lable at ScienceDirect

Current Applied Physics 11 (2011) 472e475

Contents lists avai

Current Applied Physics

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

Characterization and magnetic properties of polyethylene glycol modified NiZnferrite thin films

Ke Sun*, Zhongwen Lan, Zhong Yu, Xiaoliang NieState Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, No. 4, Section 2, North Jianshe Road, Chengdu 610054,People’s Republic of China

a r t i c l e i n f o

Article history:Received 3 March 2009Received in revised form29 July 2010Accepted 24 August 2010Available online 17 September 2010

Keywords:NiZn ferriteSolegel processThin filmsPolyethylene glycolMagnetic measurements

* Corresponding author. Tel./fax: þ86 28 83201673E-mail addresses: shmily811028@126.com, ksun@

1567-1739/$ e see front matter � 2010 Elsevier B.V.doi:10.1016/j.cap.2010.08.023

a b s t r a c t

Polyethylene glycol (PEG) modified Ni0.5Zn0.5Fe2.0O4.0 ferrite thin films were deposited on Si (100)substrate by a solegel method. The crystal structure, surface morphology and magnetic properties ofPEG-modified NiZn ferrite thin films (NZFs) were investigated by using the X-ray diffractometer (XRD),atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and vibrating samplemagnetometer (VSM). The FTIR spectrums showed that PEG molecules were on the surface of as-prepared particles, which effectively improved their dispersibility. The crystallization of NZFs improvedand crystallite size increased with increasing PEG concentration. Optimal PEG concentration couldimprove the surface morphology and reduce the roughness. Magnetic properties measurement indicatedthat compared with the sample without PEG, PEG-modified NZFs showed excellent magnetic propertieswith higher saturation magnetization and lower coercive force. Finally, PEG-modified NZFs showeda high saturation magnetization 312 emu/cm3 and low coercive force 90 Oe when PEG concentration was40 g/L.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

NiZn ferrite possesses the advantages such as high resistivityand Curie temperature, low temperature coefficient, and excellentproperties at high frequency, which can be used for microwavedevices of high performance. However, bulk NiZn ferrite compo-nents employed in discrete devices at microwave frequenciescannot be compatible with the rapid development of electronicapplications which towards downsizing, lightweight and multi-function. The thin film ferrites, incorporated into magnetic inte-grated circuits (MAGIC), are expected to replace the surfacemounted devices (SMD) in near future [1]. Therefore, the prepa-ration methods such as solegel [2e4], spin spray plating [5,6],magnetron sputtering [7] and pulsed laser deposition [8] have beenattempted to prepare the NiZn ferrite thin films (NZFs).

In comparison with other preparation methods, solegel processoffers excellent composition control, low temperature processing[2,9] and short fabrication periods at comparatively low cost.However, if agglomeration or even sedimentation appears in thesol, there will be uneven fissure in the films. So, in the preparationprocess of NiZn ferrite films, getting uniform and stable sol is one of

.uestc.edu.cn (K. Sun).

All rights reserved.

the most important factors which may affect the entire preparationprocess and the property of the final films. But to our knowledge,there are few reports on the PEG-modified NiZn ferrite thin films. Inthis study, one aim to investigate the infrared spectrums, crystalstructure, surface morphology and magnetic property of PEG-modified NZFs deposited by a solegel method. It will demonstratethat PEG can effectively prevent the colloidal particles of chelatefrom being jointed with each other. PEG also can enhance thecrystallization of NZFs, improve the surface morphology. Andoptimal PEG concentration (40 g/L) can obtain a high magnetiza-tion (312 emu/cm3) and low coercive force (90 Oe).

