Effect of Li–Cu doping on structural, electrical and magnetic properties of cobalt ferrite nanoparticles

Effect of Li–Cu doping on structural, electrical and magnetic properties of cobalt ferrite nanoparticles

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Author’s Accepted Manuscript Effect of Li-Cu doping on structural, electrical and magnetic properties of cobalt ferrite nanoparticles Saadia Rasheed, Hafiz Sartaj Aziz, Rafaqat Ali Khan, Asad Muhammad Khan, Abdur Rahim, Jan Nisar, Syed Mujtaba Shah, Farasat Iqbal, Abdur Rahman Khan www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(15)02110-0 http://dx.doi.org/10.1016/j.ceramint.2015.11.034 CERI11651

To appear in: Ceramics International Received date: 14 October 2015 Revised date: 5 November 2015 Accepted date: 6 November 2015 Cite this article as: Saadia Rasheed, Hafiz Sartaj Aziz, Rafaqat Ali Khan, Asad Muhammad Khan, Abdur Rahim, Jan Nisar, Syed Mujtaba Shah, Farasat Iqbal and Abdur Rahman Khan, Effect of Li-Cu doping on structural, electrical and magnetic properties of cobalt ferrite nanoparticles, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.11.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Effect of Li-Cu doping on structural, electrical and magnetic properties of cobalt ferrite nanoparticles Saadia Rasheeda, Hafiz Sartaj Aziza, Rafaqat Ali Khana*, Asad Muhammad Khana, Abdur Rahimb, Jan Nisarc, Syed Mujtaba Shahd, Farasat Iqbalb, Abdur Rahman Khana a

Applied and Analytical Chemistry Laboratory, Department of Chemistry, COMSATS Institute

of Information Technology, Abbottabad-22060, Khyber Pakhtunkhwa, Pakistan b

Interdisciplinary Research Center in Biomedical Materials, COMSATS Institute of Information

Technology, Lahore, Pakistan c

National Center of Excellence in Physical Chemistry, University of Peshawar, Pakistan

d

Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan

*

Corresponding author

Tel: +92-0340-9324600, Fax: +92–992-383441, E-mail: [email protected] Effect of Li-Cu doping on structural, electrical and magnetic properties of cobalt ferrite nanoparticles Saadia Rasheeda, Hafiz Sartaj Aziza, Rafaqat Ali Khana*, Asad Muhammad Khana, Abdur Rahimb, Jan Nisarc, Syed Mujtaba Shahd, Farasat Iqbalb, Abdur Rahman Khana a

Applied and Analytical Chemistry Laboratory, Department of Chemistry, COMSATS Institute

of Information Technology, Abbottabad-22060, Khyber Pakhtunkhwa, Pakistan

1

b

Interdisciplinary Research Center in Biomedical Materials, COMSATS Institute of Information

Technology, Lahore, Pakistan c

National Center of Excellence in Physical Chemistry, University of Peshawar, Pakistan

d

Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan

Abstract CoFe2-xLixCuxO4 (x= 0.00, 0.05, 0.1, 0.15, 0.2, 0.25) spinel ferrite nanoparticles have been synthesized using CTAB assisted hydrothermal method. Structural characterization of the ferrite powders was carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Employing X-ray diffraction analysis, the synthesized samples were found to be in single crystalline phase with crystallite size of 25-29 nm. The microstructural analysis revealed that the shape of particles can be changed considerably by selecting proper content of the dopants. The room temperature resistivity of all the doped samples was observed to decrease with increase in Li-Cu doping up to a value of 1.9

106 ohm.cm. The dielectric measurements showed the

normal Maxwell Wagner type dielectric dispersion due to interfacial polarization. The optimum values of saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) were found as 46emu/g, 12emu/g and 284 Oe respectively for Li-Cu content of x=0.10.

