TiO2-Doped Ni0.4Cu0.3Zn0.3Fe2O4 Nanoparticles for Enhanced Structural and Magnetic Properties.

TiO2 (0-10 wt %)-doped nanocrystalline Ni0.4Cu0.3Zn0.3Fe2O4 (Ni-Cu-Zn) ferrites were synthesized using the sol-gel route of synthesis. The cubic spinel structure of the ferrites having the Fd3m space group was revealed from the analysis of Rietveld refined X-ray diffraction (XRD) data. The secondary phase of TiO2 with a space group of I41/amd was observed within the ferrites with doping, x > 3 wt %. The values of lattice parameter were enhanced with the addition of TiO2 up to 5 wt % and reduced further for the highest experimental doping of 10 wt %. Field emission scanning electron microscopy (FESEM) images exhibit the spherical shape of the synthesized particles with some agglomeration, while the compositional purity of prepared ferrite samples was confirmed by energy-dispersive X-ray spectroscopy (EDX) and elemental mapping. The cubic spinel structure of the prepared ferrite sample was confirmed by the Raman and Fourier transform infrared (FTIR) spectra. UV-visible diffuse reflectance spectroscopy was utilized to study the optical properties of the ferrites. The value of band gap energy for the pristine sample was less than those of the doped samples, and there was a decrement in band gap energy values with an increase in TiO2 doping, which specifies the semiconducting nature of prepared ferrite samples. A magnetic study performed by means of a vibrating sample magnetometer (VSM) demonstrates that the values of saturation magnetization of the ferrites decrease with the addition of TiO2 content, and all investigated ferrites show the characteristics of soft magnetic materials at room temperature. The Mössbauer study confirms the decrease in the magnetic behavior of the doped ferrites due to the nonmagnetic secondary phase of TiO2.

Introduction

Out of the three types of ferrites, viz., spinel ferrites, hexagonal ferrites, and garnets, spinel ferrites with the common formula AFe2O4 are industrially significant magnetic materials because of their outstanding magnetic and electrical properties. The spinel ferrite, Ni–Cu–Zn, is the most imperative one having a mixed spinel structure. The nanocrystalline Ni–Cu–Zn ferrite is employed in the construction of the multilayer chip inductors (MLCI).[1−4] There are various methods of the preparation of nanocrystalline spinel ferrites, like citrate precursor, co-precipitation, sol–gel, solid-state reaction, hydrothermal, and conventional ceramic process.[5−10] Among them, the sol–gel method is the most advanced method for the production of nanocrystalline spinel ferrites due to its advantages such as requirement of a low temperature, good control over stoichiometry, and easy-to-adapt synthesis parameters.[11,12]

Titanium oxide (TiO2) has a wide range of applications in different fields including optical communication, photocatalysis, and photovoltaics.[13] TiO2-based perovskite solar cells with enhanced performance have been reported elsewhere.[14] Photoanodes prepared from nanocrystalline rutile TiO2 show better electron transfer and improved conversion efficiency of solar cells.[15] Multilayered highly efficient perovskite solar cells using mesoporous TiO2 nanostructures are reported in the literature.[16] Nanocrystalline anatase TiO2 films showing enhanced sensing performance for hazardous ammonia gas are reported in the literature.[17] The doping of Ti ions in the spinel ferrite by means of the TiO2 additive was proved to be significant for enhancement of the physical parameters of the ferrites. A study on the influence of TiO2 doping in Ni–Zn ferrite confirms that the structural and magnetic parameters like grain size, relative density, and magnetic permeability increased with a specific amount of doping within the ferrite.[18] Electrical parameters like power loss and DC resistivity were also enhanced with doping of a specific amount of TiO2 additive within the ferrite. The creation of the secondary phase of NiTiO3 and Fe2TiO5 was reported for TiO2-doped Ni ferrites,[19] as an effect of which a reduction in magnetic parameters was observed. A systematic study of TiO2-doped Mn-Zn ferrites reports that magnetic parameters like permeability and power loss of the ferrite can be enhanced by adding a suitable amount of the additive in the ferrite crystal.[6] TiO2-coated Ni ferrite nanoparticles show a decrease in saturation magnetization due to the nonmagnetic behavior of TiO2. However, TiO2-coated ferrite particles exhibit photocatalytic property and make it suitable for antimicrobial activity.[20] The TiO2–ZnFe2O4 composite system is a better material for photocatalytic activity under visible light radiation.[21] The efficiency of energy conversion of TiO2-dye-sensitized solar cell was reported to increase due to doping with ferrite particles. The rapid charge transfer properties of the ferrites facilitate efficient energy conversion.[22]

