Synthesis, structural, dielectric and magnetic properties of cobalt ferrite nanomaterial prepared by sol-gel autocombustion technique

Journal Pre-proof Synthesis, structural, dielectric and magnetic properties of cobalt ferrite nanomaterial prepared by sol-gel autocombustion technique Tulu Wegayehu Mammo, Ch Vijaya Kumari, S.J. Margarette, A. Ramakrishna, Raghavendra Vemuri, Y.B. Shankar Rao, K.L. Vijaya Prasad, Ramakrishna, N. Murali PII:

S0921-4526(19)30668-4

DOI:

https://doi.org/10.1016/j.physb.2019.411769

Reference:

PHYSB 411769

To appear in:

Physica B: Physics of Condensed Matter

Received Date: 12 August 2019 Revised Date:

7 October 2019

Accepted Date: 9 October 2019

Please cite this article as: T.W. Mammo, C.V. Kumari, S.J. Margarette, A. Ramakrishna, R. Vemuri, Y.B. Shankar Rao, K.L. Vijaya Prasad, Ramakrishna, N. Murali, Synthesis, structural, dielectric and magnetic properties of cobalt ferrite nanomaterial prepared by sol-gel autocombustion technique, Physica B: Physics of Condensed Matter (2019), doi: https://doi.org/10.1016/j.physb.2019.411769. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Synthesis, structural, dielectric and magnetic properties of Cobalt ferrite nanomaterial prepared by sol-gel autocombustion technique Tulu Wegayehu Mammo1, Ch. Vijaya Kumari2, S. J. Margarette3, A. Ramakrishna4, Raghavendra Vemuri4, Y. B. Shankar Rao5, K.L.Vijaya Prasad6, Ramakrishna7, *N. Murali7 1

Aksum University, Tigrai, Ethiopia Department of Physics, SS&N College, Narasaraopet, Guntur, India 3 Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, India – 530003 4 Department of Physics, Aditya College of Engineering and Technology (ACET), Surampalem, East Godavari, Andhra Pradesh, India. 5 Department of Physics, Anil Neerukonda Institute of Technology & Sciences, Bheemunipatnam, Visakhapatnam, India. 6 Department of ECE, Aditya College of Engineering and Technology (ACET), Surampalem, East Godavari, Andhra Pradesh, India. 7 Department of Engineering Physics, Andhra University College of Engineering (A), Andhra University, Visakhapatnam, Andhra Pradesh, India – 530003 * Corresponding author: [email protected] 2

Abstract: Cubic spinel structured CoFe2O4 nanocrystalline material has been synthesized using the sol-gel auto combustion route. Structural parameters and phase formation were studied using room temperature powder X-ray diffraction (XRD). The result shows a pure cubic spinel structured sample formation with a lattice parameter of 8.4277 Å. Field emission scanning electron microscopy (FESEM) microstructural characterization revealed the nanocrystalline structure of the so synthesized sample with non-homogeneous grain sizes and grain shapes. Fourier transform infrared (FT-IR) characterization confirmed the spinel structure formation by exhibiting the cation stretching-vibrations at the tetrahedral and octahedral sites.

The DC

resistivity measurement has been carried out using a two-probe technique and confirmed the high resistive nature of the sample.

The room temperature dielectric and AC properties were

investigated in the frequency ranges of 100 Hz – 5 MHz. The dielectric dispersion has been observed at lower frequencies. Higher magnetization value has been measured using the VSM technique. Keywords: Spinel-structured, CoFe2O4, nanocrystalline, NH3, DC resistivity. 1. Introduction 1