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Keywords: A. Powders: chemical preparation; C. Dielectric properties; C. Magnetic properties; D. Ferrites; E. Soft magnets *

Corresponding authors

Tel: +92-0340-9324600, Fax: +92–992-383441, E-mail: [email protected]

3

Introduction Material science being a fascinating field is meant to meet the requirements of ever-growing technological demands in due course of time. Varieties of new materials with diverse properties are continuously developed and produced through different synthetic routes and are made available to the consumers. In material science, nanoparticles with unique properties and applications serve the emerging technological demands and needs of electrical and magnetic devices [1]. Nanoferrites proved their worth as promising materials in applications such as highdensity magnetic storage devices, electronic devices and various biological applications [2]. Ferrites are famous for afore mentioned applications because of their low production cost, higher efficiency as compared to the other alloys and their excellent properties at nanoscale [3]. Spinel ferrites have been widely explored for their potential applications in electrical and magnetic devices [4]. Cobalt ferrite being hard magnetic material, exhibits high coercivity 5400 Oe), moderate saturation magnetization (80 emu/g), high magnetocrystalline anisotropy and high thermal and chemical stability and thus serves as promising candidate for microwave devices [5]. Electrical and magnetic properties of cobalt ferrites can be tailored by the addition of small amount of impurities for the purpose of employing them in various applications [6]. Recently, it has been found out that the saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) decrease with increase in the amount of Cu2+ in a spinel ferrite [7]. In another study relating to the influence of Cu2+ substitution on various properties of cobalt ferrite, it was observed that the electrical conduction was significantly increased [8]. Li1+ doping in cobalt ferrite enlarged lattice constants and increased the room temperature resistivity, which 4

was attributed to the charge carrier mobility [9]. It has also been reported that, magnetization in Li doped cobalt ferrite decreases (69.59emu/g to 47.71 emu/g) along with the increase in coercivity w (647 Oe to 802 Oe) due to the increase in Li content and vice-versa [10]. It is a well established fact that the synthesis methods, chemical compositions of precursors, sintering temperature and time have a profound effect on the quality and properties of spinel ferrites. Several methods have been reported for the synthesis of cobalt ferrite nanoparticles including co-precipitation [11], micro emulsion [12], spray pyrolysis [13], sol-gel auto combustion [14] and hydrothermal method [15]. We used hydrothermal method to synthesize LiCu doped cobalt ferrite because the method is low cost, environment friendly and can be used at low temperatures. Due to their low cost and enhanced magnetic properties, Li1+ doped ferrites are excellent materials for applications in high density recording media and microwave devices [10]. Similarly, Cu2+ doped ferrites showed excellent characteristics for their possible applications in sensors, as catalysts and in electronic devices because of their moderate conductivity [8]. However, literature on the systematic study of Li-Cu co-substituted cobalt ferrites is scarce. Thus the purpose of present work is to study the structural, electrical and magnetic properties of Li-Cu substituted cobalt ferrite with proposed chemical formula of CoFe22xLixCuxO4(x=0.05,

0.1, 0.15, 0.2, 0.25).

2. Experimental Procedure: The chemicals used for the synthesis of Li-Cu doped cobalt ferrite were (FeNO3)3.9H2O (Merck 98.0%), (CoNO3)2.6H2O (BDH 97.0%), LiCl (Sigma Aldrich 98.0%), (CuNO3)2.3H2O (Merck 99.0%), NaOH (Merck99.0%), CTAB (Merck 99.0%). Hydrothermal method was used for the synthesis of CoLixCuxFe2-2xO4 (x=0.05, 0.1, 0.15, 0.2, 0.25). The stoichiometric amounts of the 5

corresponding chemicals were dissolved in distilled water in order to obtain a homogeneous mixture of solution. After the addition of CTAB, the solution was stirred at room temperature for 15 minutes using a magnetic stirrer. The pH of the solution was raise to 12 by drop wise addition of 2M sodium hydroxide solution. The solution was then transferred to Teflon autoclave and was kept in an oven at 180°C for 6 hours. The precipitates thus obtained were washed with distilled water and dried at 90 °C for 15 hours. The dried powder was annealed at 750 °C for 4 hours in order to get a single crystalline phase of CoLixCuxFe2-2xO4. The as prepared samples were thermally characterized using a Simultaneous Thermogravimetric/Differential Thermal Analyzer (TG/DTA, Model: SDT Q600) to get an idea about the optimum annealing temperature. X-ray diffraction pattern (XRD) has been carried out for structural and phase purity determination using diffractometer Phillips X’Pert PRO 3040/60) that employs CuKα radiations. The scanning images were acquired in secondary electron mode using tungsten filament based scanning electron microscope (VEGA3 LM, TESCAN, Czech Republic). All samples were analyzed for room temperature DC electrical resistivity measurements using two-point probe method. We made use of RF impedance/materials analyzer (Agilent E4991A) in a range from 1MHz to 1GHz for studying dielectric properties of synthesized cobalt ferrite nanoparticles. Room temperature magnetization measurements were performed using the vibrating sample magnetometer (VSM-515AUTO). 3. Results and Discussion 3.1 Thermal Analysis Thermal analysis (298K to 1273K with the heating rate of 10K/min) was carried out for the as obtained sample from the autoclave with a proposed composition of CoFe1.9Li0.05Cu0.05O4 and the 6