In brief, the literature survey suggests that doping of a suitable amount of TiO2 additive in the ferrite is effective for enhancing the physical parameters of the ferrites. Such interesting modifications due to TiO2 doping in spinel ferrites lead to their use in a variety of applications. The Ni–Cu–Zn ferrites with a specific stoichiometric elemental composition of divalent cations with approximately 40% Ni, 30% Cu, and 30% Zn exhibits interesting physical properties, leading to a variety of applications.[23−25] It motivates us to synthesize the nanocrystalline Ni0.4Cu0.3Zn0.3Fe2O4 spinel ferrites by the sol–gel method and investigate the effects of TiO2 doping on the structural, morphological, spectroscopic, and magnetic properties of the ferrite.

Experimental Procedure

The sol–gel method was used for the preparation of nanocrystalline Ni0.4Cu0.3Zn0.3Fe2O4 ferrite doped with TiO2 additives. Analytical grade (99% pure) copper nitrate (Cu(NO3)2·6H2O), zinc nitrate (Zn(NO3)2·6H2O), nickel nitrate (Ni(NO3)2·6H2O), ferric nitrate (Fe(NO3)3·9H2O), and citric acid (C6H8O7) were utilized for the synthesis of ferrites. These nitrates and citric acid, in stoichiometric ratio, were mixed in a minimum amount of deionized water, and by adding liquid ammonia to the solution, pH 7 was achieved. Then, the solution was kept on a hot plate at 80 °C with continuous stirring until a thick, viscous gel was obtained. At this stage, the stirring was stopped while continuing the heating. The heating removes the water content from the gel, and a completely dry gel was obtained. The dry gel autoignites within a second and burns with colored fumes. The burning completes within few minutes, and a ferrite powder in the form of ash precursors was obtained. These ash precursors were ground in an agate mortar and calcined for 5 h at 400 °C in a temperature-controlled furnace. The calcined powder was mixed with TiO2 additive (0–10 wt %) and ground for 45 min before calcination at 650 °C for 4 h. The pellets with 13 mm diameter and 3 mm thickness were obtained from this powder, with a polyvinyl alcohol as a binder material. These pellets were calcined for 4 h at 400 °C to remove the binder.

Characterization Techniques

An X-ray diffractometer D2 PHASER (Bruker) was used to obtain the XRD patterns of calcined samples with Cu Kα as a radiation source having wavelength λ = 1.5405 Å inclined at diffraction angles 2θ between 20 and 80° at room temperature. Microstructures of the samples were studied using FESEM (FEI Nova Nano SEM 450) and elemental analysis from EDS (Bruker XFlash 6I30). Fourier transform infrared (FTIR) spectroscopic analysis was done using a Jasco model FT/IR 4000 series spectrometer. At room temperature, magnetic measurements were performed on a vibrating sample magnetometer (Quantum Design PPMS-VSM). The study of optical properties of synthesized samples was done on a UV–vis DRS Jasco spectrophotometer (Model V-670). The Mössbauer spectra of the ferrite samples were obtained at room temperature using 57Co as a γ-ray source in the Rh matrix. The calibration of velocity scale was performed in relation with 57Fe (in Rh). The WinNormos FIT software was utilized for the analytical fitting of the spectra.

Results and Discussion

Structural Analysis

The structural study of the ferrite samples was performed by powder X-ray diffraction, and the obtained diffraction patterns are shown in Figure . In the XRD patterns of all of the ferrites, the peaks (220), (311), (222), (400), (422), (511), and (440) were observed revealing the formation of a spinel phase within the ferrites.[26] The XRD patterns for x ≥ 3 wt % exhibit additional peaks of the TiO2 phase: (101), (103), (004), (112), (200), (105), (211), (118), and (220) at 27, 37, 38, 39, 48, 53, 55, 62, and 71°, respectively,[27] along with the peaks of the spinel phase. Using the FullProf Suite software program, the analysis of XRD data was performed by applying the Rietveld refinement technique. The parameters obtained from the Rietveld refinement, goodness of fit (χ2), expected R values (Rex), and weighted profile R-factor (Rwp) along with the relative % of the two phases (spinel and TiO2) created within the ferrites, are listed in Table .

