Journal of Magnetism and Magnetic Materials 324 (2012) 2958–2963

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Journal of Magnetism and Magnetic Materials

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n Corrnn Cor

E-m

elerman

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Effect of the number of iron oxide nanoparticle layers on the magneticproperties of nanocomposite LbL assemblies

Ilker Dincer a, Onur Tozkoparan a, Sergey V. German b, Alexey V. Markin b, Oguz Yildirim a,Gennady B. Khomutov c,d, Dmitry A. Gorin b,n, Sergey B. Venig b, Yalcin Elerman a,nn

a Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Besevler, Ankara, Turkeyb Faculty of Nano- and Biomedical Technologies, Saratov State University, Saratov 410012, Russiac Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow 119992, Russiad Institute of Nanotechnologies for Microelectronics RAS, Moscow 119992, Russia.

a r t i c l e i n f o

Article history:

Received 6 January 2012

Received in revised form

1 April 2012Available online 15 May 2012

Keywords:

Layer-by-layer assembly

Magnetite nanoparticles

Nanocomposite films

Magnetic properties

Magnetic permeability

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.04.002

esponding author. Tel.: þ79172077630.

responding author. Tel.: þ903122033426.

ail addresses: gorinda@mail.ru (D.A. Gorin),

@ankara.edu.tr (Y. Elerman).

a b s t r a c t

Aqueous colloidal suspension of iron oxide nanoparticles has been synthesized. Z-potential of iron oxide

nanoparticles stabilized by citric acid was �3573 mV. Iron oxide nanoparticles have been characterized by

the light scattering method and transmission electron microscopy. The polyelectrolyte/iron oxide nanopar-

ticle thin films with different numbers of iron oxide nanoparticle layers have been prepared on the surface of

silicon substrates via the layer-by-layer assembly technique. The physical properties and chemical composi-

tion of nanocomposite thin films have been studied by atomic force microscopy, magnetic force microscopy,

magnetization measurements, Raman spectroscopy. Using the analysis of experimental data it was

established, that the magnetic properties of nanocomposite films depended on the number of iron oxide

nanoparticle layers, the size of iron oxide nanoparticle aggregates, the distance between aggregates, and the

chemical composition of iron oxide nanoparticles embedded into the nanocomposite films. The magnetic

permeability of nanocomposite coatings has been calculated. The magnetic permeability values depend on

the number of iron oxide nanoparticle layers in nanocomposite film.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The layer-by-layer (LbL) assembly is a method of thin filmdeposition, which is often used for oppositely charged poly-mers [1]. This method was successfully applied to the preparationof thin films containing nanoparticles [2,3]. Its simplicity andreproducibility of high quality thin films make LBL assembly anattractive and alternative method in relation to many thin filmdeposition techniques such as spin coating, Langmuir–Blodgettdeposition, electrophoresis and others.

The LbL assembly containing colloid nanoparticles can bedescribed as the sequential adsorption of charged nanoparticleson oppositely charged layers of polyelectrolytes or particlesformed on the surface of solid substrates [4–13]. LbL assemblytechnique was used successfully for the preparation of less than1 mm thin nanocomposite multilayer films for different applica-tions [2–22]. The structure of these films can be controlled easilyon the nanoscale level [13].

The physical properties of this type of nanocomposite coatingswere investigated intensively [2–31]. A few articles concerned to

ll rights reserved.

the magnetic properties of polyelectrolyte/nanoparticles (PE/NP)composite films [4,5,7,10,15,16]. It was shown that the physicalproperties of the nanocomposite coatings depended on the multi-layer architecture [11,24–28].

The aim of the present study was to investigate the effect of thenumber of NP layers on magnetic properties of PE/NP nanocom-posite films prepared via LBL technique using iron oxide colloids.We fabricated and studied nanocomposite coatings which con-sisted of alternate layers of negatively charged iron oxide nano-particles and cationic polyallylamine hydrochloride molecules.

