Structural and dielectric properties of cobalt ferrite based nanocomposites

Journal Pre-proof Structural and dielectric properties of cobalt ferrite based nanocomposites Kalyani Dhabekar, K. Mohan Kant PII:

S0921-4526(20)30731-6

DOI:

https://doi.org/10.1016/j.physb.2020.412752

Reference:

PHYSB 412752

To appear in:

Physica B: Physics of Condensed Matter

Received Date: 16 October 2020 Revised Date:

25 November 2020

Accepted Date: 7 December 2020

Please cite this article as: K. Dhabekar, K.M. Kant, Structural and dielectric properties of cobalt ferrite based nanocomposites, Physica B: Physics of Condensed Matter (2021), doi: https://doi.org/10.1016/ j.physb.2020.412752. 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. © 2020 Published by Elsevier B.V.

Credit Author Statement Kalyani Dhabekar: Writing- Original draft preparation, Experimental work, Dielectric measurements

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K. Mohan Kant: Writing- Reviewing and editing, Structural characterizations

Structural and dielectric properties of cobalt ferrite based nanocomposites Kalyani Dhabekar and K. Mohan Kant* Department of Physics, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India-440010 Corresponding author address: K. Mohan Kant, Department of Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur, Maharashtra, India.

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PIN-440010, Tel. No. : 0712-2801833 / +91-8275866144

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Corresponding author e-mail: [email protected]

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Abstract

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Nanocomposites of cobalt ferrite (CoFe2O4) were synthesized using chemical co-precipitation

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route. For preparing the nanocomposites, cobalt ferrite was combined with strontium ferrite

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(C/S-1/1) and with barium ferrite (C/B-1/1) in equal weight ratio of 1:1. Dual phase Rietveld refinement procedure was employed for phase analysis of the specimens. Electrical transport

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properties were investigated within frequency scale 20 Hz-106 Hz at different temperatures

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ranging from 100°C to 400°C. Dielectric permittivity of the composites was found to increase with temperature. Polarization mechanism was attributed to Koop’s theory due to the presence of conductive and non-conductive part in ferrite composites. The ac conductivity mechanism in the composites was ascribed to Maxwell-Wagner model. With increase in temperature, C/S-1/1 depicts greater conductivity compared to C/B-1/1.

Keywords: ferrite, composite, co-precipitation, polarization, dielectric, conductivity

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1. Introduction Ferrite based nanocomposites have generated much interest due to their magnetic, electrical and electromagnetic properties and hence used in magnetic recording, electronic devices and microwave absorbers [1-3]. Currently, due to increasing demand for materials suitable for highfrequency applications, ferrites and its composites are considered as promising candidates in spintronics applications [4-5]. Most of the ferrites are non-conducting at room temperature,

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allows complete penetration of electromagnetic fields in comparison to metals, where skin depth

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hinders high frequency fields [6-7]. Ferrites primarily serve as dielectric materials due to high

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room temperature resistivity and low eddy current losses [8-9]. For microwave absorption

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applications, dielectric properties such as permittivity and dielectric loss factor play a significant

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role. The permittivity influences the width of microwave absorbing layer, while dielectric loss

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determines the decadence of electrical energy [10]. In order to achieve improved performance, low energy loss from the materials is expected [6]. Dielectric properties of various ferrite

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composite systems, such as cobalt ferrite-hafnium (CoF2-xHfxO4; x = 0.00-0.20) composite,

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BaFe12O19/Ni0.5Zn0.5Fe2O4, SrFe12O19/ Fe2O3, CoFe2O4/SrFe12O19 studied [8, 11-13]. The magnetic exchange coupling between CoFe2O4/SrFe12O19 nanocomposites has been established, yet emphasis on dielectric properties of composite systems is required to further understand their applications in microwave absorption [14]. Cobalt ferrite belongs to the class of spinel ferrites, while strontium ferrite and barium ferrite belongs to M-type hexagonal ferrites [15, 16]. The present work describes the synthesis of cobalt ferrite and nano composites by addition of strontium ferrite and barium ferrite. The present work aimed to tailor electrical properties of cobalt ferrite based magnetic nanocomposites. Systematic

