J Supercond Nov MagnDOI 10.1007/s10948-014-2505-4

O R I G I NA L PA P E R

Annealing Induced Enhancement in Magnetic Propertiesof Co0.5Zn0.5Fe2O4 Nanoparticles

Javed Iqbal · Mubasher Rajpoot · Tariq Jan ·Ishaq Ahmad

Received: 16 December 2013 / Accepted: 24 January 2014© Springer Science+Business Media New York 2014

Abstract In this paper, cobalt zinc ferrite (Co0.5Zn0.5

Fe2O4) nanoparticles (NPs) have been prepared using chem-ical co-precipitation method. In order to investigate the an-nealing induced effects on their various physical properties,the prepared samples have been annealed at 500 °C, 650 °Cand 1000 °C and then compared with as-prepared sample.X-ray diffraction (XRD) patterns of as-prepared and an-nealed samples at various temperatures exhibit single phasespinel structure. Enhancement in crystallinity and crystallitesize is observed with the increase in annealing temperature.The annealing has also greatly influence the morphologyand grain size of prepared NPs. The Co0.5Zn0.5Fe2O4 NPshave shown remarkable enhancement in magnetic momentwith increase in annealing temperature. The bandgap ener-gies of Co0.5Zn0.5Fe2O4 NPs have been measured via UVSpectrometer and observed to decrease with annealing tem-perature. FTIR spectra of the samples reveal the presence ofboth high frequency and low-frequency bands due to tetra-hedral and octahedral sites, which corroborate well with theXRD results. The observed characteristics of cobalt zinc fer-rite NPs as a function of annealing temperature are the risingcontender for many data storage and nanodevice applica-tions. Finally, the genotoxicity of prepared nanoferrites hasbeen checked via comet assay.

J. Iqbal (B) · M. Rajpoot · T. JanLaboratory of Nanoscience and Technology, Department ofPhysics, International Islamic University, Islamabad, Pakistane-mail: javed.saggu@iiu.edu.pk

I. AhmadExperimental Physics Labs, National Center for Physics,Islamabad 44000, Pakistan

Keywords Cobalt zinc ferrite · Thermal annealing · XRD ·Magnetic properties · Comet assay

1 Introduction

During the last decade, there was a growing interest in NPsof magnetic spinel ferrites due to their vast applications invarious fields such as data storage devices, sensors, nanode-vices, photoelectric devices, photocatalysis, magnetic pig-ments and microwave devices [1–3]. Among spinel ferrites,cobalt zinc ferrite NPs demonstrate improved propertiessuch as magnetostriction, magneto-optical properties, chem-ical stability and high corrosion resistivity [4]. The magneticproperties of NPs of spinel ferrites are strongly dependenton particle size and morphology. For various applicationssuch as latest data storage devices, biomedical applicationsand microwave protection, strict control over these param-eters is required [5]. Hence, the optimization of the abovementioned properties is in demand to ferrites’ community.These properties can be tailored using different synthesistechniques, varying the cobalt and zinc concentrations, ther-mal annealing and laser irradiation [6, 7]. Thermal anneal-ing is the easiest, economical and reproducible techniqueto tune these properties according to the need of applica-tions. Several synthesis approaches are followed for prepa-ration of ferrite NPs, such as matrix isolation, hydrothermalroute [7], sol–gel method [8], mechanical alloying [9] andthe co-precipitation method. Chemical co-precipitation is avery versatile, easy to handle and low cost technique for thesynthesis of ferrite NPs [10–12]. The genotoxicity of ferriteNPs is of immense interest for their applications in biomed-ical sectors. The current work deals with the synthesis ofCo0.5Zn0.5Fe2O4 NPs via the co-precipitation method, theirannealing at various temperatures characterization for anal-ysis of physical properties and genotoxicity study.

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Fig. 1 SEM micrographs ofprepared and annealed NPstaken at same resolutions: (a) asprepared, (b) at 500 °C, (c) at650 °C and (d) at 1000 °C

2 Materials and Methods

The synthesis of Co0.5Zn0.5Fe2O4 NPs was carried out viachemical co-precipitation technique. For the synthesis ofCo0.5Zn0.5Fe2O4 NPs, appropriate molar ratios of zinc ni-trate (Zn(No3)2·6H2O), cobalt sulfate (CoSo4·7H2O) andiron chloride (FeCl3·6H2O) were used. The chemicals wereused as purchased without any supplementary refinement.The molarities of iron (Fe), cobalt (Co) and zinc (Zn) werekept as 0.02, 0.01, and 0.01 M in 100 ml distilled water,respectively. The pH value of the solution was controlledbetween 11 and 11.5 by dropping 2 M NaOH solution dur-ing stirring at 70–75 °C. The salt impurities were removedthrough washing the precipitates with distilled water repeat-edly. The cleaned precipitates were placed in vacuum ovenat around 100 °C to dry overnight. In order to see the ef-fects of annealing temperature on the crystallinity and physi-cal properties, the as-prepared Co0.5Zn0.5Fe2O4 sample wasannealed at 500 °C, 650 °C and 1000 °C for 7 h in an electricfurnace.