Figure 1

Rietveld refined X-ray diffraction patterns of TiO2-doped Ni0.4Cu0.3Zn0.3Fe2O4 ferrites. Bragg’s peak positions are pointed by green- and pink-colored vertical lines at the bottom of the XRD pattern of the ferrite with x = 10 wt %.

Table 1

Discrepancy Factor (Rwp), Expected Values (Rexp), and Goodness of Fit (χ2), Spinel Phase %, and TiO2 Phase % in Ni0.4Cu0.3Zn0.3Fe2O4 Ferrites Doped with Varying wt % of TiO2 Additive

TiO2 wt %	a (Å)	D (nm)	dx (g/cm3)	LA (Å)	LB (Å)
0	8.3030	21	5.516	3.5953	2.9352
1	8.3035	33	5.664	3.5954	2.9352
2	8.3252	28	5.761	3.6048	2.9429
3	8.3772	28	6.163	3.6273	2.9613
5	8.3778	27	6.823	3. 6275	2.9615
10	8.3461	27	5.806	3.6138	2.9503

The exploration of Rietveld refinement shows that a single spinel phase having a cubic structure is formed with the Fd3m space group for the ferrites doped with x < 3 wt % of TiO2 and an additional secondary phase having a space group I41/amd is formed along with the spinel phase for the ferrites doped with x ≥ 3 wt % of TiO2. For x ≥ 3 wt % of TiO2, the secondary phase increases from 4.2 to 8.6%, while a simultaneous decrease in spinel phase from 95.8 to 91.4% was observed. It is clear that for x ≥ 3 wt %, the TiO2 additive does not enter the ferrite lattice and there is a possibility that the TiO2 molecules may surround the Fe3+ cations at the sublattice sites.[19] The values of the lattice parameter were obtained from the equation reported elsewhere[28] and are summarized in Table along with the crystalline sizes, X-ray densities, and hopping lengths. From Table , it is revealed that the values of the lattice parameter (a) are enhanced with an increase in TiO2 composition up to 5 wt %. For x < 3 wt % TiO2, the Ti4+ ions enter the lattice of the Ni–Cu–Zn ferrite, by replacing a few Fe3+ cations. Owing to the variation in the ionic radii of Ti4+ (0.61 Å) and Fe3+ (0.67 Å), expansion of the unit cell occurs and an increase in lattice parameter values is obtained. Further, for a higher content of TiO2, i.e., 3 ≤ x ≤ 5, the lattice parameter increases because of the increased molecules of TiO2 surrounding the Fe3+ nuclei at the B sublattice sites. However, for the experimental substitution limit x = 10 wt %, a small decrease in the value of lattice parameter is obtained, showing the distortion in the lattice due to the higher % of the secondary phase of TiO2 created within the ferrite.

Table 2

Parameters Obtained from the XRD Pattern of Ni0.4Cu0.3Zn0.3Fe2O4 Ferrites Doped with Different wt % of the TiO2 Additive

wt % TiO2	Rexp	Rwp	χ2	spinel phase %	TiO2 phase %
0	16.1	12.2	1.71	100	00
1	19.5	11.9	2.65	100	00
2	26.6	14.4	3.31	100	00
3	16.1	10.6	2.28	95.8	4.2
5	15.8	11.8	1.77	94.2	5.8
10	16.7	12.7	1.71	91.4	8.6

