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Current Applied Physics 14 (2014) 716e720

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Current Applied Physics

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New Mg0.5CoxZn0.5�xFe2O4 nano-ferrites: Structural elucidationand electromagnetic behavior evaluation

Maria Yousaf Lodhi a, Khalid Mahmood a, Azhar Mahmood b, Huma Malik a,Muhammad Farooq Warsi b,*, Imran Shakir c, M. Asghar a, Muhammad Azhar Khan a,*

aDepartment of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, PakistanbDepartment of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, PakistancDeanship of Scientific Research, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia

a r t i c l e i n f o

Article history:Received 3 January 2014Received in revised form2 February 2014Accepted 22 February 2014Available online 6 March 2014

Keywords:Nano-ferritesMicro-emulsion techniqueXRDDielectric propertiesMagnetic properties

* Corresponding authors. Tel.: þ92 333 5121491.E-mail addresses: azhar.khan@iub.edu.pk, azh

A. Khan).

http://dx.doi.org/10.1016/j.cap.2014.02.0211567-1739/� 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

In this work cobalt substituted magnesium zinc nanocrystalline spinel ferrites having general formulaMg0.5CoxZn0.5�xFe2O4 where x ¼ 0.1, 0.2, 0.3, 0.4, 0.5 were synthesized using micro-emulsion technique.The Co substituted samples annealed at 700 �C and characterized by various characterization tech-niques, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), dielectricmeasurements and vibrating sample magnetometer (VSM). XRD analysis confirmed single phase spinelstructure and the crystalline size calculated by Scherrer’s formula found to be in 21.38e45.5 nm range.The lattice constant decreases as substitution of Co is increased. The decrease in lattice constant isattributed to the smaller ionic radius of cobalt as compared to zinc ion. The FTIR spectra reveled twoprominent frequency bands in the wave number range 400e600 cm�1 which confirm the cubic spinelstructure and completion of chemical reaction. The dielectric parameters were observed to decreasewith the increased Co contents. The peaking behavior was observed beyond 1.8 GHz. The frequencydependent dielectric properties of all these nanomaterials have been explained qualitatively inaccordance with Koop’s phenomenological theory. Magnetic studies revealed that the coercivity (Hc)attains maximum value of 818 Oe at w21 nm. The increasing trend of magnetic parameters (coercivityand retentivity) is consistent with crystallinity. The crystallite size is small enough to attain consid-erable signal to noise ratio in high density recording media. The optimized magnetic parameterssuggest that the material with composition Mg0.5Co0.5Fe2O4 may have potential applications in highdensity recording media.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Spinel nano-ferrites are materials of today’s research due totheir amazing structural, dielectric, electrical and magneticproperties [1]. These properties make ferrites an ideal candidatefor technical applications such as magnetic recording devices,magnetic storage devices, magnetic resonance imagingenhancement, catalysis, sensors and pigments [2e5]. The spinel-type ferrites have closed pack structure with chemical formulaAB2O4, where A and B represent metallic ions located at differentsites i.e. A type metallic ions located at tetrahedral sites and Btype located at octahedral sites [6,7]. MgeCoeZn nano-ferrite is

ar_manais@yahoo.com (M.

an interesting material because of its high coercivity, moderatesaturation magnetization, good chemical stability and mechanicalhardness; therefore it can found applications in magnetic drugdelivery and high density information storage [8,9]. It is acceptedfact that magnetic and dielectric properties of MgeCoeZn ferritesare strongly depended upon the composition and cation distri-bution. For example MgeCoeZn ferrites can exhibit ferromag-netic, anti-ferromagnetic and paramagnetic behavior according tothe occupancy of cations in A- and B-sites, which can be incor-porated into the spinel lattice of the parent magnetic structure[10]. Furthermore the dielectric, electrical and magnetic proper-ties of ferrites can also be enhanced significantly by using nano-particles synthesis approach [12]. There are different synthesistechniques available in the literature for the synthesis of MgeCoeZn nano-ferrites like solegel [13], hydrothermal [14], mechano-chemical [15], precursor [16] and auto-emulsion method [17].Therefore the careful variation of Co and nano-crystalline

Fig. 2. Effect of Co concentration on the lattice constant and crystalline size ofMg0.5CoxZn0.5�xFe2O4 nano-ferrites.