For the nowadays electronic technological advances, it would be almost unthinkable without the use of magnetic oxides. Be it cores for inductors and transformers, discs or tapes, thin films for high-density computer memories, nonreciprocal microwave control devices, antennas for home electronics, microwave antireflection coatings, permanent magnets, and so forth, all are possible as a result of discovery and modifications of magnetic materials with dielectric properties (or vice versa) [1 – 4]. Ferrites are a large class of magnetic oxides with incredible dielectric and ferrimagnetic properties, and it is such combined electrical and magnetic properties with possible modifications that let these materials be very much applicable in almost all technological fields. Based on their crystal structure, they can be spinel, garnet, and hexagonal ferrite types [5, 6]. Bragg determined for the first time that spinel structured ferrites have the structure of natural spinel MgAl2O4 [4, 7]. These types of ferrites usually are soft types with a general formula MFe2O4 with M = Co, Ni, Zn, Cu, or other metals [7, 8]. In this present work, due to myriads of properties that could be tuned up or uncovered, the spinel structured CoFe2O4 was synthesized and studied using different analytical techniques. In general, CoFe2O4 with its high coercivity, moderate saturation magnetization, large magnetocrystalline anisotropy, high chemical, and structural stabilities, and also being made to be a solid solution has given it the material of choice for lots of applications [9, 10]. The nanoparticles of CoFe2O4 have been reported to have a coercivity of about 500 – 2000 Oe at room temperature [10]. Many synthesis techniques have been employed in preparing the nanostructured CoFe2O4: sonochemical reactions [11], mechanochemical synthesis [12, 13], the hydrothermal method [14], coprecipitation [15, 16], and sol-gel routes [17 – 19].

The Particle size, crystallinity,

composition, site occupancy, etc., play a significant role in determining the structural, electrical, magnetic, and chemical properties of nano-ferrite particles [20].

Therefore, the synthesis

techniques play a central role in determining the properties of CoFe2O4 and hence its applications as well. As stated, due to its numerous applications, cobalt ferrite materials have been studied extensively, although still important properties could be explored. Pham D. Thang, et al., have used the complexometric synthesis method to prepared magnetic fine particles of cobalt ferrite materials. They have investigated the structural and magnetic properties of the calcined and sintered powder samples of CoFe2O4; and their analysis emphasized on the impact of temperature on so mentioned properties of this material and hence they have summarized that the 2

calcination temperature determine the size of the particles and also as the calcination temperature increases, the coercivity of the samples decrease [21-22]. The impact of starting solution acidity on structural, microstructural, and magnetic properties of powder CoFe2O4 prepared by solution combustion method has been investigated by B. Pourgolmohammad, et al. They have shown a decrease in the specific surface area as the PH of the sample increases; whereas the coercivity increased by the increase in the PH value due to the decrease in crystallite size [23]. Jyoti Sharma, et al., studied the dielectric properties of sol-gel synthesized nanocrystalline CoFe2O4 in the frequency range of 100 – 5.85 GHz. At lower frequencies, which they considered to be in the range of 100 – 120 MHz, the dielectric constant exhibits dispersion with frequency due to the space charge polarization. Whereas in the higher frequency ranges of 3.95 – 5.85 GHz, they reported almost frequency independent dielectric constant and an increase in the dielectric loss tangent with a peak at 4.525 GHz [24]. Furthermore, extensive studies of partially substituted CoFe2O4 materials by divalent and also trivalent cations have been reported with interesting outcomes [25]. Therefore, in this present research, we report the synthesis of CoFe2O4 nanomaterial using a sol-gel auto combustion route with applications of ammonia as a neutralizing agent and citric acid as a fuel agent. We think that all the synthesis conditions at hand here will result in tuned values of electrical and dielectric properties of this material. Besides, to the best of our knowledge, most reports lack detailed calculations of the DC electrical resistivity; and also different analytic techniques have been employed to study the structural, electrical, dielectric, and magnetic properties.