Magnetic properties of nanocomposite LbL films were studiedusing temperature dependent and magnetic field dependent mag-netization measurements. These measurements were complemen-ted with atomic force microscopy (AFM), magnetic force microscopy(MFM). Iron oxide nanoparticles were characterized by dynamiclight scattering (DLS) and transmission electron microscopy (TEM)measurements. The chemical composition of nanocomposite coat-ings was studied by Raman spectroscopy technique.

2. Experimental section

The following materials were used for the film preparation:poly(allylamine hydrochloride) (PAH, MW¼70,000, Sigma), wasdissolved in aqueous solution of 0.15 M sodium chloride; watersolution of poly(ethylenimine) (PEI, MW¼600,000–1,000,000,

Fig. 1. The distribution of hydrodynamic diameter of iron oxide nanoparticle

measured using dynamic light scattering (DLS).

Fig. 2. TEM images of the iron oxide nanoparticles. Scale bar is 50 nm.

I. Dincer et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2958–2963 2959

Sigma), with the concentration of 2 mg/ml. Iron oxide colloidalaqueous suspension (Fig. 1) was prepared with a particle massconcentration of 5 mg/ml at pH¼5.9. In this work the methoddescribed previously by R. Massart was used for iron oxidenanoparticles synthesis [17]. The processes of synthesis andstabilization were carried out in a special device under nitrogenatmosphere at temperature 20 1C. 0.65 g of FeCl3 and 0.24 g ofFeCl2 were added to 100 ml (0.1 M) of NaOH under rapidmechanical stirring and the mixture was allowed to stir for30 min. With the aid of a constant magnet the black solid productwas decanted. The sediment was dispersed in the solution of citricacid in water at concentration 20 g/l and then was stirred rapidly.This operation was carried out five times using different volumeof water solution of citric acid consequently 40, 20, 20, 10, 10 ml.Finally, colloid was dialyzed for 3 day. The iron oxide nanoparti-cles were stabilized by citric acid and revealed a negative chargeas the Z-potential value of the nanoparticles in aqueous suspen-sion was �3573 mV at pH 6.470.1 (Malvern Zetasizer NanoZS). The distribution of hydrodynamic diameter of iron oxidenanoparticles measured by dynamic light scattering (DLS) ispresented in Fig. 1. The mean size of the iron oxide nanoparticlesmeasured by DLS (Malvern Zetasizer Nano ZS) was 1476 nm (seeFig.1) and was in good agreement with transmission electronmicroscopy (TEM, Jeol JEM-100B Transmission Electron Micro-scope operating at 100 kV) data of about 10 nm (see Fig. 2).

The nanocomposite films were formed on the (1 1 1) surfaceof Si wafers with a natural oxide layer. The wafers were firstsubjected to the standard RCA cleaning process. The surfaceof silicon wafer with a natural oxide layer being immersedin water may be partially covered by the hydroxyl groups.The resulting silicon substrate surface has been found to benegatively charged in water at neutral pH [18,19]. To increasethe efficiency of polyelectrolyte adhesion to the substrate surface

and simultaneously decrease its initial roughness, one layer ofpolyethylenimine (PEI) was deposited on silicon wafers [19]. Thethickness of the PEI precursor layer was found to be about 2 nmby X-ray reflectometry and ellipsometry [19] and by AFM [18].Multilayers were prepared by LbL self-assembly technique viaalternate treating a substrate in solutions of PAH and iron oxidenanoparticles which had positive and negative Z-potential values,respectively. Each freshly deposited layer was washed three timesby deionized water before starting the next deposition step inorder to discard some charged molecules or nanoparticles whichhad not been adsorbed on the substrate surface. The obtainednanocomposite films were dried in air at room temperature.Deposition of multilayers on the silicon substrate was carriedout by the automatic setup ‘‘POLIION-1M’’ [20,21]. The followingmultilayer compositions were prepared: (Sample 1) PEI/(ironoxide/PAH)6; (Sample 2) PEI/(iron oxide/PAH)11; (Sample 3) PEI/(iron oxide/PAH)16.

