Composites Science and Technology 72 (2012) 908–914

Contents lists available at SciVerse ScienceDirect

Composites Science and Technology

journal homepage: www.elsevier .com/ locate /compsci tech

Preparation of porous magnetic nanocomposites using corncob powders astemplate and their applications for electromagnetic wave absorption

Xue-Gang Chen a,⇑, Ji-Peng Cheng b, Shuang-Shuang Lv c, Ping-Ping Zhang d, Shu-Ting Liu a, Ying Ye a,⇑a Department of Ocean Science and Engineering, Zhejiang University, Hangzhou 310058, PR Chinab Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310028, PR Chinac Department of Nonmetallic Research, Zhejiang Institute of Geology & Mineral Resources, Hangzhou 310007, PR Chinad Second Institute of Oceanography, SOA, Hangzhou 310012, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 August 2011Received in revised form 1 March 2012Accepted 3 March 2012Available online 10 March 2012

Keywords:A. Functional compositesD. X-ray diffraction (XRD)B. Magnetic propertiesA. NanocompositesE. Heat treatment

0266-3538/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compscitech.2012.03.001

⇑ Corresponding authors. Tel.: +86 571 8820639(X.G. Chen), tel.: +86 571 87984550; fax: +86 571 88

E-mail addresses: chenxg83@zju.edu.cn (X.-G.(Y. Ye).

Strong absorption, low density, and thin matching thickness are important parameters for electromag-netic (EM) wave absorbers. In this study, we prepared novel porous magnetic nanocomposites usingcorncob powders as template. The presence of corncob will significantly decrease the bulk density ofsamples from more than 4.0 to about 0.55 g cm�3. The porous structures remarkably decreased the per-mittivity (e) and permeability (l) and enhanced the impendence matching between the absorber and air.The porous magnetic nanocomposites exhibit enhanced absorption for EM waves at thin matching thick-ness. The optimum thickness is only 1.0–1.4 mm, with bandwidth of RL < �5 dB of about 8 GHz, coveringthe half X-band and the whole Ku-band. The areal density of magnetic absorbers at this study is onlyabout 0.7–1.0 kg m�2 at thickness of 1.0–1.4 mm, much lower than the reported values of other magneticabsorbers. Due to the strong absorption at low density and thin matching thickness, the porous magneticnanocomposites prepared using corncob powders as template are promising light-weight EM waveabsorbers.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Corn is one of the major crops in the world, sharing equalimportance with rice and wheat. Corncob is the agriculturalbyproduct of corn produced by peeling off the corn kernels. About800 million tons of corn is produced per annum all over the world,which giving about 150 million tons of corncob [1]. The majorcompositions of corncob include cellulose, hemicellulose, lignin,and so on [2]. Therefore, the researches on the utilization of corn-cob focused on the extracting of organic compounds and prepara-tion of absorbers [3,4]. For instance, high concentration ethanol [5]and xylose [6] can be produced from pre-treated corncob. Likeother herb husks, corncob can also be activated by acid treatment[7] or calcination [8,9], the resulting products activated carbon ex-hibit strong adsorption capacities. Nevertheless, corncob in agri-culture is usually abandoned or burnt in air, not only wasted theresources in corncob, but also polluted the environment. Therefore,the utilization of corncob is still an important issue for the treat-ment of agricultural wastes.

ll rights reserved.

9; fax: +86 571 88208890208890 (Y. Ye).Chen), gsyeying@zju.edu.cn

Electromagnetic (EM) wave absorbers are widely used indomestic and military fields, either to protect the human beingfrom EM wave irradiation or to protect the military equipmentfrom being detected by radar waves [10,11]. Ideal EM wave absorb-ers should exhibit strong absorption and broad bandwidth for EMwaves on the basis of thin matching thickness and low density[11,12]. The requirements of strong absorption and broad band-width can be achieved by traditional magnetic or magnetic/carbonabsorbers such as ferrite [13–17], carbonyl iron [18–20], carbonnanotube nanocomposites [21–23], and so on. However, decreas-ing the matching thickness and density is still a challenge for theEM wave absorbers. Various methods have been applied to prepareEM wave absorbers with wide-band strong absorption and lowdensity, among which preparation of porous magnetic nanocom-posites is an efficient way [24–27]. For example, Liu et al. fabri-cated porous carbon/Co nanocomposites using a sol–gel method[28], which present enhanced EM wave absorption with low den-sity. Porous Fe3O4/Fe/SiO2 nanorods were prepared using a seriesof heat treatment, showing excellent EM wave absorption proper-ties [29].