M.Y. Lodhi et al. / Current Applied Physics 14 (2014) 716e720 717

synthesis of MgeCoeZn ferrites is still a key topic of study amongthe researchers and still needs to be investigated thoroughly inorder to enhance magnetic and dielectric properties.

In this paper, we have reported micro-emulsion method tosynthesize Mg0.5CoxZn0.5�xFe2O4 nano-ferrites. The crystalline sizedecreases appreciably with the addition of Co ions, confirmed byXRD measurements. VSM measurements elucidated that magneticproperties improved significantly with the increasing concentrationof Co contents in the samples. The coercivity of all the substitutedsamples was found to be a few hundred oersteds which is one of thenecessary conditions for recording media.

2. Experimental

The Mg0.5CoxZn0.5�xFe2O4 (x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5) nano-crystalline ferrites were prepared using micro-emulsion method asreported in the literature [10]. Following chemicals were used forthe preparation of Mg0.5CoxZn0.5�xFe2O4 nanocrystalline ferrites:Fe(NO3)2$9H2O (SigmaeAldrich, 98%), Zn(NO3)2 (SigmaeAldrich,98%), CoNO3 (SigmaeAldrich, 99%), MgNO3 (Sigma Aldrich, 99%),cetyltrimethyl ammonium bromide ((C16H33)N(CH3)3Br), aqueousammonia (NH4OH), 35 wt%, BDH, deionized water (conductivityw2e4 S). Required volumes of metal salts solutions having con-centrations 0.15 M were mixed and stirred on a magnetic hot plateat 50e60 �C. Aqueous solution of CTAB (100 mL, 0.45 M) was usedas surfactant. The pH value was adjusted by using 2 M ammoniasolution and the pH value was maintained 11e12 for all the mate-rials. The reaction mixture was further stirred for 4e5 h. The pre-cipitates were washed with deionized water until the pH reducedto 7.Water was evaporated in the oven at 100 �C, and annealingwascarried out at 700 �C for 7 h in a temperature controlled mufflefurnace Vulcan A-550 at a heating rate 15 �C/min. The obtainedmaterials were grinded into powder form and were characterizedby various techniques. The powder XRD analysis was carried out toconfirm the purity of the synthesize materials using Philips X’ PertPRO 3040/60 diffractometer with Cu Ka as radiation source. FTIRspectra were recorded on Nexus 470 spectrometer. The hysteresisloops measurements were carried out at room temperature usingvibrating sample magnetometer (VSM) Lakeshore-74071. Thedielectric measurements were performed using LCR meter at roomtemperature.

Fig. 1. XRD patterns for Mg0.5CoxZn0.5�xFe2O4 (x ¼ 0.1, 0.2, 0.3, 0.4, 0.5) nano-ferrites.

3. Results and discussion

3.1. Structural analysis

Fig. 1 displays the typical XRD spectra of cobalt substitutedmagnesium zinc nanoferrites with different composition (x¼ 0, 0.1,0.2, 0.3, 0.4 and 0.5). We have observed following peaks and suc-cessfully indexed as (111), (200), (220), (311), (222), (400), (422)(511) (440), (620) and (533) which are the characteristics planes ofsingle phase cubic spinel structure [18,19] indicating the solubilityof cations into their respective lattice sites. The crystallite size of allsamples were found using Scherrer’s formula [20,21]:

D ¼ 0:9 l

b cos q; (1)

where D is crystallite size, b is full width at half maximum (FWHM)of the most intense (311) peak, l is X-ray wavelength and q isdiffraction angle. The crystallite size is found to decreases from45.5 nm to 21.38 nm as the concentration of Co increases from x¼ 0to x¼ 0.5 as shown in Fig. 2. The lattice constant “a” decreases from8.378 to 8.366�A by Co2þ substitution. The cell volume has also beenobserved to decrease from 588 to 585 �A. The decrease in latticeconstant is attributed to the smaller ionic radius of Co2þ (0.74 �A)[22] than zinc Zn2þ (0.82 �A) [23]. Since smaller cations (Co2þ) arereplaced by the larger cations (Zn2þ) and hence decrease in latticeconstant is justified. The bulk densities of all samples were foundsmaller than X-ray densities. This indicates the existence of pores innano-crystallites which were formed and developed during thesample preparation or sintering the samples. The porosity valueswere found in the range of 0.68e0.73. It is also clear from Table 1that bulk density decreases with the increase of Co concentration,which may be due the fact that Co has lower atomic weight thanthe Zn ion.

Table 1Lattice constant (a), cell volume, bulk density, X-ray density, porosity and crystallitesize of the (MgCoxZn1�xFe2O4) nanoferrites.

Co content (x) x ¼ 0.0 x ¼ 0.1 x ¼ 0.2 x ¼ 0.3 x ¼ 0.4 x ¼ 0.5

Lattice constant (�A) 8.378 8.376 8.375 8.372 8.369 8.366Cell volume (�A)3 588.05 587.63 587.42 586.79 586.16 585.53Bulk density (g/cm3) 2.52 2.33 2.27 2. 21 2.14 2.10X-ray density (g/cm3) 4.83 4.81 4.79 4.78 4.77 4.77Porosity 0.48 0.52 0.53 0.54 0.56 0.56Crystalline size (nm) 41.08 38.34 33.69 30.79 25.50 21.30

Table 2The values of various magnetic parameters of “MgCoxZn1�xFe2O4” nanoferrites.

Sr.No

Coconcentration

Coercivity“Hc”(Oe)

Retentivity“Mr”(emu/g)

Magnetization“Ms” (emu/g)

Magnetic moment(Bohr magneton)

1 0 76.82 1.286 12.60 0.492 0.1 114.29 2.16 16.67 0.653 0.2 181.50 3.348 15.49 0.614 0.3 329.15 5.421 16.09 0.635 0.4 253.97 5.391 21.33 0.836 0.5 818.81 6.860 14.68 0.57

M.Y. Lodhi et al. / Current Applied Physics 14 (2014) 716e720718

3.2. Fourier transform infrared spectroscopy analysis

FTIR spectroscopy is used in order to investigate the cationicdistribution, the completion of chemical reaction, deformation inthe spinel structure due to the incorporation of foreign ions [24].Fig. 3 shows the representative FTIR spectra of Mg0.5CoxZn0.5�x-

Fe2O4 nano-ferrites for x ¼ 0 and 0.5. Both samples consist of twoprominent frequency bands around 400 and 600 cm�1 indicatingthat both samples have single phase cubic spinel structure [25]. Inferrites, the metal cations are situated in two different sublatticeswhich have tetrahedral and octahedral sites [26]. It is reported inliterature that in cobalt substituted zinc ferrites, the tetrahedral siteis occupied by Zn ion while cobalt and iron partially occupy boththe tetrahedral and octahedral sites [27]. Therefore presence of FTIRbands at 429, 424, 446 cm�1 and 517, 520 cm�1 are clearly indicatesthe occupation of Zn atoms on the tetrahedral site while Co on theoctahedral site. The band at 1129 cm�1 is due to formation of Cosubstituted spinel ferrites [28]. The absorption band at 2920 cm�1

is due to the stretching mode of HeOeH bending vibration of freeor absorbed water which implies that the hydroxyl groups areretained in ferrites [29].

