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Ceramics International 40 (2014) 9439–9444www.elsevier.com/locate/ceramint

Influence of cobalt substitution on the magnetic properties of zincnanocrystals synthesized via micro-emulsion route

Huma Malika, Azhar Mahmoodb, Khalid Mahmooda, Maria Yousaf Lodhia,Muhammad Farooq Warsib,n, Imran Shakirc,d, Hassan Wahabe, M. Asghara,

Muhammad Azhar Khana,nn

aDepartment of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, PakistanbDepartment of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan

cDeanship of Scientific Research, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi ArabiadInterdisciplinary Research Centre in Biomedical Materials (IRCBM), COMSATS Institute of Information Technology (CIIT), Lahore, Pakistan

ePhysics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan

Received 21 January 2014; received in revised form 5 February 2014; accepted 6 February 2014Available online 19 February 2014

Abstract

A series of nanocrystalline spinel ferrites with chemical formula CoxZn1�xFe2O4 (x¼0, 0.2, 0.4, 0.6, 0.8, and 1.0) were synthesized by themicro-emulsion technique. The samples were annealed at 700 1C for 7 h at the rate of 15 1C/min. The cubic spinel structure was systematicallyexamined by X-ray diffraction and Fourier transform infrared spectroscopy. The lattice constant decreased from 8.39 to 8.34 Å as the Coconcentration increased from x¼0 to x¼1 because of the smaller ionic radii of Co as compared to Zn. The dielectric properties of the synthesizednanomaterials were studied in the frequency range 1–3 GHz. The dielectric properties were influenced by Co substitution and were explained bythe electron hopping mechanism between Fe2þ and Fe3þ ions. Vibrating sample magnetometry was used to study the magnetic parameters ofthese nanoferrites. M–H loops of the materials show CoxZn1�xFe2O4 nanocrystals to exhibit ferrimagnetic behavior at room temperature. Themagnetic properties are explained in terms of cation distribution and grain size effect. Thus high Ms (21.29 emu/g) and Hc (1382.4 Oe) exhibitedby the CoFe2O4 nanocrystals are favorable for their potential use in high density recording media applications.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Nanocrystalline ferrites; Co-substitution; Micro-emulsion; XRD; M–H loops

1. Introduction

Since last two decades, the nanomaterials have beenextensively studied worldwide because of their unique proper-ties that are not exhibited by the corresponding bulk materials.Among various nanomaterials, the spinel ferrite (MFe2O4,M¼Ni, Co, Mn, Zn, etc.) nanocrystals have become immen-sely popular magnetic materials due to some unique propertiesexhibited by these materials. These properties include largemagnatocrystalline anisotropy, high coericivity, mechanical

10.1016/j.ceramint.2014.02.01514 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author.ing author. Tel.: þ92 62 9255461.ss: azhar_manais@yahoo.com (M.A. Khan).

harness and chemical stability [1–3]. These specific propertiesmake them useful in various technical applications such asmagnetic recording devices, magnetic storage devices, mag-netic resonance imaging enhancement, catalysis, drug targetingand gas sensors [4–7]. Furthermore, nanoferrites are gainingsubstaintial interest among the research community as theyexhibit a relationship between magnetic properties and theircrystal structure. It is reported in literature that small crystallinesize significantly improved the electrical and magnetic proper-ties of ferrites [8]. Therefore different synthesis techniques likehydrothermal treatment [9], emulsion method [10], co-precipitation[11], mechano-chemical method [12], micro-wave sintering [13],electrochemical synthesis [14], sol–gel synthesis [14] and chemicalmicro-emulsion method [15] are frequently used for the synthesis

