Journal of Magnetism and Magnetic Materials 398 (2016) 20–25

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Journal of Magnetism and Magnetic Materials

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journal homepage: www.elsevier.com/locate/jmmm

Effect of zinc concentration on the structural and magnetic propertiesof mixed Co–Zn ferrites nanoparticles synthesized by sol/gel method

M. Ben Ali a,b,n, K. El Maalam a,b, H. El Moussaoui a, O. Mounkachi a, M. Hamedoun a,nn,R. Masrour d, E.K. Hlil c, A. Benyoussef a,b

a MAScIR Foundation, Institute of Nanomaterials and Nanotechnologies, Materials & Nanomaterials Center, B.P., 10100 Rabat, Moroccob Laboratory of Magnetism and the Physics of the high Energies, URAC 12, Department of Physics, B.P. 1014, Faculty of Science, Mohammed V University,Rabat, Moroccoc Institut Néel, CNRS-UJF, B.P. 166, 38042 Grenoble Cedex, Franced Laboratory of Materials, Processes, Environment and Quality, Cady Ayyed University, National School of Applied Sciences, PB 63 46000, Safi, Morocco

a r t i c l e i n f o

Article history:Received 21 April 2015Received in revised form20 June 2015Accepted 23 August 2015Available online 25 August 2015

Keywords:Magnetic propertiesMixed ferritesSol/gel method

x.doi.org/10.1016/j.jmmm.2015.08.09753/& 2015 Elsevier B.V. All rights reserved.

esponding author at: MAScIR Foundation, Inhnologies, Materials & Nanomaterials Center,responding author.ail addresses: m.benali06@gmail.com (M. Bendoun@mascir.com (M. Hamedoun).

a b s t r a c t

Synthesization of zinc-substituted cobalt ferrites nano-particles Co1�xZnxFe2O4 (x¼0.0–0.3) has beenachieved by the sol/gel method. The characterization of the synthesized nano-particles has been done byX-ray diffractometry (XRD), transmission electron microscopy (TEM) and Fourier transform infraredspectroscopy (FITR). The relation between the composition and magnetic properties has been in-vestigated by Magnetic Properties Measurement System (MPMS). The results revealed that the nano-particles size is in the range of 11–28 nm. It was found that the zinc substitution in cobalt ferrite in-creases saturation magnetization from 60.92 emu/g (x¼0) to 74.67 emu/g (x¼0.3). Nevertheless, zincconcentrations cause a significant decrease in coercivity.

& 2015 Elsevier B.V. All rights reserved.

stitute of Nanomaterials andB.P., 10100 Rabat, Morocco.

Ali),

1. Introduction

Magnetic nano-particles are of great technological importancebecause of their use in various hi-tech applications. Spinel ferrites(AB2O4; a tetrahedral and B octahedral sites) are one of the most in-teresting Magnetic nano-particles because of their applications invarious fields, such as high-frequency systems, electronic circuits,

M. Ben Ali et al. / Journal of Magnetism and Magnetic Materials 398 (2016) 20–25 21

power delivering devices, electromagnetic interference suppression,and in biotechnology [1]. In recent years, cobalt ferrite (CoFe2O4) islargely studied in the search for improved properties and new appli-cations; this is due to its high coercitivity, moderate saturation mag-netizations and large magnetostrictive coefficient [2–4]. These prop-erties make it a very promising magnetic material for a variety ofapplications in new technological areas, such as magnetic drug de-livery, hyperthermia treatment, microwave device and high-densityinformation storage [5–7]. Several preparation techniques have beenused for the synthesis of ferrites nano-particles, which exhibit novelproperties. Gyergyek et al. [8] has reported the synthesis method effecton particles sizes and magnetic properties in Cobalt ferrite using threedifferent techniques: co-precipitation, microemulsion method andthermal decomposition. The results show a significant variation ofsaturation magnetization and coercitivity. The magnetic properties canbe modified either by varying the size of the nano-particles [9], or bytuning the concentration of magnetic phases in this ferrite [10–13]. Inthis context, Zinc substituted mixed ferrites is widely treated by sev-eral authors, Veena Gopalan et al. [14] have reported a study on Mn–Znmixed ferrite and found that the magnetization decreases with zincsubstitution and have supposed an existence of a metastable cationdistribution with possible surface effects. Sanjeev Kumar et al. [15]have synthesized Ni–Zn ferrite by the inverse microemulsion techni-que and found changes in magnetic behavior due to cation distribu-tion and interparticles interactions.