Atomic Force Microscopy (AFM) images were taken by using aNT-MDT-SOLVER-PRO M instrument operating in the tappingmode with NSG01/Co tips. The surface was scanned at 2.08 Hzwith 256 lines per image resolution and the filter technique wasapplied to the images persecuted. To optimize the sensitivity ofMagnetic Force Microscopy (MFM), the MFM measurements wereperformed in tapping mode by using two pass techniques. In thefirst scan, the AFM image was taken in tapping mode to give areference height line. The MFM scan was performed taking intoaccount this reference line with a lifted cantilever which wasdriven at its resonance frequency. The phase shift was detecteddue to magnetic interactions between the tip and the sample.

The magnetic measurements were carried out using a Vibrat-ing Sample Magnetometer EV9 from MicroSense at the tempera-tures 193 and 473 K in an applied magnetic field up to 2 T. Themagnetic field was created by a water cooled electromagnet andwas oriented parallel to the film surface. From the magnetichysteresis loops, the correct demagnetization values correspond-ing to the sample signals were obtained by subtracting thediamagnetic signal of the substrate from the total registeredsignal.

All Raman spectra were obtained with Ntegra Spectra Ramanmicroscope system, using a solid state laser ‘‘Blue Cobalt’’(l0¼473 nm; laser power at spot, 3.5 mW). Raman measure-ments were carried out using a �100 objective with a numericalaperture of 0.90 and lateral resolution �1 mm. An exposure timewas equal to 60 s and the samples were glued to the glass surface.

The Raman study of iron oxide nanoparticles was carried outimmediately and in a week. Changing in spectra was not found.Nanocomposite coatings were fabricated during three days usingiron oxide nanoparticle colloid after 6 weeks since colloid pre-paration. MFM and then magnetic measurements were carriedout in two weeks after the nanocomposite coatings preparation.Raman spectra of nanocomposite coating were measured in twoweeks after study of their magnetic properties.

3. Results and discussions

Fig. 3 Shows the topography and phase images obtained byAFM and MFM measurements of the polyelectrolyte/iron oxidenanoparticle nanocomposites. AFM images were processed by theSPM data visualization and analysis tool Gwyddion 2.5 softwarein order to obtain the roughness (Ra, Rrms), the thickness ofcomposite films (dth), the diameter of nanoparticle aggregates(Dagg), the surface concentration of nanoparticle aggregates (Nagg),and the average distance between aggregates (Lagg). From AFMmeasurements we found that the average roughness (Ra) valueswere 6.5, 6.7 and 5.9 nm (Table 1) for sample 1 (PEI/(iron oxide/

Fig. 3. Topography images (a), (c) and (e) and MFM phase images under zero

magnetic field (b), (d) and (f) for the samples. (a), (b) Sample with the following

structure PEI (iron oxide/PAH)6; (c), (d) sample with structure PEI(iron oxide/

PAH)11; (e), (f)–sample with structure PEI(iron oxide/PAH)16).

Table 1The results of AFM study of the samples (PEI(iron oxide/PAH)6, PEI(iron oxide/

PAH)11 and PEI(iron oxide/PAH)16). The roughness (Ra, Rrms), the thickness of the

composite films (dth), the diameter of the nanoparticle aggregates (Dagg), the

surface concentration of a nanoparticle aggregates (Nagg), the average distance

between the aggregates (Lagg) as determined from AFM.

Sample number (number of iron

oxide nanoparticle layers (N))

Ra,

(nm)

Rrms,

(nm)

dth,

(nm)

Dagg,

(nm)

Nagg,

(mm2)

Lagg,

(nm)

1 (6) 6.5 8.5 63 131 16 116

2 (11) 6.7 9.2 97 139 22 76

3 (16) 5.9 7.5 106 128 25 72

Table 2The results obtained by the analysis of MFM measurements for the samples

(PEI(iron oxide/PAH)6, PEI(iron oxide/PAH)11 and PEI(iron oxide/PAH)16). The

diameter of magnetic aggregates (Daggm ), the surface concentration of the magnetic

aggregates (Naggm ), the average distance between the magnetic aggregates (Lagg

m ) as

determined from MFM.