However, these methods usually use activated carbon as tem-plate or require rigorous reagents and procedures, which increasedthe preparation cost of porous magnetic materials. In this study,we prepared novel low-density porous magnetic (including Feand FeNi) nanocomposites using corncob powders as template.

Fig. 1. XRD patterns of (a) porous Fe nanocomposite, (b) porous FeNi nanocom-posite, and (c) FeNi alloy prepared without using corncob powders.

X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908–914 909

We examined the complex permittivity (e) and permeability (l),and calculated the EM wave absorption of the porous magneticnanocomposites as a function of thickness. The use of corncobpowders is found to be essential for the EM wave absorption ofporous magnetic nanocomposites which showing enhanced reflec-tion loss (RL) and absorption bands at thin matching thickness.

2. Materials and methods

2.1. Materials

Corncob powders were obtained from Hangzhou, China, andwas washed thoroughly by water and dried at 60 �C before usage.Ferric nitrate (Fe(NO3)3�9H2O, AR), Nickel Nitrate (Ni(NO3)2�6H2O,AR), Citric acid (C6H8O7, AR), and Dimethylformamide (DMF) werepurchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai,China. Polyamic acid (PAA) was obtained from Guangcheng plasticCo., Ltd., Changzhou, China. All reagents are used without furtherpurification.

2.2. Preparation of porous magnetic nanocomposites

In a typical procedure, 20 g ferric nitrate (11.6 g ferric nitrateand 8.4 g nickel nitrate for the preparation of porous FeNi nano-composite) and 20 g citric acid were dissolved in 50 mL deionizedwater under continuous stirring. A certain amount (20 g) corncobpowders were added into the mixture. After stirred vigorously,the product was dried in a temperature-controlled oven at 80 �Cfor 1 h and then heated at 140 �C for 1 h. The obtained sampleswere calcined at 500 �C for 2 h under air and then reduced at450 �C for 30 min under mixed atmosphere of 200 mL min�1 H2

and 500 mL min�1 N2. After cooled to room temperature andcoated by polyimide (PI, prepared from cyclo-dehydration of PAAusing DMF as solvent) [30,31], porous Fe (or porous FeNi) nano-composite was prepared.

In order to investigate the effects of corncob powders on thestructure, morphology, and EM characteristics of magnetic sam-ples, we prepared FeNi alloy without using corncob powders astemplate, while other conditions are kept constant.

2.3. Characterizations

The phase purity and crystal structure of the samples weredetermined by a D/max 2550 X-ray diffractometer (XRD) (Rigaku,Japan) with Cu Ka radiation (k = 0.15406 nm) at a scan rate of0.02 s�1. The operation voltage and current were maintained at36 kV and 34 mA, respectively. The surface morphologies and ele-mental compositions of the samples were studied by an S-4800scanning electron microscope (SEM) and energy disperse spectros-copy (EDS) (Hitachi, Japan) at accelerating voltage of 5.0 kV. Thecomplex permittivity and permeability were determined by aHP8720ES vector network analyzer at EM wave frequency of 2–18 GHz and thickness of 2 mm. The filling rate of the samples is60% with paraffin as substrate.

3. Results and discussion

3.1. XRD patterns

Fig. 1 shows the XRD patterns of the samples. The porous Fenanocomposite is mainly composed by Fe (Iron, JCPDF # 06-0696) and Fe3O4 (magnetite, JCPDF # 72-2303). The relative inten-sity and sharply shape of the corresponding peaks indicate thatboth Fe and Fe3O4 are well crystallized, and the content of Fe ismuch higher than that of Fe3O4. Both porous FeNi and FeNi alloy

that prepared without using corncob exhibit similar diffractionpatterns, which can be indexed to well-crystallized FeNi (Taenite,JCPDF # 47-1417) with characteristic peaks of 111, 200, and 220.EDS results show that all three samples exhibit 18–23 atomic per-cent of carbon, which may be derived from the carbonization of cit-ric acid and corncob powders.