3.3. Magnetic studies

The hysteresis loops of all the compositions of Mg0.5CoxZn0.5�x-

Fe2O4 nanoferrites were recorded against applied magnetic field inthe range of �10,000 Oe to 10,000 Oe and are shown in Fig. 4. Thehysteresis loops obtained for Mg0.5CoxZn0.5�xFe2O4 nanoferrites areobserved tobenarrowthat confirm the softmagnetic natureof thesenanoferrites. The magnetic parameters such as saturation magne-tization (Ms), retentivity (Mr) and coercivity (Hc)were obtained fromthese hysteresis curves by using reported formulas (Table 2). Themagnetic moment has been calculated by using the formula [30],

hB ¼ M �Ms

5585; (2)

whereM is molecular weight of the sample andMs is the saturationmagnetization. In a cubic system of ferrimagnetic spinels, themagnetic order is due to a super exchange interaction mechanismoccurring between the metal ions in the A and B sublattices [31]. InZnFe2O4, Zn2þ preferably occupies the tetrahedral A sites and Fe3þ

occupies the octahedral sites. The CoFe2O4 has inverse spinelstructure in which Co2þ occupy octahedral sites and Fe3þ ions areequally distributed in tetrahedral and octahedral sites, with their

Fig. 3. FTIR spectra of representative Mg0.5Co0.5Fe2O4 and Mg0.5Zn0.5Fe2O4 nano-ferrites.

spin in the opposite direction. The magnetic properties greatlyinfluenced due to Co2þ (3d7) ions having orbital moment of 3.7 mBper formula unit [32]. On the other hand, Mg2þ ions can exist inboth sites but strongly prefer to occupy octahedral site. The mag-netic moment of Mg2þ is zero, so in MgFe2O4 magnetic couplingspurely originate from the magnetic moment of Fe cations [33]. Themagnetic moment per formula unit, according to Neel’s two sub-lattice model is given by the difference of magnetic moment of eachsublattice and can be expressed as [34]:

M ¼ MA �MB; (3)

whereMA andMB are the A- and B-sublattice magnetic moments inmB respectively. The variations ofMs, Hc andMr with cobalt contentsare shown in Fig. 5. This could be due to large magnetic moment ofCo2þ (with magnetic moment of 3 mB) ions which are beingsubstituted by Zn i.e. diamagnetic having no magnetic moment. AsCoþ2 ions go to octahedral sites so Fe3þ ions will go to tetrahedral orA sites in order to maintain charge neutrality, and hence netmagnetization increase. The values of coercivity and retentivity arefound to increase by increasing Co contents. The increase in coer-civity with Co substitution is due to the fact that Co2þ ion has largemagneto crystalline anisotropy because of three unpaired electronsand strong LeS coupling [35]. The contribution to magneto crys-talline anisotropy from Fe3þ ions having five unpaired electrons isdifferent in all three systems. The Mg2þ and Zn2þ ions have nounpaired electrons and lead to zero total electron spin. It has beenobserved that for “Mg0.5Zn0.5Fe2O4” composition the hysteresisloop is typical S shaped with very low coercivity (Hc z 76.82 Oe)and maximum value of coercivity (Hc z 818.81 Oe) is obtained for“Mg0.5Co0.5Fe2O4” composition. These values are comparable toearlier reported data such as Ali et al. [36].

The saturation magnetization and coercivity of CoxZn1�xFe2O4prepared by microwave method was increased by increasing substi-tution of cobalt and it was reported that this is due to the anisotropic

Fig. 4. Hysteresis loops for Mg0.5CoxZn0.5�xFe2O4 nano-ferrites.

Fig. 5. The variation of Ms, Hc and Mr with cobalt content for Mg0.5CoxZn0.5�xFe2O4

nano-ferrites. Fig. 6. The variation of dielectric constant with frequency of Mg0.5CoxZn0.5�xFe2O4

nanoferrites.

Fig. 7. The variation of dielectric loss with frequency of Mg0.5CoxZn0.5�xFe2O4

nanoferrites.

M.Y. Lodhi et al. / Current Applied Physics 14 (2014) 716e720 719

nature andmagnetic character of cobalt. Asmore Co ions replacewithZn ions by increasing the x, the coercivity and the saturation magne-tization increases and the sample with x ¼ 0.8, Co0.8Zn0.2Fe2O4,showed ferromagnetic behavior at room temperature.