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electrical transport measurements on cobalt ferrite and the composites were performed and analyzed the behavior with suitable Maxwell-Wagner model [17]. 2. Experimental details 2.1 Materials In the present work, high purity precursors (≥99.9%) such as, cobalt (II) nitrate hexahydrate (Co(NO3)2∙6H2O), strontium (II) nitrate (Sr(NO3)2), barium (II) nitrate (Ba(NO3)2), iron (III)

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nitrate nonahydrate (Fe(NO3)3∙9H2O) were used. Sodium hydroxide (NaOH) was used as

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precipitating agent.

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2.2 Synthesis of CoFe2O4, SrFe12O19 and BaFe12O19 nanoparticles

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For the synthesis of low dimensional cobalt ferrite (CoFe2O4), strontium ferrite (SrFe12O19) and

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barium ferrite (BaFe12O19) chemical co-precipitation route was employed. For synthesis of

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CoFe2O4 nanoparticles, precursor solutions of Co(NO3)2∙6H2O and Fe(NO3)3∙9H2O were prepared in 1:2 molar ratios respectively [14]. 4 M NaOH solution was added to the precursor

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solution until the pH reached to 12. The reaction temperature was maintained at 80℃ for 2 hours.

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Similarly solutions of Sr(NO3)2 and Fe(NO3)3∙9H2O (Sr+2:Fe+3 = 1:10) were prepared to synthesis SrFe12O19 nanoparticles [18]. 2.5 M NaOH solution was added drop wise to attain pH value of 12. The reaction was carried out at 90℃ for 4 hours, leads to precipitation. The obtained precipitate was washed several times with ethanol and distilled water. For preparation of BaFe12O19 nanoparticles, solutions of Ba(NO3)2 and Fe(NO3)3∙9H2O in the molar ratio of 1:10.5 was prepared [19]. Excess amount of barium nitrate was required due to limited solubility of barium hydroxide in water [20].

The pH of resulting solution was

maintained at 12 by drop wise addition of 5 M solution of NaOH. The resultant precipitate

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solution was maintained at 90℃ for 6 hours. On completion of reaction time, the obtained solution was washed with solution of ethanol and distilled water. All obtained products were dried at their respective synthesis temperatures. For formation of pure hexaferrite phase, both SrFe12O19 and BaFe12O19 nanoparticles were annealed at 900℃ for 4 hours. 2.3 Synthesis of CoFe2O4-SrFe12O19 and CoFe2O4-BaFe12O19 nanocomposites

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The synthesized SrFe12O19 and BaFe12O19 nanoparticles were mixed with CoFe2O4 in weight

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ratio 1:1. The prepared nanocomposites were annealed at 850℃ for 3 hours to produce CoFe2O4-

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SrFe12O19 and CoFe2O4-BaFe12O19 nanocomposites. The CoFe2O4-SrFe12O19 and CoFe2O4-

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BaFe12O19 nanocomposites were labeled as C/S-1/1 and C/B-1/1 respectively. The obtained

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samples were pressed into pellets having thickness of 1 mm using hydraulic press. For

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impedance measurements, electrodes were established using silver paste. Phase analysis of the prepared samples was performed by using X-ray diffractometer (Bruker radiation ( = 1.5406 Å). The obtained X-ray diffraction patterns

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AXS D8 Advance) with Cu

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were analysed using Rietveld refinement by Fullprof Suite program. Electrical transport measurements were performed using Wayne-Kerr Precision 6500B Impedance Analyser within the frequency range of 20 Hz-106 Hz from 50℃ to 400℃ in step intervals of 50℃. 3. Results and discussion 3.1 Phase analysis The refined X-ray diffraction patterns for CoFe2O4 nanoparticles, C/S-1/1 and C/B-1/1 were presented in figure 1. The open circles and solid black lines represent observed intensities and estimated intensities respectively. The observed differences between observed and calculated intensities are depicted below. The vertical columns represent allowed Braggs reflections. The