weight losses are shown in Fig 1. Clearly, TG/DTG curves show three prominent weight losses in the studied range. A gradual weight loss at and around 373K is due to the evaporation of absorbed water because of hygroscopic nature of samples [16]. A sharp weight loss occurred between 500 and 573K, which reveals the thermal decomposition of CTAB molecule in this temperature range [17]. Around 873K, the weight loss may be attributed to the decomposition of the metal hydroxides in the precursor powder that possibly convert to the respective oxides [18]. No further weight loss above ~900K confirms our faith in the mechanism that for getting single crystalline phase, the optimum temperature for annealing the synthesized samples should be above 900K. Therefore, we annealed our samples at a temperature of 1023K that is well above 900K and hence tried to reduce possibility of extra phases in our final product. 3.2. Structural Analysis Spinel structure of substituted cobalt ferrites CoFe2-2xLixCuxO4 (x=0.00, 0.05, 0.1, 0.15, 0.2, 0.25) synthesized by CTAB assisted hydrothermal method has been confirmed by the X-ray diffraction (XRD) as shown in Fig. 2 (representative samples). The characteristic peaks with miller indices of (220), (311), (400), (511) and (440), perfectly matched with the standard pattern (00-001-1121) of cobalt ferrite thus confirming the successful synthesis of single crystalline cubic phase. The lattice constant was determined by using the equation, , where‘d’ is the inter planar distance and (hkl) are the Miller indices. A continuous increase in the lattice constant ‘a’ as shown in Table 1, can be explained on the basis of Vegard’s law that describes the change in lattice parameters on the basis of ionic radii of the substituted ions [19]. Hence the replacement of Fe3+ (0.64Å) [20] with large ionic radii ions such as Cu2+ (0.73 Å) [21] and Li1+ (0.74Å) [9] resulted in the increase of the lattice parameter ‘a’. A regular 7

increase in cell volume (Vcell) (Table 1) with increasing content of Cu-Li ions in the crystal structure is attributed to the increase in lattice constant. The average crystallite sizes of all samples were calculated using Debye-Scherer equation

, where ‘k’ is shape

factor, ‘λ’ is the wavelength of X-rays, and ‘β’ is the full width at half maximum of the corresponding peak. The crystallite sizes thus calculated were found in the range from 24 to 29nm as shown in Table 1. The X-ray density was calculated using the formula,



,

where ‘M’ is the molar mass of the sample, ‘a’ is the lattice constant and ‘NA’ is Avogadro’s number. The calculated X-ray density decreased regularly with increasing in amount of dopants (Table 1), which is due to the constant decrease in molar mass of the sample. 3.3 Morphology Fig. 3 shows the scanning electron microscopy images for CoFe2O4 (a, b) and CoFe1.5 Li0.25Cu0.25O4 (c, d) samples. It can be seen for undoped samples (Fig 3a, b) that the particles have more or less irregular shape with broad size distribution, however revealing a coagulation phenomenon. The particles shape changed significantly by doping with Li-Cu ions and the particles adopt needle shaped morphology (Fig 3c, d). This change in shape of particles can be noticed very well when a comparison is made at a scale of 50µm for both samples (Fig 3b, d). One can make use of this needle shaped pattern in various fields including catalysis, photonics, imaging, sensing etc. 3.4 Electrical Resistivity Measurements Room temperature electrical resistivity measurements for Li-Cu doped CoFe2O4 were recorded using two-point probe method. Fig. 4 shows the variation in the room temperature electrical