H. Malik et al. / Ceramics International 40 (2014) 9439–94449440

of ferrite nanomaterials. It is also found that type and compositionof substituted metal ions in host ferrite play a very important role intailoring the properties of parent ferrite materials [16]. Differentmetallic ions substituted in nanoferrites have been reported in theliterature to improve their magnetic properties. For example Rajjabet al. [17] prepared Ni–Co magnesium ferrites by the micro-emulsion method. They observed an improvement in coericivityand saturation magnetization by increasing the concentration ofNi–Co in the host structure. Wu et al. [18] synthesizedCo0.35Mg0.65Fe2O4 nanoferrites by the solid state reaction methodwith spinel cubic structure. Their magnetic characterization indi-cated the specific saturation magnetization of Co0.35Mn0.65Fe2O4

obtained at 773 K was 66.14 A m2 kg�1. Liu et al. [19] reportedthe preparation of ferrites with composition Co1þxFe2�xO4

(0.0rxr2.0) by chemical co-precipitation. Magnetic measure-ments indicated that the saturation moment M of the samples at10 K decreased from a maximum of 3.16 mB/formula for x¼0.0 toa minimum of 0.0 mB/formula for x¼2.0, with a local minimum atx¼1.4. Therefore the proper selection and composition variationof substituted metallic cations in the parent ferrites materials are avery important area of research and still need to be investigatedthoroughly in order to enhance electromagnetic properties.

In the present article, we have demonstrated the effect of Cosubsitution on the various properties of zinc based spinelferrites synthesized by the micro-emulsion technique and alsoaim to enhance magnetic parameters (coercivity, remanenceand saturation magnetization) to make these nanocrystalssuitable for recording media devices fabrication.

Fig. 1. XRD patterns of CoxZn1�xFe2O4 (x¼0, 0.2, 0.4, 0.6, 0.8, and 1.0)nanoferrites.

0.0 0.2 0.4 0.6 0.8 1.0

8.34

8.35

8.36

8.37

8.38

8.39

Latti

ce C

onst

ant(A

0 )

Co-Concentration

18

21

24

27

30

33

36

39

42

Cry

stal

line

Size

(nm

)

Fig. 2. The variations of lattice constant and crystallite size for CoxZn1�xFe2O4

nanoferrites.

2. Experimental

The CoxZn1�xFe2O4 (x¼0, 0.2, 0.4, 0.6, 0.8, and 1.0)nanocrystals were prepared using the micro-emulsion technique[9]. Following chemicals were used for the preparation ofCoxZn1�xFe2O4 nanocrystalline ferrites – Fe(NO3)2 � 9H2O(Sigma-Aldrich, 98%), ZnNO3 (Sigma-Aldrich, 98%), CoNO3

(Sigma-Aldrich, 99%), cetyltrimethylammoniumbromide ((C16H33)N(CH3)3Br) (Amresco, 98%), aqueous ammonia and deionisedwater (conductivity�2 to 4 S). Required volumes of metal saltssolutions having concentrations 0.15 M were prepared, mixed andstirred on a magnetic hot plate. The temperature of all reactionsmixtures were maintained at 50–60 1C. Aqueous solution ofCTAB (100 mL, 0.45 M) was added and the pH value wasadjusted using aqueous ammonia (2 M). The pH value wasmaintained at 11–12 for all the compositions. The reactionmixtures were further stirred for 4–5 h. The precipitates werewashed with deionized water until the pH reduced to 7. Thedrying was carried out in the oven at 100 1C, and annealing wascarried out at 700 1C for 7 h in a temperature controlled mufflefurnace Vulcan A-550 at a heating rate of 15 1C/min. Theobtained materials were grinded into powder form and werecharacterized by various techniques. The powder XRD analysiswas performed using a Philips X'Pert PRO 3040/60 diffract-ometer with CuKα as radiation source. FTIR spectra wererecorded on a Nexus 470 spectrometer. The dielectric measure-ments were carried out using a LCR meter at room temperature.

The hysteresis loops measurements were recorded at 298 K usinga vibrating sample magnetometer VSM Lakeshore-74071.