In this work, we have synthesized Co1�xZnxFe2O4 (x¼0, 0.05, 0.1,0.2 and 0.3) using a sol/gel process. The obtained nanoparticles havemany potential applications, including their use for the preparation offerrofluid, for the energy conversion application utilizing the magne-tically induced convection for thermal dissipation. The effect of zinc

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ght (

%)

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eigh

t (%

/°C

)

200 40020

40

60

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100

Temperature (°

Wei

ght (

%)

Fig. 1. Thermogravimetric analysis for C

substitution on the structure and magnetic properties of mixed Co–Znferrite has been investigated by diffraction XRD, transmission electronmicroscopy (TEM), and superconducting quantum interference device(SQUID). All results will be presented and discussed.

2. Experiments

Iron(III) nitrate nonahydrate (Fe(NO3)3, H2O), Cobalt(II) nitratehexahydrate (Co(NO3)2, 6H2O), Zinc(II) nitrate hexahydrate (Zn(NO3)2,6H2O), citric acid and ethanol were the precursors used to synthesizeCo1�xZnxFe2O4. The chemicals were all of analytical reagent gradeor equivalent and they were used as received without furtherpurification.

The samples were prepared according to the sol/gel route. In-itially, a stoichiometric amount of nitrate precursors were dis-solved in 50 ml of ethanol under constants stirring at room tem-perature for 30 min, an amount of citric acid (stabilizing agent) isadded into the reactive mixture with maintaining a molar ratio of1:2 between metal nitrates and citric acid. The solution was thensonicated using an ultrasonic processor for 60 min at a tempera-ture of 60 °C; the solution was thereafter heated to evaporate thesolvent excess, concentrated and transformed into a gel. Finally,the samples were calcined at a temperature of 400 °C for 6 h.

All samples were analyzed by X-ray diffraction (Model: D8Discover Bruker AXS) using Cu Kα radiation (λCu¼1.5407 Å). Themagnetic properties of the samples were studied in by MagneticProperties Measurement System (MPMS-7XL), with a QuantumDesign XL-SQUID magnetometer. The morphology and particlesize of the as prepared samples were determined by transmission

100 200 300 400 500 600 700

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Temperature (°C)

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ght (

%)

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eigh

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/°C

)

600 800

C)

0,0

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Der

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eigh

t (%

/°C

)

o1�xZnxFe2O4 (x¼0.0, 0.2 and 0.3).

20 30 40 50 60 70

0

2000

4000

6000

8000

10000

12000

14000

16000

440

511

400

311

220

inte

nsity

2θ

Fig. 2. X-ray diffraction patterns of Co1�xZnxFe2O4 samples sintered at 400 °C withx varied from 0.0 to 0.3.

Table 1The values of lattice constant, particles sizes and IR absorption bands (ν1,ν2) ofCo1�xZnxFe2O4 (x¼0; 0.1; 0.2) samples at room temperature.

Composition Lattice const (Å) DXRD (nm) ν1 (cm�1) ν2 (cm�1)

CoFe2O4 8.497 11.7 540.3 323.4Co0.95Zn0.05Fe2O4 8.477 21.3 550.0 330.7Co0.9Zn0.1Fe2O4 8.468 23.7 545.2 318.7Co0.8Zn0.2Fe2O4 8.522 22.1 548.4 331.2Co0.7Zn0.3Fe2O4 8.521 28.5 542.0 300.8

M. Ben Ali et al. / Journal of Magnetism and Magnetic Materials 398 (2016) 20–2522

electron microscopy (TEM) observations with an acceleratingvoltage of 200 kV. The infrared ABB-Bomen FTLA 2000 was per-formed over the range 4000–250 cm�1.

Fig. 3. TEM image of (a) CoFe2O4, (b) Co0.9Zn0.1Fe2O4,

3. Results and discussion

Thermogravimetric Analysis (TG/DTA, TA Instruments–PfeifferVacuum) equipment was used to determine the thermal decom-position and weight loss of the precursor powder, by varying thetemperature from 25 °C up to 800 °C with a heating rate of 10 °C/min. Fig. 1 shows the TGA curve. The first weight loss step is in thetemperature range of 150–230 °C. It corresponds to the decom-position of acetates [16]; and the exothermic peak at 300–350 °Cin the DTA curve is presumed to be associated with the crystal-lization of spinel phase. In order to have comparable samples wedecided to anneal all the samples at a temperature of 400 °C.