2. Experimental procedures

2.1. Sample preparation

Ni0.5Zn0.5Fe2.0O4.0 thin films (NZFs) were deposited on Si (100)substrate by a solegel method. Stoichiometric quantities ofanalytical grade Zn(CH3COO)2$2H2O, Ni(CH3COO)2$4H2O and Fe(NO3)3$9H2O were dissolved in 2-methoxyethanol to form a mixedsolution. After the solution was stirred for 1 h, the acetic acid wasadded to adjust the concentration of the solution to 0.2 mol/L.Meanwhile, polyethylene glycol (PEG, C.R., molecular weightw2000) varied from 0 g/L to 60 g/L in steps of 10 g/L was added inthe reaction mixture. The prepared solution was then continuously

Fig. 2. XRD patterns of NZFs with different PEG concentrations: (a) 0 g/L, (b) 40 g/L and(c) 60 g/L.

K. Sun et al. / Current Applied Physics 11 (2011) 472e475 473

stirred for 2 h and placed at room temperature for 36 h to form thestable solegel precursors for following processes. First, the wetfilms were deposited by a spin coating method on the substrate ofSi(100) with 4000 rpm for 30 s. Second, the wet films were dried at100 �C for 10 min to remove the mixed solvents. Then, the opera-tions of spin coating and drying were repeated to get the requiredfilm thickness of 140 nm. In the end, the dried films were annealedat 700 �C for 1 h in air and cooled slowly in the furnace.

2.2. Sample characterization

Infrared spectrums were recorded by using an NICOLET MX-1FTIR for the air-dried precursor under 90 �C in KBr medium in therange of 500e4000 cm�1. The phase identification of the thin filmswas performed by the Philips X’Pert PRO X-ray diffractometer(XRD), with Cu Ka radiation. The surface morphology of the filmswas analyzed by atomic force microscopy (AFM), and magneticmeasurements by the TOEI VSM-5S-15 vibrating sample magne-tometer (VSM) at room temperature.

3. Results and discussion

3.1. FTIR characterization

Fig. 1 shows the Fourier transform infrared spectroscopy (FTIR)spectrums of the dried NiZn ferrites gel with 60 g/L and withoutPEG. The adsorbed water is featured by bands at 3420 and1620e1630 cm�1, which are assigned to OeH stretching andHeOeH bending modes of vibration [10e12]. For the dried gel,band of OeH stretching vibration is observed at 3400e3450 cm�1,but the intensity of this band is different. For the gel with 60 g/LPEG, the intensity of the band around 3400 cm�1 is weaker than thegel without PEG. The bands around 600 cm�1 are observed in thetwo samples which are caused by the stretching vibration due tothe interactions produced between the oxygen and the cationoccupying the octahedral and tetrahedral sites. Spectrums of theas-prepared nanoparticles consist of bands at 2870 cm�1 wereattributed to symmetric and asymmetric methylene (eCH2e) [13].The bands observed around 1600 cm�1 are due to the bendingvibration of the carboxyl group. The carbonate CeO vibrations of

Fig. 1. FTIR spectrums for Ni0.5Zn0.5Fe2.0O4.0 ferrite gel powder dried at 135 �C:(a) without PEG; (b) with 60 g/L PEG.

the interlamellar carbonates are present at 1440 and 660 cm�1. Theabsorption band at 1100 cm�1 produced by the CeOeC stretchingindicates the presence of linkage in the absorbed moleculesbecause of PEG [14]. So the PEG molecules are on the surface of as-prepared particles, which means that PEG can effectively preventthe colloidal particles of chelate from being jointedwith each other.

3.2. Phase characterization

The XRD patterns of NZFs with various PEG concentrations areshown in Fig. 2. It is observed that NZFs with different PEGconcentrations show cubic spinel structure and without unidenti-fied peaks. As the content of PEG increases, the diffraction intensityof cubic spinel phase becomes stronger. Obviously, PEG enhancesthe mass-transfer and ions-diffusion, the amount of spinel phasethus increases and the arrangement of crystal cells inside grainsbecomes more regular. Moreover, the crystallite sizes of NZFs withdifferent PEG concentrations are listed in Table 1. The crystallite sizeis determined from the diffraction peak broadening with the use ofthe Scherrer equation:

D ¼ 0:9lb cos q

(1)

where l is the wavelength of the target Cu Ka 0.15405 nm, b is thefull width at half maximum of diffracted (311) peak. It is observedthat crystallite size increases with increasing PEG. The reasonableexplanation is that PEG can increase the attraction among thepolymer chains by coordination, resulting in the aggregation ofnanoparticles to large diameter.