3. Results and discussion

3.1. XRD analysis

The powder XRD patterns for all CoxZn1�xFe2O4 nanofer-rites compositions are shown in Fig. 1. The identification ofthe phase was carried out by comparing the observed structuralparameters with the standard diffraction patterns. Followingpeaks were observed in all the samples and successfullyindexed as (111), (220), (311), (222), (400), (422), (511),(440) (620), (533) and (622) planes of cubic spinal phase ofCoxZn1�xFe2O4 [20,21]. All peaks showed perfect match withstandard patterns of ICDD # (00-014-0181) that indicated thatthe synthesized materials possess single phase cubic spinelstructure. The crystallite size for all compositions of Cox

Table 1Lattice constant (a), cell volume, bulk density, X-ray density, porosity and crystallite size for CoxZn1�xFe2O4 nanocrystals.

Composition Lattice constant (Å) Volume (Å)3 Bulk density (g cm�3) X-ray density (g cm�3) Porosity Crystallite size (nm)

ZnFe2O4 8.39 590.6 3.7 5.42 0.31 41Zn0.8Co0.2Fe2O4 8.38 588.5 3.6 5.41 0.33 36Zn0.6Co0.4Fe2O4 8.37 586.4 3.55 5.40 0.34 33Zn0.4Co0.6Fe2O4 8.36 584.3 3.53 5.40 0.34 28Zn0.2Co0.8Fe2O4 8.35 582.2 3.37 5.39 0.37 24CoFe2O4 8.34 580.1 3.21 5.38 0.40 19

Fig. 3. The FTIR spectrum of CoxZn1�xFe2O4 nanoferrite.

H. Malik et al. / Ceramics International 40 (2014) 9439–9444 9441

Zn1�xFe2O4 was determined by the Sherrer formula (Fig. 2).The crystalline size decreased from 41 nm to 19 nm as the Cocontent increased from x¼0 to x¼1. Fig. 2 also demonstratedthat lattice constant decreased as the Co content increased inCoxZn1�xFe2O4 nanoferrites. The decrease in lattice constantis referred to the smaller ionic radius of Co2þ (0.74 Å) [22] ascompared to Zn2þ (0.82 Å) [23]. Since smaller ions (Co2þ )replace the larger ions (Zn2þ ) which ultimately results in thereduction of lattice constant. Similar linear variation of thelattice constant has been observed by Koseoglu et al. [24]. X-ray density for all samples was also calculated using Eq. (1)[25] and is shown in Table 1.

ρX ¼ZM

NAVcellð1Þ

where M is the molecular weight, NA is the Avogadro number,and V is the cell column.

The bulk density was calculated by using Eq. (2) [25].

ρm ¼ m

πr2hð2Þ

In Eq. (2), the “ρm” is the measured bulk density, “m”denotes the mass of the pellet, “r” is the radius, and “h” isthickness of the pellet.

Table 1 indicates that true density ρX�ray is higher than thebulk densityρm, for all the compositions. This might be owingto the existence of pores in these samples. This argument wasconfirmed by determining the porosity for all samples. Thecalculated values of porosity show increasing trend withincreasing Co doping that justified higher values of truedensity as compared to bulk density.

Fig. 4. Magnetic hysteresis loops for CoxZn1�xFe2O4 (x¼0, 0.2, 0.4, 0.6, 0.8and 1.0) nanoferrites measured at room temperature.

3.2. FTIR analysis

Fig. 3 displays the FTIR spectrum of representativeZn1�xCoxFe2O4 nanoferrite recorded in the range of400–4000 cm�1 at 298 K. The two main absorption bands(ν1) and (ν2) corresponding to the stretching vibration of thetetrahedral and octahedral sites are found around 550 cm�1

and 400 cm�1 respectively [26]. In Zn1�xCoxFe2O4 nanofer-rites the tetrahedral site is occupied by Zn ion while cobalt andiron partially occupy both the tetrahedral and octahedral sitesas they are divalent ions. In Fig. 3 the bands observed at 418,428, 480 cm�1 are assigned to octahedral group complexeswhile the band 526 cm�1 is attributed to the tetrahedral groupcomplexes [27].