Sample number

(number of iron oxide

nanoparticle layers N)

Daggm (nm) Nagg

m (mm�2) Laggm (nm)

1(6) 141 13 134

2(11) 128 20 94

3(16) 114 25 86

I. Dincer et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2958–29632960

PAH)6), sample 2 (PEI/(iron oxide/PAH)11) and sample 3 (PEI/(iron oxide/PAH)16), respectively.

This is not typical for this type of nanocomposite layersbecause roughness usually increases with the growth of numbersof iron oxide layers [8,11–16,20,21]. The different type of thedependence can be explained by lower magnitude of Z potentialfor iron oxide nanoparticles used in our experiments. We havereceived earlier similar results during AFM study of microcapsuleshells with embedded iron oxide nanoparticles [22]. The thick-ness of composite layers dth increases with increasing the numberof nanoparticle layers (Table 1). But a slight difference bet-ween the thickness dth of samples with 11 and 16 layers of ironoxide nanoparticles was observed along with decreasing theroughness of nanocomposite coatings. This fact can be explainedby filling the cavities formed on the surface of nanocompositecoatings via adsorption of iron oxide nanoparticles from nano-particulate colloid suspension. The size of a cavity is proportionalto the roughness of nanocomposite films. The average roughnessof a nanocomposite film containing 11 layers of iron oxide

nanoparticles is close to the average size of a single nanoparticle(Table 1). This fact is confirmed by the results obtained bythe calculation of the nanoparticle aggregates diameter (Dagg),the surface concentration of nanoparticle aggregates (Nagg),and the average distance between aggregates (Lagg). It was notedthat the increase of the surface concentration of nanoparticleaggregates (Nagg) correlated with the decrease of the averagedistance between aggregates (Lagg) and the growth of the numberof iron oxide nanoparticle layers N. The dependence of thenanoparticle aggregates diameter (Dagg) on the number (N) ofnanoparticle layers is of a nonmonotonous character and hasmaximum at 11 layers of iron oxide nanoparticle layers. Theanalysis of MFM images is presented in Table 2. In contrast to theresults of AFM images, the diameter of magnetic aggregates ofnanoparticles Dagg

m calculated using the results of MFM imagesdecreased with increasing the number of layers N. This fact can beexplained taking into account only the suggestion on the differentchemical composition of aggregates. The iron oxide nanoparticlescan consist of a mix of magnetite and maghemite phases[21,23,29,30]. The volume fraction of maghemite increases withthe growth of layer number and therefore the size of areas withhigher magnetic permeability is less than the size of aggregates.But the surface concentration of the magnetic aggregates (Nagg

m )and the average distance between magnetic aggregates (Lagg

m )decrease with increasing the number of iron oxide nanoparticlelayers. The similar behavior for surface concentration of nano-particle aggregates (Nagg) and the average distance between them(Lagg) was observed during the analysis of AFM images.

We used the NSG01/Co magnetic tips for MFM measurementsand calculated the phase shift (DF) using the following equation[32–34]:

DF¼p2�F ð1Þ

However, the magnetic force gradient values were also calcu-lated by using the following equation and the results were givenin Fig. 4.

DF¼Q

k

dFZ

dzð2Þ

Here, k is the spring constant and its value is about 5 N/m. Q isthe quality factor and its value is about 315.

The MFM images of the sample 1, sample 2 and sample3 obtained by MFM under a zero magnetic field are shown inFig. 3(b), (d) and (f), respectively. With increasing the number ofmagnetic layers, the areas containing iron oxide nanoparticleaggregates were seen more clearly (Fig. 3(c)).

