Influence of Co2+ on structural and electromagnetic properties of Mg–Zn nanocrystals synthesized via co-precipitation route

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Influence of Co2 þ on structural and electromagnetic properties of Mg-Zn nanocrystals synthesized via co-precipitation route Muhammad Amir Rafiq, Muhammad Azhar Khan, M. Asghar, S.Z. Ilyas, Imran Shakir, Muhammad Shahid, Muhammad Farooq Warsi

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Cite this article as: Muhammad Amir Rafiq, Muhammad Azhar Khan, M. Asghar, S.Z. Ilyas, Imran Shakir, Muhammad Shahid, Muhammad Farooq Warsi, Influence of Co2 þ on structural and electromagnetic properties of Mg-Zn nanocrystals synthesized via coprecipitation route, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.04.141 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.

Influence of Co2+ on structural and electromagnetic properties of Mg-Zn nanocrystals synthesized via co-precipitation route Muhammad Amir Rafiqa, Muhammad Azhar Khana, M. Asghar*a, S. Z. Ilyasb, Imran Shakirc, Muhammad Shahidd, Muhammad Farooq Warsi*d a

Department of Physics, The Islamia University of Bahawalpur, Bahawalpur-63100, Pakistan

b

Department of Physics, Allama Iqbal Open University, Islamabad, Pakistan

c

Deanship of Scientific Research, College of Engineering, P.O. Box 800, King Saud University, Riyadh 11421, Saudi Arabia

d

Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur-63100, Pakistan

*Corresponding Author: [email protected] Phone: +92 62 9255473, Fax: +92 62 9255474

Abstract Co-precipitation method was used to synthesize the nanoparticles of cobalt substituted zinc. magnesium ferrites having formula “Mg0.6-0.5xZn0.4-0.5xCoxFe2O4” where Fabricated samples were annealed at 750˚C for 6 hours. The cubic spinel structure of Mg0.60.5xZn0.4-0.5xCoxFe2O4 nanocrystals was confirmed by Fourier transformed infra-red (FTIR) and X-ray diffraction (XRD). Lattice parameter, crystalline size, cell volume, X-ray density, bulk density and porosity were also determined using XRD data. Lattice parameter exhibits overall decreasing trend (0.824-0.817 nm) with cobalt content; it is due to the substitution of cobalt (having smaller ionic radii) with magnesium and zinc ions. Cation distribution among A and B sites were studied by FTIR spectrum. Vibrating sample magnetometery (VSM) was used to investigate magnetic properties of as prepared nanoparticles. Coercivity exhibits the inverse relation to crystalline size. Lowest value of coercivity (47.722 Oe) was obtained for the sample having x=0.15. Dielectric constant, dielectric loss and dielectric tan loss were inversely related with the frequency. Key Words: Nano-ferrites; XRD; Dielectric parameters; Coercivity. 1 Introduction Magnesium-zinc ferrites are very helpful for reducing hysteresis loses [1], high frequency devices fabrication [2-4], high density media storage devices, magnetic reading and recording data [4], sensor devices[1, 2], and bio-medical applications [3]. Nanoparticles of spinel ferrites are potential candidates with respect to research as well as industrial point of view due to their distinctive and remarkable magnetic, electrical, dielectric and structural

properties [3]. As compared to other ferrites, spinel ferrites having chemical formula “MFe2O4” (where M is a divalent metal ions) are important magnetic material because they have high resistivity, low dielectric losses[2], good thermal stability[4] and high saturation magnetization. Magnetic parameters of Mg-Zn ferrites strongly depend on particle size which depends on sintering time, sintering temperature and grown techniques[5]. Because of unquenched orbital angular momentum, Co2+ ions are recognized to transform the magneto-crystalline anisotropy. Therefore the substitution of cobalt in Mg-Zn ferrites makes some important modification which enhances magnetic and dielectric properties [6]. Lodhi et al. described that magnetic and dielectric properties of Mg-Zn-Co ferrites depend upon chemical composition and cation distribution between octahedral and tetrahedral sites [3]. It is already reported that magnetic and electrical properties of ferrites have inverse relation with crystalline size [7]. To synthesize ferrites in nano-scale, there are a number of synthetic approaches to synthesize nanoferrites such as sol-gel [8-11], micro-emulsion [3], solid state [12], micro wave sintering [13], novel combustion [14], two step direct micro-emulsion [15], co-precipitation [16, 17], hydrothermal [18], mechano-chemical [19] etc. In this present work, we choose, co-precipitation route to prepare magnesium-zinc-cobalt ferrites having chemical formula “Mg0.6-0.5xZn0.4-0.5xCoxFe2O4” where (with step size 0.05) because it is less toxic, economic and environment friendly growth technique. Effect of cobalt doping on Mg-Zn ferrites has been studied to enhance the magnetic and electrical properties to make these nanoferrites suitable for high frequency electronic devices. 2 Experimental 2.1 Synthesis “Mg0.6-0.5xZn0.4-0.5xCoxFe2O4” where