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resistivity of CoFe2-2xLixCuxO4(x=0.05, 0.1, 0.15, 0.2, 0.25) with the corresponding values shown in Table 1. The room temperature electrical resistivity values of all the Li-Cu doped samples were observed to decrease as compared to the un-doped cobalt ferrite (Table 1). In case of ferrites, it is an established fact that the same element existing in more than one valence states, distributed over the crystallographic sites are generally responsible for the conduction phenomenon [22]. In addition, the conductivity in ferrites is supposed to take place due to impurities (more distinct at low temperatures), due to the polaron hopping and the magnetic ordering in the crystal structure. Moreover, the shifting of polaron to the adjacent lattice sites having same element with different ionization states also contribute to the conduction phenomenon in ferrites. In spinel ferrites, the hopping mechanism is more pronounced at the octahedral-B sites as these sites are at close distance to each other in comparison to that of A-B sites while no hopping takes place between A-A sites [23]. Having all this discussion and looking at the variations of electrical resistivity in Fig. 4, the trend can be explained as follows. It has been reported earlier that normally Li1+ prefers to occupy octahedral site in spinel ferrites but in the presence of ions having strong preference for B-site (Cu, Cr, V, Ni), Li1+ migrates to tetrahedral site [7, 24] thus replaces Fe3+ ions at these sites. Moreover, Cu2+ has strong preference to substitute at B site and hence plays an important role in replacing Fe3+ ions on octahedral sites [8]. Thus, when Li1+ and Cu2+ ions are co-substituted in cobalt ferrite, Li1+ occupies tetrahedral site and Cu2+ takes octahedral site. Therefore, the decrease in resistivity can be assigned on one hand to the increased concentrations of Fe2+/Fe3+ pairs at octahedral sites because of Li1+ ions preferences towards tetrahedral sites and on the other hand, to the accumulation of Cu1+/Cu2+ at octahedral sites. The Cu1+/Cu2+ pair itself has started to take part in the hopping mechanism of electron through the following mechanism: 9

As n–type conduction takes place by hopping of electrons between Fe2+ and Fe3+ ions, therefore, p-type conduction due to the hole exchange between Cu2+ and Cu1+ ions could not be totally overruled [25].

Thus the Li-Cu substitution in cobalt ferrite tends to decrease the electrical resistivity for all the samples and was found to be 1.9

106 ohm.cm at x= 0.1. At concentration x= 0.15, a slight

increase in resistivity was observed, which may be due to the fact that Li1+ ions present at tetrahedral sites started shifting to occupy some octahedral sites, thus resulting into a decrease in the hopping probability and in turn a trivial increase in resistivity. Above x=0.15, again a decrease in resistivity was observed. 3.5 Dielectric Measurements: The main factors influencing the dielectric properties of ferrite nanoparticles include the synthesis method, grain size and the cation distribution. Fig 5 and 6 show the room temperature variation of dielectric constant and dielectric loss as a function of frequency in a range from 1 MHz to 3 GHz. Dielectric dispersion is observed in all the samples under study where the real part έ) and imaginary part (

decrease with increase in frequency due to space charge effect.

Maxwell- Wagner interfacial type polarization phenomenon can be used to explain the dispersion of dielectric constant with frequency that is in good agreement with the Koop’s

10

phenomenological theory [26]. The polarization decreases with increase in applied frequency and after certain frequency value, it becomes constant. The decrease in the dielectric constant values reveals that the polarization mechanism in ferrites is same as exhibited in case of conduction process. The displacement of electron in direction of applied field due to electronic exchange between

, determines the polarization in ferrite [27]. Beyond a certain

frequency, the exchange of electrons between ferric and ferrous ions cannot follow the applied field, which results in decrease of polarization. Thus, the dielectric constant values decreases with increase in frequency [28]. The imperfection in crystal lattice and impurities in ferrites cause the polarization to lag behind the applied field and this fact contributes towards the dielectric loss [29]. The loss factor is usually caused by domain wall resonance. Losses are found to decrease with increase in the frequencies because of the inhibition of domain walls movement and thus the magnetization tends to change by rotation. Moreover, a rapid decrease in dielectric loss value is observed at low frequencies, however, when the frequency increases the rate of decrease in dielectric loss turns out to be slow and becomes independent of frequency. Therefore, the low loss values at higher frequencies make the samples promising candidate for high frequency applications [12]. This can be explained based on the fact that in low frequency regions due to grain boundaries, resistivity is high and more energy is required for the exchange of electron between Fe2+ and Fe3+ ions. Thus, loss is high at low frequencies but at high frequencies, less energy is required for electron exchange between Fe3+ ions at octahedral sites because resistivity is low due to grains. Other factors involved in high dielectric loss values at low frequencies may be the impurities, moisture or crystal defects. The dielectric constant and dielectric loss values when calculated at 0.5 GHz,