The toxicity measurements of Co0.5Zn0.5Fe2O4 NPswere carried out via an in-vitro test. The as-prepared NPswere dispersed in distilled water by sonication. The ob-

tained suspension was subsequently distilled with phosphatebuffered saline (PBS) and sodium citrate to obtain concen-trated solution of 0.1 mg/10 ml. About 10 µl of the pre-pared suspension was added to wells containing 990 µl ofcells.

3 Results and Discussions

3.1 Morphological Study

The morphology and grain size of the as-preparedCo0.5Zn0.5Fe2O4 and samples annealed at various temper-atures have been studied by means of the scanning elec-tron microscopy (SEM) and are shown in Fig. 1. Uni-formly distributed spherical NPs are observed, which re-flects the significance of the co-precipitation techniqueused for the preparation of ferrite NPs. The average grainsizes of the prepared samples are found to increase from21 to 130 nm with the increase in annealing tempera-ture. The morphology is greatly influenced by anneal-ing temperature and can be seen in Fig. 1. The particlesbecome flatter with the increase in annealing tempera-ture.

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Fig. 2 XRD patterns of CoZnferrites NPs annealed at varioustemperatures

Table 1 Summary of annealing effects on crystallite size (Dxrd), lat-tice parameter (a), volume (V ), measured density (ρm), X-ray den-sity (ρx), porosity (P ), grain size, coercivity (Hc), saturation magneti-

zation (Ms), remanence (Mr) and energy bandgap (Eg) of synthesizedcobalt zinc ferrite nanoparticles

Parameters DXRD(Å)

a

(Å)V

(Å3)ρm(g cm−3)

ρx(g cm−3)

P

(fraction)Grain size(nm)

Hc(Oe)

Ms(emu/g)

Mr(emu/g)

Mr/Ms(S/R)

Eg(eV)

As prepared 245 8.17 545.3 4.46 3.87 0.12 21 96.88 19.32 4.14 0.21 1.71

500 °C 360 8.37 586.4 4.10 3.78 0.08 38 91.13 43.37 9.10 0.20 1.69

650 °C 407 8.39 590.6 4.0 3.76 0.06 44 135.3 53.99 15.16 0.28 1.66

1000 °C 417 8.41 594.8 3.97 3.75 0.05 130 113.5 66.58 19.08 0.29 1.59

3.2 Structural Study

X-ray diffraction (XRD) patterns of prepared ferrites sam-ples annealed at various temperatures exhibit a single phasespinel structure without additional impurity phase as shownin Fig. 2. One extra peak appears at 32o in the sample an-nealed at 650 °C which can be indexed to zincite [13]. XRDanalysis confirms the formation of cubic spinel structures,as (220), (311), (222), (400), (331), (422), (511), (440) and(531) are the cubic unit planes which reflect the cubic spinelstructure. The plane (311) shows the exact spinel phase. Av-erage crystallite sizes of samples have been calculated usingDebye–Scherrer formula [14]:

DXRD = 0.9λ

β cos θ(1)

where λ represents the wavelength in Å of the X-rays used,θ represents Bragg’s angle, β is FWHM measured in radi-ans, and the crystallite size in Å is represented by DXRD.In the current work, the crystallite size is found to be inthe range of 245–417 Å. XRD analysis indicates that thecrystallite size increases as annealing temperature of thesamples increases, and intense diffraction features become

stronger due to the growth of grains. The characteristic pa-rameters of as prepared Co0.5Zn0.5Fe2O4 and annealed sam-ples at 500 °C, 650 °C and 1000 °C are summed up in Ta-ble 1.