3.4. Dielectric studies

One of the most significant properties of ferrites is the dielectricbehavior which depends on the preparation conditions, composi-tion, sintering temperature, and the crystallite size [11,37,38]. Thedielectric constant of Mg0.5CoxZn0.5�xFe2O4 nanoferrites wasdetermined by using the formula [39]:

ε0 ¼ cd

εoA; (4)

where d is the thickness of the pellets, c is the capacitance of thepellets, εo is the permittivity of free space andA is the cross-sectionalarea of the flat surface of the pellets inm2. The variation of dielectricconstant (ε) with respect to frequency has been shown in Fig. 6. Thevalue of dielectric constant for all the samples decreases byincreasing frequency. The decrease in dielectric constant withincreasing frequency is due the fact that at higher frequencies anyeffect contributing to polarization is found to show lagging behindthe applied field. When frequency is increased beyond a certainfrequency limit, the electron hopping cannot follow the electric fieldfluctuations and causes decrease in the dielectric constant. Koop’s[40] revealed that at low frequency the dielectric constant originatesfrom the grain boundaries having a high dielectric constant due tohigh resistivity at the grain boundary region. Whereas, at high fre-quency the dielectric constant comes from the grains having a smallvalue of dielectric constant due to lowresistivity. Itmaybe concludedthat the electrons interchange between Feþ2 and Feþ3 ions results inthe direction of an applied electric field, which is responsible forelectrical polarization in ferrites. The other reasons of higher value ofdielectric constant at low frequencies may be due to dislocations,voids, and defects presents in the crystal structure of nano-ferrites.The dielectric loss (tan d) was calculated using the relation [41]:

tan d ¼ ε00

ε0 ; (5)

where tan d is the loss angle, ε00 is the imaginary part of the

dielectric constant and is a quantity of the absorption of energy bythe dielectric from the alternating field ε

0 is the real part of the

dielectric constant. The graph of dielectric loss tangent verses fre-quency is depicted in Fig. 7. The dielectric loss factor decreases byincreasing frequency. It is reported in literature that the values ofdielectric loss depend on number of factors, such as Feþ2 contents,structural homogeneity and stoichiometry, which in turn dependson the synthesis methods and composition. The decrease ofdielectric loss tangent with increase in frequency is due to the factthat there is strong association between dielectric behavior of fer-rites and the conduction mechanism. In ferrites, the conductionmechanism is considered due to hopping of holes in n-type andhopping of electrons in p-type. A maximum loss may be observed,when the hopping frequency and frequency of externally appliedelectric field are almost equal [42].

4. Conclusion

The effect of cobalt substitution on magnesium zinc nano-crystalline ferrites was studied. X-ray diffraction confirms the for-mation of single-phase crystalline structure and crystallite size inthe range 21.38e45.5 nm. The crystallite size is small enough to

M.Y. Lodhi et al. / Current Applied Physics 14 (2014) 716e720720

achieve reasonable signal to noise ratio in high density recordingmedia. FTIR spectra of the spectrum exhibited twomain absorptionbands, thereby confirming the spinel structure. The decrease indielectric constant with increasing frequency is observed. Thedecrease in loss tangent by increasing frequencymay be ascribed tothe MaxwelleWagner polarization and conduction mechanism inthese nanostructured ferrites. Magnetic study concludes that thevalues of coercivity and retentivity are increased by increasing Cocontents. The increase in coercivity with Co substitution is due tothe reason that Co2þ ion has large magneto crystalline anisotropybecause of three unpaired electrons.

The optimized magnetic parameters suggest the possible use ofthese nanomaterials in high density recording media.

Acknowledgments

The authors are thankful to the Islamia University of Bahawalpur(Pakistan), Higher Education Commission (HEC) of Pakistan (Projectno. PD-IPFP/HRD/HEC/2013/1912) and Deanship of ScientificResearch at King Saud University (RGP-VPP-312).

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