Table 1Crystallite sizes of NZFs with various PEG concentrations.

CPEG (g/L) 0 40 60D (nm) 25.1 30.6 32.3

Fig. 3. AFM images of the PEG-modified NZFs: (a) 0 g/L, (b) 40 g/L and (c) 60 g/L.

K. Sun et al. / Current Applied Physics 11 (2011) 472e475474

3.3. Surface morphology observation

Fig. 3 shows the surface morphology of NZFs with different PEGconcentrations. There are few regular crystallites on the surface ofNZFs without PEG (see Fig. 3a) and the crystallization is the worst.The poor crystallization has been confirmed in the previous anal-ysis of XRD (see Fig. 2a). After adding PEG, the surface morphologyof NZFs can be improved and the crystallites becomemore uniform.When PEG concentration reaches 40 g/L, the crystallite size ishomogeneous, surface morphology is smooth and the crystallitesize is about 40 nm (see Fig. 3b). While PEG concentration exceeds40 g/L, abnormal crystallites appear and surface morphologybecomes coarse (see Fig. 3c). For NZFs without PEG in the solution,metal ions like Fe3þ, Ni2þ and Zn2þ chelate and may directly becombined with each other, resulting in uneven distribution of themetal ions. Therefore, the crystallization is poor and the crystallitesare few. Since PEG has large chain molecular structure, it canenclose the colloidal particles resulted from the reaction betweenmetal ions and acetic acid, then avoid the direct touch of thosecolloids [15]. Therefore, the addition of PEG changes the

Table 2Rrms values of NZFs with various PEG concentrations.

CPEG (g/L) 0 20 40 60Rrms (nm) 3.95 2.75 1.67 3.78

homogeneity of the coating solution. However, large amount of PEGis not always expected because of the accompanying increase inNZFs roughness. Simultaneously abnormal crystallites will also beintroduced. Here, the roughness of NZFs with various PEGconcentrations can be quantitatively identified by the root-mean-squared roughness (rms) Rrms (see Table 2). Rrms is given by thestandard deviation of the data from the AFM image, and deter-mined using the standard definition as follows:

Fig. 4. Hysteresis loop of NZFs with 40 g/L PEG.

Fig. 5. Saturation magnetization (Ms) and coercivity (Hc) of NZFs as a function of PEGconcentration.

K. Sun et al. / Current Applied Physics 11 (2011) 472e475 475

Rrms ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXN

n¼1

ðzn � zÞ2

N � 1

vuuuut(2)

where zn represents the height of the nth data, z equals to the meanheight of zn in AFM topography, andN is the number of the data. It isobserved from Table 2 that both too low and too high concentra-tions of PEG can lead to NZFs with larger Rrms values. Therefore,40 g/L PEG is the optimal concentration here.

3.4. Magnetic properties measurement

Fig. 4 shows the hysteresis loop of NZFs with 40 g/L PEG. It isobserved the hysteresis loop of NZFs is that of a typical ferrimagnet.The saturation magnetization (Ms) and coercive force (Hc) of NZFsas a function of PEG concentration are demonstrated in Fig. 5. Aninitial increase followed by a subsequent decrease inMs is observedwith the increase of PEG. However, Hc decreases. The maximumMsis 312 emu/cm3 when the PEG concentration is 40 g/L. Magneticproperties of ferrites are sensitively dependent on the structure,composition, defects, crystallite size, internal strain and cationdistribution. From previous analyses (Figs. 2 and 3), crystallizationenhances, crystallite size increases and surface morphologyimproves with increasing PEG, which can lower the ferrite defects.Thus, Ms increases and Hc decreases. However, excessive PEG alsoproduces abnormal crystallites and large Rrms value, which impairsthe magnetization values. Thus, excessive PEG goes against themagnetization process, resulting in a decrease in Ms. The crystal-lization, crystallite size and morphology of NZFs with PEG are