The comparison of AFM (Fig. 3(a), (c), (e)) and MFM (Fig. 3(b),(d), (f)) images was shown. Due to the AFM images, the roughnessof the nanocomposite coating surface is associated with thecluster formation during the nanocomposite coating growth.These results are in good agreement with previously obtaineddata [19–21,23].

Fig. 4. The magnetic force gradient of the samples PEI(iron oxide/PAH)6 (a), PEI (iron oxide/PAH)11 (b) and PEI(iron oxide/PAH)16 (c) as a function of the magnetic field. For

each sample, five different points have been chosen for the field dependent magnetic force gradient graphics.

Table 3The results of the calculation based on the magnetization measurements of the samples (PEI(iron oxide/PAH)6, PEI(iron oxide/PAH)11 and PEI(iron oxide/PAH)16): MS

(193 K)—saturation magnetization at 193 K; MS (473 K)—saturation magnetization at 473 K; m (193 K)—magnetic permeability at 193 K; m (473 K)—magnetic

permeability at 473 K.

Sample number (number of

iron oxide nanoparticle layers N)

MS (193 K) MS (473 K) m (193 K) m (493 K)

(emu/cm3) (emu/g) (emu/cm3) (emu/g)

1(6) 286.91 57.38 128.92 25.78 5.4 2.6

2(11) 361.03 72.35 110.05 22.05 5.6 3.1

3(16) 318.59 63.97 286.14 57.45 4.8 3.9

I. Dincer et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2958–2963 2961

Fig. 4 Shows the magnetic field dependence of a magneticforce gradient of the samples. However, for all the samples,magnetic force gradients increase with increasing a magneticfield for five different points which were chosen to determine theeffect of the applied magnetic field on the sample. This relationdepends on the following equation:

dF

dz¼

24mSmP

z5ð3Þ

As a result of ascending applied magnetic field values themagnetic moments of the samples increased. Due to this relation,the magnetic force gradient values were increased. To ascertainmagnetic aggregates sizes and shapes, it was necessary to getMFM measurements. Using contrast differences in MFM images itis easy to determine locations of magnetic nanoparticle aggre-gates on the surface (Fig. 3).

The calculation of saturation magnetization MS was carried outbased on M(H) curves. The mass and the volume of iron oxidenanoparticles in the nanocomposite coatings were used to deter-mine MS (see Table 3).

The product of the nanocomposite coating volume (Vcoating) andthe volume fraction of iron oxide nanoparticles gives us thevolume of iron oxide nanoparticles in the nanocomposite coatings.The volume of the nanocomposite coating (Vcoating) is a product ofthe surface area (A) and the thickness of the coatings (dth). Weused the thickness of the coatings (dth) obtained by AFM measure-ments (Table 1). The surface area (A) of the samples was alsomeasured. The volume fractions of iron oxide nanoparticles in thenanocomposite coatings were calculated using the Bruggeman (Br)

approximations based on the ellipsometry data [21]. The ellipso-metry method is used for characterization of the nanocompositecoating volume as a whole, but not only of the surface ofnanocomposite coatings. The values of the volume fractions ofthe iron oxide nanoparticles in the nanocomposite coatings are0.3770.02 for 6 layers, 0.4570.02 for 11 layers and 0.7570.12for 16 layers of iron oxide nanoparticles. The relative error value ofvolume of iron oxide nanoparticles in the coating is less than 15%.The surface area (A) determination makes a big contribution torelative error of nanoparticle volume. Mass of iron oxide nano-particles was calculated as the product of volume of iron oxidenanoparticles in the coating and density of magnetite (5.15 g/cm3).

Fig. 5(a)–(c) shows the magnetic field dependence of themagnetization at 193 and 473 K for samples 1–3, respectively.The magnetization versus applied magnetic field-M(H) curvespresented in Fig. 5 are characterized by a typical sigmoidal shapewith a very small (approximately zero) hysteresis loop in the lowfield region. The saturation magnetization-MS values depend onthe number of magnetic layers at both temperatures (see Table 3).The MS values for an iron oxide nanoparticle have a goodcorrelation with the data obtained from the earlier study [35].

