogenous agglomerated spherical in size. The energy dispersive X-ray (EDX) spectroscope confirms the compositions of the elements in the prepared sample as per the stoichiometry ratio. The FTIR spectra show the two strong bands observed around 410 and 547 cm−1, which have been assigned to the metal–oxygen stretching in tetrahedral and octahedral, respectively. The UV–vis spectroscopy measurement signifies the increase in the energy bandgap from 1.61 to 1.96 eV with increasing the Cr3+ ions, which are explained based on its high-electrical resistivity as compared to the electrical sensitivities of the others in the host matrix. The room temperature magnetic measurement shows that there is an observed decrease in saturation magnetization (Ms) from 42.02 to 32.11 emu/g, remnant magnetization (Mr) from 18.59 to 5.69 emu/g, and coercive field from 1,530.43 to 115.5 G with the dopant. The hysteresis loop observed becomes narrower indicating that the prepared sample is a more soft magnetic material that has technological applications, such as transformer core, high microwave absorbing devices, etc.



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*Source: 5103278-2023-02-08.xml*