2. Experimental 2.1. Sample preparation For our sample preparation, we used the following AR graded metal nitrates, citric acid, and ammonia purchased from Merck Company: Co(NO3)2.6H2O (99%), Fe(NO3)3.9H2O (99.5%), C6H8O7.H2O (99.7%), and ammonia (28 – 30% for analysis). A stoichiometric amount of the metal nitrates and citric acid have been mixed and stirred using a magnetic stirrer. This stirring dwelt for about an hour and then the magnetic stirrer was set on to heat the mixture, and another 1 hour heating with gradual increasing the temperature of heating was done. After one hour of heating, ammonia was added drop-wise to keep the sample neutral. As time goes on, the 3

sample was gradually changed to a very viscous solution which then with the increase in temperature ignited to give a porous and voluminous substance. Using mortar and pestle the obtained as-burnt sample has been ground for about half an hour. This ground sample was put into the muffle box furnace to be pre-heated at 500 ºC for about 2 hrs. Again, after another half, an hour grinding half of this sample was annealed at 1100 ºC for 3hrs. The other half of the sample was ground with the addition of about 6 drops of polyvinyl alcohol (PVA) to make a pellet. The pellet was sintered at 1150 ºC for 4 hrs and after being rubbed with emery paper and pasted with silver paste with another heat treatment it was made ready for electrical characterizations.

2.2. Characterization Techniques For structural characterization and phase identification, we have used the PANalytical XPert PRO diffractometer with CuKα radiation and λ = 1.5402 Å also with a continuous scan step size of 0.008. The Bragg's diffraction angle 2θ ranges from 10° to 80° scanned at room temperature. The unit cell lattice parameter was calculated by the least square fitting method from the d-spacing and (hkl) values. Furthermore, the crystallite size of the sample was obtained from the XRD pattern by applying Scherrer's equation.

The particle microstructure and

morphology of the powder sample was obtained from field emission scanning electron microscopy (FESEM) image using Carl Zeiss, EVOMA 15, Oxford Instruments, Inca Penta FETx3.JPG instrument. The bond formation and stretching-vibration modes of the cations and all other constituent groups of the sample were studied using Fourier transform infrared spectroscopy (FT-IR). FT-IR data was obtained from IR Prestige21 Shimadzu by employing the KBr pressed pellet technique. The DC electrical resistivity of the so-synthesized material was measured using a two-probe technique in the temperature ranges of 303 – 488 K. Using 5 HP4192A impedance analyser, the dielectric and AC electrical conductivity properties were investigated at the frequency ranges of 100 – 5 MHz. A room temperature vibrating sample magnetometer has been employed to study magnetic properties.

3. Results and Discussion

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3.1. XRD analysis: Powder X-ray diffraction (XRD) measurement of the as-synthesized sample is shown in Figure 1. From the figure, it can be seen that all the sharp and clear narrow peaks of the XRD pattern confirm the formation of a single-phase cubic spinel structure of CoFe2O4 material with Fd-3m space group (JCPDS card no. 22-1086). The peaks also tell about the clear crystalline form of the synthesized sample. From the peaks (111), (220), (311), (400), (422), (511) and (440) and the values of their corresponding Bragg angles, the lattice parameter was calculated using unit cell software. Besides, the crystallite size and cell volume have been calculated and the results are presented in Table 1. The crystallite size was calculated using Debye-Scherrer's formula: where β is the peak full width at half maximum (in radians) at the observed peak angle θ, k is the crystallite shape factor (was considered 0.94) and λ is the X-ray wavelength. The crystallite size calculation has been carried out using the highest peak (311). The results thus obtained are in close agreement with the previously reported works [6, 24]. The addition of ammonia affects the size of the nano-sized materials as reported previously [5]. Moreover, the obtained lattice parameter is slightly higher for a nanocrystalline material and this might be due to the environmental situation, i.e., the measured room temperature might have been relatively higher which will affect the lattice parameter expansion.