3.2. SEM characterizations

The SEM images of the samples are shown in Fig. 2. When usingcorncob powders as template, the obtained Fe and FeNi nanocom-posites exhibit similar porous structures. The porous nanocompos-ites maintained the initial plate-like structures of corncobpowders. The diameter of Fe and FeNi particles in the nanocompos-ites is about 100 nm, forming pores with diameter of 20–200 nm.When without using corncob powders as template, however, theresulting FeNi particles are melted together, without showingany pores. The diameter of a single FeNi alloy is about 200–500 nm, growing according to the VLS mechanism [32]. It is indi-cated that during the heat treatment, corncob will be decomposedto carbon and releasing numerous gases, which separated the mag-netic particles and construct porous structures. The magneticnanocomposites will benefit from the porous structures, becausethe presence of pores will significantly decrease the bulk densityof samples and enhance their EM wave absorption via scatteringeffect [33].

3.3. Complex permittivity and permeability

Relative complex permittivity (e = e0 � je00) and complex perme-ability (l = l0 � jl00) are essential parameters for the EM waveabsorbers. e0 and l0 are the dielectric constant and real magneticpermeability of absorber, while the imaginary part suggests thedielectric or magnetic loss for EM waves [34]. As shown in Fig. 3,the e0 of porous Fe and porous FeNi decrease slightly with theincreasing of frequency. On the contrast, the e00 of porous Fe andporous FeNi increase with frequency. The e of magnetic particlesis usually predominated by various polarizations, and orientationalpolarization is the most promising mechanism in this studyaccording to the curves [35–37]. The e0 and e00 of porous FeNi arerelative higher than that of porous Fe and exhibit a peak at fre-quency of 13–14 GHz, which may be attributed to the dielectricresonance effect [38]. The dielectric loss tangents (tande = e00/e0)of samples were calculated from the measured e0 and e00. The tande

of porous Fe decreases with frequency at 2–13 GHz and increasessignificantly thereafter. The tande of porous FeNi increases withfrequency and shows a peak at about 14 GHz. According to thetande values, the dielectric loss of porous Fe and porous FeNi

Fig. 2. SEM images of (a and b) porous Fe nanocomposite, (c and d) porous FeNi nanocomposite, and (e and f) FeNi alloy prepared without using corncob as template.

910 X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908–914

increases with frequency and porous Fe present higher dielectricloss for EM waves at 2–13 GHz.

The l0 of porous magnetic nanocomposite decreases slightlywith the increasing of frequency. However, the l00 and magneticloss tangent (tandm = l00/l0) increases with frequency firstly andexhibit maximums at frequency of about 4–12 GHz. The magneticloss types include hysteresis loss, eddy current effect, natural res-onance, and so on [39]. According to the l values, the magnetic lossof porous Fe and FeNi nanocomposites is manly induced by eddycurrent loss and natural resonance effect. Both l00 and tandm valuesof porous FeNi are higher than that of porous Fe, indicating thatporous FeNi exhibit higher magnetic loss for EM waves.

When without using corncob powders as template, however,the as-prepared FeNi alloy shows much higher e, l, and loss tan-gent than that of porous FeNi (Fig. 4), which may be ascribed tothe melting and adhesion of FeNi particles. The conductive net-work formed by melting and adhesion will significantly improvethe conductivity, and therefore increased the e, l, and loss tangent.Both e and l of FeNi alloy decrease with the increasing frequencyand exhibit some peaks. The peaks of tandm appear at frequency of6.2 GHz, 9.2 GHz, and 12.5 GHz, which are the natural resonancepeaks of FeNi alloy. The C value with formula of C = l00(l0)�2 f�1,which was suggested by Wu et al. [40] indicates that the magneticloss of FeNi alloy is predominated by natural resonance effect.

3.4. EM wave absorption properties

According to the transmission line theory, the RL values of sin-gle-layer EM wave absorbers can be evaluated from the measured eand l using the following equations [41]:

Zin ¼ Z0

ffiffiffiffile

rtanh j

2pfdc

ffiffiffiffiffiffilep� �

ð1Þ

RLðdBÞ ¼ 20 logZin � Z0

Zin þ Z0

�������� ð2Þ

where Zin and Z0 are the impedance of absorber and air, respec-tively. e and l are the complex permittivity and permeability of ab-sorber, respectively. f is the frequency of EM wave. d is the thicknessof the absorber. c is the velocity of light. Fig. 5 shows the calculatedthree-dimensional and color-filling patterns of RL values for porousFe and porous FeNi prepared using corncob powders as template.Both porous Fe and porous FeNi absorbers present weak absorption(RL < �5 dB) for EM waves at thickness of less than 1 mm. With theincreasing of thickness, the absorption band with maximum RLmove to lower frequencies. The maximum RL values of porous Feand porous FeNi absorbers are �37.9 dB and �40.8 dB, respectively.Both porous Fe and porous FeNi absorbers show low RL (�2.5 dB to�10 dB) but wide-band absorption for EM waves at high frequency(8–18 GHz) and high thickness (4–10 mm). To obtain the optimumthickness and frequency of porous magnetic absorbers, we analyzedthe variations of bandwidth of RL as a function of thickness. Asshown in Fig. 6a and b, the bandwidth of RL < �10 dB (B10) for bothporous Fe and porous FeNi is 0 at thickness of less than 0.7 mm. TheB10 values increase dramatically with thickness from 0.7 mm to1.3 mm and decrease thereafter. The maximum B10 values for por-ous Fe and porous FeNi absorbers are 3.6 GHz at 1.3 mm and2.8 GHz at 1.0 mm, respectively. The variation of bandwidth ofRL < �5 dB (B5) for porous FeNi is similar to the B10 values. TheB5 value of porous FeNi absorber increases with thickness from

Fig. 3. Frequency dependence of complex permittivity, permeability, and loss tangent for porous Fe (hollow patterned lines) and porous FeNi (solid patterned lines) preparedusing corncob powders as template. The filling rate is 60% using paraffin as substrate.

Fig. 4. Frequency dependence of electromagnetic parameters for FeNi alloy prepared without using corncob powders as template. The filling rate is 60% using paraffin assubstrate.

X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908–914 911

Fig. 5. Three-dimensional and color-filling patterns of RL for (a and b) porous Fe and (c and d) porous FeNi nanocomposites at filling rate of 60% using paraffin as substrate.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Effects of thickness on the bandwidth of (a) RL < �5 dB and (b) RL < �10 dB, (c) maximum RL for porous Fe (hollow patterned lines) and porous FeNi (solid patternedlines), (d) frequency dependence of RL for porous Fe (hollow patterned lines) and porous FeNi (solid patterned lines) at optimum thickness.

912 X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908–914

0 GHz at thickness of 0.1–0.6 mm to 8 GHz at 1.1 mm. It starts todecrease thereafter and maintains at about 1 GHz at thickness of7.7–10 mm. The B5 values of porous Fe absorber, however, exhibitthree peaks with the increasing of thickness. The maximum B5 va-lue of porous Fe absorber is 6.8 GHz at 1.4 mm. All B5 values of por-ous Fe absorber are greater than 3 GHz at thickness of 1.2–10 mm.

Fig. 6c shows the maximum RL of porous Fe and porous FeNiabsorbers as a function of thickness. We can conclude that porousFeNi exhibits higher maximum RL value (�40.8 dB), but porous Feabsorber shows wide range thickness of RL < �20 dB. According tothe higher B5, B10, and maximum RL values, it is suggested thatporous Fe is more suitable for the application in EM wave absorb-

ers. The optimum thickness for porous Fe and porous FeNi absorb-ers are 1.3–1.4 mm and 1.0–1.2 mm, respectively. At the optimumthickness, as shown in Fig. 6d, the optimum EM wave frequencyshould be 10–18 GHz, showing RL values of less than �5 dB.

When without using corncob powders as template, as shown inFig. 7, the resulting FeNi alloy exhibit much lower RL values thanthat of porous FeNi absorber. Although the e, l, and loss tangentof FeNi alloy are much higher than that of porous FeNi, the extre-mely poor impendence matching caused by the huge difference be-tween e and l prevented the EM waves from entering into theabsorber. As a result, the maximum RL for EM waves at 2–18 GHz with thickness of 0.1–10 mm is only about �5 dB.

Fig. 7. (a) Frequency dependence of RL at 2 mm and (b) three-dimensional pattern of RL for FeNi alloy prepared without using corncob powders. The filling rate is 60% withparaffin as substrate.

Table 1Comparisons between magnetic absorbers of this study and other porous EM wave absorbers.