For further investigations of the structural properties ofCo0.5Zn0.5Fe2O4 nanoparticles, the measured density (ρm),X-ray density (ρx) and porosity of the prepared sampleshave been calculated using standard relations as reported inthe literature [15]. Table 1 indicates that both ρm and ρx de-crease with the increasing values of annealing temperature.This might be attributed to the difference in atomic massesof cobalt and zinc, which affects the density of NPs due toan increase in volume of prepared samples with annealingtemperature [15]. The porosity of ferrite NPs is found to de-crease with the increase in annealing temperature as shownin Table 1, which might possibly be due to the increase ingrain size.

3.3 Compositional Study

Chemical and elemental investigations of the as-preparedsample have been performed through EDX and XRF, respec-tively. Chemical peaks of the as-prepared Co0.5Zn0.5Fe2O4

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NPs are shown in Fig. 3(a). The contents of each elementhas been calculated theoretically and compared with experi-mental results, which are observed to be in good agreement.

Fig. 3 (a) EDX and (b) XRF spectra of as prepared CoZn ferritesnanoparticles

XRF plots confirm that the prepared nanoferrites are com-posed of cobalt, zinc and iron in specific ratio as shown inFig. 3(b).

3.4 Magnetic Study

The variation of magnetization (M) as a function of ap-plied field (H ) at room temperature for Co0.5Zn0.5Fe2O4

NPs annealed at different temperatures (i.e., 0 °C, 500 °C,650 °C and 1000 °C) exhibit very clear M–H loops asshown in Fig. 4. The contracted M–H loops identify thesoft magnetic nature of the samples. The M–H loop of theCo0.5Zn0.5Fe2O4 NPs is broadened with the increase in an-nealing temperature as shown in the inset of Fig. 4.

The variation in coercivity (Hc) with annealing tempera-ture is shown in Fig. 5(a), which indicates that the anneal-ing temperature significantly influences the Hc of preparednanoferrites. Hc has sinusoidal behavior and exhibits signifi-cant values ranging between 96.88 and 113.5 Oe for synthe-sized ferrite NPs. The saddle points reflect variation in Hc

with annealing temperature which can be explained on thebasis of the domain structure and anisotropy of crystal [16].Hc initially decreases slightly with annealing temperatureand then increases remarkably reaching its maximum valueof 135.8 Oe at 650 °C. Upon further annealing, the value ofHc decreases significantly at 1000 °C. This variation in thevalues of Hc may possibly be attributed to the crystallite sizeeffects. Recently, it is reported that, below a critical crystal-lite size, Hc increases with the increase in crystallite size dueto surface effects [17]. And above the critical crystallite size,Hc tends to decrease with increase in crystallite size. By theincrease in crystallite size, the number of grain boundariesincreases, and consequently Hc of NPs decreases. Hence,

Fig. 4 M–H loops of CoZnferrites nanoparticles

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Fig. 5 (a) Coercivity as a function of annealing temperature. (b) Plotsof Ms and Mr versus annealing temperature

the decrease in Hc with the increasing annealing tempera-ture can be attributed to a decrease in the anisotropy fieldthat in response decreases the domain wall energy [17, 18].

Figure 5(b) clearly shows that both the saturation mag-netization (Ms) and remanence (Mr) increase with increas-ing values of the annealing temperature. This increase in Ms

and Mr for prepared ferrite NPs with annealing temperaturecan be explicated by Neel’s theory [19]. Both Ms and Mr

increase due to a steady increase in particle size and crys-tallinity as shown in Fig. 5(b). In order to further explorethe magnetic properties of ferrite nanoparticles, the ratioMr/Ms has been calculated. The trend of Mr/Ms of syn-thesized Co0.5Zn0.5Fe2O4 NPs indicates that at room tem-perature a considerable amount of NPs are still superparam-agnetic in the absence of externally applied magnetic fieldas shown in Table 1. Thus, by varying the annealing temper-

Fig. 6 Diffuse reflectance spectra of annealed Co0.5Zn0.5Fe2O4nanoparticles

ature of ferrite NPs, the magnetic properties of the preparedCo0.5Zn0.5Fe2O4 NPs can be tuned.

3.5 Optical Study

Optical properties of the prepared ferrite NPs annealed atvarious temperatures have been investigated using diffusereflectance spectroscopy (DRS). The percentage of diffusereflectance of ferrite NPs is found to increase with the in-crease in annealing temperature as shown in Fig. 6. The in-crease in the percentage of diffuse reflectance is due to thecoloration effect on the optical properties of the preparednanoferrites [20].

The energy bandgap (Eg) of the synthesized ferrite NPshas been determined by extrapolation of the linear part ofthe reflectance spectra towards the x-intercept. Table 1 in-dicates that Eg of the prepared Co0.5Zn0.5Fe2O4 NPs de-creases with annealing temperature and it comes out to be1.69, 1.66 and 1.59 eV for 500, 650 and 1000 °C, respec-tively. This decrease in Eg might possibly be attributed tothe increase in crystallite size with the increase in annealingtemperature.