X-ray diffraction of cobalt zinc ferrite samples with x¼0; 0.05,0.1, 0.2 and 0.3 are shown in Fig. 2, the patterns reveal the pre-sence of peaks characteristic of the cubic phase of spinel ferrite inall samples, the observed diffraction peaks perfectly match withthose of CoFe2O4 (JCPDS Card no 22-1086). The absence of anyadditional peaks related to impurities indicates the high purity ofour samples. The lattice parameter was calculated using the fol-lowing relations,

a d h k l2 2 2 2 12( )= [ + + ]

The lattice constant was found in the range of 8.49–8.52 (Ta-ble 1). The values of lattice parameters change with zinc con-centration, this behavior could be attributed to the substitution ofCo cation (of radius 0.75 Å) by a larger cation which is Zn2þ (ofradius 0.82 Å) [17,18]. A similar type of variation is also observedin the case of Cu1�xZnxFe2O4 [19] where the lattice parameterincreases with increasing Zinc content. The particles size wascalculated from the most intense peak 311 using the well knownScherrer formula [20].

D0.9cosDRX

λβ θ

=

where λ is the wavelength (Cu Ka), β is the full width to half-maximum (FWHM) of line broadening and θ is the Bragg angle ofdiffraction. The particles size calculated are listed in Table 1, theparticles sizes of the prepared samples were obtained between 11

(c) Co0.8Zn0.2Fe2O4, (d) Co0.7Zn0.2Fe2O4, samples.

0 10 20 30 400

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grain size (nm)

num

ber o

f par

ticle

s

5 10 15 20 25 30 35 40 450

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ber o

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10 20 30 40 50 60 700

5

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35

40

grain size (nm)

num

ber o

f par

ticle

s

Fig. 4. Curve of grain size distribution of (a) CoFe2O4, (b) Co0.9Zn0.1Fe2O4, (c) Co0.8Zn0.2Fe2O4, (d) Co0.7Zn0.2Fe2O4, samples.

M. Ben Ali et al. / Journal of Magnetism and Magnetic Materials 398 (2016) 20–25 23

and 28 nm.To reveal the local microstructure of the particles, Transmission

electron microscopy analysis was carried out. Fig. 3 present theresults for Co1�xZnxFe2O4 samples with x¼0, 0.1, 0.2 and 0.3. Theimage reveals that particles are in nanometer range with sphericalshape and good dispersion. The particles sizes have been esti-mated using Image analysis algorithm (Fig. 4), the fitted curveindicates the presence of a population of particles centered around17, 20, 16 and 30 nm for CoFe2O4, Co0.9Zn0.1Fe2O4, Co0.8Zn0.2Fe2O4,and Co0.7Zn0.2Fe2O4, respectively. These values are in good agree-ment with the size obtained from the DRX patterns.

The FTIR analysis was performed at room temperature in therange of 250–3000 cm�1 for detecting chemical bonds present inthe samples and ensuring the formation of ferrite phase.

Generally, there are two main characteristic absorption peaksin FTIR spectra of the spinel ferrites, which are related to intrinsicstretching-vibrations of the oxygen bonds with metal cations inpositions A and B [21,22]. Where The first band in the range of500–650 cm�1 corresponds to the stretching vibrations of themetal at the tetrahedral site (M[4]2O). The other band appearedaround 300–450 cm�1 is attributed to the octahedral-metalstretching (M[6]2MO).

The infrared spectra of all the mixed cobalt–zinc ferrites are

shown in Fig. 5. It can be seen from the figures that the IR spec-trum of mixed cobalt–zinc ferrites exhibit the two expected bandsin a spinel structure, ν1 with the higher wavenumber observed inthe range of 540–550 cm�1 and ν2 with lower wavenumber ob-served in the range of 300–300 cm�1. The band positions of theinvestigated samples are tabulated in Table 1. It is clear from Ta-ble 1, that the frequency band shift with the variation of Zincconcentration, this may be due to the re-distribution of cations;Co, Fe and Zn on both sites [15]. The same observation has beenmade by Azhar Khan et al. [23] in the case of cobalt dopedterbium.

Cobalt ferrite exhibits a hard magnetic behavior [24] with arelatively high coercivity and high magnetization [2]. However,magnetic properties of this spinel ferrite depend strongly on theparticle size [25,26], cation distribution [27] and doping [28].

In our work, the magnetic measurement of mixed Co1�xZnxFe2O4

nanoparticles, with varied x from 0 to 0.3, was done by MagneticProperties Measurement System. The hysteresis loops at 300 K forthese samples are shown in Fig. 6. The saturation magnetization (Ms)was obtained from hysteresis loop at H¼60000 Oe. The positive re-manent magnetization (Mr) was obtained from hysteresis loop atH¼0 Oe. The coercivity field was obtained from hysteresis loop atM¼0. The saturation magnetization (Ms), remanent magnetization

800 700 600 500 400 3000

20

40

60

80

100

Wavenumber (cm-1)

Inte

nsity

Fig. 5. FTIR band of Co1�xZnxFe2O4 ferrite nanocrystals with x varied from 0.0 to0.3.