Samples Bulk density(g cm�3)

Thickness(mm)

Bandwidth of RL < �5 dB(GHz)

Bandwidth of RL < �10 dB(GHz)

Maximum RL(dB)

Areal density of absorber(kg m�2)

Porous Fe usingcorncob

0.53 ± 0.05 1.4 6.8 3.0 �21.86 0.952.0 4.6 2.2 �21.01 1.365.3 4.6 0.8 �37.85 3.60

Porous FeNi usingcorncob

0.56 ± 0.06 1.1 8.0 2.6 �12.29 0.772.0 3.6 1.6 �18.82 1.392.5 2.6 1.2 �40.80 1.74

FeNi alloy 4.1 ± 0.2 2.0 – – �5.20 5.64Porous cement/EPS

[26]1.108 20 / 6.2 �15.27 /

Porous carbon fiber[27]

1.28 2.9 / 10.9 �15.53 /

X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908–914 913

Consequently, the use of corncob powders is essential for theEM wave absorption of magnetic absorbers (Table 1). First, porousstructures will be fabricated when using corncob powders as tem-plate, which will significantly decrease the bulk density of thematerials and absorbers. The density is of great importance tothe EM wave absorbers especially in military fields. The bulk den-sities of porous Fe and porous FeNi are only 0.53 ± 0.05 g cm�3 and0.56 ± 0.06 g cm�3, respectively, which are much lower than that ofFeNi alloy (4.1 ± 0.2 g cm�3) prepared without using corncob pow-ders. When applied as EM wave absorbers, the calculated arealdensities of porous Fe and porous FeNi absorbers at optimumthickness are 0.95 kg m�2 and 0.77 kg m�2, respectively, muchlower than the reported values of magnetic EM wave absorbers[42]. Second, the porous magnetic nanocomposites prepared usingcorncob powders exhibit much higher EM wave absorption be-cause of their improved impendence matching and scattering ef-fect of porous materials. The bandwidth of RL < �5 dB, bandwidthof RL < �10 dB and maximum RL of porous Fe and porous FeNiabsorbers are much higher than that of the counterpart. Further-more, we compared our results with other porous EM waveabsorbers. As shown in Table 1, although the bandwidths of porouscarbon fiber and porous cement composite are larger than that ofthis study, their bulk densities and matching thicknesses are muchhigher either. Therefore, this study provides an important methodto fabricate porous magnetic EM wave absorbers with low density,strong absorption, and thin matching thickness.

4. Conclusions

In summary, we have fabricated novel porous magnetic nano-composites using corncob powders as template in this study. Theuse of corncob powders is found to be essential for the propertiesof magnetic nanocomposites. First of all, the presence of corncobsignificantly decreased the bulk density of the samples, whichfacilitated the applications for EM wave absorbers. The second,

the porous structures decreased both e and l values of the samplesand the impendence matching between the absorber and air wasenhanced as a result. The porous magnetic nanocomposites exhibitmuch stronger absorption for EM waves than the FeNi alloy thatprepared without using corncob powders. The matching thicknessof the porous magnetic absorbers is only 1.0–1.4 mm, with arealdensity of only about 0.7–1.0 kg m�2. The absorption bands ofRL < �5 dB covers half X-band and the whole Ku-band. This studynot only fabricated novel porous magnetic nanocomposites withstrong EM wave absorption at low density and thin matchingthickness, but also utilized the agricultural waste – corncob. Theprepared porous magnetic nanocomposites are potential light-weight EM wave absorber with bright future.

Acknowledgement

This research is supported by the ‘‘Fundamental Research Fundsfor the Central Universities’’, People’s Republic of China.

References

[1] Hettinga WG, Junginger HM, Dekker SC, Hoogwijk M, McAloon AJ, Hicks KB.Energy Policy 2009;37(1):190–203.

[2] Donnelly BJ, Helm JL, Lee HA. Cereal Chem 1973;50:548–52.[3] Robinson T, Chandran B, Nigam P. Environ Int 2002;28(1–2):29–33.[4] Wu FC, Tseng RL, Juang RS. Environ Technol 2001;22(2):205–13.[5] Liu K, Lin X, Yue J, Li X, Fang X, Zhu M, et al. Bioresour Technol

2010;101(13):4952–8.[6] Chen Y, Dong B, Qin W, Xiao D. Bioresour Technol 2010;101(18):6994–9.[7] Vaughan T, Seo CW, Marshall WE. Bioresour Technol 2001;78(2):133–9.[8] Ioannidou O, Zabaniotou A. Renew Sust Energy Rev 2007;11(9):1966–2005.[9] Chen XG, Lv SS, Zhang PP, Zhang L, Ye Y. J Therm Anal Calorim

2011;104(3):1055–62.[10] Wessapan T, Srisawatdhisukul S, Rattanadecho P. J Heat Transfer

2011;133(5):51101.[11] Chen XG, Ye Y, Cheng JP. J Inorg Mater 2011;26(5):449–57.[12] Park KY, Lee SE, Kim CG, Han JH. Compos Sci Technol 2006;35(1):49–56.[13] Huang XG, Chen JA, Zhang J, Wang LX, Zhang QT. J Alloys Compd

2010;506(1):347–50.[14] Song J, Wang L, Xu N, Zhang Q. J Rare Earth 2010;28(3):451–5.