3.6 FTIR Measurements

Infrared (IR) spectra of Co–Zn ferrites NPs annealed at vari-ous temperatures have been obtained in the frequency rangeof 4000–400 cm−1 and shown in Fig. 7. Two main broadmetal–oxygen bands are important in the IR spectra of allspinel ferrites [21]. The first one is related to the presenceof metal ions at tetrahedral site and is usually observed inthe IR frequency range of 580–610 cm−1. The second oneis observed in lower frequency range, i.e., 385–450 cm−1,which corresponds to octahedral metal ions stretching vi-brations [19]. In our case, the existence of absorption bandsaround 580–610 and 410–440 cm−1 confirms the formation

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of single phase ferrite NPs. The intensity of the tetrahedralstretching vibration band is higher as compared to that of oc-tahedral stretching vibration band in all samples. This maybe attributed to the shorter bond length of tetrahedral clus-ter and longer bond length of octahedral cluster. The bandsaround 3420–3440 and 1630–1650 cm−1 are assigned toO–H stretching and H–O–H bending modes of vibrations,respectively. The results obtained from FTIR are in goodagreement with those obtained from XRD patterns.

Fig. 7 FTIR spectra of CoZn nanoferrites annealed at different tem-peratures

3.7 Toxicity Measurements

DNA fragmentation has been analyzed as a marker for geno-toxicity of the as-prepared Co0.5Zn0.5Fe2O4 NPs using thealkaline single cell gel electrophoresis or comet assay. Thepurpose of this study is to investigate the genotoxic effects offerrite NPs on human peripheral blood cells. With this inten-tion, a human peripheral blood sample was treated with theas prepared sample for 24 h. In the current study, DNA dam-age caused by the 10 µg/ml concentration of the as-preparedCo0.5Zn0.5Fe2O4 NPs is significant. Figure 8 shows thecomet assay results for the human leucocytes obtained froman in vitro test of the as prepared Co0.5Zn0.5Fe2O4 NPs. Thegenotoxicity of Co0.5Zn0.5Fe2O4 NPs seems to be due to thegeneration of reactive oxygen species (ROS) that are con-sidered as a major source of spontaneous DNA damage. It isobserved that nanoferrites cause low level damage between39 % and 44 % for 10 µg/ml concentration in human periph-eral blood cells.

4 Conclusions

Spinel Co0.5Zn0.5Fe2O4 NPs have been successfully syn-thesized by a low temperature, high purity and easily repro-ducible chemical co-precipitation route. Different synthesisparameters which are found to be significant, have been op-timized in order to get controlled size nanoparticles. Theas-prepared and annealed Co0.5Zn0.5Fe2O4 NPs have single

Fig. 8 Genotoxicity of CoZnferrites NPs via comet assay:(a) normal cells, (b) damagedcells, (c) slightly damaged cells

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phase crystallinity with clear diffraction patterns. It has beenobserved that the width of diffraction peaks contracts as theannealing temperature increases, which may be due to theincrease in grain size of ferrite NPs. EDX and XRF micro-plots have confirmed that the prepared Co0.5Zn0.5Fe2O4

spinel NPs are composed of cobalt, zinc and iron in the spe-cific ratio. Furthermore, the magnetic moment is found tobe enhanced significantly due to annealing of the preparedCo0.5Zn0.5Fe2O4 NPs. The hysteresis loops obtained frommagnetic measurements using VSM show the soft magneticnature of the prepared Co0.5Zn0.5Fe2O4 nanoparticles. Thebehavior of Mr/Ms indicates that at room temperature a con-siderable amount of NPs are still superparamagnetic in theabsence of an externally applied magnetic field. The opticalproperties have shown tailoring in bandgap as a function ofthe annealing temperature. Hence, it is concluded that theannealing temperature has a significant effect on the physi-cal properties of Co0.5Zn0.5Fe2O4 NPs. Finally, the toxicityof the as-prepared Co0.5Zn0.5Fe2O4 NPs has been found tobe low towards human peripheral cells.

Acknowledgement This work was funded by the HEC Project IPFP(Grant No. PM-IPFP/HRD/HEC/2011/3386) and HEC Ph.D. Scholarfunding (Tariq Jan). The cooperation of Dr. Arshad from NUST is alsoacknowledged.

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