3.2. FESEM studies: The morphology and microstructure of the crystalline materials synthesized are shown in Figure 2. The electrical and magnetic properties of ferrites are strongly dependent on the morphology of the particles. The image depicts that topographically the powder sample is porous and having irregularly shaped grains distributed non-homogenously. Besides, morphology shows some degree of particle agglomeration which could be inferred to the magnetic nature of the sample so synthesized [26]. Moreover, it exhibits coral-like features 6 indicative of the nanocrystalline nature of the samples. The particle sizes are almost nonuniform and also these particles are larger as they are aggregations of nanocrystallites of magnetic material. The large surface area of the nanocrystalline composition of the synthesized material helps in the growth of large grain sizes due to the coalescence of these crystallites. Not only is the material composition determines the grain size growth of this material, but also the sintering temperature and synthesis route do have a significant effect. Thus, from the FESEM grain size measurements, grains of size in the range of up to 799 nm have been obtained as 5

measured by the software associated with the FESEM. Such larger grain growth might have been in response to the relatively larger annealing temperature and magnetic properties of the constituent crystallites. The pore structures are also clearly visible in this FESEM image. The EDS spectrum is shown in Figure 3, also confirms that Co: Fe ratio is nearly 1:2 indicating that the stoichiometric proportion is maintained. Besides, the peaks from the EDS spectra have been assigned to the constituent elements. Thus, the EDS hints that the constituent elements come together to form CoFe2O4. Table 2 summarizes elemental composition and ranges of grain sizes. 3.3. FT-IR analysis: To confirm the results from XRD analysis, room temperature FT-IR measurement of the synthesized sample has been carried out and the result is depicted in Figure 4. The FT-IR measurement lets us explore the spinel phase formation of cobalt nanoparticles. Moreover, FT-IR spectra not only inform the positions of the divalent (Co2+) and trivalent (Fe3+) ions but also the vibration modes as well [26]. Thus, the measured FT-IR spectrum shows that the bands at lower frequency (around 400 cm-1 – υ2) are composed of stretching vibrations of the octahedral groups (Co2+ - O2- ) [10] whereas the higher frequency bands (around 600 cm-1 – υ1) are due to vibration of tetrahedral groups (Fe3+ - O2- ). These band groups could be attributed to the formation of the ferrite phase [26 – 29]. The absorption bands in the ranges of 1319.35 – 2115.43 cm-1 could be assigned to N-O and C-O stretching and bonding. Whereas, those bands ranging from 3501.73 to 3961.96 cm-1 can be inferred to O-H stretching bands. These bands may be due to some powder from the sample in the wall of the beaker, not being fired enough to remove N2 and CO2 completely [10]. Of course, these higher frequency absorption bands are not visible as intense peaks in the figure but understandably they exist.

3.4. DC electrical resistivity: Parts of the electrical properties of the so synthesized CoFe2O4 material were investigated through a two-probe DC resistivity study.

The DC electrical

resistivity properties of materials help in extracting valuable information on the behavior of electrical charge carriers, which leads to an explanation of the conduction mechanism in ferrites [30]. For instance, ferrites with a high variation of resistivity over a large range of temperatures are ideal for application in sensor technology [31]. In the present work, a pelletized sample of nanocrystalline CoFe2O4 material was put in digitally controlled temperature variable heater of 6

the two-probe electrical circuit.

The resistivity measurements were carried out from a

temperature of 303 K to 488 K. Therefore, the resistivity ρdc can be expressed as. Figure 5 shows the variation of lnρ with 103 /T K. The scales are made to be as shown in the figure for simplification purposes. As depicted in the figure, it can be concluded that the DC electrical resistivity decreases linearly with temperature, which is a prominent feature of ferrites as semiconductors. The hoping could be exhibited by Fe2+ and Fe3+ cations at the octahedral sites. From the variation of the DC resistivity with temperature, it could also be simple to understand the involvement of small polaron hopping as conduction means in this material. From the present ferrite material – CoFe2O4, a DC electrical resistivity of the order up to

107

Ω.cm has been achieved at lower temperatures; and for higher temperatures, it dropped to the orders of 104 Ω.cm.