The average crystallite size (D) for the synthesized ferrite nanoparticles was investigated from the peak with the maximum intensity, i.e., the peak (311) of XRD pattern using Scherer’s equation[29]where θ is Bragg’s angle, λ is the wavelength of X-rays, and β is the full width at half-maximum (FWHM). The computed crystallite size (D) values (Table ) decreased with an increase in TiO2 doping composition. These values lie between 20 and 33 nm, which indicates that the prepared particles are the nanoparticles. The agglomeration of the particles was reported as the reason behind the increase in the particle size of the nanoferrites.[30] In the present study, the particle size of TiO2-doped Ni–Cu–Zn ferrites is larger than that of the pristine Ni–Cu–Zn ferrite, revealing the occurrence of agglomeration for the doped samples of ferrites. The variation in particle size is not much compared to that in the lattice parameter values. The reason behind is that, in nanoferrites, the particle size depends upon temperature and time of annealing.[31] In the present TiO2-doped Ni–Cu–Zn ferrites, both these factors, i.e., time and temperature of annealing, were kept constant for all of the ferrite samples, owing to which, a little variation in particle sizes was reported. The X-ray density (dx) was investigated from the relation[32]where N is Avogadro’s number and M is the molecular weight. It is found that there is an increment in X-ray density with the doping of TiO2 molecules in Ni–Cu–Zn ferrites. This is because of the increasing amount of doping of TiO2, which is adding its molecular weight (47.867 amu) to the molecular weight of the Ni0.4Cu0.3Zn0.3Fe2O4 ferrite, due to which the molecular weight of the doped ferrites is increasing. The increase in molecular weight increases the density (dx). Furthermore, the values of bulk density are smaller than the X-ray density. The variation in bulk and X-ray densities is because of the formation of pores in the prepared samples.

The hopping lengths LB and LA at the octahedral and tetrahedral sites, respectively, were obtained by employing the following equations from the literature[33]It was found that the hopping lengths at LA and LB show similar performance to that of the lattice constant.

Field Emission Scanning Electron Microscopy

The FESEM images of Ni–Cu–Zn ferrites for x = 0, 1, 3, and 10 wt % TiO2 additive are shown in Figure . These images reveal that all of the prepared particles are spherical in shape and are agglomerated due to magnetic interactions. The average grain size is in between 42 and 50 nm. The elemental mapping and EDX were carried out, which confirms the existence of elements like Zn, Ni, Fe, Cu, O, and Ti in the selected samples (Figure ).

Figure 2

SEM images of TiO2-doped Ni0.4Cu0.3Zn0.3Fe2O4 ferrites.

Figure 3

Elemental mapping and EDX pattern of TiO2-doped Ni0.4Cu0.3Zn0.3Fe2O4 ferrites.

Raman Spectroscopy

Figure illustrates the Raman spectra of TiO2-doped Ni–Cu–Zn ferrites in the frequency range 100–800 cm–1 at room temperature. According to the group theory, only five Raman-active modes are possible for spinel ferrites, viz., A1g, Eg, and three F2g modes.[34,35] The modes above 600 cm–1 and below 600 cm–1 are associated with metal–oxygen (M–O) (symmetrical stretching) bonding at tetrahedral sites and metal–oxygen bonding (symmetrical, antisymmetrical bending) at octahedral sites, respectively.[36,37] The Raman bands observed for Ni–Cu–Zn ferrite doped with TiO2 are depicted in Table . The Raman bands found at 150, 190, 318–320, 463–469, and 666–685 cm–1 correspond to F2g(1), F2g(2), Eg, F2g(3), and A1g modes. From Figure and Table , it is obvious that the wavenumber position of the bands does not change for x = 1, 2, and 3 wt % TiO2 composition. The wavenumber positions of bands were slightly changed, and also the F2g (2) band is observed for x = 10 wt % TiO2 composition. This slight difference in Raman bands with a different content of TiO2 in Ni–Cu–Zn ferrites is linked with the rearrangement of cations between the octahedral and tetrahedral sites.

Figure 4

Raman spectra of Ni0.4Cu0.3Zn0.3Fe2O4 ferrites doped with varying wt % TiO2 additive.

Table 3

Raman Modes with Wavenumbers for Ni0.4Cu0.3Zn0.3Fe2O4 Doped with TiO2 Additive

		Raman shift (cm–1)
sr. no. of the peak in the spectra	Raman modes	x = 1 wt %	x = 2 wt %	x = 3 wt %	x = 10 wt %
1	F2g(1)	149	150	145	150
2	F2g(2)	313	312	322	170
3	Eg	480	480	485	310
4	F2g(3)	638	637	646	458
5	A1g	689	688	697	664