The data presented in Table 3 display the nonmonotonicdependence of MS on the number of iron oxide nanoparticlelayers at 193 K. This dependence has maximum at 11 iron oxidenanoparticle layers. The maximum value of MS is 72.35 emu/g.This value has a good agreement with data obtained for themagnetite nanoparticles [35] and is less than MS value for bulkmagnetite [36,37]. As usual, the value of MS for nanoparticles isless than MS value for bulk materials [35]. MS values are

Fig. 5. The M(H) curves of the samples at 193 and 473 K. (a) For sample with

structure PEI (iron oxide/PAH)6, (b) for sample with structure PEI (iron oxide/

PAH)11, (c) for sample with structure PEI (iron oxide/PAH)16. & shows the

magnetization of the magnetic film and substrate at 193 K. The line shows the

magnetization of the diamagnetic substrate at 193 K. ’ shows the magnetization

of ferromagnetic films at 193 K. The stars show the magnetization of ferromag-

netic films at 473 K.

Fig. 6. Raman spectra of the nanocomposite films. (a) Film with structure PEI(iron

oxide/PAH)6; (b) film with structure PEI (iron oxide/PAH)11; (c) film with structure

PEI (iron oxide/PAH)16; (d) silicon substrate.

I. Dincer et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2958–29632962

73.5 emu/g [36] and 92 emu/g [36] or 84 emu/g [37] for bulkmaghemite and for bulk magnetite, respectively. The reducedmagnitude of MS values for iron oxide nanoparticles was obtaineddue to the existence of a spin disordered surface layer [37].Decreasing of the size of nanoparticles leads to the surfacevolume ratio increasing and therefore the effect of the spindisordered surface layer is increased on the MS values.

We obtained rather high saturation magnetization for sample 3 athigh temperature. It was very close to the saturation magnetizationwhich we measured at lower temperature (193 K) and made thissample very important from the application point of view.

The magnetic permeability m values calculated from M(H)curves at 193 K and 473 K are given in Table 3.

The magnetic permeability was calculated by using the follow-ing equation [21]:

m¼ 1þ4pM

Hð4Þ

Here, M is the magnetization and H is the magnetic field. Themagnetic permeability of the samples is of a nonmonotonouscharacter and has maximum at 11 layers of iron oxide nanopar-ticles. The dependence of magnetic permeability value on thenumber of iron oxide layers can be explained using onlythe suggestion on different chemical composition of differentsamples. Therefore, it was necessary to study the chemicalcomposition of samples obtained. The optimal method for the

determination of chemical composition of nanostructured mate-rials is Raman spectroscopy [29–31].

The obtained Raman spectra of (iron oxide/PAH)n self-assembled films are shown in Fig. 6. The silicon substrate servesas an internal standard for the spectral measurements, usingRaman shift of Si band at 521 cm [38] (Figs. 6, 7). In Fig. 6 one cansee two characteristic peaks: at 670 cm and around 700 cmcorresponding to magnetite [29,30] and maghemite [31], respec-tively. The peak at 670 cm overlaps the Raman shift for silicon at670 cm, however, in spectra of the films containing iron oxide theintensity of that peak is significantly higher than the peak at615 cm for silicon. Thus, it is established that magnetite andmaghemite phases are present in the nanocomposite films.

Iron oxide nanoparticles with Raman spectra containing onlythe peak at 670 cm (Fig. 7) were used for nanocomposite coatingpreparation. This means that newly prepared colloid suspensionof iron oxide nanoparticles consists only of magnetite phase(Fig. 7) [29,30].

Atmospheric oxygen can interact with iron oxide nanoparticlesembedded in the nanocomposite coating and it induces thetransition from the magnetite Fe3O4 to the maghemite g-Fe2O3.As a result the mixture of magnetite and maghemite phases(Fe3O4-d) is present in the nanocomposite films.