11

showed that the dielectric constant and dielectric loss was maximum at x =0.05 (Table 1). The maximum dielectric loss was observed in the frequency range of 1.5 GHz to 3 GHz indicating the liability of this material’s use in microwave devices in this frequency range. 3.6. Magnetic Measurements The room temperature magnetic measurements were carried out using vibrating sample magnetometer and the hysteresis loops are shown in Fig. 7. The parameters determined by these measurements include saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) and are shown in Table 1 for all Li-Cu doped samples. It was observed that the saturation magnetization increases up to 46emu/g Li-Cu for content of x=0.10 but a decrease is observed above x=0.1 and it reaches up to 12emu/g at x=0.25. There is constant decrease observed in the remnant magnetization (Mr) from 15emu/g to 5 emu/g with increasing the Li-Cu content from x= 0.05 to 0.2. In ferrites, the super-exchange interaction between cations conciliated by oxygen ions keeps all the cations present at one location (A-site) in one direction and for the other site (B-site) in opposite direction. Thus, the total magnetic moment is calculated by the difference of magnetic moment of B-site cations and A-site cations. The magnetic moment per formula unit is generally calculated by using an equation as follows.

The theoretical magnetic moments for Fe3+, Cu2+ and Li1+ are 5 µB, 1 µB, and 0 µB. It is evident that the Li1+(0 µB) ion has tetrahedral site preference in spinel structure while Cu2+(1 µB) has

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octahedral site preference. Thus the substitution of both the ions will replace Fe3+(5µB) ions at tetrahedral as well as octahedral site, reducing the net magnetic moment of the crystal structure. Along with this, the replacement of Fe3+ ions with Li1+ and Cu 2+ions also results in reduction of super exchange interaction between A and B sites [30]. Thus, the decrease in net magnetic moment as well as super-exchange interaction between the sites resulted in decrease saturation magnetization at higher Li-Cu content level. This reduced value of remnant magnetization may be due to two factors i.e. reduction in magnetocrystalline anisotropy of the structure with increase in dopant concentration and the presence of hematite α-Fe2O3) impurity phase in the sample may also result in the reduction of remanence [31]. The coercivity (Hc), is a parameter that is dependent on the saturation magnetization and can also be effected or changed by the heat treatment or deformation, critical diameter and domain structure. The coercivity can be calculated by using the Brown’s relation,

where K1, is the magneto-crystalline anisotropy of the crystal and Ms is the saturation magnetization. Thus it is clear from the relation above that the coercivity is directly related to the magneto-crystalline anisotropy [32]. Therefore, the substitution of Li1+ and Cu2+ in cobalt ferrite reduces the magneto-crystalline anisotropy, which in turn decreases the coercivity, thus shifting the properties of cobalt ferrite to soft magnetic ferrite. From Table 1 is indicative of the fact that the coercivity is 455Oe in pure cobalt ferrite sample that has been reduced constantly up to the 221Oe by Li-Cu doping at x=0.2

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4. Conclusions Li-Cu doped cobalt ferrites were successfully prepared by hydrothermal method with varying concentrations of dopants. The X-ray diffraction studies clearly showed the formation of single phase spinel structure. The lattice parameter was found to increase with increase in dopant content. The Li-Cu substitution in cobalt ferrite was found to have a significant effect on both the structural and magnetic properties. The average crystallite size was found out to be in the range of 25-29 nm while a gradual increase in the cell volume was also noticed. The room temperature electrical resistivity of the synthesized samples was observed to decrease up to 1.9

106 ohm.cm

at content level of x =0.1 making these materials suitable for their use in line transmissions and microwave devices. The dielectric properties showed the typical phenomenon of ferrites and a decrease with increase in frequency confirmed the Maxwell-Wagner interfacial polarization. The magnetic measurements showed the decrease in coercivity Hc, saturation magnetization Ms and remnant magnetization Mr with increasing Li-Cu content level. Thus shifting the properties of cobalt ferrite towards soft magnetic ferrite make these materials a promising candidate for high frequency applications. Acknowledgement COMSATS Institute of information technology, Abbottabad is highly acknowledged for providing necessary facilities to complete the project.