Fourier Transform Infrared (FTIR) Spectroscopy

Figure displays the FTIR spectra of the Ni–Cu–Zn ferrite doped with various compositions of TiO2. The spectra were obtained between 400 and 1000 cm–1. The two strong peaks ν1 (577.08–624.52 cm–1) and ν2 (397.32–424.72 cm–1) are observed in each sample related to the vibrations of the metal ion–oxygen bonds in tetrahedral and octahedral sites, respectively.[38] The formation of such peaks evidenced that the spinel ferrite structure was formed in our studied ferrite samples. The variation in the ν1 and ν2 peaks was attributed to the difference in bond lengths (Fe–O) associated with the tetrahedral and octahedral sites.[39] From Figure and the attached table, it is found that there is a small variation in peak positions with TiO2 doping in Ni–Cu–Zn ferrite because of the redistribution or migration of cations between the tetrahedral and octahedral sites in the Ni–Cu–Zn ferrite doped with TiO2 additive. It is found that the frequency ν1 for the pristine sample is less than those for the doped samples. For doped ferrites, ν1 decreased continuously with increasing doping till the 5 wt % dopant level. For the dopant level ≤2 wt %, the decrease in ν1 indicates the occupancy of Ti4+ ions at octahedral sites due to which the Fe3+ ions migrated to tetrahedral sites and bond lengths were increased, and as a consequence, a decrease in vibrational frequencies (ν1) was reported.[30] The possible cause behind the decrease in ν1 for dopant level >2 wt % is the deposition of TiO2 molecules around the Fe3+ cations due to which the bond lengths were increased, and consequently, a decrease in ν1 value is reported. An apparent increase in ν1 for the highest experimental doping of 10 wt % was due to the creation of lattice distortions by the TiO2 phase with a higher concentration (Table ). The vibration frequency ν2 increased up to 2 wt % doping of TiO2, clearly indicating Ti4+ occupancy at the octahedral sites. However, for TiO2 doping >2 wt %, ν2 decreases till 5 wt % and then suddenly increases for 10 wt %. It clearly points toward the fact that the occupancy of Ti4+ at octahedral sites is not proportional to the doping (%), i.e., Ti4+ does not substitute the Fe3+ ion at the octahedral sites for higher doping levels (>2 wt %), and finally, a larger change in the value of ν2 confirms the creation of a strong TiO2 phase for 10 wt % doping. Thus, the occurrence of spinel phase and a secondary phase of TiO2 molecules is confirmed by the FTIR analysis.

Figure 5

FTIR spectra of TiO2-doped Ni0.4Cu0.3Zn0.3Fe2O4 ferrites. The attached table shows the values of the IR vibration frequencies ν1 and ν2 possessed by the ferrites.

UV–Vis Diffuse Reflectance Spectroscopy

The optical properties of Ni–Cu–Zn ferrites doped with different concentrations of TiO2 additives were investigated using a UV–vis diffuse reflectance spectrophotometer. The measurements were conducted between 200 and 800 nm, and the obtained plots are displayed in Figure . The obtained spectra of the studied ferrites show absorption in the visible region. The absorption coefficients α were investigated from the Kubelka–Munk function given by the formula[40]where R is the reflectance value, F(R) is the Kubelka–Munk function, and α is the absorption coefficient. Moreover, in the Tauc relation,[41,42] values of the absorption coefficient were utilized to obtain band gap energy (Eg). The relation is given bywhere A is the proportionality constant, h is Planck’s constant, and υ is the frequency of light. The band gap energy values of all selected samples were computed by depicting the graph of (ahυ)2 versus hυ as shown in Figure .

Figure 6

UV–vis absorbance spectra of TiO2-doped Ni0.4Cu0.3Zn0.3Fe2O4 ferrites.

Figure 7

Tauc plots of Ni0.4Cu0.3Zn0.3Fe2O4 ferrites doped with varying content of TiO2 additive.

The band gap energies (Eg) were deduced by extrapolating the linear portions of these graphs. The band gap energy values of Ni–Cu–Zn ferrites doped with different contents of TiO2 additive are represented in Table . It is evident from Table that the band gap energy value of the pristine sample is less than the doped samples, and there is decrement in the values of band gap energies with an increase in the content of TiO2 in the doped Ni–Cu–Zn ferrite. This is due to the variation in the crystallite size of the studied samples. It also shows that the doped ferrite samples exhibit a semiconducting nature. Another reason for the reduction in Eg values for doped ferrites is the presence of a secondary phase of TiO2, which creates lattice defects. Due to these lattice defects, the bonding of electrons with nucleus becomes loose and a lower energy is needed for the electrons to leave from the outermost shell.[43] The reduction in band gap energies of the ferrites is reported in the literature.[44]