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Bragg’s reflections corresponding to all indexed planes of CoFe2O4 were refined using Fd 3m space group. A Pseudo- voigt function was used to generate the peak pattern. The position of O atom was fixed during refinement, while position of Co and Fe atoms were refined. The various refined parameters extracted from diffraction patterns for CoFe2O4 nanoparticles, C/S-1/1 and

C/B-1/1 (figure 1) are listed in Table 1. The crystallite size for CoFe2O4 nanoparticles calculated by Scherrer’s formula and by Rietveld method was observed as 20 nm and 17 nm respectively.

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The bond lengths Fe − O and Co − O were found to be 2.0370 Å and 1.8071 Å respectively. The

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bond angles ∠O − Co − O , ∠Co − O − Fe and ∠O − Fe − O were 109.43°, 123.05° and 86.83°

RA−O + RB −O

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R=

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two cations [15] was calculated as:

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respectively. Figure 2 depicts the generated crystal structure. Effective bond length ( R) between

[ RA−O + RB −O ]

1/2

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where, RA−O and RB −O are bond lengths of cation A (Co+2) and B (Fe+3) with oxygen ion

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respectively. Thus, the bond lengths, effective bond length obtained was 1.961Á. For C/S-1/1

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and C/B-1/1, dual phase refinement method was employed. In C/S-1/1, individual phase of CoFe2O4 and SrFe12O19 were refined in accordance with space groups Fd 3m and P63 / mmc respectively. Subsequently, all parameters corresponding to CoFe2O4 and SrFe12O19 were fixed during refinement, dual phase refinement was employed. Similar procedure was followed for C/B-1/1, in which BaFe12O19 (space group P63 / mmc ) was present. The average crystallite size of cobalt ferrite, strontium ferrite and barium ferrite in the prepared composites was found to be 20 nm, 50 nm and 70 nm respectively. The diffraction patterns for C/S-1/1 and C/B-1/1 reveals presence of individual peaks corresponding to both the phases of composites. Existence of pure phases confirms independent structural phase in the prepared composites. The obtained weight

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fractions of cobalt and strontium in composite C/S-1/1 were 71.8% and 28.2% respectively. In C/B-1/1, weight fraction of cobalt is 68.7% and that of barium is 31.3%. 3.2 Dielectric Studies 3.2.1 Impedance analysis

The electrical behavior of CoFe2O4 nanoparticles, C/S-1/1 and C/B-1/1 was studied by complex impedance spectroscopy (CIS) within frequency range 20 Hz-106 Hz from 50°Cto 400°C. In

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CIS, the parameter Z known as electrical impedance is obtained as

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Z = Z ′ + iZ ′′

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where, Z ′ and Z ′′ represent the respective real and imaginary impedance. Figure 3 depicts the

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variation of real impedance with applied frequency for CoFe2O4 nanoparticles, C/S-1/1 and C/B-

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1/1. It is observed that with increase in temperature, magnitude of Z ′ decreases for CoFe2O4 and

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the prepared composites C/S-1/1 and C/B-1/1. Composites reveal semiconducting behavior,

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which may arise due to release of trapped charges leading to ease in mobility [21]. Higher

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impedance is observed in C/S-1/1 and C/B-1/1 compared to CoFe2O4 (figures 3(b) and 3(c)). For As synthesized composites consists of equal weight proportions of strontium ferrite, barium ferrite and cobalt ferrite. Hence, strontium ferrite and barium ferrite acting as solute particles in C/S-1/1 and C/B-1/1 respectively. Thus, creation of scattering centers by solute particles results in higher values of impedance compared to parent compounds [22]. Similarly, C/B-1/1 exhibits an increase in magnitude of Z ′ compared to C/S-1/1, attributed to the larger size of Ba+2 ion [16]. The larger size of Ba+2 ion leads to more effective scattering in C/B-1/1, leading to