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[16] R. Kambale, P. Shaikh, N. Harale, V. Bilur, Y. Kolekar, C. Bhosale, K. Rajpure, Structural and magnetic properties of Co1−xMnxFe2O4 0≤ x≤ 0.4) spinel ferrites synthesized by combustion route, Journal of Alloys and Compounds, 490 (2010) 568-571. [17] U. Kurtan, R. Topkaya, A. Baykal, M. Toprak, Temperature dependent magnetic properties of CoFe2O4/CTAB nanocomposite synthesized by sol–gel auto-combustion technique, Ceramics International, 39 (2013) 6551-6558. [18] A.M.R.d.F. Teixeira, T. Ogasawara, M.C.d.S. Nóbrega, Investigation of sintered cobalt-zinc ferrite synthesized by coprecipitation at different temperatures: a relation between microstructure and hysteresis curves, Materials Research, 9 (2006) 257-262. [19] C.S. Kim, S.W. Lee, S.I. Park, J.Y. Park, Y.J. Oh, Atomic migration in Ni-Co ferrite, Journal of Applied Physics, 79 (1996) 5428-5430. [20] A. Farea, S. Kumar, K.M. Batoo, A. Yousef, C.G. Lee, Structure and electrical properties of Co0.5CdxFe2.5−xO4 ferrites, Journal of alloys and Compounds, 464 (2008) 361-369. [21] M.J. Iqbal, M.N. Ashiq, Physical and electrical properties of Zr-Cu substituted strontium hexaferrite nanoparticles synthesized by co-precipitation method, Chemical Engineering Journal, 136 (2008) 383-389. [22] N. Sivakumar, A. Narayanasamy, J.-M. Greneche, R. Murugaraj, Y. Lee, Electrical and magnetic behaviour of nanostructured MgFe2O4 spinel ferrite, Journal of Alloys and Compounds, 504 (2010) 395-402.

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0≤ x≤ 0.15) Multiferroic System,

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Figure 1

Figure 1

19

Figure 2

Figure 2

20

Figure 3

Figure 3 (a, b)

21

Figure 3 (c, d)

22

Figure 4

Figure 4

23

Figure 5

Figure 5

24

Figure 6

Figure 6

25

Figure 7

Figure 7

26

Table 1 Structural, electrical and magnetic parameters calculated for CoFe2-2xLixCuxO4 samples Parameters

Li-Cu Content, x 0.00

0.05

0.10

0.15

0.20

0.25

Lattice constant (a/Ǻ)

8.33

8.33

8.35

8.34

8.36

8.36

Crystallite size (D/nm)

26

28

25

25

29

26

Cell Volume (V/Ǻ3)

578

572

580

582

584

584

X-ray density (g/cm3)

5.3

5.3

5.2

5.2

5.1

5.0

Resistivity ρ/106 ohm.cm)

28

8.1

1.9

5.0

3.1

2.2

Dielectric constant εˊ ) @0.5GHz

13

14.9

4.68

5.13

4.47

5.11

Dielectric loss (ɛ ˊ ˊ ) @ 0.5 GHz

5.76

6.13

0.97

1.05

3.13

0.60

Saturation Magnetization, Ms (emu/g)

39

43

46

20

13

12

Remnant Magnetization, Mr (emu/g)

15

10

12

8

5

7

Coercivity, Hc (Oe)

455

373

284

258

221

225

20

Figure Captions Figure 1 Themogravimetric analysis and Derivative of thermogravimetric analysis (TGA/DTG) of CoFe1.9Li0.05Cu0.05O4 ferrite precursor Figure 2 X-ray diffraction patterns for the nanoparticles of CoFe2-2xLixCuxO4 Figure 3 Scanning electron images of (a, b) CoFe2O4 (c, d) CoFe1.5Li0.25Cu0.25O4 Figure 4 Variation in the room temperature resistivity for CoLixCuxFe2-xO4 spinel ferrites versus Li-Cu content x Figure 5 Variations of dielectric constant εˊ ) versus frequency (GHz) for CoLixCuxFe2-2xO4 samples Figure 6 Variation of dielectric loss εˊ ˊ ) versus frequency (GHz) for CoLixCuFe2-2xO4 samples Figure 7 Hysteresis loop for the nanoparticles of CoLixCuxFe2-2xO4 spinel ferrites

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