better than those of NZFs without PEG, thus PEG-modified NZFsshow higher Ms and lower Hc.When the PEG concentration is 40 g/L, Ms of NZFs achieves its maximum (312 emu/cm3) and Hc reacheslow value (90 Oe). References 7 and 16 reported that NZFs preparedby magnetron sputtering process showed excellent magneticproperties. Desai et al. [7] got the highest Ms (250 emu/cm3) andlow Hc (80Oe) at 800 �C. Gao et al. [16] obtained the highest Ms(360 emu/cm3) and low Hc (100 Oe) at 850 �C. It shows that NZFsprepared by solegel method can get excellent magnetic propertiesat lower annealing temperature.

4. Conclusions

PEG-modified NiZn ferrite thin films have been deposited on Si(100) substrate by a solegel method. The FTIR shows that PEGmolecules are on the surface of as-prepared particles. The crystal-lization of NZFs enhances and crystallite size increases withincreasing PEG concentration. Optimum PEG concentrationimproves the surface morphology and reduces the roughness,resulting in an increase in Ms and a decrease in Hc. Finally, whenPEG concentration is 40 g/L, NZFs possess higher Ms (312 emu/cm3)and lower Hc (90 Oe).

References

[1] M. Yamaguchi, K. Ishhara, K.I. Arai, IEEE Trans. Magn. 29 (1993) 3222e3224.[2] F. Liu, T.L. Ren, C. Yang, L.T. Liu, A.Z. Wang, J. Yu, Mater. Lett. 60 (2006)

1403e1406.[3] S.Y. Bae, C.S. Kim, Y.J. Oh, J. Appl. Phys. 85 (1999) 5226e5228.[4] K. Sun, Z.W. Lan, Z. Yu, X.L. Nie, L.Z. Li, C.Y. Liu, J. Magn. Magn. Mater. 320

(2008) 1180e1183.[5] N. Matsushita, T. Nakamura, M. Abe, J. Appl. Phys. 93 (2003) 7133e7135.[6] C.M. Fu, H.S. Hsu, Y.C. Chao, N. Matsushita, M. Abe, J. Appl. Phys. 93 (2003)

7127e7129.[7] M. Desai, S. Prasad, N. Venkataramani, I. Samajdar, A.K. Nigam, N. Keller,

R. Krishnan, E.M. Baggio-Saitovitch, B.R. Pujada, A. Rossi, J. Appl. Phys. 91(2002) 7592e7594.

[8] C.N. Chinnasamy, S.D. Yoon, A. Yang, A. Baraskar, C. Vittoria, V.G. Harris,J. Appl. Phys. 101 (09M517) (2007) 1e3.

[9] K. Sun, Z.W. Lan, X.L. Nie, Z. Yu, X.N. Zhao, L.Z. Li, Yadian Yu Shengguang 31(2009) 709e711.

[10] M.S. Hegde, D. Larcher, L. Dupont, B. Beaudoin, K. TekaiaElhsissen,J.M. Tarascon, Solid State Ionics 93 (1996) 33e35.

[11] L.B. Newalkar, S. Komarneni, H. Katsuki, Mater. Res. Bull. 36 (2001)2347e2355.

[12] M.G. Han, Y. Ou, W.B. Chen, L.J. Deng, J. Alloys Compd. 474 (2009) 185e189.[13] S. Thimmaiah, M. Rajamathi, N. Singh, P. Bera, F. Meldrum, N. Chandrasekhar,

R. Seshadri, J. Mater. Chem. 11 (2001) 3215e3221.[14] J. Wang, C.R. Zhang, J. Feng, J. Inorg. Mater. 20 (2005) 435e441.[15] F.X. Cheng, Z.Y. Peng, Z.G. Xu, C.S. Liao, C.H. Yan, Thin Solid Films 339 (1999)

109e113.[16] J.H. Gao, Y.T. Cui, Z. Yang, Mater. Sci. Eng., B 110 (2004) 111e114.