increased impedance. Gradual decrease in Z ′ is observed with increase in frequency. The dominant nature of grain boundaries having resistive nature at low frequencies may contribute to reduction in Z ′ with increase in frequency. With increasing frequency, conducting grains are 6

more active leading to decrease in impedance. The existence of a linear region is discerned which increases with rise in temperature (Figure 3). The ease of movement of charge carriers at higher temperatures leads to rise in dc resistance [21]. The impedance curves tend to merge at higher frequencies, implying the existence of space charge in the prepared specimens (Figure 3) [21].

Nyquist plots for CoFe2O4 nanoparticles, C/S-1/1 and C/B-1/1 at different temperatures were

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shown in figure 4. A decrease in radius of semicircular arcs is observed with rise in temperature,

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confirms the semiconducting nature of the prepared C/S-1/1 and C/B-1/1 nanocomposites. The

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Nyquist plots obtained at various temperatures were fitted with an electrical equivalent circuit

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using EIS spectrum analyzer. The fitted plots for CoFe2O4, C/S-1/1 and C/B-1/1 at 250°C were

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presented in figure 5. The equivalent circuit is a parallel combination of two R − CPE networks

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connected in series (inset of figure 5). The resistances R1 and R2 were due to contribution of CoFe2O4 and SrFe12O19 in C/S-1/1 respectively. Similarly, in case of C/B-1/1, the contribution is

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due to CoFe2O4 and BaFe12O19. CPE1 and CPE2 are constant phase elements related to the

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resistance and capacitance as C = Q1/ n R(1−n )/ n where n is 0 for resistor and 1 for pure capacitor [13]. Units of CPE exponent n is s n Ω −1 and Q is CPE coefficient. 3.2.2 Conductivity analysis

Variation of conductivity with applied frequency for CoFe2O4, C/S-1/1 and C/B-1/1 is shown in figure 6 in a temperature range of 100°C to 400°C. The conductivity curves reveal a frequency independent linear region, depicting dc conductivity. The linear region extends up to 105 Hz exhibiting a gradual increase in conductivity, thereafter with increase in temperature. The increase in conductivity is marked by non-linear region depicting ac conductivity. Contribution

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of both dc and ac parts of conductivity contribute to total conductivity, and illustrated by Jonscher Power law equation [23]:

σ t = σ dc + σ ac σ t = σ dc + Bω n where, σ t is total conductivity, σ dc and σ ac are dc and ac conductivity respectively, ω is

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frequency, n is frequency exponent representing the degree of interaction of mobile ions with the

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conductivity and dc resistivity are tabulated in table 2.

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lattice and B is constant that determines the strength of polarizability. The obtained values of dc

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The mechanism of ac conductivity in CoFe2O4 nanoparticles, C/S-1/1 and C/B-1/1 was explained

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on the basis of Maxwell-Wagner two layer model [17]. In case of CoFe2O4 nanoparticles, Fe ions may present in more than one valence state leading to hopping of electrons [21]. Simultaneously

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electron hopping occurs from Co+2 to Co+3. At lower frequencies, mobility of electrons occurs

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by occupying the neighboring vacant sites. The long range movement is enhanced leading to dc

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conductivity in the linear region (figure 6). The electrons relax by hopping back and forth in respective sites with increase in frequency, may lead to rise in active conduction depicted by dispersive non-linear region in ac conductivity. Both strontium ferrite and barium ferrite belongs to the class of M-type hexaferrites [16]. In Mtype hexaferrites, conduction is solely due to the electron hopping between divalent Fe ions to the ions present in trivalent state [12]. The Fe atoms are distributed among three octahedral sites, one tetrahedral site and one bipyramidal site in strontium ferrite and barium ferrite [16]. The distance between two Fe ions at octahedral sites is less than distance between a Fe ion present at octahedral site and another one present at tetrahedral site. The hopping probability between Fe ions present at two different octahedral sites is large. Hopping probability between two 8

tetrahedral sites is almost zero. Fe+3 ions are present at tetrahedral sites and Fe+2 ions preferentially occupy the octahedral sites [12]. The frequency exponent n as calculated from the fitted curves in figure 6 was plotted with varying temperature in figure 7. It is observed that with increase in temperature, n decreases and attains a minimum. Subsequently, n increases after reaching minimum, which is a characteristic signature of overlapping large polaron tunneling (OLPT) mechanism. Overlapping large polaron