Table 4

Values of Absorption Bands (υ1 and υ2) and Band Gap Energy for Ni0.4Cu0.3Zn0.3Fe2O4 Doped with the TiO2 Additive

	absorption bands cm–1
wt % TiO2	υ1	υ2	Eg (eV)
0	578.96	397.29	1.115
1	624.02	403.72	1.528
2	624.79	405.08	1.502
3	620.03	403.16	1.464
5	581.18	399.48	1.424
10	623.53	414.76	1.411

Magnetic Properties

The magnetic properties of the ferrites were explored at room temperature using a vibrating sample magnetometer. The hysteresis loops for TiO2-doped Ni–Cu–Zn nanocrystalline spinel ferrites are given in Figure , which illustrates the change in magnetization with the applied magnetic field. The magnetization curves for all of the samples exhibit normal characters of soft magnetic materials. From the hysteresis loops of the studied ferrite samples, values of remanence magnetization (Mr), coercivity (Hc), and saturation magnetization (Ms) were obtained and are summarized in Table .

Figure 8

Room-temperature magnetic hysteresis loops for TiO2-doped Ni–Cu–Zn ferrites. The inset shows the variations of coercivity with varying TiO2 doping.

Table 5

Magnetic Parameters of Ni0.4Cu0.3Zn0.3Fe2O4 Ferrites Doped with Different Concentrations of the TiO2 Additive

wt % TiO2	Ms (emu/g)	ηB (μB)	Hc (Oe)	Mr (emu/g)
0	71.684	3.052	40.874	7.278
1	71.227	3.114	46.592	8.624
2	70.920	3.115	51.224	6.652
3	67.672	3.182	51.329	7.653
5	66.166	3.196	51.630	7.768
10	63.566	3.434	53.424	9.692

It is obvious from Table that the saturation magnetization values are declining with the addition of TiO2 due to the nonmagnetic behavior of the titanium ions. This can be explained according to Neel’s theory and the superexchange interaction mechanism.[45] For TiO2 doping level x ≤ 0.2 wt %, the presence of a single spinel phase within the ferrites clearly indicates that the nonmagnetic Ti4+ ions were substituted in the Ni–Cu–Zn lattice. Since Ti4+ ions have strong preference toward the B site, they occupy the site by replacing the Fe3+ ions that weakens the exchange interaction between A and B sites. As a result, a decrement in the saturation magnetization with an increment in the composition of TiO2 in Ni–Cu–Zn nanocrystalline spinel ferrites is observed[46] for doping level x ≤ 0.2 wt %. Further, the decrement in saturation magnetization is also due to the decline in the crystallite size of the prepared samples. The decrement in the values of saturation magnetization for doping level x > 2 wt % was due to the accumulation of nonmagnetic TiO2 molecules surrounding the magnetic Fe3+ cations. The decrease in saturation magnetization for doping level (wt %) 3 ≥ x ≤ 10 is due to the increase in the nonmagnetic phase of TiO2 (Table ). The coercivity (Hc) values are enhanced with an increase in the content of TiO2 in Ni–Cu–Zn ferrites, which is linked with the reduction in crystallite size.[47] However, remanent magnetization does not show regular variation with TiO2 doping.

The values of ηB were investigated experimentally using the equation[48]where Ms is the saturation magnetization and Mw is the molecular weight of the sample. The obtained values of ηB are given in Table . The increase in oxygen vacancies due to TiO2 doping is evident from this increase in ηB values.[49] In ferrites, a change in ηB values depends on the concentration of oxygen vacancies, and there is a direct relation between the number of vacancies and magneton number.[50]