3.5. Dielectric properties: From AC measurements using HP-4192A impedance analyzer, the dielectric constant was calculated using the relation [24], where, Cp is the capacitance, d is the thickness of pellet, A is the cross-section area of the flat surface of the pellet, and εo is the permittivity of free space (8.85 x 10−12 F/m) [25]. The dispersion of the dielectric constant at lower frequencies and then remaining being constant at higher frequencies could be attributed to space charge polarization of the Maxwell-Wagner two-layer model which is in line with the Koop's phenomenological theory. According to this model, polycrystalline ferrites are composed of poorly conducting grain boundaries and well-conducting grains. At lower frequencies the grain boundaries are 9 more effective insulators and electrons reaching these boundaries by hopping between Fe2+ and Fe3+ will be piled and accumulated there and hence the region will be polarized resulting in the high dielectric constant of the material at these lower frequencies. As the frequency increases, the polarization decreases because, beyond a certain frequency of the external field, the frequency of electron exchange between Fe3+ and Fe2+ ions cannot follow the alternating field frequency [24]. Figures 6 (a) – (c) depict the room temperature variation of the dielectric parameters with the applied frequency on our synthesized material – CoFe2O4. The points mentioned in the above paragraph hold for this material too. As shown in Figure 6 (a), the real part of the dielectric constant (ɛ') decreases with frequency; and even exhibiting dispersion at lower frequencies. This dispersion property is due to the space charge polarization. Similar 7

behaviors are observed for the imaginary part of the dielectric constant. The slight difference will be the degree of response to the applied AC electric field. The decrease of the dielectric loss (ɛ'') with frequency, in this case, is sharp as shown in Figure 6 (b). At four particular lower frequencies (100, 500, 1000, and 3000 Hz) of the applied electric field, the values of the dielectric parameters are shown in Table 3. In the table, it is shown that the loss tangent (tan δ) decreases with frequency; and this is straightforward that at low frequency more energy is required for hopping of charge carriers so that the energy loss will be high. Moreover, the sharp decrease in dielectric loss (ɛ'') can be confirmed from the results in the Table.3 Furthermore, the loss tangent (tan δ) shows a peak and this peaking behavior is explained by the Rezlescu model. The peak position is the maximum dielectric loss of the material. That is, at peak frequency, the material should gain the maximum energy so that it keeps the hopping of the charge carriers at the same frequency to the applied field. Therefore, the condition is given as ωτ = 1, where ω = 2πfmax, τ is the relaxation time [33].

3.6. AC conductivity: Figure 7 shows the room temperature AC conductivity variation of the as-synthesized CoFe2O4 nanocrystalline material. As shown in the figure, the AC conductivity increases with frequency and hence the conduction mechanism could be attributed to the Verwey electron hopping mechanism. Verwey describes that at room temperature the conduction in ferrites is 10 due to impurities that enhance electron hopping whereas at high temperature it could be due to polaron hopping. Moreover, Verwey adds that the room temperature electron hopping mechanism is exhibited by the electron exchange between ions (Fe3+ + e- → Fe2+) of the same element having different valence states and distributed randomly in crystallographically equivalent lattice sites [34].

Not only are the electrons hopping between Fe3+ and Fe2+

responsible for conduction in this cobalt ferrite nanocrystalline material, but also the hole hopping between Co3+ and Co2+ as well. Besides, the frequency acts as a power source for the hopping of holes and electrons in this system. As the frequency increases, localized charges (or trapped charges) will be liberated and become free so that they involve in the conduction process [35]. At the highest frequency, there is even a sharp increase in conductivity as shown in the Figure. 3.7. 8