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tunneling mechanism suggests an occurrence of conduction process by phonon assisted electron

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hopping generally termed as polaron hopping. The hopping occurs at sites with more than one

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valence state. In large polaron tunneling, distorted polaron cloud present at two different sites

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3.2.3 Permittivity measurements

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overlap each other thus, reducing the polaron hopping energy [24].

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The dielectric measurements were carried out in frequency range 20 Hz-106 Hz at different temperatures ranging from 100°C to 400°C. An increase in dielectric constant is observed with

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rise in temperature (figure 8). Due to increase in temperature, the charge carriers are able to

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follow the varying ac field and piling up near interfaces leading to increase in polarization. The increase in polarization with temperature corresponds to increased dielectric constant for the prepared samples. Lower dielectric constant for C/B-1/1 compared to C/S-1/1 is due to lower conductivity of the former as observed from figure 6(c). Due to lower conductivity of C-B/1/1 the charge carriers have low hopping frequency leading to decreasing polarization and consequently decreasing permittivity. With increase in frequency, dielectric constant decreases due to the inability of charge carriers to follow the changing alternating field and leads to decrease in space charge polarization and lower dielectric constant. Figure 9 shows the variation of dielectric loss factor (ε '') with frequency for CoFe2O4 nanoparticles, C/S-1/1 and C/B-1/1.

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Value of ε '' was observed to be large at low frequencies and begins to increase with increase frequency. The decrease in ε '' at higher frequencies may be attributed to higher charge carrier density distribution [12]. Higher values of ε '' at lower frequencies may be ascribed to crystal defects and impurities [12]. The fast rising trend of ε '' at low frequencies is attributed to polarization mechanism associated with thermally activated conduction of mobile charge carriers.

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The dispersive nature of dielectric curves is explained on the basis of Koop’s theory [25].

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According to Koop’s theory, a dielectric structure consists of one layer of conducting materials,

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and a second layer of relatively poor conducting materials. The former contribution is effective

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at high frequency region, while later is effective at low frequency region. In low frequency

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region, for determining the dielectric polarization of ferrites, surface polarization dominates

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compared to ionic or electronic polarization due to predominance of poor conducting layer [25]. The variation of dielectric loss with applied frequency for CoFe2O4 nanoparticles, C/S-1/1 and

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C/B-1/1 is shown in figure 10. At lower frequency, a high dielectric loss for all the prepared

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samples is observed which decreases gradually with increase in frequency. The high dielectric loss is attributed to impurities and crystal defects in the prepared samples [12]. With increase in frequency, the polarization lags behind the changing alternating field which causes reduction in charge displacement leading to decrease in dielectric loss. 4. Conclusions

CoFe2O4 nanoparticles and composites C/S-1/1 and C/B-1/1 were synthesized employing chemical co-precipitation synthesis route. X-ray diffraction patterns were refined in accordance with space groups Fd 3m and P63 / mmc for cubic and hexagonal structures respectively. For the prepared composites, dual phase Rietveld refinement was performed, revealed the weight

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fractions of cobalt and strontium in the composite C/S-1/1 were 71.8% and 28.2% respectively. In C/B-1/1, weight fraction of cobalt was found to be 68.7% and that of barium was 31.3%. The conductivity mechanism was explained on the basis of Maxwell-Wagner model. With increase in temperature, conductivity in C/S-1/1 was observed to be greater than C/B-1/1. The dielectric polarization in the prepared samples was established based on Koop’s theory in which polarization of charge carriers in C/B-1/1 was found to be less compared to C/S-1/1.