(with step size 0.05) nano-ferrites were

fabricated by using co-precipitation method [16, 20]. The following chemicals were used for the fabrication of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4 ferrites nanoparticles; Aqueous Ammonia NH4OH (BDH, 35%), (CH3COO)2Mg.4H2O (BDH, 99%), CH3COO)2Co.4H2O (BDH, 98%), ZnCl2 (Merck Germany, 98%), and [Fe(NO3]3 9H 2O (Merck Germany, 98%). 0.1M solutions of Mg, Zn and Co was prepared in distilled water, on the other hand 0.2M solution of Fe was also prepared. These solutions were mixed according to calculated ratio and placed on hot plate for stirring and heated up to 55˚C. Initially, all solutions were acidic having pH about 23. Furthermore, heating of solutions was stopped and pH of all reactions was raised to 10 to form basic media by adding aqueous ammonia in the solutions. After achieving basic media, all mixtures were put on magnetic stirring for 6 hours at room temperature. After completing the process of stirring, all mixtures were left as such for 12 hours; the precipitates were formed and settled down. All the six types of particles, thus grown were washed with distilled water (H2O) until all mixtures became neutral, having pH level approximately equal to 7.0. All beakers containing different compositions were placed in oven at 80˚C for drying. Grown precipitates were grinded with mortar and pestle. Grinded nanoparticles of all samples were

sintered at 750˚C for 6 hours by using fully controlled and automatic Muffle Furnace Vulcan A-550. 2.2 Analysis and Characterization Fabricated nanocrystalline ferrites having chemical formula “Mg0.6-0.5xZn0.4-0.5xCoxFe2O4” were characterized by various techniques. Formation of single phase spinel structure and related properties were evaluated by X-ray diffractometer (model Philips PW 1710 using Cu Kα radiation having wavelength 1.5414 Å) and fourier transform infrared spectrometer (Nexus 470). Magnetic properties were studied by vibrating sample magnetometery (Lakeshore-74071) at 300K whereas LCR meter (Wayne Ker WK6500B) was used to investigate dielectric parameters in frequency range 1MHz to 3 MHz at room temperature. 3 Results and discussion 3.1 XRD X-ray diffraction patterns of cobalt substituted Mg-Zn ferrites are shown in figure 1. In the XRD patterns, prominent peaks were observed from following planes (111), (200), (220), (311), (222), (400), (422), (511), (440), (620) and (533); which confirm that all grown samples are comprised of single phase FCC spinel structure without showing any other additional phase. Such diffraction peaks were already reported for face centered cubic spinel ferrites in literature [3, 6, 21]. Grain sizes of nanoferrites were evaluated by using well known formula named after a physicist P. Scherrer [3, 6, 22]; (1) Where peak and

is wavelength of X-ray (1.5414 Å),

is full width half maxima (FWHM) of intense

is diffraction angle of that peak. It has been observed from Table 1 that grain size

decreases with cobalt contents “x” and it reaches minimum value (30.94nm) at x=0.05. It is because, the substitution of smaller radii of cobalt ions with magnesium-zinc ions. Further addition of cobalt contents causes an increase in grain size; it may be due to the migration of Fe3+ cationic to tetrahedral sites from octahedral sites to maintain charge neutrality [3, 7]. Mean lattice constant has been estimated by using following function [23]; (2) Where “ ” is the d spacing and h, k, l are the miller indices of the planes associated with particular peaks. An appreciable decrease has been observed in lattice constant (8.256-8.17 Å) of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4 with cobalt contents. Initially lattice constant shows decreasing tendency for first two samples (x=0.00, 0.05). It may be due to the fact that replacement of magnesium-zinc ions by cobalt ions having smaller ionic radii as compared to magnesium-zinc [3, 7]. Further increase in x, the lattice constant increases; because the

segregation of cobalt ions at the grain boundaries [24]. Bulk density of each pallet of all composition has been calculated by using following equation [11, 22]; (3) Where “m” is the mass of pallet, “ ” radius of pallet and h is the thickness of pallet. X-ray density has been estimated by using following relation [22]; (4) Where “M” is molar weight of sample, “NA” Avogadro’s No (