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

-80

-60

-40

-20

0

20

40

60

80

M(e

mu/

g)

H (T)

Fig. 6. Room temperature magnetization cures of Co1�xZnxFe2O4 samples sinteredat 400 °C with x varied from 0.0 to 0.3.

Table 2The values of the saturation magnetization (Ms) and the coercitif field (Hc) for eachsample.

Composition Ms (emu/g) Hc (Oe)

CoFe2O4 60.92 2000Co0.95Zn0.05Fe2O4 63.99 1600Co0.9Zn0.1Fe2O4 68.01 800Co0.8Zn0.2Fe2O4 72.17 220Co0.7Zn0.3Fe2O4 74.67 170

Table 3Values of the coercitif field (Hc) of CoFe2O4 by several synthesis methods.

Synthesis method Hc (Oe) Reference

Co-precipitation 800 [8]Co-precipitation 850 [30]Co-precipitation in reverse microemulsion 700 [8]Thermal decomposition 1900 [8]Combustion 2002 [30]Sol/gel (citrate method) 2000 This work

0,00 0,05 0,10 0,15 0,20 0,25 0,3050

55

60

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75

80

x

Ms

(em

u/g)

0

250

500

750

1000

1250

1500

1750

2000

2250

Hc

(Oe)

Fig. 7. Variation of magnetization and Hc values with zinc concentration.

M. Ben Ali et al. / Journal of Magnetism and Magnetic Materials 398 (2016) 20–2524

(Mr) and coercivity (Hc) are listed in Table 2.The hysteresis curves show that for the undoped cobalt ferrite

the coercivity is about 2000 Oe, this value may be due to the highanisotropy constant of the Cobalt ferrite (Table 3) [29]. However,several authors associate the high value of coercitivity to the

synthesis method. In the studies of Saso Gyergyek et al. [8], thecoercitivity value varies from 700 to 1900 Oe by changing thesynthesis method, according to the authors this variation is at-tributed to the difference of cation distribution in the samples.Similar studies were reported by Houshiar et al. [30]. It was alsoobserved that the value of coercivity shows a significant decreasefrom 2000 Oe to 170 Oe with increasing the zinc substitution, thedecrease in coercivity with zinc concentration is reported in Fig. 7.This variation can be attributed to the reduction of domain wallenergy caused by the weak magnetocrystalline anisotropy of thezinc [31].

The saturation magnetization (Ms) for x¼0 is about 60,92 emu/g with an average grain size of 11.7, while the value reported in theliterature for bulk sample is about 81 emu/g [32]. However, thesubstitution of cobalt by zinc shows an increase in saturationmagnetization up to reach 74.67 emu/g for x¼0.3 (Fig. 7). Ac-cording to Blanco-Gutierrez et al. [33], the bulk ZnFe2O4 has anantiferromagnetic behavior with normal spinel structure(Zn2þ)Td[Fe3þFe3þ]OhO2�

4 in which all the Zn2þ cations are lo-cated in tetrahedral sites (A) and Fe3þ cations are in octahedralsites (B) with antiparallel moments. Thereby the substitution ofnonmagnetic ion which has a preferential A site occupancy resultsin the reduction of the exchange interaction between A and B sites.Hence by varying the degree of zinc substitution it is possible tovary magnetic properties of the fine particles.

Similar studies were reported by Nandapure et al. [10] in thecase of Zn–Ni mixed ferrite, were saturation magnetization in-creases due to the addition of Zn content in NiFe2O4. In this sense,the substitution of Zn2þ ions at tetrahedral sites (A) in place ofFe3þ ions into an inverse cobalt ferrite [34] lead to the decrease ofmagnetic moment in tetrahedral A-site. However, A-site magneticmoment is less than B-site magnetic moment and the net mag-netic moment increases up to 74.67 emu/g for x¼0.3. However,the magnetization decreases after a certain zinc concentration dueto Yafet-Kittel arrangement [35]. In this context a lower magne-tizations value was reported by Yadav et al. for Co1�xZnxFe2O4

M. Ben Ali et al. / Journal of Magnetism and Magnetic Materials 398 (2016) 20–25 25

with x¼0.5 [36].

4. Conclusions

Particles of zinc-substituted cobalt ferrites have been success-fully prepared by sol/gel methods; the spinel crystalline structurewas verified by DRX, the addition of the zinc does not change thecrystalline phase of the particles and the results matches perfectlywith the standard reported data. Therefore, there are significantchanges in other physical properties. It was found that the parti-cles size calculated from DRX results were between 11–28 nm; andthat the lattice constant varies with zinc concentration. Thecoercivity shows a significant decrease from 2000 Oe to 170 Oewith increasing the zinc amount while the saturation magnetiza-tion increases with the addition of a small concentration of Zinc.

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