3.6 Magnetic properties: The magnetic properties of the synthesized nanocrystal material were analyzed using a vibrating sample magnetometer (VSM) at room temperature. The properties of most magnetic materials could be best described using the hysteresis loop which is obtained from VSM data. Figure 8 shows the M-H curve of the prepared CoFe2O4 nanocrystals. The saturation magnetization of the synthesized cobalt ferrite nanocrystals has been obtained at room temperature from the hysteresis loop to be 94.505 emu/g. This value is relatively higher as compared to the previous reports for bulk CoFe2O4 material [8, 37]; and this might be due to larger grain sizes obtained. The magnetic behavior of our synthesized nanocrystals measured by VSM can be attributed to the competition of ferromagnetic ions such as Fe3+ and Co2+ ions as non-magnetic transition metal ions in the occupancy of the tetrahedral and octahedral sites. Since the coercivity (Hc) of a magnetic material is a measure of its magnetocrystalline anisotropy, the small nanoparticles, which have close to zero coercivity and no remanence become a single domain with little anisotropy energy. This is a characteristic of superparamagnetic nanocrystals. The coercivity (Hc) of the synthesized nanocrystals has been obtained from Figure 8 and the data obtained from VSM gives 891.58 Oe value. Thus, these nanocrystals show ferromagnetic behavior at room temperature. Such properties make this material favorable for wide engineering applications such as drug delivery, bioseparation, 11 and magnetic refrigeration systems [36]. Table 4 summarizes the calculated and tuned values of magnetic parameters.

4. Summary: Spinel structured nanocrystalline CoFe2O4 material has been synthesized using a sol-gel auto combustion technique. Citric acid has been used as a chelating agent and ammonia as a neutralizing agent. Phase formation has been confirmed by the XRD analysis and also FTIR spectroscopic results, and it turned out that pure phase with characteristic stretching vibrations has been obtained. FESEM with EDS were employed to study the microstructures and elemental compositions, respectively. Larger grain sizes have been observed from the FESEM images. The DC electrical resistivity calculation has been presented, and it was measured using the twoprobe technique and confirmed the higher resistive nature of this material. The dielectric and 9

electrical properties were studied using HP-4192A impedance analyser in the frequency ranges of 100 – 5 MHz. The dielectric parameters show a dispersion relation at lower frequencies which is attributed to the space charge polarization.

The AC conductivity increases with

frequency as the frequency powers the generation of more charge carriers.

The room

temperature magnetic properties obtained from VSM resulted in higher values of magnetization which might be due to larger grain sizes of the synthesized material. In general, due to synthesis conditions, tuned values of the measured electrical, dielectric and magnetic parameters have been obtained.

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Table 1: Structural parameters of synthesized CoFe2O4. a (Å)

Volume Cell (Å)3

8.4277 598.5954

Crystallite size (nm)

Space Group

X-ray density (g/cm3)

28.29

Fd-3m

5.2063

Bulk Porosity density (%) (g/cm3) 4.268

18.03

Table 2: Elemental composition obtained from EDS and grain size of CoFe2O4. Element O Fe Co Grain size

Weight % Atomic % 27.57 57.50 47.26 28.24 25.17 14.25 ~ 799 nm

Table 3: Values of the dielectric parameters at different frequencies. Frequency (Hz) 100 500 1000 3000

ɛ' 86.7763 29.5585 25.1257 21.1733

ɛ'' 291.420 67.5048 36.8803 13.9595

tanδ 3.35829 2.28377 1.46783 0.65929

Table 4: Magnetic parameters of CoFe2O4 measured at room temperature. Saturation Remanent Coercivity Remanence Composition magnetization Magnetization Hc (Oe) ratio = Mr/Ms Ms(emu/g) Mr(emu/g) CoFe2O4 94.505 29.95 891.58 0.3169

Figure 1: XRD pattern of CoFe2O4 synthesized at 1100 °C.

Figure 2: FESEM image of CoFe2O4 synthesized at 1100 ºC.

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Figure 3: EDS Spectrum of CoFe2O4 nanoparticles.

Figure 4: FT-IR spectra of CoFe2O4 material

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Figure 5: Graph of lnρ vs. 103/T for nanocrystalline CoFe2O4 material

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Figure 6: Variation of the a) real part (ɛ'), b) imaginary part (ɛ''), and c) loss tangent (tan δ) of the dielectric constant with frequency of CoFe2O4 material.

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Figure 7: Variation of AC conductivity with frequency for CoFe2O4material.

Figure 8: Hysteresis curve of CoFe2O4 nanopowder at room temperature.

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