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Conflict of interest

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It is hereby declared that the authors do not have any conflict of interest.

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List of Tables

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Table 1: Refined parameters for CoFe2O4, C/S-1/1 and C/B-1/1. Here; RP is profile factor, RB is

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Braggs R factor, RF is crystallographic factor, Rwp is weighed profile factor and χ 2 is goodness of fit.

CoFe2O4

C/S-1/1

C/B-1/1

12.20

-

-

5.20

-

-

4.31

-

-

Rwp (%)

3.40

3.31

4.20

χ2

1.92

2.50

3.10

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RP (%)

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RB (%) RF (%)

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Refined Parameters

Table 2: Values of dc conductivity (σ dc ) and dc resistivity ( ρ dc ) for (a) CoFe2O4 nanoparticles, (b) C/S-1/1 and (c) C/B-1/1 at different temperatures. Temperature

CoFe2O4

C/S-1/1

(°C)

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C/B-1/1

σ dc (Ω −1cm −1 ) ρdc (Ωcm) σ dc (Ω −1cm −1 ) ρdc (Ωcm) σ dc (Ω −1cm −1 ) ρdc (Ωcm) 1.39×10-7

7.19×106

1.68×10-7

5.95×106

4.8×10-10

2.08×109

150

1.82×10-6

5.49×105

2.01×10-7

4.97×106

1.36×10-9

7.35×108

200

9.46×10-6

1.05×105

8.54×10-6

1.17×105

3.78×10-9

2.64×108

250

5.31×10-5

1.88×104

3.45×10-5

2.89×105

1.02×10-8

9.80×107

300

1.76×10-4

5.68×103

6.93×10-5

1.44×104

2.85×10-8

3.50×107

350

2.34×10-4

4.27×103

1.33×10-4

7.51×103

2.48×10-7

4.03×106

400

9.51×10-4

1.05×103

1.52×10-4

6.57×103

7.76×10-7

1.28×106

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100

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List of Figures

Figure 1: Rietveld refined X-ray diffraction patterns of (a) CoFe2O4 nanoparticles, (b) C/S-1/1 and (c) C/B-1/1.

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Figure 2: Generated unit cell of CoFe2O4 from refinement parameters.

Figure 3: Variation of real part of impedance with frequency for (a) CoFe2O4 nanoparticles, (b) C/S-1/1 and (c) C/B-1/1.

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Figure 4: Nyquist plots for (a) CoFe2O4 nanoparticles, (b) C/S-1/1 and (c) C/B-1/1 at different

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temperatures. (Inset: Enlarged view of Nyquist plots from temperature range 250°C to 400°C.)

Figure 5: Fitted Cole-Cole plots to equivalent circuit model for (a) CoFe2O4 nanoparticles, (b) C/S-1/1 and (c) C/B-1/1 at 250°C. Solid line represents fitted result to the measured data. 3

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Figure 6: Variation of ac conductivity with frequency at different temperatures for (a) CoFe2O4 nanoparticles, (b) C/S-1/1 and (c) C/B-1/1. Solid red line indicates curve fitting using Power law

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equation.

Figure 7: Variation of frequency exponent n with temperature for (a) CoFe2O4 nanoparticles, (b) C/S-1/1 and (c) C/B-1/1. 4

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Figure 8: Variation of real part of dielectric constant with frequency for (a) CoFe2O4

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nanoparticles, (b) C/S-1/1 and (c) C/B-1/1 at different temperatures.

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Figure 9: Variation of imaginary part of dielectric constant with frequency for (a) CoFe2O4

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nanoparticles, (b) C/S-1/1 and (c) C/B-1/1 at different temperatures.

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Figure 10: Variation of dielectric loss with frequency for (a) CoFe2O4 nanoparticles, (b) C/S-1/1

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and (c) C/B-1/1 at different temperatures.

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Conflict of interest

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It is hereby declared that the authors do not have any conflict of interest.