3.3. VSM analysis

Hysteresis loops for all compositions of CoxZn1�xFe2O4

nanocrystallites measured at 298 K in an applied field up to10 kOe are shown in Fig. 4. It was observed that themagnetization increases with increase in external magnetic

H. Malik et al. / Ceramics International 40 (2014) 9439–94449442

field strength at low field region and attains the maximumvalue for field �10 kOe. It can be seen that all magnetizationcurves are saturated at higher field region and the hysteresiscurves for samples are S shaped that are the characteristics offerromagnetic materials. In a cubic system of spinel ferrites themagnetic order is mainly constructed due to super exchangeinteractions between metal ions of sub-lattices A and B. InZnFe2O4, Zn

2þ preferably occupies the tetrahedral (A) sitesand Fe3þ prefers the octahedral sites (B). CoFe2O4 has aninverse spinel structure in which Co2þ occupies octahedral siteand Fe3þ ions equally occupied tetrahedral and octahedralsites, with their spins in the opposite direction. The magneticproperties of spinel ferrites depend on the types of cation andthe distribution of cations between the two interstitial sites ofspinel lattice. The distribution of cations between A and B sitedepend on preparation method, the type of bonding and ionicradius. The magnetic properties mainly depend on the Co2þ

(3d7) ions having orbital moment of 3.7 mB/formula unit [17].On the other hand magnetic moment of Zn is zero, therefore inZnFe2O4 magnetic couplings purely originate from the mag-netic moment of Fe cations. The magnetic moment per formulaunit, according to Neel's two sub-lattice model is given by thedifference of magnetic moment of each sub-lattice.

M ¼MA�MB ð3ÞIn the above equation the MA and MB are the A- and B-sub-

lattice magnetic moments in μB respectively [19]. Fig. 5revealed the effect of Co-content on coericivity (Hc), saturationmagnetization (Ms) and retentivity (Mr) of CoxZn1�xFe2O4

nanoferrites. The increase in the saturation magnetization andthe coercivity of the nanocrystalline ferrites by increasing theCo-content may be attributed to the magnetic character and theanisotropic nature of cobalt. Since more Co ions replace withZn ions by increasing the “x”, the saturation magnetization andthe coercivity increase. It was also observed that saturationmagnetization first increases upto cobalt content x¼0.6 andthen decreases for x¼0.8. The increase in saturation magne-tization is directly related to the enhancement in magneticCo2þ ions replacing Zn2þ ions that are non-magnetic innature, which affects the super exchange interactions [8,9].The decrease in the saturation magnetization value for x=0.8 is

Fig. 5. The compositional variations of magnetic parameters for CoxZn1�x-

Fe2O4 nanoferrites measured at room temperature.

due to the migration of Fe3þ ion from B sites to A sites. Theincrease in coercivity may be attributed to the increase inmagnetocrystalline anisotropy with increasing Co contents[28–30]. A small number of Co2þ ions enter in the spinallattice, leading to the appearance of the spin-orbital couplingthat determines the magnetic anisotropy in the ferrites. There-fore as the cobalt content increases, the magneto-crystallineanisotropy increases, this decreases the domain wall energyresulting in high coercive force. The magnetic moment performula unit in Bohr magnetron (mB) is calculated by thefollowing equation [21]:

μB ¼M �Ms

5585ð4Þ

In the above equation, M is the molecular weight and Ms isthe saturation magnetization. The magnetic parameters for allthe compositions are given in Table 2.