Mössbauer Spectroscopy

Mössbauer spectra measured at room temperature for the typical samples of Ni–Cu–Zn ferrites doped with 0, 2, 5, and 10 wt % of TiO2 are shown in Figure . It is obvious from the Mössbauer spectra that all of the samples exhibit well-defined Zeeman split sextets, one of them corresponding to Fe3+ ions at the tetrahedral A sites and the second one corresponding to the Fe3+ ions at the octahedral B site. No central paramagnetic contribution from the paramagnetic-Zn ions is observed in any of the samples, revealing the ordered magnetic structure and the long-range magnetic interactions in all of the samples. Saturation magnetization is known to be directly proportional to hyperfine field. Hyperfine field (Hhf) does not change much with TiO2 doping (Table ) and can be qualitatively explained on the basis of Neel’s superexchange interactions.[51] Thus, it can be considered that TiO2 does not enter the Ni–Cu–Zn spinel ferrite and not replacing any of the ions in the spinel lattice. Thus, a decrease in saturation magnetization could be related to an increase in the nonmagnetic phase of TiO2 in the composition. It is evident from Table that the isomer shift (IS) at the B site is greater than that at the A site and could be related to the large band separation of Fe3+–O2– for the B-site ions compared to the A-site ions. The range of values of IS indicated that Fe ions exist in the 3+ valence state with high-spin configuration.[52] The linewidth (Γ) was found to increase by a small margin for A site with TiO2 doping. The increased broadening A site with TiO2 doping may be attributed to the increasing number of surrounding TiO2 around the Fe3+ nuclei at A sites. Nearly zero values of QS within the experimental error are an indication of the fact that the cubic symmetry is retained between the Fe3+ ions and its surrounding Ni, Cu, and Zn ions even after TiO2 doping in the Ni–Cu–Zn spinel crystal.

Figure 9

Room-temperature Mössbauer spectra for the typical samples with 0, 2, 5, and 10% of TiO2-doped Ni0.4Cu0.3Zn0.3Fe2O4 ferrites.

Table 6

Linewidth (Γ), Isomer Shift (IS), Quadrupole Splitting (QS), Hyperfine Magnetic Field (Hhf), and Relative Area (A) in Percentage of the Tetrahedral and Octahedral Sites of Fe3+ Ions for TiO2-Doped Ni0.4Cu0.3Zn0.3Fe2O4 Ferrite Derived from Mössbauer

sample (%)	iron site	Γ (mm/s)	IS (mm/s)	QS (mm/s)	Hhf (T)	A (%)
0	sextet A	0.69 ± 0.02	0.29 ± 0.01	–0.02± 0.01	47.5 ± 0.1	67
	sextet B	1.07± 0.07	0.32 ± 0.02	0.02 ± 0.01	42.4 ± 0.1	33
2	sextet A	0.73± 0.03	0.29 ± 0.01	0.03 ± 0.02	47.1 ± 0.1	70
	sextet B	0.73 ± 0.03	0.41 ± 0.02	–0.01± 0.01	41.9 ± 0.1	30
5	sextet A	0.74± 0.02	0.29 ± 0.01	0.01± 0.02	47.1 ± 0.1	72
	sextet B	0.74 ± 0.02	0.36 ± 0.02	0.07± 0.04	41.6 ± 0.1	28
10	sextet A	0.76± 0.01	0.29 ± 0.01	–0.02± 0.02	47.5 ± 0.1	71
	sextet B	0.76 ± 0.01	0.29 ± 0.02	–0.05± 0.02	42.7 ± 0.1	29

Conclusions

Nanocrystalline Ni–Cu–Zn ferrites (Ni0.4Cu0.3Zn0.3Fe2O4) doped with varying wt % of TiO2 additive were successfully synthesized by the sol–gel method. The analysis of XRD data through Rietveld refinement has revealed the creation of cubic spinel phase having the Fd3m space group within the ferrites. Further, with the addition of TiO2 in the Ni0.4Cu0.3Zn0.3Fe2O4 ferrite, a secondary phase with a space group I41/amd was observed for x ≥ 0.3 wt % doping. There is an increment in the values of lattice parameter up to 5 wt % of TiO2 doping, and for higher doping of 10 wt % TiO2, it declines. The FESEM morphology depicts the agglomeration of spherical-shaped particles at a few places. The compositional purity of prepared ferrites was confirmed by elemental mapping and energy-dispersive X-ray spectroscopy. From the Raman and FTIR spectra, the spinel cubic structure of the investigated ferrite samples along with the creation of the secondary phase of TiO2 (for x ≥ 0.3 wt %) was confirmed. From the Tauc plots, the values of band gap energy for the studied samples were investigated, and it is obvious that the band gap energy value of the pure sample is less than the doped samples and there is a decrement in band gap energy with an increase in TiO2 content in Ni–Cu–Zn ferrites. Magnetic study depicts that the saturation magnetization was reduced with an increase in the TiO2 concentration. Also, all of the samples exhibit normal characters of soft magnetic materials at room temperature. Mössbauer spectra analysis suggested that the TiO2 ions do not accommodate in the ferrite lattice and the decrease in saturation magnetization is mainly related to the increase in the nonmagnetic phase of TiO2 in the ferrites.