914 X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908–914

[15] Costa ACFM, Diniz AP, Silva VJ, Kiminami RHGA, Cornejo DR, Gama AM, et al. JAlloys Compd 2009;483(1–2):563–5.

[16] Shen G, Xu Z, Li Y. J Magn Magn Mater 2006;301(2):325–30.[17] Wu KH, Ting TH, Liu CI, Yang CC, Hsu JS. Compos Sci Technol

2008;68(1):132–9.[18] Youh MJ, Wu HC, Lin WH, Chiu SC, Huang CF, Yu HC, et al. J Nanosci

Nanotechnol 2011;11(3):2315–20.[19] Qing YC, Zhou WC, Jia S, Luo F, Zhu DM. Appl Phys A 2010;100(4):1177–81.[20] Feng YB, Qiu T, Shen CY, Li XY. IEEE Trans Magn 2006;42(3):363–8.[21] Zhao DL, Zhang JM, Li X, Shen ZM. J Alloys Compd 2010;505(2):712–6.[22] Park KY, Han JH, Lee SB, Kim JB, Yi JW, Lee SK. Compos Sci Technol 2009;69(7–

8):1271–8.[23] Gui X, Ye W, Wei J, Wang K, Lv R, Zhu H, et al. J Phys D: Appl Phys

2009;42(7):75002.[24] Tang X, Tian Q, Zhao B, Hu K. Mater Sci Eng A 2007;445–446:135–40.[25] Wang T, He J, Zhou J, Ding X, Zhao J, Wu S, et al. Microporous Mesoporous

Mater 2010;134(1–3):58–64.[26] Guan H, Liu S, Duan Y, Zhao Y. Cem Concr Compos 2007;29(1):49–54.[27] Xie W, Cheng HF, Chu ZY, Chen ZH, Zhou YJ. J Inorg Mater 2009;24(2):320–4.[28] Liu QL, Zhang D, Fan TX. Appl Phys Lett 2008;93(1):13110.[29] Chen YJ, Gao P, Zhu CL, Wang RX, Wang LJ, Cao MS, et al. J Appl Phys

2009;106(5):54303.

[30] Angelo RW, Poliak RM, Susko JR. Polyimide coating process and material. USPatent; 1980.

[31] Chen XG, Lv SS, Zhang PP, Cheng JP, Liu ST, Ye Y. J Magn Magn Mater2012;324(9):1745–51.

[32] Givargizov EI. J Cryst Growth 1975;31:20–30.[33] Zhou J, He J, Li G, Wang T, Sun D, Ding X, et al. J Phys Chem C

2010;114(17):7611–7.[34] Yusoff AN, Abdullah MH, Ahmad SH, Jusoh SF, Mansor AA, Hamid SAA. J Appl

Phys 2002;92(2):876–82.[35] Neelakanta PS. J Phys: Condens Matter 1990;2:4935.[36] Watts PCP, Hsu WK, Barnes A, Chambers B. Adv Mater 2003;15(7–8):600–3.[37] Zhang XF, Dong XL, Huang H, Wang DK, Lv B, Lei JP. Nanotechnology

2007;18:275701.[38] Shi XL, Cao MS, Yuan J, Fang XY. Appl Phys Lett 2009;95(16):163108.[39] Wallace JL. IEEE Trans Magn 1993;29(6):4209–14.[40] Wu M, Zhang YD, Hui S, Xiao TD, Ge S, Hines WA, et al. Appl Phys Lett

2002;80(23):4404–6.[41] Naito Y, Suetake K. IEEE Trans Microw Theory Technol 1971;19(1):65–72.[42] Lv R, Kang F, Gu J, Gui X, Wei J, Wang K, et al. Appl Phys Lett

2008;93(22):223105.