) and “Vcell” is the

3

volume of unit cell (a ). Porosity of samples were evaluated by using following relation [25]; (5) It has been observed from Table 1 that X-ray densities of all samples are greater than bulk densities; this indicates the presence of pores space in grown samples which were produced during fabrication or sintering process[13]. 3.2 FTIR Ferrites characteristics base absorption bonds

and

(516.8-532.2 cm-1 and 447.4-

470.5cm-1 respectively) have been observed in FTIR-spectrum (Figure 2) of Mg0.6-0.5xZn0.4and 0.5xCoxFe2O4 Nano-precipitate which confirm the formation of spinel ferrites [3]. bonds associated with tetrahedral and octahedral sites respectively [26]. It is already reported that bond length and wavenumber (or vibrational frequency) are inversely related [1]. It can be noticed that bond length of tetrahedral site increases with cobalt contents. Whereas octahedral bond length decreases with cobalt contents; it may be due to the migration of Fe3+ ions form octahedral to tetrahedral site [1]. 3.3 VSM Vibrating sample magnetometery (VSM) technique was used to evaluate magnetic properties of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4. Variation in coercivity “Hc” of spinel ferrites is strongly dependent on crystalline size; there is inverse relation between crystalline size and coercivity “Hc” of spinel ferrites, same behavior is reported in literature [11]. This inverse relation between coercivity and crystalline size can be observed clearly from figure 3. 3.4 Dielectric measurements Dielectric behavior of spinel ferrites depends upon chemical composition of the material, grain size, annealing temperature and fabrication methodology[3]. Dielectric constant “ ” was determined with the help of following equation[3]:

(6) Where “A” is cross-section area of pellet, “c” is capacitance of the pallet, “ ” is permittivity of the free space and “d” is the thickness of the pallet. Effect of frequency on dielectric constant “ ” of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4 has been shown in figure 4; dielectric constant “ ” for all fabricated samples shows decreasing trend with the frequency of applied electric field. Initially, value of dielectric constant is greater; this is due to the fact that at low frequency, dielectric constant is associated with the grain boundaries which have high resistivity. Further increase in frequency, dielectric constant decreases because at high frequency, the dielectric constant is correlated with the grain (instead of grain boundaries) which has low resistivity [3, 7]. Therefore it can be concluded that penetration of electric field into the dielectric material is related inversely with dielectric constant [27]. Variation in dielectric constant ε ′ with frequency can also be justified on the basis of electrons hopping mechanism between the ions. There is an exchange of electrons taking place between Fe3+ and Fe2+ ions in the direction of external field which produces polarization in ferrites, for that reason dielectric constant ε ′ reduces with frequency of the external applied electric field [6]. Dielectric loss “ ε ′′ ” is the total amount of absorbed energy by the sample from applied electric field. Effect on dielectric loss due to frequency also shows decreasing trend like dielectric constant shown in figure 5. Peelamedu et al. described that material with high resistivity normally have low dielectric losses [28]. Resonance peaks have been observed at high frequency; therefore Mg0.6-0.5xZn0.4-0.5xCoxFe2O4 nanoparticles are potential candidates for high frequency applications [27]. Dielectric tan loss was evaluated by using the equation as given bellow [3]; (7) Where ε ′ is the (real part of) dielectric constant, ε ′′ is the dielectric loss (imaginary part) and is the loss angle. Dielectric tan loss as function of frequency of applied electric field was calculated at room temperature. The inverse relation between dielectric tan loss and frequency is shown in figure 6. This variation in dielectric loss with respect to frequency may be due to several factors; such as conduction mechanism (hopping of electron ), materials composition of sample, annealing temperature, grown between technique and particle size [3, 7]. 4. Conclusion Mg0.6-0.5xZn0.4-0.5xCoxFe2O4 nanoparticles were prepared successfully with the help of coprecipitation method. X-ray diffraction patterns and FTIR spectra confirmed the successful growth of single phase spinel nanoparticles. Lattice constant as a function of cobalt contents decreases up to 8.17 Å. Minimum particle size has been achieved for Mg0.6-0.5xZn0.40.5xCoxFe2O4 at x=0.05. Vibration sample magnetometery (VSM) was used to evaluate the

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Table 1: Grain size, lattice constant, cell volume, bulk density, X-ray density and porosity of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4. Cobalt content “x”

x=0.00

x=0.05

x=0.1

x=0.15

x=0.2

x=0.25

Lattice constant (A)

8.246

8.246

8.246

8.246

8.246

8.246

560.695

560.695

560.695

560.695

560.695

560.695

Bulk density (g/cm3)

3.31

3.31

3.31

3.31

3.31

3.31

X-ray Density (g/cm3)

5.13

5.13

5.13

5.13

5.13

5.13

Porosity

0.355

0.355

0.355

0.355

0.355

0.355

Grain size (nm)

37.9

37.9

37.9

37.9

37.9

37.9

Cell volume (A3)

Figure1: XRD patterns Nano-precipitate of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4.

Figure 2: FTIR-spectrum of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4 Nanoprecipitate.

Figure 3: Inverse relationship between coercivity and crystalline size of Mg0.6-0.5xZn0.4-

0.5xCoxFe2O4.

Figure 4: Variation of relative permittivity with frequency of Mg0.6-0.5xZn0.4-0.5xCoxFe2O4 Nano-crystals.

Figure 5: Variation of dielectric loss with frequency at room temperature.

Figure 6: Variation of dielectric tan loss with frequency at room temperature.