3.4. Dielectric properties

The dielectric dispersion in ferrites can be explained on thebasis of interfacial polarization as predicted by the Maxwell–Wagner model. Dielectric structure of ferrites consists of wellconducting grains separated by the grain boundaries which arepoor conductors. The hopping of electron between Fe2þ andFe3þ results in the pile up of electrons at the grain boundariesand produce polarization in ferrites. However, as the frequencyof the externally applied electric field is increased, dielectricconstant decreases and then becomes constant because theelectron hopping between Fe2þ and Fe3þ does not follow theapplied electric field at very high frequencies [28]. Rabinkinand Novikova pointed out that polarization in ferrites is asimilar process to that conduction. The electron exchangebetween Fe2þ2Fe3þ results in the local displacement ofelectrons in the direction of applied field that determines thepolarization. Polarization decreases with the increase offrequency and then reaches a constant value. It is due to thefact that beyond a certain frequency of external field, theelectrons exchange Fe2þ2Fe3þ cannot follow the alternatingfield [28]. Fig. 6 shows that the dielectric constant behavior islargely affected by Co-contents. The decrease in dielectricconstant with increasing Co content for samples with x¼0.4,0.6 and 1.0 may be due to the migration of Fe3þ ions fromoctahedral site to tetrahedral site that decreases the hoppingprocess. However, the increase in dielectric constant forsample with x¼0.8 may be attributed to the formation ofFe2þ ions on the octahedral sites. The formation of Fe2þ ionsin octahedral site increases the electron exchange betweenFe2þ and Fe3þ and hence enhances the polarization [31].The dielectric loss in ferrites mainly originates from electron

hopping and defect dipoles. The electron hopping contributesto the dielectric loss only in low frequency range. The responseof electron hopping is decreased with the increase in frequencyand hence the dielectric loss decreases in high frequency rangeas shown in Fig. 7. In the high frequency range the chargeddefect dipoles contribute to the dielectric loss. The dielectricloss also exhibits peaking behavior that is depicted in Fig. 7.

Table 2Saturation value, remanance, coercivity and magnetic moment for CoxZn1�xFe2O4.

Composition Saturation value “Ms”(emu/g) Remanance “Mr”(emu/g) Coercivity “Hc” (Oe) Magnetic moment “μB”

ZnFe2O4 3 0.28 95.5 0.12Co0.2Zn0.8Fe2O4 5.51 0.52 101.8 0.23Co0.4Zn0.6Fe2O4 14.90 2.78 145.2 0.63Co0.6Zn0.4Fe2O4 20.89 6.05 352.1 0.88Co0.8Zn0.2Fe2O4 7.85 2.82 528.1 0.33CoFe2O4 21.29 11.51 1382.4 0.89

Fig. 6. Dielectric constant for “CoxZn1�xFe2O4” nanoferrites (x¼0, 0.2, 0.4,0.6, 0.8, and 1.00).

Fig. 7. Dielectric loss as a function of frequency for “CoxZn1�xFe2O4”

nanoferrites (x¼0, 0.2, 0.4, 0.6, 0.8, and 1.0).

H. Malik et al. / Ceramics International 40 (2014) 9439–9444 9443

Furthermore, the loss peaks in Fig. 7 shift to high frequencyside with increase in Co-contents. This may be due to the factthat cobalt substitution prefers to go at octahedral site thatstrengthens the dipole–dipole interactions and consequently itrestricts the rotation of the dipoles.

4. Conclusions

CoxZn1�xFe2O4 nanocrystals have been successfullysynthesized by the micro-emulsion method. X-ray diffractionand Fourier transform infrared spectroscopy confirm the crystalstructure of cubic spinel. The lattice parameter decreases withincreasing Co content due to smaller ionic radius of Co2þ thanZn2þ ion. The room temperature dielectric constant and losstangent decrease with the increase in frequency indicates thenormal dielectric behavior for all the samples. The magneticproperties such as remanence (Mr), coercivity (Mc) andsaturation magnetization (Ms) were calculated from hysteresisloops. Magnetization was found to exist in the range of3–21.29 emu/g, remanence from 0.28 to 11.51 emu/g andcoercivity from 95.5 to 1382.4 Oe. These enhancements inthe magnetic properties are due to the larger magnetic momentof cobalt as compared to zinc. The optimized magneticparameters suggest the utility of the nanocrystals in recordingmedia applications.

Acknowledgments

The authors are thankful to the Islamia University ofBahawalpur (Pakistan), Higher Education Commission(HEC) of Pakistan (Project no. PD-IPFP/HRD/HEC/2013/1912), the Deanship of Scientific Research at King SaudUniversity (RGP-VPP-312) and the International Foundationfor Science Sweden (Grant no.: W/5408-1).

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