Raman spectroscopy allows us to establish the qualitative trendof values of magnetite and maghemite phases in their dependenceon the iron oxide nanoparticle layers. We can conclude definitely,that the volume fraction of maghemite in the nanocomposite filmsdepends on the nanoparticle layer value using the analysis of theRaman spectra data presented in Table 4. That behavior ofmaghemite volume fraction can be explained using the followingsuggestion. It takes more time to form a higher number ofnanoparticle layers, therefore we have more time for oxidation ofnanoparticles. The oxidation process takes place also after theformation of a nanocomposite coating, because the thickness ofcoating (the number of layers) influences the ability of a nano-composite coating to protect the magnetite phase against theoxidation. Both processes led to a nonmonotonic dependence ofmagnetic permeability on the number of iron oxide nanoparticlelayers. The dependence of magnetic permeability on nanoparticlelayers has maximum at 11 iron oxide nanoparticle layers.

Increasing of magnetic permeability with increasing of nano-particle layer number at high temperature can be explained by

Fig. 7. Raman spectra of the iron oxide nanoparticles and the nanocomposite

films. (a) Film with structure PEI(iron oxide/PAH)16; (b) aqueous colloid of

nanoparticles based on magnetite/maghemite composite; (c) aqueous colloid of

magnetite nanoparticles in a week after the iron oxide colloid preparation;

(d) silicon substrate.

Table 4The intensity of lines I (670 cm) and peak area S (670 cm) responsible for

magnetite and intensity of lines I (700 cm) and peak area S (700 cm) responsible

for maghemite in the Raman spectra of the samples: PEI(iron oxide/PAH)6,

PEI(iron oxide/PAH)11 and PEI(iron oxide/PAH)16.

Sample number

(number of iron

oxide nanoparticle

layers N)

I (670 cm�1) I (700 cm�1) S (670 cm�1) S (700 cm�1)

1(6) 724 465 34385 25528

2(11) 602 413 32169 21185

3(16) 367 268 19178 15749

I. Dincer et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2958–2963 2963

changing the maghemite volume fraction as a result of thermaltreatment during the magnetization measurements at high tem-perature (473 K) which can lead to oxidation of magnetite tomaghemite [29,30]. Of course, for rather thick films with highernumber of layers the increase of maghemite fraction is smaller thanthat for the samples with low amount of formed layers because theefficiency of oxygen diffusion to magnetite nanoparticles dependson the overall thickness of nanocomposite coatings.

Thus, the magnetic permeability of nanocomposite films withiron oxide nanoparticles is higher than 1 (Table 3). It is necessaryto take this fact into account under the study of physical proper-ties of nanocomposite films containing iron oxide nanoparticles,particularly in the calculation of the volume fraction of iron oxidenanoparticles using the refractive index data, as in Ref.21.

4. Summary and conclusions

It has been established that the magnetic permeability valuedepends on the number of iron oxide nanoparticle layers in ananocomposite film and that the dependence is of nonmonotoniccharacter with maximum. We have shown that this dependence canbe explained using the analysis of the Raman spectroscopy data. Theiron oxide nanoparticle phase in nanocomposite coatings is amixture of magnetite and maghemite phases. The magnetite and

maghemite phases depend on a number of iron oxide nanoparticlelayers because the iron oxide nanoparticles are oxidized frommagnetite to maghemite. Those results should be taken into accountwhen the LbL assembly method is used to create nanocompositecoatings with iron oxide nanoparticles. These nanocomposite filmshave good perspectives for applications in electronics to createantireflective coatings and also for biomedical applications to createcoatings with remote control of physical properties using alternativemagnetic field or microwave radiation, which is very important forfabrication of new generation substrates in tissue engineering andadvanced drug delivery systems.

Acknowledgments

This work was supported by TUBITAK (Grand No: 209T054),RFBR (10-08-91219-CT), RFBR (11-08-12058-ofi-m-2011).

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